Publication Library



Hybrid Simulation Method for a Structure Subjected to Fire and Its Application to a Steel Frame

Authors: Xuguang Wang; Robin E. Kim, Ph.D.; Oh-Sung Kwon, Ph.D., M.ASCE; and Inhwan Yeo, Ph.D.
DOI: 10.1061/(ASCE)ST.1943-541X.0002113

Abstract: This paper presents a hybrid fire simulation method for civil structures in which a critical element subject to fire is experimentally tested while the remaining structural system is numerically analyzed simultaneously. The proposed method is different from previous approaches in that it is fully validated with full-scale specimen subjected to high temperature and that it is an automated displacement controlled test with deformation error compensation. The two substructures (i.e., an experimental model and a numerical model) are integrated through network to enforce displacement compatibility and force equilibrium. Then, the developed simulation method is applied to a fire simulation of a steel moment resisting frame where one of the columns is assumed to be under temperature load following ISO 834-11:2014 fire curve. The results show that the proposed hybrid simulation method can replicate the numerical prediction, and thus can be applied to more challenging structural systems such as the structural behavior under fire load, which is computationally difficult using numerical models.



Multi-Objective Optimal Design of a Building Envelope and Structural System Using Cyber-Physical Modeling in a Wind Tunnel

Authors: Michael L. Whiteman, Pedro L. Fernández-Cabán, Brian M. Phillips, Forrest J. Masters, Jennifer A. Bridge, and Justin R. Davis

Abstract: This paper explores the use of a cyber-physical systems (CPS) “loop-in-the-model” approach to optimally design the envelope and structural system of low-rise buildings subject to wind loads. Both the components and cladding (C&C) and the main wind force resisting system (MWFRS) are considered through multi-objective optimization. The CPS approach combines the physical accuracy of wind tunnel testing and efficiency of numerical optimization algorithms to obtain an optimal design. The approach is autonomous: experiments are executed in a boundary layer wind tunnel (BLWT), sensor feedback is monitored and analyzed by a computer, and optimization algorithms dictate physical changes to the structural model in the BLWT through actuators. To explore a CPS approach to multi-objective optimization, a low-rise building with a parapet wall of variable height is considered. In the BLWT, servo-motors are used to adjust the parapet to a particular height. Parapet walls alter the location of the roof corner vortices, reducing suction loads on the windward facing roof corners and edges, a C&C design load. At the same time, parapet walls increase the surface area of the building, leading to an increase in demand on the MWFRS. A combination of non-stochastic and stochastic optimization algorithms were implemented to minimize the magnitude of suction and positive pressures on the roof of a low-rise building model, followed by stochastic multi objective optimization to simultaneously minimize the magnitude of suction pressures and base shear. Experiments were conducted at the University of Florida Experimental Facility (UFEF) of the National Science Foundation’s (NSF) Natural Hazard Engineering Research Infrastructure (NHERI) program.



Improvement of Real-Time Hybrid Simulation Using Parallel Finite-Element Program

Authors: Li-Qiao Lu, Jin-Ting Wang, and Fei Zhu

Abstract: This paper proposes a novel framework to efficiently calculate a large-scale finite element (FE) numerical substructure in real-time hybrid simulation (RTHS). It is composed of a non-real-time Windows computer and a real-time Target Computer. The Windows computer is used to solve the FE numerical substructure by parallel computing in soft real-time, while the real-time Target Computer generates displacement signals for the controller in real time. Based on the proposed framework, a RTHS with numerical substructure simulated in Windows environment is developed. It is demonstrated that the computational efficiency of the RTHS could be greatly improved by parallel programming.



Computational Challenges in Real-Time Hybrid Simulation of Tall Buildings under Multiple Natural Hazards

Authors: C. Kolay, J.M. Ricles, T.M. Marullo, S. Al-Subaihawi, S.E. Quiel

Abstract: The essence of real-time hybrid simulation (RTHS) is its ability to combine the benefits of physical testing with those of computational simulations. Therefore, an understanding of the real-time computational issues and challenges is important, especially for RTHS of large systems, in advancing the state of the art. To this end, RTHS of a 40-story (plus 4 basement stories) tall building having nonlinear energy dissipation devices for mitigation of multiple natural hazards, including earthquake and wind events, were conducted at the NHERI Lehigh Experimental Facility. An efficient implementation procedure of the recently proposed explicit modified KR-? (MKR-?) method was developed for performing the RTHS. This paper discusses this implementation procedure and the real-time computational issues and challenges with regard to this implementation procedure. Some results from the RTHS involving earthquake loading are presented to highlight the need for and application of RTHS in performance based design of tall buildings under earthquake hazard.



Evaluation of integration methods for hybrid simulation of complex structural systems through collapse

Authors: Maikol Del Carpio R., M. Javad Hashemi, and Gilberto Mosqueda

Abstract: This study examines the performance of integration methods for hybrid simulation of large and complex structural systems in the context of structural collapse due to seismic excitations. The target application is not necessarily for real-time testing, but rather for models that involve large-scale physical sub-structures and highly nonlinear numerical models. Four case studies are presented and discussed. In the first case study, the accuracy of integration schemes including two widely used methods, namely, modified version of the implicit Newmark with fixed-number of iteration (iterative) and the operator-splitting (non-iterative) is examined through pure numerical simulations. The second case study presents the results of 10 hybrid simulations repeated with the two aforementioned integration methods considering various time steps and fixed-number of iterations for the iterative integration method. The physical sub-structure in these tests consists of a single-degree-of-freedom (SDOF) cantilever column with replaceable steel coupons that provides repeatable highly nonlinear behavior including fracture-type strength and stiffness degradations. In case study three, the implicit Newmark with fixed-number of iterations is applied for hybrid simulations of a 1:2 scale steel moment frame that includes a relatively complex nonlinear numerical substructure. Lastly, a more complex numerical substructure is considered by constructing a nonlinear computational model of a moment frame coupled to a hybrid model of a 1:2 scale steel gravity frame. The last two case studies are conducted on the same porotype structure and the selection of time steps and fixed number of iterations are closely examined in pre-test simulations. The generated unbalance forces is used as an index to track the equilibrium error and predict the accuracy and stability of the simulations



Servo-hydraulic actuator in controllable canonical form: Identification and experimental validation

Authors: Amin Maghareh, Christian E. Silva, Shirley J. Dyke

Abstract: Hydraulic actuators have been widely used to experimentally examine structural behavior at multiple scales. Real-time hybrid simulation (RTHS) is one innovative testing method that largely relies on such servo-hydraulic actuators. In RTHS, interface conditions must be enforced in real time, and controllers are often used to achieve tracking of the desired displacements. Thus, neglecting the dynamics of hydraulic transfer system may result either in system instability or sub-optimal performance. Herein, we propose a nonlinear dynamical model for a servo-hydraulic actuator (a.k.a. hydraulic transfer system) coupled with a nonlinear physical specimen. The nonlinear dynamical model is transformed into controllable canonical form for further tracking control design purposes. Through a number of experiments, the controllable canonical model is validated.



Model-based framework for multi-axial real-time hybrid simulation testing

Authors: Gaston A. Fermandois and Billie F. Spencer, Jr.

Abstract: Real-time hybrid simulation is an efficient and cost-effective dynamic testing technique for performance evaluation of structural systems subjected to earthquake loading with rate-dependent behavior. A loading assembly with multiple actuators is required to impose realistic boundary conditions on physical specimens. However, such a testing system is expected to exhibit significant dynamic coupling of the actuators and suffer from time lags that are associated with the dynamics of the servo-hydraulic system, as well as control-structure interaction (CSI). One approach to reducing experimental errors considers a multi-input, multi-output (MIMO) controller design, yielding accurate reference tracking and noise rejection. In this paper, a framework for multi-axial real-time hybrid simulation (maRTHS) testing is presented. The methodology employs a real-time feedback-feedforward controller for multiple actuators commanded in Cartesian coordinates. Kinematic transformations between actuator space and Cartesian space are derived for all six-degrees-of freedom of the moving platform. Then, a frequency domain identification technique is used to develop an accurate MIMO transfer function of the system. Further, a Cartesian-domain model-based feedforward-feedback controller is implemented for time lag compensation and to increase the robustness of the reference tracking for given model uncertainty. The framework is implemented using the 1/5th-scale Load and Boundary Condition Box (LBCB) located at the University of Illinois at Urbana-Champaign. To demonstrate the efficacy of the proposed methodology, a single-story frame subjected to earthquake loading is tested. One of the columns in the frame is represented physically in the laboratory as a cantilevered steel column. For real time execution, the numerical substructure, kinematic transformations, and controllers are implemented on a digital signal processor. Results show excellent performance of the maRTHS framework when six-degrees-of-freedom are controlled at the interface between substructures



Real time hybrid simulation with online model updating: An analysis of accuracy

Authors: Ge Ou, Shirley J. Dyke, and Arun Prakash

Abstract: In conventional hybrid simulation (HS) and real time hybrid simulation (RTHS) applications, the information exchanged between the experimental substructure and numerical substructure is typically restricted to the interface boundary conditions (force, displacement, acceleration, etc.). With additional demands being placed on RTHS and recent advances in recursive system identification techniques, an opportunity arises to improve the fidelity by extracting information from the experimental substructure. Online model updating algorithms enable the numerical model of components (herein named the target model), that are similar to the physical specimen to be modified accordingly. This manuscript demonstrates the power of integrating a model updating algorithm into RTHS (RTHSMU) and explores the possible challenges of this approach through a practical simulation. Two Bouc–Wen models with varying levels of complexity are used as target models to validate the concept and evaluate the performance of this approach. The constrained unscented Kalman filter (CUKF) is selected for using in the model updating algorithm. The accuracy of RTHSMU is evaluated through an estimation output error indicator, a model updating output error indicator, and a system identification error indicator. The results illustrate that, under applicable constraints, by integrating model updating into RTHS, the global response accuracy can be improved when the target model is unknown. A discussion on model updating parameter sensitivity to updating accuracy is also presented to provide guidance for potential users



Shake table real?time hybrid simulation techniques for the performance evaluation of buildings with inter?story isolation

Authors: Ruiyang Zhang, Brian M. Phillips, Shun Taniguchi, Masahiro Ikenaga, Kohju Ikago

Abstract: Interstory isolation systems have recently gained popularity as an alternative for seismic protection, especially in densely populated areas. In inter?story isolation, the isolation system is incorporated between stories instead of the base of the structure. Installing inter?story isolation is simple, inexpensive, and disruption free in retrofit applications. Benefits include nominally independent structural systems where the accelerations of the added floors are reduced when compared to a conventional structural system. Furthermore, the base shear demand on the total structure is not significantly increased. Practical applications of inter?story isolation have appeared in the United States, Japan, and China, and likewise new design validation techniques are needed to parallel growing interest. Real?time hybrid simulation (RTHS) offers an alternative to investigate the performance of buildings with inter?story isolation. Shake tables, standard equipment in many laboratories, are capable of providing the interface boundary conditions necessary for this application of RTHS. The substructure below the isolation layer can be simulated numerically while the superstructure above the isolation layer can be tested experimentally. This configuration provides a high?fidelity representation of the nonlinearities in the isolation layer, including any supplemental damping devices. This research investigates the seismic performance of a 14?story building with inter?story isolation. A model?based acceleration?tracking approach is adopted to control the shake table, exhibiting good offline and online acceleration tracking performance. The proposed methods demonstrate that RTHS is an accurate and reliable means to investigate buildings with interstory isolation, including new configurations and supplemental control approaches.



Development and Verification of Distributed Real-Time Hybrid Simulation Methods

Authors: Xin Li; Ali I. Ozdagli; Shirley J. Dyke, A.M.ASCE; Xilin Lu; and Richard Christenson, M.ASCE

Abstract: Hybrid simulation combines numerical simulation and physical testing, and is thus considered to be an efficient alternative to traditional testing methodologies in the evaluation of global performance of large or complex structures. Real-time hybrid simulation (RTHS) is performed when it is important to fully capture rate-dependent behaviors in the physical substructure. Although the demand to test more complex systems grows, not every laboratory has the right combination of computational and equipment resources available to perform largescale experiments. Distributed real-time hybrid simulation (dRTHS) facilitates testing that is to be conducted at multiple geographically distributed laboratories while utilizing the Internet to couple the substructures. One major challenge in dRTHS is to accommodate the unpredictable communication time delays between the various distributed sites that occur as a result of Internet congestion. Herein, a dRTHS framework is proposed where a modified Smith predictor is adopted to accommodate such communication delays. To examine and demonstrate the sensitivity of the proposed framework to communication delays and to modeling errors, parametric analytical case studies are presented. Additionally, the effectiveness of this dRTHS framework is verified through successful execution of multisite experiments. The results demonstrate that this framework provides a new option for researchers to evaluate the global response of structural systems in a distributed real-time environment.



Analysis of actuator delay and its effect on uncertainty quantification for real-time hybrid simulation

Authors: Cheng Chen, Weijie Xu, Tong Guo and Kai Chen

Abstract: Uncertainties in structure properties can result in different responses in hybrid simulations. Quantification of the effect of these uncertainties would enable researchers to estimate the variances of structural responses observed from experiments. This poses challenges for real-time hybrid simulation (RTHS) due to the existence of actuator delay. Polynomial chaos expansion (PCE) projects the model outputs on a basis of orthogonal stochastic polynomials to account for influences of model uncertainties. In this paper, PCE is utilized to evaluate effect of actuator delay on the maximum displacement from real-time hybrid simulation of a single degree of freedom (SDOF) structure when accounting for uncertainties in structural properties. The PCE is first applied for RTHS without delay to determine the order of PCE, the number of sample points as well as the method for coefficients calculation. The PCE is then applied to RTHS with actuator delay. The mean, variance and Sobol indices are compared and discussed to evaluate the effects of actuator delay on uncertainty quantification for RTHS. Results show that the mean and the variance of the maximum displacement increase linearly and exponentially with respect to actuator delay, respectively. Sensitivity analysis through Sobol indices also indicates the influence of the single random variable decreases while the coupling effect increases with the increase of actuator delay.



Force-Based Frame Element Implementation for Real-Time Hybrid Simulation Using Explicit Direct Integration Algorithms

Authors: Chinmoy Kolay, A.M.ASCE ; and James M. Ricles

Abstract: Existing state determination procedures for force-based finite elements use either an iterative scheme at the element level or a noniterative scheme at the element level that relies on an iterative solution algorithm for the global equilibrium equations. The former cannot ensure convergence in real-time computations, whereas the latter requires an implicit direct integration algorithm; therefore, these procedures are not applicable to real-time hybrid simulation (RTHS) utilizing an explicit direct integration algorithm. A new procedure is developed based on a fixed number of iterations and an unconditionally stable explicit model-based integration algorithm. If the maximum number of iterations is reached, element resisting forces are corrected to re-establish compatibility, and unbalanced section forces are carried over to and corrected in the next time step. This procedure is used in the numerical simulation and RTHS of an earthquake-excited two-story reinforced concrete building. Results show that an accurate solution can be obtained even without performing any iteration. The influence of the model-based parameters of the integration algorithm on the stability and accuracy of the RTHS is also studied



Mixed Force and Displacement Control for Testing Base-Isolated Bearings in Real-Time Hybrid Simulation

Authors: Narutoshi Nakata , Richard Erb, and Matthew Stehman

Abstract: This paper presents a robust mixed force and displacement control strategy for testing of base isolation bearings in real-time hybrid simulation. The mixed-mode control is a critical experimental technique to impose accurate loading conditions on the base isolation bearings. The proposed mixed-mode control strategy consists of loop-shaping and proportional-integral-differential controllers. Following experimental validation, the mixed-mode control was demonstrated through a series of real-time hybrid simulation. The experimental results showed that the developed mixed-mode control enables accurate control of dynamic vertical force on the base isolation bearings during real-time hybrid simulation.



Analysis of hybrid viscous damper by real time hybrid simulations

Authors: Mark Laier Brodersen, Ge Ou, Jan Høgsberg, Shirley Dyke

Abstract: Results from real time hybrid simulations are compared to full numerical simulations for a hybrid viscous damper, composed of a viscous dashpot in series with an active actuator and a load cell. By controlling the actuator displacement via filtered integral force feedback the damping performance of the hybrid viscous damper is improved, while for pure integral force feedback the damper stroke is instead increased. In the real time hybrid simulations viscous damping is emulated by a bang-bang controlled Magneto-Rheological (MR) damper. The controller activates high-frequency modes and generates drift in the actuator displacement, and only a fraction of the measured damper force can therefore be used as input to the investigated integral force feedback in the real time hybrid simulations.



Real-time hybrid testing with equivalent force control method incorporating Kalman filter

Authors: Pengfei Shi1, Bin Wu, Billie F. Spencer Jr., Brian M. Phillips and Chia-Ming Chang

Abstract: The equivalent force control (EFC) method has been developed for real-time hybrid testing to replace the numerical iteration of implicit integration with a force-feedback control loop. With this control loop, the EFC method can also compensate for the time delay in real-time hybrid testing. However, the delay compensation effect of the EFC can be influenced by factors such as noises in the measured displacement. This paper discusses the influence of the measurement noises on real-time hybrid testing with the EFC. The Kalman filter is proposed to filter the noises in the measured actuator displacement for improved performance. A higher proportional gain in the PID controller, which improves the effect of time delay compensation of the EFC method, can be allowed without losing stability when incorporating the Kalman filter. A series of real-time hybrid tests were conducted, and the results validated that the EFC method with Kalman filter can effectively compensate for the time delay.



Comparison of explicit integration algorithms for real-time hybrid simulation

Authors: Fei Zhu1, Jin-Ting Wang, Feng Jin1, Yao Gui

Abstract: Real-time hybrid simulation (RTHS) combines physical experimentation with numerical simulation to evaluate dynamic responses of structures. The inherent characteristics of integration algorithms change when simulating numerical substructures owing to the response delay of loading systems in physical substructures. This study comprehensively investigates the effects of integration algorithms on the delay-dependent stability and accuracy of multiple degrees-of-freedom RTHS systems. Seven explicit integration algorithms are considered; and the discrete-time root locus technique is adopted. It is found that the stability of RTHS system is mainly determined by the time delay rather than the integration algorithms, whereas its accuracy mainly depends on the accuracy characteristic of the applied integration algorithm itself. An unconditionally stable integration algorithm cannot always guarantee good stability performance; and the inherent accuracy or numerical energy dissipation of integration algorithms should be taken into account in RTHSs. These theoretical findings are well verified by RTHSs.



Development of a ten?element hybrid simulation platform and an adjustable yielding brace for performance evaluation of multi?story braced frames subjected to earthquakes

Authors: Saeid Mojiri, Oh?Sung Kwon, Constantin Christopoulos

Abstract: This paper presents a ten?element hybrid (experimental?numerical) simulation platform, referred to as UT10, which was developed for running hybrid simulations of braced frames with up to ten large?capacity physical brace specimens. This paper presents the details of the development of different components of UT10 and an adjustable yielding brace (AYB) specimen, which was designed to perform hybrid simulations with UT10. As the first application of UT10, a five?story buckling?restrained braced frame and a special concentrically braced frame (BRBF and SCBF) were designed and tested with AYB specimens and buckling specimens representing the braces. Cyclic tests of the AYB, one? and three?element hybrid simulations of the BRBF, and four?element hybrid simulations of the SCBF inside the UT10 confirmed the functionality of UT10 for running hybrid simulations on multiple specimens. The tests also indicated that AYB was capable of producing a stable hysteretic response with characteristics similar to BRBs. Comparison of the results of the hybrid simulations of the BRBF and SCBF with their fully numerical models showed that the modeling inaccuracies of the yielding braces could potentially affect the global response of the multi?story braced frames further emphasizing the need for experimental calibration or hybrid simulation for achieving more accurate response predictions. UT10 provides a simple and reconfigurable platform that can be used to achieve a realistic understanding of the seismic response of multi?story frames with yielding braces, distinguish their modeling limitations, and improve different modeling techniques available for their seismic response prediction.



A framework for multi-platform simulation of reinforced concrete structures

Authors: Vahid Sadeghian, Oh-Sung Kwon, Frank Vecchio

Abstract: This study presents a framework for multi-platform analysis and hybrid simulation of reinforced concrete (RC) structures. In this approach, each subpart of the structure, based on its mechanical characteristics, is modelled using the most suitable finite element analysis tool or represented with a test specimen. The proposed framework combines all the substructure modules and takes into account the interactions between them by satisfying compatibility and equilibrium requirements. The main contribution of the study lies in demonstrating the effectiveness of multi-platform modelling in accurate and practical analysis of complex RC structures or multi-disciplinary RC systems with a particular focus on shear behaviour. Three application examples including a wide-flange shear wall, a three-storey frame with critical joints, and a soil-structure interaction simulation are discussed in detail. It is concluded that the multi-platform analysis can compute the behaviour of such structures with a level of accuracy that was previously difficult to achieve with most single-platform analysis software.



Small-scale multi-axial hybrid simulation of a shear-critical reinforced concrete frame

Authors: Vahid Sadeghian, Oh-Sung Kwon, Frank Vecchio

Abstract: This study presents a numerical multi-scale simulation framework which is extended to accommodate hybrid simulation (numerical-experimental integration). The framework is enhanced with a standardized data exchange format and connected to a generalized controller interface program which facilitates communication with various types of laboratory equipment and testing configurations. A small-scale experimental program was conducted using a six degree-of-freedom hydraulic testing equipment to verify the proposed framework and provide additional data for small-scale testing of shearcritical reinforced concrete structures. The specimens were tested in a multi-axial hybrid simulation manner under a reversed cyclic loading condition simulating earthquake forces. The physical models were 1/3.23-scale representations of a beam and two columns. A mixed-type modelling technique was employed to analyze the remainder of the structures. The hybrid simulation results were compared against those obtained from a large-scale test and finite element analyses. The study found that if precautions are taken in preparing model materials and if the shear-related mechanisms are accurately considered in the numerical model, small-scale hybrid simulations can adequately simulate the behaviour of shear-critical structures. Although the findings of the study are promising, to draw general conclusions additional test data are required.



Application of hybrid?simulation to fragility assessment of the telescoping self?centering energy dissipative bracing system

Authors: Viswanath Kammula, Jeffrey Erochko, Oh?Sung Kwon, Constantin Christopoulos

Abstract: Substructure hybrid simulation has been the subject of numerous investigations in recent years. The simulation method allows for the assessment of the seismic performance of structures by representing critical components with physical specimens and the rest of the structure with numerical models. In this study the system level performance of a six?storey structure with telescoping self?centering energy dissipative (T?SCED) braces is validated through pseudo?dynamic (PsD) hybrid simulation. Fragility curves are derived for the T?SCED system. This paper presents the configuration of the hybrid simulation, the newly developed control software for PsD hybrid simulation, which can integrate generic hydraulic actuators into PsD hybrid simulation, and the seismic performance of a structure equipped with T?SCED braces. The experimental results show that the six?storey structure with T?SCED braces satisfies performance limits specified in ASCE 41.



Model updating method for substructure pseudo?dynamic hybrid simulation

Authors: Oh?Sung Kwon, Viswanath Kammula

Abstract: Substructure hybrid simulation has been actively investigated and applied to evaluate the seismic performance of structural systems in recent years. The method allows simulation of structures by representing critical components with physically tested specimens and the rest of the structure with numerical models. However, the number of physical specimens is limited by available experimental equipment. Hence, the benefit of the hybrid simulation diminishes when only a few components in a large system can be realistically represented. The objective of the paper is to overcome the limitation through a novel model updating method. The model updating is carried out by applying calibrated weighting factors at each time step to the alternative numerical models, which encompasses the possible variation in the experimental specimen properties. The concept is proposed and implemented in the hybrid simulation framework, UI?SimCor. Numerical verification is carried out using two?DOF systems. The method is also applied to an experimental testing, which proves the concept of the proposed model updating method.



Probabilistic robust design of control systems for high-fidelity cyber–physical testing

Authors: Thomas Sauder, Stefano Marelli, Asgeir J. Sørensen

Abstract: Cyber–physicalempirical methods consist in partitioning a dynamical system under study into a set of physical and numerical substructures that interact in real-time through a control system. In this paper, we define and investigate the fidelity of such methods, that is their capacity to generate systems whose outputs remain close to those of the original system under study. In practice, fidelity is jeopardized by uncertain and heterogeneous artefacts originating from the control system, such as actuator dynamics, time delays and measurement noise. We present a computationally efficient method, based on surrogate modelling and active learning techniques, to (1) verify that a cyber–physical empirical setup achieves probabilistic robust fidelity, and (2) to derive fidelity bounds, which translate to absolute requirements to the control system. For verification purposes, the method is first applied to the study of a simple mechanical system. Its efficiency is then demonstrated on a more complex problem, namely the active truncation of slender marine structures, in which the substructures’ dynamics cannot be described by an analytic solution.



Active truncation of slender marine structures: Influence of the control system on fidelity

Authors: Thomas Sauder, Stefano Marelli, Kjell Larsen, Asgeir J. Sørensen
DOI: doi here

Abstract: Performing hydrodynamic model testing of ultra-deep water floating systems at a reasonable scale is challenging, due to the limited space available in existing laboratories and to the large spatial extent of the slender marine structures that connect the floater to the seabed. In this paper, we consider a method based on real-time hybrid model testing, namely the active truncation of the slender marine structures: while their upper part is modelled physically in an ocean basin, their lower part is simulated by an adequate numerical model. The control system connecting the two substructures inevitably introduces artefacts, such as noise, biases and time delays, whose probabilistic description is assumed to be known. We investigate specifically how these artefacts influence the fidelity of the active truncation setup, that is its capability to reproduce correctly the dynamic behaviour of the system under study. We propose a systematic numerical method to rank the artefacts according to their influence on the fidelity of the test. The method is demonstrated on the active truncation of a taut polyester mooring line.



A Novel Hybrid Testing Approach for Piping Systems of Industrial Plants

Authors: Oreste S. Bursi, Giuseppe Abbiati, Md S. Reza

Abstract: The need for assessing dynamic response of typical industrial piping systems subjected to seismic loading motivated the authors to apply model reduction techniques to experimental dynamic substructuring. Initially, a better insight into the dynamic response of the emulated system was provided by means of the principal component analysis. The clear understanding of reduction basis requirements paved the way for the implementation of a number of model reduction techniques aimed at extending the applicability range of the hybrid testing technique beyond its traditional scope. Therefore, several hybrid simulations were performed on a typical full-scale industrial piping system endowed with a number of critical components, like elbows, Tee joints and bolted flange joints, ranging from operational to collapse limit states. Then, the favourable performance of the L-Stable Real-Time compatible time integrator and an effective delay compensation method were also checked throughout the testing campaign. Finally, several aspects of the piping performance were commented and conclusions drawn.



Hybrid simulation of a multi?span RC viaduct with plain bars and sliding bearings

Authors: Giuseppe Abbiati, Oreste S. Bursi, Philippe Caperan, Luigi Di Sarno, Francisco Javier Molina, Fabrizio Paolacci, Pierre Pegon

Abstract: This paper deals with the seismic response assessment of an old reinforced concrete viaduct and the effectiveness of friction?based retrofitting systems. Emphasis was laid on an old bridge, not properly designed to resist seismic action, consisting of 12 portal piers that support a 13?span bay deck for each independent roadway. On the basis of an OpenSEES finite element frame pier model, calibrated in a previous experimental campaign with cyclic displacement on three 1:4 scale frame piers, a more complex experimental activity using hybrid simulation has been devised. The aim of the simulation was twofold: (i) to increase knowledge of non?linear behavior of reinforced concrete frame piers with plain steel rebars and detailing dating from the late 1950s; and (ii) to study the effectiveness of sliding bearings for seismic response mitigation. Hence, to explore the performance of the as built bridge layout and also of the viaduct retrofitted with friction?based devices, at both serviceability and ultimate limit state conditions, hybrid simulation tests were carried out. In particular, two frame piers were experimentally controlled with eight?actuator channels in the as built case while two frame piers and eight sliding bearings were controlled with 18?actuator channels in the isolated case. The remaining frame piers were part of numerical substructures and were updated offline to accurately track damage evolution.



Nonlinear heterogeneous dynamic substructuring and partitioned FETI time integration for the development of low?discrepancy simulation models

Authors: Oreste S. Bursi, Giuseppe Abbiati, Enrico Cazzador, Pierre Pegon, Francisco J. Molina

Abstract: This article presents a novel approach to model validation and to the calibration of complex structural systems, through the adoption of heterogeneous (numerical/physical) simulation based on dynamic substructuring (HDS). HDS isolates the physical sub?system (PS) that contains the key region of nonlinear behavior of interest and is tested experimentally, separate from the remainder of the system, that is, the numerical sub?system (NS), which is numerically simulated. A parallel partitioned time integrator based on the finite element tearing and interconnecting method plays a central role in solving the coupled system response, enabling a rigorous and stable synchronization between sub?systems and a realistic interaction between PS and numerical sub?system response. This feature enhances the quality of benchmarks for validation and calibration of low?discrepancy models through virtual structural testing. As a proof of concept, we select an old reinforced concrete viaduct, subjected to seismic loading. Several HDS were conducted at the European Laboratory for Structural Assessment in Ispra (Italy) considering two physical piers and related concave sliding bearings as PSs of the heterogeneous system. As a result, the benefit of employing HDS to set benchmarks for model validation and calibration is highlighted, by developing low?discrepancy FE models of critical viaduct components.



A composite experimental dynamic substructuring method based on partitioned algorithms and localized Lagrange multipliers

Authors: Giuseppe Abbiati, Vincenzo La Salandra, Oreste S. Bursi, Luca Caracoglia

Abstract: Successful online hybrid (numerical/physical) dynamic substructuring simulations have shown their potential in enabling realistic dynamic analysis of almost any type of non-linear structural system (e.g., an as-built/isolated viaduct, a petrochemical piping system subjected to non-stationary seismic loading, etc.). Moreover, owing to faster and more accurate testing equipment, a number of different offline experimental substructuring methods, operating both in time (e.g. the impulse-based substructuring) and frequency domains (i.e. the Lagrange multiplier frequency-based substructuring), have been employed in mechanical engineering to examine dynamic substructure coupling. Numerous studies have dealt with the above-mentioned methods and with consequent uncertainty propagation issues, either associated with experimental errors or modelling assumptions. Nonetheless, a limited number of publications have systematically cross-examined the performance of the various Experimental Dynamic Substructuring (EDS) methods and the possibility of their exploitation in a complementary way to expedite a hybrid experiment/numerical simulation. From this perspective, this paper performs a comparative uncertainty propagation analysis of three EDS algorithms for coupling physical and numerical subdomains with a dual assembly approach based on localized Lagrange multipliers. The main results and comparisons are based on a series of Monte Carlo simulations carried out on a five-DoF linear/non-linear chain-like systems that include typical aleatoric uncertainties emerging from measurement errors and excitation loads. In addition, we propose a new Composite-EDS (C-EDS) method to fuse both online and offline algorithms into a unique simulator. Capitalizing from the results of a more complex case study composed of a coupled isolated tank-piping system, we provide a feasible way to employ the C-EDS method when nonlinearities and multi-point constraints are present in the emulated system.



Comparison between real?time dynamic substructuring and shake table testing techniques for nonlinear seismic applications

Authors: Charles-Philippe Lamarche, Robert Tremblay, Pierre Léger, Martin Leclerc, Oreste S. Bursi

Abstract: Results from real?time dynamic substructuring (RTDS) tests are compared with results from shake table tests performed on a two?storey steel building structure model. At each storey, the structural system consists of a cantilevered steel column resisting lateral loads in bending. In two tests, a slender diagonal tension?only steel bracing member was added at the first floor to obtain an unsymmetrical system with highly variable stiffness. Only the first?storey structural components were included in the RTDS test program and a Rosenbrock?W linearly implicit integration scheme was adopted for the numerical solution. The tests were performed under seismic ground motions exhibiting various amplitude levels and frequency contents to develop first and second mode?dominated responses as well as elastic and inelastic responses. A chirp signal was also used. Coherent results were obtained between the shake table and the RTDS testing techniques, indicating that RTDS testing methods can be used to successfully reproduce both the linear and nonlinear seismic responses of ductile structural steel seismic force resisting systems. The time delay introduced by actuator?control systems was also studied and a novel adaptive compensation scheme is proposed.



Rosenbrock?based algorithms and subcycling strategies for real?time nonlinear substructure testing

Authors: Oreste S. Bursi, Chuanguo Jia, Leonardo Vulca, Simon A. Neild, David J. Wagg

Abstract: In this paper, Rosenbrock?based algorithms originally developed for real?time testing of linear systems with dynamic substructuring are extended for use on nonlinear systems. With this objective in mind and for minimal overhead, both two? and three?stages linearly implicit real?time compatible algorithms were endowed with the Jacobian matrices requiring only one evaluation at the beginning of each time step. Moreover, these algorithms were improved with subcycling strategies. In detail, the paper briefly introduces Rosenbrock?based L?Stable Real?Time (LSRT) algorithms together with linearly implicit and explicit structural integrators, which are now commonly used to perform real?time tests. Then, the LSRT algorithms are analysed in terms of linearized stability with reference to an emulated spring pendulum, which was chosen as a nonlinear test problem, because it is able to exhibit a large and relatively slow nonlinear circular motion coupled to an axial motion that can be set to be stiff. The accuracy analysis on this system was performed for all the algorithms described. Following this, a coupled spring?pendulum example typical of real?time testing is analysed with respect to both stability and accuracy issues. Finally, the results of representative numerical simulations and real?time substructure tests, considering nonlinearities both in the numerical and the physical substructure, are explored. These tests were used to demonstrate how the LSRT algorithms can be used for substructuring tests with strongly nonlinear components.



Monolithic and partitioned time integration methods for real-time heterogeneous simulations

Authors: Oreste S. Bursi, Zhen Wang, Chuanguo Jia, Bin Wu

Abstract: Real-time (RT) heterogeneous simulations define a class of hybrid numerical–experimental techniques based on dynamic substructuring and capable of simulating the non-linear response of an emulated mechanical system. With this objective in mind, we present two direct coupling algorithms endowed with subcycling, capable of ensuring the continuity of acceleration between non-overlapping subdomains. In greater detail, firstly we introduce monolithic Rosenbrock L-stable algorithms and, in view of the analysis of complex emulated systems, we recall a recent direct parallel algorithm. Secondly, we propose an improved parallel version of the progenitor algorithm together with its solution procedure. Consequently, in order to reduce drift, we introduce a mass-orthogonal velocity projection characterized by a non-negative energy dissipation. Moreover, both a convergence analysis on a SDoF test problem and simulations on single- and four-DoF systems are presented. Lastly, a novel test rig devised to perform nonlinear substructured RT tests is introduced and a few test results are presented.



Actuator dynamics compensation based on upper bound delay for real?time hybrid simulation

Authors: Bin Wu, Zhen Wang, Oreste S. Bursi

Abstract: Real?time hybrid simulation represents a powerful technique capable of evaluating the structural dynamic performance by combining the physical simulation of a complex and rate?dependent portion of a structure with the numerical simulation of the remaining portion of the same structure. Initially, this paper shows how the stability of real?time hybrid simulation with time delay depends both on compensation techniques and on time integration methods. In particular, even when time delay is exactly known, some combinations of numerical integration and displacement prediction schemes may reduce the response stability with conventional compensation methods and lead to unconditional instability in the worst cases. Therefore, to deal with the inaccuracy of prediction and the uncertainty of delay estimation, a nearly exact compensation scheme is proposed, in which the displacement is compensated by means of an upper bound delay and the desired displacement is picked out by an optimal process. Finally, the advantages of the proposed scheme over conventional delay compensation techniques are shown through numerical simulation and actual tests.



An effective online delay estimation method based on a simplified physical system model for real-time hybrid simulation

Authors: Zhen Wang, Bin Wu, Oreste S. Bursi, Guoshan Xu, Yong Ding

Abstract: Real-Time Hybrid Simulation (RTHS) is a novel approach conceived to evaluate dynamic responses of structures with parts of a structure physically tested and the remainder parts numerically modelled. In RTHS, delay estimation is often a precondition of compensation; nonetheless, system delay may vary during testing. Consequently, it is sometimes necessary to measure delay online. Along these lines, this paper proposes an online delay estimation method using least-squares algorithm based on a simplified physical system model, i.e., a pure delay multiplied by a gain reflecting amplitude errors of physical system control. Advantages and disadvantages of different delay estimation methods based on this simplified model are firstly discussed. Subsequently, it introduces the least-squares algorithm in order to render the estimator based on Taylor series more practical yet effective. As a result, relevant parameter choice results to be quite easy. Finally in order to verify performance of the proposed method, numerical simulations and RTHS with a buckling-restrained brace specimen are carried out. Relevant results show that the proposed technique is endowed with good convergence speed and accuracy, even when measurement noises and amplitude errors of actuator control are present.



Performance-based earthquake evaluation of a full-scale petrochemical piping system

Authors: Oreste S. Bursi, Md S. Reza, Giuseppe Abbiati, Fabrizio Paolacci

Abstract: Assessment of seismic vulnerability of industrial petrochemical and oil & gas piping systems can be performed, beyond analytical tools, through experimental testing as well. Along this line, this paper describes an experimental test campaign carried out on a full-scale piping system in order to assess its seismic behaviour. In particular, a typical industrial piping system, containing several critical components, such as elbows, a bolted flange joint and a Tee joint, was tested under different levels of realistic earthquake loading. They corresponded to serviceability and ultimate limit states for support structures as suggested by modern performance-based earthquake engineering standards. The so called hybrid simulation techniques namely, pseudo-dynamic and real time testing with dynamic substructuring, were adopted to perform seismic tests. Experimental results displayed a favourable performance of the piping system and its components; they remained below their yielding, allowable stress and allowable strain limits without any leakage even at the Near Collapse Limit State condition for the support structure. Moreover, the favourable comparison between experimental and numerical results, proved the validity of the proposed hybrid techniques alternative to shaking table tests.



Pseudodynamic tests with substructuring of a full?scale precast box?modularized structure made of reinforced concrete shear walls

Authors: Guoshan Xu, Zhen Wang, Bin Wu, Oreste S. Bursi, Xiaojing Tan, Qingbo Yang, Long Wen, Hongbin Jiang

Abstract: A novel horizontal and vertical wall?to?wall and wall?to?floor connection methods for precast box?modularized structure with reinforced concrete shear walls (PBSRCSWs) are proposed in this paper. The entailing behavior of the proposed connections and the seismic performance of one full?scale six?story PBSRCSWs were experimentally studied by means of pseudodynamic substructure tests. In order to improve the relevant experimental accuracy, we presented and validated one versatile testing platform Hytest, combined with external displacement feedback control (EDFC; Hytest with EDFC). In greater detail, it was shown from the pseudodynamic substructure test results that the proposed Hytest with EDFC can effectively impose the desired displacements on the specimens rather than on the actuators. Moreover, both the horizontal and vertical wall?to?wall connections proposed for the PBSRCSWs exhibited a favorable behavior whilst the PBSRCSWs subjected to earthquake records showed an excellent seismic performance.



Hybrid simulation of structural systems with online updating of concrete constitutive law parameters by unscented Kalman filter

Authors: Zhu Mei, Bin Wu, Oreste S. Bursi, Ge Yang, Zhaoran Wang

Abstract: Online model updating in hybrid simulation (HS) can represent an effective technique to reduce modeling errors of parts numerically simulated, that is, numerical substructures, especially when only a few critical components of a large system can be tested, that is, physical substructures. As a result, in an enhanced HS with online model updating, parameters of constitutive relationship can be identified based on experimental data provided by physical substructures and updated in numerical substructures. This paper proposes a novel method to identify constitutive parameters of concrete laws with unscented Kalman filter (UKF). In order to implement UKF, parts of the source codes of the OpenSEES software were modified to compute estimated measurements. Prior to experimental HS, a parametric study of UKF constitutive law parameters was conducted. As a result, the effectiveness of the UKF combined with OpenSEES was validated through numerical simulations, a monotonic loading test on a concrete column and real?time HSs of a reinforced concrete frame run with both standard and model?updating techniques based on UKF.



An improved equivalent force control algorithm for hybrid seismic testing of nonlinear systems

Authors: Zhen Wang, Bin Wu, Guoshan Xu, Oreste S. Bursi

Abstract: The equivalent force control (EFC) algorithm is a hybrid seismic testing method based on both an implicit integration algorithm and force feedback control. As it performs the computation of the numerical substructure with a fixed sampling number and some evaluations are not necessary, the EFC method is believed to be time?consuming for seismic testing of nonlinear systems with complicated numerical substructure model. In order to tackle this problem, the EFC method with varying sampling number (vEFC) has been conceived. The analysis of the vEFC method has shown that 2 traditional pseudodynamic testing (PDT) variants on the basis of implicit time integration schemes and numerical iteration, that is, the IPDT1 method and the IPDT2 method, can be recovered from the vEFC method. Moreover, the advantages of the vEFC method, such as fast response rate and compensation for control errors and possible slippage, are demonstrated.



HyTest: Platform for Structural Hybrid Simulations with Finite Element Model Updating

Authors: Ge Yang, Bin Wu, Ge Ou, Zhen Wang, Shirley Dyke
DOI: 10.1016/j.advengsoft.2017.05.007

Abstract: Hybrid simulation has been demonstrated to be a powerful method to evaluate the system-level dynamic performance of structure. With the numerical substructure analyzed with finite element software and the difficult-to-model components tested with an experimental substructure, complex structures with sophisticated behaviors can readily be examined through a hybrid simulation. To coordinate and synchronize the substructures in hybrid simulation, software is required. In recent studies, model updating has been integrated into hybrid simulation to improve testing accuracy by updating the numerical model during the analysis. However, online model updating scheme requires some modifications in the typical hybrid simulation testing procedure, and this greater complexity is entailed in its implementation regarding the collaboration of identification algorithms with existing hybrid simulation software. To address this issue and broaden the utilization of hybrid simulation with model updating, an existing platform named HyTest originally for conventional hybrid simulation is extended for this purpose. This version of HyTest facilitates the online identification of material constitutive parameters using experimental measurements in its finite element based identification module. It also includes a data center with a uniform data transmission protocol to incorporate different substructures and modules. A numerical example is used to demonstrate the online identification of material parameters for concrete and steel models in a reinforced column, and to verify the accuracy of the identification module. Lastly the effectiveness of HyTest in conducting hybrid simulation with model updating is validated using actual hybrid tests on a steel frame.



Predictive stability indicator: a novel approach to configuring a real-time hybrid simulation

Authors: Amin Maghareh, Shirley Dyke, Siamak Rabieniaharatbar, and Arun Prakash
DOI: 10.1002/eqe.2775

Abstract: Real-time hybrid simulation (RTHS) is an effective and versatile tool for the examination of complex structural systems with rate dependent behaviors. To meet the objectives of such a test, appropriate consideration must be given to the partitioning of the system into physical and computational portions (i.e., the configuration of the RTHS). Predictive stability and performance indicators (PSI and PPI) were initially established for use with only single degree-of-freedom systems. These indicators allow researchers to plan a RTHS, to quantitatively examine the impact of partitioning choices on stability and performance, and to assess the sensitivity of an RTHS configuration to de-synchronization at the interface. In this study, PSI is extended to any linear multi-degree-of-freedom (MDOF) system. The PSI is obtained analytically and it is independent of the transfer system and controller dynamics, providing a relatively easy and extremely useful method to examine many partitioning choices. A novel matrix method is adopted to convert a delay differential equation to a generalized eigenvalue problem using a set of vectorization mappings, and then to analytically solve the delay differential equations in a computationally efficient way. Through two illustrative examples, the PSI is demonstrated and validated. Validation of the MDOF PSI also includes comparisons to a MDOF dynamic model that includes realistic models of the hydraulic actuators and the control-structure interaction effects. Results demonstrate that the proposed PSI can be used as an effective design tool for conducting successful RTHS.



Adaptive multi-rate interface: development and experimental verification for real-time hybrid simulation

Authors: Amin Maghareh, Jacob P. Waldbjørn, Shirley J. Dyke, Arun Prakash, and Ali I. Ozdagli
DOI: 10.1002/eqe.2713

Abstract: Real-time hybrid simulation (RTHS) is a powerful cyber-physical technique that is a relatively cost-effective method to perform global/local system evaluation of structural systems. A major factor that determines the ability of an RTHS to represent true system-level behavior is the fidelity of the numerical substructure. While the use of higher-order models increases fidelity of the simulation, it also increases the demand for computational resources. Because RTHS is executed at real-time, in a conventional RTHS configuration, this increase in computational resources may limit the achievable sampling frequencies and/or introduce delays that can degrade its stability and performance. In this study, the Adaptive Multi-rate Interface rate-transitioning and compensation technique is developed to enable the use of more complex numerical models. Such a multirate RTHS is strictly executed at real-time, although it employs different time steps in the numerical and the physical substructures while including rate-transitioning to link the components appropriately. Typically, a higher-order numerical substructure model is solved at larger time intervals, and is coupled with a physical substructure that is driven at smaller time intervals for actuator control purposes. Through a series of simulations, the performance of the AMRI and several existing approaches for multi-rate RTHS is compared. It is noted that compared with existing methods, AMRI leads to a smaller error, especially at higher ratios of sampling frequency between the numerical and physical substructures and for input signals with highfrequency content. Further, it does not induce signal chattering at the coupling frequency. The effectiveness of AMRI is also verified experimentally.



Method for Real-Time Hybrid Model Testing of ocean structures: Case study on horizontal mooring systems

Authors: S.A.Vilsen, T.Sauder, A.J.Sørensen, M.Føre
DOI: 10.1016/j.oceaneng.2018.10.042

Abstract: This paper presents a method for Real-Time Hybrid Model testing (ReaTHM testing) of ocean structures. ReaTHM testing is an extension to traditional hydrodynamic model-scale testing, where the system under study is partitioned into physical and numerical substructures. The physical and numerical subsystems are connected in real-time through a control system. Based on experience with various ReaTHM tests, a general method for ReaTHM testing of ocean structures has been proposed. An experimental case study was carried out to illustrate the proposed method. The study was conducted in a state-of-the-art hydrodynamic laboratory, where a physical cylindrical buoy was placed in a still-water basin. Horizontal mooring loads from a numerical mooring system, which were modelled using the nonlinear finite element software RIFLEX were actuated onto the physical substructure. System performance was verified through comparison with a physical horizontal mooring system consisting of physical springs.



Kalman estimation of position and velocity for ReaTHM testing applications

Authors: Eirill Bachmann Mehammer, Martin Føre, Thomas Sauder, Valentin Bruno Chabaud, and Thomas Parisini
DOI: 10.1088/1742-6596/1104/1/012008

Abstract: Offshore wind power research is a rapidly growing field, because of the present climate crisis and increasing focus on renewable energy. Model testing plays an important role in the risk and cost analysis associated with offshore wind turbines (OWTs). The real-time hybrid model testing concept (ReaTHM testing) solves important challenges related to model testing of OWTs, such as achieving an accurate modelling of the wind field, and the occurrence of scaling issues when modelling wind and waves simultaneously. However, ReaTHM test set-ups are generally sensitive to noise, signal loss and inaccuracies in sensor values. The present study is focused on the design and implementation of a state estimator able to accurately estimate the position and velocity of floating structures, while taking disturbances into account. By combining the information received from several different sensors with mathematical models, the estimator provides smooth and reliable position and velocity estimates for ReaTHM testing applications. The main objective of the present study is to develop a kinematic state space model that could represent the motion of any floating structure in six degrees of freedom (6-DOF). The kinematic model is implemented in MATLAB, and acceleration time series obtained with numerical simulations are used as inputs. The computed outputs agree with the corresponding simulated motions. A Kalman estimator based on the state space model is designed, implemented and tested against virtual data from the numerical model, with artificially added disturbances. Sensitivity analyses addressing the robustness towards noise, time delays, signal loss and uncertainties are performed to identify the limits of the estimator. The estimator is demonstrated to be robust to most types of disturbances. Further, the state estimator is tested against physical data from laboratory experiments. Good agreement between the physically measured and the estimated states is observed.



Real-time aerodynamics hybrid simulation: Wind-induced effects on a reduced-scale building equipped with full-scale dampers

Authors: Teng Wu, Wei Song
DOI: 10.1016/j.jweia.2019.04.005

Abstract: As buildings are designed to be taller and more slender, they become lighter and more flexible with less inherent damping. If left uncontrolled, excessive wind-induced building response can cause serious safety and serviceability issues. Additional damping provided by adding an auxiliary damping system to the tall building is considered as one of the most cost-effective means to suppress the wind-induced response. Typically, the performance of these damping systems is evaluated experimentally with scaled damper and building models. However, the simplified small-scale dampers may not truly reflect the complex behavior of the full-scale damping systems. To realize the effective reduction of the wind-induced response of tall buildings, a real-time aerodynamics hybrid simulation (RTAHS) methodology that can offer improved response evaluation of a tall building integrated with an auxiliary damping system is introduced in this study. In this novel dynamic testing approach, the accurate evaluation of wind-induced tall building response is achieved by interacting an aeroelastic model of the tall building with the numerical model of the full-scale damper via interfacing actuators during the wind-tunnel tests. The feasibility and simulation accuracy of the proposed dynamic testing technique in the wind tunnel is numerically demonstrated by two case studies involving the wind-induced response reduction of a tall building equipped with both small-scale and full-scale damper properties.



Real-Time Hybrid Simulation and Experiment for Aeroelastic Testing of Flexible Wings

Authors: Weihua Su, Wei Song, and Vincent Hill
DOI: 10.2514/6.2019-2032

Abstract: The concept of hybrid simulation and experiment for aeroelastic testing is introduced in this paper. In a hybrid simulation, a coupled aeroelastic system is “broken down” into an aerodynamic simulation subsystem and a structural vibration subsystem. The coupling between structural dynamics and aerodynamics is still maintained by the real-time communication between the two subsystems. As the vibration of the testing article (a wing member or a full aircraft) is actuated by actuators, a hybrid aeroelastic simulation/experiment can eliminate the sizing constraint of the conventional aeroelastic testing performed within a wind-tunnel. It also significantly saves the cost of the wind-tunnel testing, especially when a fatigue study is conducted. However, several critical technical problems need to be addressed in both the aerodynamic simulation and vibration testing to enable a hybrid simulation in the teal time. This paper will prove the concept of hybrid simulation/experiment and discuss some of the critical problems underlying the hybrid simulation/experiment.



Design and Performance Evaluation of an Optimal Discrete-Time Feedforward Controller for Servo-Hydraulic Compensation

Authors: Saeid Hayati, and Wei Song
DOI: 10.1061/(ASCE)EM.1943-7889.0001399

Abstract: Servo-hydraulic actuators exhibit frequency-dependent variations of amplitude and delay during real-time hybrid simulation (RTHS). Effective compensation techniques to overcome these variations is a crucial component for the successful implementation of RTHS. Most of the existing compensation techniques have demonstrated effective performance under excitations with relatively low frequency bandwidth. To further advance the servo-hydraulic compensation for broader frequency bandwidth, this paper presents the design and performance evaluation of an optimal discrete-time model-based feedforward controller under inputs with broader frequency bandwidth as high as 0–30 Hz. As a compensation technique has not been fully explored in RTHS, the model-based design of discrete-time domain compensation techniques introduces the new technical challenge of inverting nonminimum phase systems. This paper identifies this new challenge by providing detailed supporting derivation, and explains the use of a digital filtering technique—a finite impulse response (FIR) filter—to address this new challenge, and the development process of the proposed FIR compensator using different optimization schemes. Furthermore, this paper demonstrates the compensation performance of the proposed FIR compensator, both numerically and experimentally, under reference inputs with various bandwidths, including bandlimited white noises with frequency bandwidth as high as 0–30 Hz. For comparison purposes, several existing feedforward compensation techniques are also implemented and compared with the proposed FIR compensator. Based on this study, it is found that the proposed FIR compensator technique not only provides excellent compensation performance under various bandwidths, but also offers great flexibility in its formulation by varying the model order with desired compensation performance and computational demands.



An optimal discrete-time feedforward compensator for real-time hybrid simulation

Authors: Saeid Hayati and Wei Song
DOI: 10.12989/sss.2017.20.4.483

Abstract: Real-Time Hybrid Simulation (RTHS) is a powerful and cost-effective dynamic experimental technique. To implement a stable and accurate RTHS, time delay present in the experiment loop needs to be compensated. This delay is mostly introduced by servo-hydraulic actuator dynamics and can be reduced by applying appropriate compensators. Existing compensators have demonstrated effective performance in achieving good tracking performance. Most of them have been focused on their application in cases where the structure under investigation is subjected to inputs with relatively low frequency bandwidth such as earthquake excitations. To advance RTHS as an attractive technique for other engineering applications with broader excitation frequency, a discrete-time feedforward compensator is developed herein via various optimization techniques to enhance the performance of RTHS. The proposed compensator is unique as a discrete-time, model-based feedforward compensator. The feedforward control is chosen because it can substantially improve the reference tracking performance and speed when the plant dynamics is well-understood and modeled. The discrete-time formulation enables the use of inherently stable digital filters for compensator development, and avoids the error induced by continuous-time to discrete-time conversion during the compensator implementation in digital computer. This paper discusses the technical challenges in designing a discrete-time compensator, and proposes several optimal solutions to resolve these challenges. The effectiveness of compensators obtained via these optimal solutions is demonstrated through both numerical and experimental studies. Then, the proposed compensators have been successfully applied to RTHS tests. By comparing these results to results obtained using several existing feedforward compensators, the proposed compensator demonstrates superior performance in both time delay and Root-Mean-Square (RMS) error.



Development of a hybrid simulation controller for full?scale experimental investigation of seismic retrofits for soft?story woodframe buildings

Authors: Xiaoyun Shao, Weichiang Pang, Chelsea Griffith, Ershad Ziaei, John van de Lindt
DOI: 10.1002/eqe.2704

Abstract: Hybrid simulations of a full?scale soft?story woodframe building specimen with various retrofits were carried out as part of the Network for Earthquake Engineering Simulation Research project – NEES?Soft: seismic risk reduction for soft?story woodframe buildings. The test structure in the hybrid simulation was a three?story woodframe building that was divided into a numerical substructure of the first story with various retrofits and a full?scale physical substructure of the upper two stories. Four long?stroke actuators, two at the second floor and two at the roof diaphragm, were attached to the physical substructure to impose the simulated seismic responses including both translation and in?plane rotation. Challenges associated with this first implementation of a full?scale hybrid simulation on a woodframe building were identified. This paper presents the development and validation of a scalable and robust hybrid simulation controller for efficient test site deployment. The development consisted of three incremental validation phases ranging from small?scale, mid?scale, to full?scale tests conducted at three laboratories. Experimental setup, procedure, and results of each phase of the controller development are discussed, demonstrating the effectiveness and efficiency of the incremental controller development approach for large?scale hybrid simulation programs with complex test setup.



Hybrid simulation for system-level structural response

Authors: Justin Adam Murray, Mehrdad Sasani, Xiaoyun Shao
DOI: 10.1016/j.engstruct.2015.09.018

Abstract: Hybrid simulations combine physical and analytical components into a single simulation to evaluate theresponse of a structure, often under seismic ground motion. This allows an experiment to be conducted inwhich structural components with complex response can be modeled experimentally and morewell-known components can be represented within an analytical model. The coordination softwareUI-SimCor, developed by the MUST-SIM NEES facility at the University of Illinois at Urbana–Champaign, is a hybrid simulation tool which performs the dynamic analysis and other software andhardware coordination tasks for hybrid simulations. In many hybrid simulations, including those thathave used UI-SimCor, analytical models with few effective degrees of freedom are typically used. In sim-ulations where system-level behaviors and the response of the analytical components are of importance,a more detailed analytical system is needed. This changeover to a more complex analytical system andincrease in general complexity of the hybrid simulation can cause various issues within the UI-SimCorframework. This study discusses the difficulties and issues that arise from having large and complex ana-lytical substructures in hybrid simulation, and the effective mitigation or solutions to those problems.



Real-Time Hybrid Simulation with Online Model Updating: Methodology and Implementation

Authors: Xiaoyun Shao; Adam Mueller ; and Bilal Ahmed Mohammed
DOI: 10.1061/(ASCE)EM.1943- 7889.0000987

Abstract: Hybrid simulations have shown great potential for economic and reliable assessment of structural seismic performance by combining physical experimentation on part of the structural system and numerical simulation of the remaining structural components. Current hybrid simulation practices often use a fixed numerical model without considering the possible availability of a more-accurate model obtained during hybrid simulation through an online model updating technique. To address this limitation and improve the reliability of numerical models in hybrid simulations, this paper presents a method and an implementation procedure of conducting real-time hybrid simulation (RTHS) with online model updating. The Unscented Kalman Filter (UKF) was adopted as the parameter identification algorithm applied to the Bouc-Wen model that defines the hysteresis of the experimental substructure. The identified parameters are then used to update the models of the numerical substructures during RTHS. A parametric study of the UKF system model parameters is carried out first to determine the optimum values to be used in the verification experiments. Then RTHS of a three-story steel shear building model is conducted and the effectiveness of online model updating in RTHS and the proposed implementation procedure is demonstrated. Guidelines for implementing the UKF for online model updating in RTHS and research needs for further development are discussed.



Real?time force control for servo?hydraulic actuator systems using adaptive time series compensator and compliance springs

Authors: Yunbyeong Chae, Ramin Rabiee, Abdullah Dursun, Chul?Young Kim
DOI: 10.1002/eqe.2994

Abstract: Servo?hydraulic actuators have been widely used for experimental studies in engineering. They can be controlled in either displacement or force control mode depending on the purpose of a test. It is necessary to control the actuators in real time when the rate?dependency effect of a test specimen needs to be accounted for under dynamic loads. Real?time hybrid simulation (RTHS) and effective force testing (EFT) method, which can consider the rate?dependency effect, have been known as viable alternatives to the shake table testing method. Due to the lack of knowledge in real?time force control, however, the structures that can be tested with RTHS and EFT are fairly limited. For instance, satisfying the force boundary condition for axially stiff members is a challenging task in RTHS, while EFT has a difficulty to be implemented for nonlinear structures. In order to resolve these issues, this paper introduces new real?time force control methods utilizing the adaptive time series (ATS) compensator and compliance springs. Unlike existing methods, the proposed force control methods do not require the structural modeling of a test structure, making it easy to be implemented especially for nonlinear structures. The force tracking performance of the proposed methods is evaluated for a small?scale steel mass block system with a magneto?rheological damper subjected to various target forces. Accuracy, time delay, and resonance response of these methods are discussed along with their force control performance for an axially stiff member. Overall, a satisfactory force tracking performance was observed by using the proposed force control methods.



Real?time hybrid simulation for an RC bridge pier subjected to both horizontal and vertical ground motions

Authors: Yunbyeong Chae, Jinhaeng Lee, Minseok Park, Chul?Young Kim
DOI: 10.1002/eqe.3042

Abstract: It is well known that real?time hybrid simulation (RTHS) is an effective and viable dynamic testing method. Numerous studies have been conducted for RTHS during the last 2 decades; however, the application of RTHS toward practical civil infrastructure is fairly limited. One of the major technical barriers preventing RTHS from being widely accepted in the testing community is the difficulty of accurate displacement control for axially stiff members. For such structures, a servo?hydraulic actuator can generate a large force error due to the stiff oil column in the actuator even if there is a small axial displacement error. This difficulty significantly restricts the implementation of RTHS for structures such as columns, walls, bridge piers, and base isolators. Recently, a flexible loading frame system was developed, enabling a large? capacity real?time axial force application to axially stiff members. With the aid of the flexible loading frame system, this paper demonstrates an RTHS for a bridge structure with an experimental reinforced concrete pier, which is subjected to both horizontal and vertical ground motions. This type of RTHS has been a challenging task due to the lack of knowledge for satisfying the time?varying axial force boundary condition, but the newly developed technology for real?time force control and its incorporation into RTHS enabled a successful implementation of the RTHS for the reinforced concrete pier of this study.



Experimental study on the rate-dependency of reinforced concrete structures using slow and real-time hybrid simulations

Authors: Yunbyeong Chae, Minseok Park, Chul-Young Kim, Young SukPark
DOI: 10.1016/j.engstruct.2016.11.065

Abstract: A great number of studies have been conducted to study the loading rate effect on the behavior of reinforced concrete (RC) structures. A majority of these studies, however, are focused on the component behavior of an RC specimen by imposing a predefined cyclic displacement history on the specimen without considering the interaction of the specimen with the entire structural system. In this study, the rate-dependency effect of an RC pier on the global response of a bridge is experimentally investigated using the slow and real-time hybrid simulations. The RC pier is used to support a two-span prestressed concrete girder bridge. The nonlinear response of the bridge under earthquake loads is accounted for by physically testing the RC pier in a laboratory, while the upper structural system of the bridge including the bridge deck and girders are analytically modeled. A dynamic servo-hydraulic actuator is connected to the top of the pier to transfer the inertial force of the bridge deck and girders to the pier. Due to the lack of knowledge in real-time force control, the axial load effect on the dynamic response of the RC pier is not considered in this study. Prior to conducting the hybrid simulations, predefined cyclic displacement tests are conducted for the bridge pier specimens with the same displacement history, but with different rates, in order to investigate any change in strength and energy dissipation capacity of the RC pier. Then, a series of slow and real-time hybrid simulations are conducted to investigate the rate-dependency effect on the seismic response of the bridge. The results from the predefined cyclic displacement tests and hybrid simulations are provided and discussed along with the observation from these tests.



A Computationally Efficient Algorithm for Real?Time Tracking the Abrupt Stiffness Degradations of Structural Elements

Authors: Ying Lei, Huan Zhou, Zhi?Lu Lai
DOI: 10.1111/mice.12217

Abstract: Real?time structural identification and damage detection are necessary for on?line structural damage detection and optimal structural vibration control during severe loadings. Frequently, structural damage can be reflected in the stiffness degradation of structural elements. In this article, a time?domain three?stage algorithm with computational efficiency is proposed for real?time tracking the onsets, locations, and extents of abrupt stiffness degradations of structural elements using measurements of structural acceleration responses. Structural dynamic parameters before damage are recursively estimated in stage I. Then, the time instants and possible locations of degraded structural elements are detected by tracking the errors between the measured data and the corresponding estimated values in stage II. Finally, the exact locations and extents of stiffness degradations of structural elements are determined by solving simple constrained optimization problems in stage III. Both numerical examples and an experimental test are used to validate the proposed algorithm for real?time tracking the abrupt stiffness degradations of structural elements in linear or nonlinear structures using measurements of structural acceleration responses polluted by noises.



Parametric model of servo-hydraulic actuator coupled with a nonlinear system: Experimental validation

Authors: Amin Maghareh, Christian E. Silva, Shirley J. Dyke
DOI: 10.1016/j.ymssp.2017.11.009

Abstract: Hydraulic actuators play a key role in experimental structural dynamics. In a previous study, a physics-based model for a servo-hydraulic actuator coupled with a nonlinear physical system was developed. Later, this dynamical model was transformed into controllable canonical form for position tracking control purposes. For this study, a nonlinear device is designed and fabricated to exhibit various nonlinear force-displacement profiles depending on the initial condition and the type of materials used as replaceable coupons. Using this nonlinear system, the controllable canonical dynamical model is experimentally validated for a servo-hydraulic actuator coupled with a nonlinear physical system.



Robust integrated actuator control: experimental verification and real?time hybrid?simulation implementation

Authors: Ge Ou, Ali Irmak Ozdagli, Shirley J. Dyke, Bin Wu
DOI: 10.1002/eqe.2479

Abstract: In this paper, we propose a new actuator control algorithm that achieves the design flexibility, robustness, and tracking accuracy to give real?time hybrid?simulation users the power to achieve highly accurate and robust actuator control. The robust integrated actuator control (RIAC) strategy integrates three key control components: loop shaping feedback control based on H? optimization, a linear?quadratic?estimation block for minimizing noise effect, and a feed?forward block that reduces small residual delay/lag. The combination of these components provides flexible controller design to accommodate setup limits while preserving the stability of the H? algorithm. The efficacy of the proposed strategy is demonstrated through two illustrative case studies: one using large capacity but relatively slow actuator of 2500?kN and the second using a small?scale fast actuator. Actuator tracking results in both cases demonstrate that the RIAC algorithm is effective and applicable for different setups. Real?time hybrid?simulation validation is implemented using a three?DOF building frame equipped with a magneto?rheological damper on both setups. Results using the two very different physical setups illustrate that RIAC is efficient and accurate.



Enabling role of hybrid simulation across NEES in advancing earthquake engineering

Authors: Daniel Gomez; Shirley J. Dyke; Amin Maghareh
DOI: 10.12989/sss.2015.15.3.913

Abstract: Hybrid simulation is increasingly being recognized as a powerful technique for laboratory testing. It offers the opportunity for global system evaluation of civil infrastructure systems subject to extreme dynamic loading, often with a significant reduction in time and cost. In this approach, a reference structure/system is partitioned into two or more substructures. The portion of the structural system designated as 'physical' or 'experimental' is tested in the laboratory, while other portions are replaced with a computational model. Many researchers have quite effectively used hybrid simulation (HS) and real-time hybrid simulation (RTHS) methods for examination and verification of existing and new design concepts and proposed structural systems or devices. This paper provides a detailed perspective of the enabling role that HS and RTHS methods have played in advancing the practice of earthquake engineering. Herein, our focus is on investigations related to earthquake engineering, those with CURATED data available in their entirety in the NEES Data Repository.



Experimental implementation of predictive indicators for configuring a real-time hybrid simulation

Authors: Fangshu Lin, Amin Maghareh, Shirley J. Dyke, Xilin Lua
DOI: 10.1016/j.engstruct.2015.07.040

Abstract: Real-time hybrid simulation (RTHS) is gaining acceptance as an efficient and cost-effective method for realistic structural evaluation. Advances in real-time computing and control methods have enabled research in the development of this novel methodology to progress rapidly. However, to explore effectiveness and accuracy, and thus build broader confidence in the use of this method as an alternative to shake table testing, there is a need to better understand and address the key features that determine the success of an RTHS. Here we discuss the design and analysis of a SDOF RTHS case study conducted in Purdue University’s Intelligent Infrastructure Systems Lab (IISL). We examine the key factors that determine the success, through configuration of the test using predictive indicators, design of an appropriately effective actuator controller, and a thorough comparison with shake table testing. The reference structure chosen for this case study is a single story, moment resisting frame structure. This particular specimen is of lab scale and well-known component properties, making it a suitable choice for such an investigation. However, noise, control–structure interaction and damping introduce numerous challenges typically faced in establishing an effective RTHS configuration. We investigate two key issues that lead to the design of a successful RTHS, specifically the partitioning between numerical and physical substructure for stability and performance, and the actuator motion control algorithm. Predictive indicators are demonstrated to be particularly helpful for properly configuring an RTHS experiment to meet a researcher’s specified objectives. Furthermore a direct comparison is conducted to examine the ability of RTHS to replicate a shake table test. The results demonstrate that with proper partitioning and actuator control design, successful RTHS can be implemented despite unfavorable transfer system properties.



Large-Scale Real-Time Hybrid Simulation for Evaluation of Advanced Damping System Performance

Authors: Anthony Friedman, Shirley J. Dyke, Brian Phillips, Ryan Ahn
DOI: 10.1061/(ASCE)ST.1943-541X.0001093

Abstract: As magnetorheological (MR) control devices increase in scale for use in real-world civil engineering applications, sophisticated modeling and control techniques may be needed to exploit their unique characteristics. Here, a control algorithm that utilizes overdriving and backdriving current control to increase the efficacy of the control device is experimentally verified and evaluated at large scale. Real-time hybrid simulation (RTHS) is conducted to perform the verification experiments using the nees@Lehigh facility. The physical substructure of the RTHS is a 10-m tall planar steel frame equipped with a large-scale MR damper. Through RTHS, the test configuration is used to represent two code-compliant structures, and is evaluated under seismic excitation. The results from numerical simulation and RTHS are compared to verify the RTHS methodology. The global responses of the full system are used to assess the performance of each control algorithm. In each case, the reduction in peak and root mean square (RMS) responses (displacement, drift, acceleration, damper force, etc.) is examined. Beyond the verification tests, the robust performance of the damper controllers is also demonstrated using RTHS.



Time delay effects on large-scale MR damper based semi-active control strategies

Authors: Y-J Cha, A K Agrawal, S J Dyke
DOI: 10.1088/0964-1726/22/1/015011

Abstract: This paper presents a detailed investigation on the robustness of large-scale 200 kN MR damper based semi-active control strategies in the presence of time delays in the control system. Although the effects of time delay on stability and performance degradation of an actively controlled system have been investigated extensively by many researchers, degradation in the performance of semi-active systems due to time delay has yet to be investigated. Since semi-active systems are inherently stable, instability problems due to time delay are unlikely to arise. This paper investigates the effects of time delay on the performance of a building with a large-scale MR damper, using numerical simulations of near- and far-field earthquakes. The MR damper is considered to be controlled by four different semi-active control algorithms, namely (i) clipped-optimal control (COC), (ii) decentralized output feedback polynomial control (DOFPC), (iii) Lyapunov control, and (iv) simple-passive control (SPC). It is observed that all controllers except for the COC are significantly robust with respect to time delay. On the other hand, the clipped-optimal controller should be integrated with a compensator to improve the performance in the presence of time delay.



Cyber-Physical Codesign of Distributed Structural Health Monitoring with Wireless Sensor Networks

Authors: Gregory Hackmann, Weijun Guo, Guirong Yan, Zhuoxiong Sun, Chenyang Lu, Shirley Dyke
DOI: 10.1109/TPDS.2013.30

Abstract: Our deteriorating civil infrastructure faces the critical challenge of long-term structural health monitoring for damage detection and localization. In contrast to existing research that often separates the designs of wireless sensor networks and structural engineering algorithms, this paper proposes a cyber-physical codesign approach to structural health monitoring based on wireless sensor networks. Our approach closely integrates 1) flexibility-based damage localization methods that allow a tradeoff between the number of sensors and the resolution of damage localization, and 2) an energy-efficient, multilevel computing architecture specifically designed to leverage the multiresolution feature of the flexibility-based approach. The proposed approach has been implemented on the Intel Imote2 platform. Experiments on a simulated truss structure and a real full-scale truss structure demonstrate the system's efficacy in damage localization and energy efficiency.



Parametric identification of a servo-hydraulic actuator for real-time hybrid simulation

Authors: Yili Qian, Ge Ou, Amin Maghareh, Shirley J.Dyke
DOI: 10.1016/j.ymssp.2014.03.001

Abstract: In a typical Real-time Hybrid Simulation (RTHS) setup, servo-hydraulic actuators serve as interfaces between the computational and physical substructures. Time delay introduced by actuator dynamics and complex interaction between the actuators and the specimen has detrimental effects on the stability and accuracy of RTHS. Therefore, a good understanding of servo-hydraulic actuator dynamics is a prerequisite for controller design and computational simulation of RTHS. This paper presents an easy-to-use parametric identification procedure for RTHS users to obtain re-useable actuator parameters for a range of payloads. The critical parameters in a linearized servo-hydraulic actuator model are optimally obtained from genetic algorithms (GA) based on experimental data collected from various specimen mass/stiffness combinations loaded to the target actuator. The actuator parameters demonstrate convincing convergence trend in GA. A key feature of this parametric modeling procedure is its re-usability under different testing scenarios, including different specimen mechanical properties and actuator inner-loop control gains. The models match well with experimental results. The benefit of the proposed parametric identification procedure has been demonstrated by (1) designing an H? controller with the identified system parameters that significantly improves RTHS performance; and (2) establishing an analysis and computational simulation of a servo-hydraulic system that help researchers interpret system instability and improve design of experiments.



Establishing a stability switch criterion for effective implementation of real-time hybrid simulation

Authors: Amin Maghareh, Shirley J. Dyke, Arun Prakash and Jeffrey F. Rhoads
DOI: 10.12989/sss.2014.14.6.1221

Abstract: Real-time hybrid simulation (RTHS) is a promising cyber-physical technique used in the experimental evaluation of civil infrastructure systems subject to dynamic loading. In RTHS, the response of a structural system is simulated by partitioning it into physical and numerical substructures, and coupling at the interface is achieved by enforcing equilibrium and compatibility in real-time. The choice of partitioning parameters will influence the overall success of the experiment. In addition, due to the dynamics of the transfer system, communication and computation delays, the feedback force signals are dependent on the system state subject to delay. Thus, the transfer system dynamics must be accommodated by appropriate actuator controllers. In light of this, guidelines should be established to facilitate successful RTHS and clearly specify: (i) the minimum requirements of the transfer system control, (ii) the minimum required sampling frequency, and (iii) the most effective ways to stabilize an unstable simulation due to the limitations of the available transfer system. The objective of this paper is to establish a stability switch criterion due to systematic experimental errors. The RTHS stability switch criterion will provide a basis for the partitioning and design of successful RTHS.



Establishing a predictive performance indicator for real?time hybrid simulation

Authors: Amin Maghareh, Shirley J. Dyke, Arun Prakash, Gregory B. Bunting
DOI: 10.1002/eqe.2448

Abstract: Real?time hybrid simulation (RTHS) is increasingly being recognized as a powerful cyber?physical technique that offers the opportunity for system evaluation of civil structures subject to extreme dynamic loading. Advances in this field are enabling researchers to evaluate new structural components/systems in cost?effective and efficient ways, under more realistic conditions. For RTHS, performance metric clearly needs to be developed to predict and evaluate the accuracy of various partitioning choices while incorporating the dynamics of the transfer system, and computational/communication delays. In addition, because of the dynamics of the transfer system, communication delays, and computation delays, the RTHS equilibrium force at the interface between numerical and physical substructures is subject to phase discrepancy. Thus, the transfer system dynamics must be accommodated by appropriate actuator controllers. In this paper, a new performance indicator, predictive performance indicator (PPI), is proposed to assess the sensitivity of an RTHS configuration to any phase discrepancy resulting from transfer system dynamics and computational/communication delays. The predictive performance indicator provides a structural engineer with two sets of information as follows: (i) in the absence of a reference response, what is the level of fidelity of the RTHS response? and (ii) if needed, what partitioning adjustments can be made to effectively enhance the fidelity of the response? Moreover, along with the RTHS stability switch criterion, this performance metric may be used as an acceptance criteria for conducting single?degree?of?freedom RTHS.



Hybrid Testing of the Stiff Rocking Core Seismic Rehabilitation Technique

Authors: Derek Slovenec, Alireza Sarebanha, Michael Pollino, Gilberto Mosqueda

Abstract:The use of a stiff rocking core (SRC) has been proposed as a seismic rehabilitation technique to mitigate soft-story response in low-rise to midrise steel concentrically braced frame (CBF) structures. This technique uses a stiff, elastic “spine” to provide corrective lateral forces at the onset of soft-story response but otherwise remains passive for the first mode vibration response. Yielding link element can also be incorporated in the SRC-to-structure connection to dissipate energy and reduce overall building drift. An experimental testing program was performed to investigate the fundamental behaviors of the SRC rehabilitation technique applied to two approximately 1/3-scale prototype CBFs representative of modern and older design practices. Hybrid testing methods were used to simulate building dynamics, the influence of gravity framing, and response of upper stories for a midrise prototype building. Each prototype frame was subjected to two seismic ground motions to evaluate cumulative damage followed by quasi-static cyclic testing to failure. The results from these tests indicate that the SRC is effective at mitigating soft-story response by vertically redistributing lateral demands throughout the structure.



DesignSafe: New Cyberinfrastructure for Natural Hazards Engineering

Authors: Ellen M. Rathje, Clint Dawson, Jamie E. Padgett, Jean-Paul Pinelli, Dan Stanzione, Ashley Adair, Pedro Arduino, Scott J. Brandenberg, Tim Cockerill, Charlie Dey, Maria Esteva, Fred L. Haan Jr., Matthew Hanlon, Ahsan Kareem, Laura Lowes, Stephen Mock, and Gilberto Mosqueda.
DOI: 10.1061/(ASCE)NH.1527-6996.0000246

Abstract: Natural hazards engineering plays an important role in minimizing the effects of natural hazards on society through the design of resilient and sustainable infrastructure. The DesignSafe cyberinfrastructure has been developed to enable and facilitate transformative research in natural hazards engineering, which necessarily spans across multiple disciplines and can take advantage of advancements in computation, experimentation, and data analysis. DesignSafe allows researchers to more effectively share and find data using cloud services, perform numerical simulations using high performance computing, and integrate diverse datasets so that researchers can make discoveries that were previously unattainable. This paper describes the design principles used in the cyberinfrastructure development process, introduces the main components of the DesignSafe cyberinfrastructure, and illustrates the use of the DesignSafe cyberinfrastructure in research in natural hazards engineering through various examples.



Assessment of Numerical and Experimental Errors in Hybrid Simulation of Framed Structural Systems through Collapse

Authors: M. Javad Hashemi, Gilberto Mosqueda, Dimitrios G. Lignos, Ricardo A. Medina, Eduardo Miranda
DOI: 10.1080/13632469.2015.1110066

Abstract: Hybrid simulation can provide significant advantages for large-scale experimental investigations of the seismic response of structures through collapse, particularly when considering cost and safety of conventional shake table tests. Hybrid simulation, however, has its own challenges and special attention must be paid to mitigate potential numerical and experimental errors that can propagate throughout the simulation. Several case studies are presented here to gain insight into the factors influencing the accuracy and stability of hybrid simulation from the linear-elastic response range through collapse. The hybrid simulations were conducted on a four-story two-bay moment frame with various substructuring configurations. Importantly, the structural system examined here was previously tested on a shake table with the same loading sequence, allowing for direct evaluation of the hybrid simulation results. The sources of error examined include: (1) computational stability in numerical substructure; (2) setup and installation of the physical specimen representing the experimental substructure; and (3) the accuracy of the selected substructuring technique that handles the boundary conditions and continuous exchange of data between the subassemblies. Recommendations are made regarding the effective mitigation of the various sources of errors. It is shown that by controlling errors, hybrid simulation can provide reliable results for collapse simulation by comparison to shake table testing.



Large-Scale Hybrid Simulation of a Steel Moment Frame Building Structure through Collapse

Authors:Maikol Del Carpio Ramos, Gilberto Mosqueda, M. Javad Hashemi
DOI: 10.1061/(ASCE)ST.1943-541X.0001328

Abstract:The implementation of two series of hybrid simulations that aim to trace the system-level seismic response of a four-story steel moment frame building structure through collapse is presented. In the first series of tests, a half-scale 1½-bay by 1½-story physical substructure of a special steel moment-resisting frame is considered, while in the second series the physical substructure corresponds to the gravity framing system with a similar-sized specimen. An objective of these tests is to demonstrate the potential of hybrid simulation with substructuring as a cost-effective alternative to earthquake simulators for large-scale system-level testing of structural frame subassemblies. The performance of a recently developed substructuring technique and time-stepping integration method for hybrid simulation are evaluated when employed with large and complex numerical substructures exhibiting large levels of nonlinear response. The substructuring technique simplifies the experimental setup by reducing the number of required actuators while adequately approximating the boundary conditions including lateral displacements and axial loads on columns. The test method was found to be reliable with capabilities to provide insight into experimental behavior of structural subassemblies under realistic seismic loading and boundary conditions.



Hybrid simulation of a structure to tsunami loading

Authors: Bahareh Forouzan, Dilshan SP Amarsinghe Baragamage, Koushyar Shaloudegi, Narutoshi Nakata, Weiming Wu

Abstract:A new hybrid simulation technique has been developed to assess the behavior of a structure under hydrodynamic loading. It integrates the computational fluid dynamics and structural hybrid simulation and couples the fluid loading and structural response at each simulation step. The conventional displacement-based and recently developed force-based hybrid simulation approaches are adopted in the structural analysis. The concept, procedure, and required components of the proposed hybrid simulation are introduced in this article. The proposed hybrid simulation has been numerically and physically tested in case of a coastal building impacted by a tsunami wave. It is demonstrated that the force error in the displacement-based approach is significantly larger than that in the force-based approach. The force-based approach allows for a more realistic and reliable structural assessment under tsunami loading.



Evaluation of frequency evaluation index based compensation for benchmark study in real-time hybrid simulation

Authors: Weijie Xu, Cheng Chen, Tong Guo, Menghui Chen

Abstract: Actuator control plays an essential role to achieve stable and accurate real-time hybrid simulation (RTHS) results. Delay compensation is often used to minimize the desynchronization at the interface between numerical and experimental substructures. In this study, a new delay compensation method is proposed for RTHS, which integrates the inverse compensation method (IC) and frequency-domain evaluation index (FEI). Window technique is utilized to enable FEI for calculation of almost instantaneous time delay and the IC parameter is then adjusted accordingly for optimal compensation. The performance of this windowed FEI compensation (WFEI) is evaluated and compared with that of the IC and the adaptive inverse compensation (AIC) through computational simulations of a benchmark model with different initial estimates of time delay. It is demonstrated that the WFEI compensation not only provides accurate actuator control when initial estimated time delay deviates from actual values but also have good robustness under unpredicted uncertainties of the servo-hydraulic system.



Stability Analysis of Real-Time Hybrid Simulation for Time-Varying Actuator Delay Using the Lyapunov Krasovskii Functional Approach

Authors: Liang Huang, Cheng Chen, Tong Guo, Menghui Chen

Abstract:In a real-time hybrid simulation (RTHS), the actuator delay in experimental results might deviate from actual structural responses and even destabilize the real-time test. Although the assumption of a constant actuator delay helps simplify the stability analysis of RTHS, experimental results often show that the actuator delay varies throughout the test. However, research on the effect of time-varying delay on RTHS system stability is very limited. In this study, the Lyapunov-Krasovskii functional is introduced for the stability analysis of RTHS system. Two stability criteria are proposed for a linear system with a single constant delay and a time-varying delay. It is demonstrated that (1) the stable region of a time-varying delay system shrinks with the increase of the first derivative of time-varying delay; and (2) the stable region of the time-varying delay system is smaller than that of constant-time-delay system. Computational simulations were conducted for RTHS systems with a single degree of freedom to evaluate the proposed criteria. When the experimental specimen is an ideal elastic spring, the stability region of RTHS system with time-varying delay is shown to depend on the stiffness partition, structural natural period, and damping ratio. Significant differences in stability regions indicate that the time-varying characteristics of actuator delay should be considered for stability analysis of RTHS systems.



Localized evaluation of actuator tracking for real-time hybrid simulation using frequency-domain indices

Authors:Weijie Xu , Tong Guo , Cheng Chen

Abstract:Accurate actuator tracking plays an important role in real-time hybrid simulation (RTHS) to ensure accurate and reliable experimental results. Frequency-domain evaluation index (FEI) interprets actuator tracking into amplitude and phase errors thus providing a promising tool for quantitative assessment of real-time hybrid simulation results. Previous applications of FEI successfully evaluated actuator tracking over the entire duration of the tests. In this study, FEI with moving window technique is explored to provide post-experiment localized actuator tracking assessment. Both moving window with and without overlap are investigated through computational simulations. The challenge is discussed for Fourier Transform to satisfy both time domain and frequency resolution for selected length of moving window. The required data window length for accuracy is shown to depend on the natural frequency and structural nonlinearity as well as the ground motion input for both moving windows with and without overlap. Moving window without overlap shows better computational efficiency and has potential for future online evaluation. Moving window with overlap however requires much more computational efforts and is more suitable for post-experiment evaluation. Existing RTHS data from Network Earthquake Engineering Simulation (NEES) is utilized to further demonstrate the effectiveness of the proposed approaches. It is demonstrated that with proper window size, FEI with moving window techniques enable accurate localized evaluation of actuator tracking for real-time hybrid simulation.



Delay Estimation for Reliability Interpretation of Real-Time Hybrid Simulation Involving Viscous Dampers Towards Seismic Hazard Mitigation

Authors: Samuel Richardson , Cheng Chen , Jose Valdovinos , Wenshen Pong , Kai Chen

Abstract: Laboratory experiments play a critical role in earthquake engineering research for seismic safety evaluation of civil engineering structures. Servo-hydraulic actuators play a vital role to maintain the compatibility on boundaries between the analytical and experimental substructures in a real-time hybrid simulation. Previous study has indicated that actuator delay could significantly affect the accuracy of real-time hybrid simulation involving viscous dampers. Identifying the amount of actuator delay therefore is critical for reliability assessment of experimental results to properly interpret the performance of viscous dampers for seismic hazard mitigation. In this study a frequency domain based approach is applied for real-time hybrid simulation of viscous dampers with the presence of actuator delay. Computational simulations are conducted to assess the accuracy of the approach for estimating the delay when the substructures develop nonlinear behavior for reliability interpretation of real-time hybrid simulation.



Analysis of actuator delay and its effect on uncertainty quantification for real-time hybrid simulation

Authors: Cheng Chen, Weijie Xu, Tong Guo, Kai Chen
DOI: 10.1007/s11803-017-0409-6

Abstract:Uncertainties in structure properties can result in different responses in hybrid simulations. Quantification of the effect of these uncertainties would enable researchers to estimate the variances of structural responses observed from experiments. This poses challenges for real-time hybrid simulation (RTHS) due to the existence of actuator delay. Polynomial chaos expansion (PCE) projects the model outputs on a basis of orthogonal stochastic polynomials to account for influences of model uncertainties. In this paper, PCE is utilized to evaluate effect of actuator delay on the maximum displacement from real-time hybrid simulation of a single degree of freedom (SDOF) structure when accounting for uncertainties in structural properties. The PCE is first applied for RTHS without delay to determine the order of PCE, the number of sample points as well as the method for coefficients calculation. The PCE is then applied to RTHS with actuator delay. The mean, variance and Sobol indices are compared and discussed to evaluate the effects of actuator delay on uncertainty quantification for RTHS. Results show that the mean and the variance of the maximum displacement increase linearly and exponentially with respect to actuator delay, respectively. Sensitivity analysis through Sobol indices also indicates the influence of the single random variable decreases while the coupling effect increases with the increase of actuator delay.



Real-time system support for hybrid structural simulation

Authors: David Ferry, Gregory Bunting, Amin Maghareh, Arun Prakash, Shirley Dyke, Kunal Agrawal, Chris Gill, Chenyang Lu
DOI: 10.1145/2656045.2656067

Abstract: Real-time hybrid simulation (RTHS) is an important tool in the design and testing of civil and mechanical structures when engineers and scientists wish to understand the performance of an isolated component within the context of a larger structure. Performing full-scale physical experimentation with a large structure can be prohibitively expensive. Instead, a hybrid testing framework connects part of a physical structure within a closed loop (through sensors and actuators) to a numerical simulation of the rest of the structure. If we wish to understand the dynamic response of the combined structure, this testing must be done in real-time, which significantly restricts both the size of the simulation and the rate at which it can be conducted. Adding parallelism to the numerical simulation can enable both larger and higher frequency real-time simulations, potentially increasing both the accuracy and the control stability of the test. We present a proof-of-concept exploration of the execution of real-time hybrid simulations (an exemplar of a more general class of cyber-mechanical systems) with parallel computations. We execute large numerical simulations within tight timing constraints and provide a reasonable assurance of timeliness and usability. We detail the operation of our system, its design features, and show how parallel execution could enable qualitatively better experimentation within the discipline of structural engineering.



Global EDF scheduling for parallel real-time tasks

Authors: Jing Li, Zheng Luo, David Ferry, Kunal Agrawal, Christopher Gill, Chenyang Lu
DOI: 10.1007/s11241-014-9213-9

Abstract: As multicore processors become ever more prevalent, it is important for real-time programs to take advantage of intra-task parallelism in order to support computation-intensive applications with tight deadlines. In this paper, we consider the global earliest deadline first (GEDF) scheduling policy for task sets consisting of parallel tasks. Each task can be represented by a directed acyclic graph (DAG) where nodes represent computational work and edges represent dependences between nodes. In this model, we prove that GEDF provides a capacity augmentation bound of 4?2/m and a resource augmentation bound of 2?1/m. The capacity augmentation bound acts as a linear-time schedulability test since it guarantees that any task set with total utilization of at most m/(4?2m) where each task’s critical-path length is at most 1/(4?2/m) of its deadline is schedulable on m cores under GEDF. In addition, we present a pseudo-polynomial time fixed-point schedulability test for GEDF; this test uses a carry-in work calculation based on the proof for the capacity bound. Finally, we present and evaluate a prototype platform—called PGEDF—for scheduling parallel tasks using global earliest deadline first (GEDF). PGEDF is built by combining the GNU OpenMP runtime system and the LITMUS-RT operating system. This platform allows programmers to write parallel OpenMP tasks and specify real-time parameters such as deadlines for tasks. We perform two kinds of experiments to evaluate the performance of GEDF for parallel tasks. (1) We run numerical simulations for DAG tasks. (2) We execute randomly generated tasks using PGEDF. Both sets of experiments indicate that GEDF performs surprisingly well and outperforms an existing scheduling techniques that involves task decomposition.



Equivalent force control combined with adaptive polynomial?based forward prediction for real?time hybrid simulation

Authors: Huimeng Zhou, David J. Wagg, Mengning Li

Abstract: The equivalent force control method uses feedback control to replace numerical iteration and solve the nonlinear equation in a real?time hybrid simulation via the implicit integration method. During the real?time hybrid simulation, a time delay typically reduces the accuracy of the test results and can even make the system unstable. The outer?loop controller of the equivalent force control method can eliminate the effect of a small time delay. However, when the actuator has a large delay, the accuracy of the test results is reduced. The adaptive forward prediction method offers a solution to this problem. Thus, in this paper, the adaptive polynomial?based forward prediction algorithm is combined with equivalent force control to improve the test accuracy and stability. The new method is shown to give good stability properties for a specimen with nonlinear stiffness by analyzing the location of the poles of the discrete transfer system. Simulations with linear and nonlinear specimens are then presented to demonstrate the effectiveness of this method. Finally, experimental results with a linear stiffness specimen and a magneto?rheological damper are used to demonstrate that this method has better accuracy than the equivalent force control method with nonadaptive delay compensation.



Hybrid fire testing in a non-linear environment using a proportional integral controller

Authors: Elke Mergny, Thomas Gernay, Guillaume Drion, Jean-Marc Franssen
DOI: 10.1108/JSFE-09-2018-0022

Abstract: Purpose – The purpose of this paper is to propose a new framework based on linear control system theory and the use of proportional (P) controller and proportional integral (PI) controller to address identified stability issues and control the time properties in hybrid fire testing. Design/methodology/approach – The paper approaches hybrid fire testing as a control problem. It establishes the state equation to give the general stability conditions. Then, it shows how P and PI controllers can be incorporated in the system to maintain stability. A virtual hybrid fire testing is performed on a 2D steel frame for validation and to compare the performance of the controllers. Findings – Control system theory provides an efficient framework for hybrid fire testing and rigorously formulate the stability conditions of the system. The use of a P-controller stabilises the process, but this controller is not suitable for continuous change of stiffness of the substructures. In contrast, a PI-controller handle the stiffness changes. The results of a virtual hybrid fire testing of a 2D steel frame shows that the PI-controller succeeds in reproducing the global behaviour of the frame, even if the surrounding structure is non-linear and subjected to fire. Originality/value – The paper provides a rigorous formulation of the general problem of hybrid fire testing and shows the interest of a PI controller to control the process under varying stiffness. This methodology is a step forward for hybrid fire testing because it allows capturing the global behaviour of a structure, including where the numerical substructure behaves nonlinearly and is subjected to fire.



Cyber-physical structural optimization using real-time hybrid simulation

Authors:RuiyangZhang, Brian M.Phillips, Pedro L.Fernández-Cabán, Forrest J.Masters
DOI: 10.1016/j.engstruct.2019.05.042

Abstract: Traditionally, structural optimization is a numerical process; candidate designs are created and evaluated through numerical simulation (e.g., finite element analysis). However, when dealing with complex structures that are difficult to model numerically, large errors could exist between the numerical model and the physical structure. In this case, the optimization is less meaningful because the optimal results are associated with the numerical model instead of the physical structure. Experiments can be included in the optimization algorithm to represent complex structures or components. However, the time and cost limitations are prohibitive when iteratively constructing and evaluating complete structural systems. Real-time hybrid simulation (RTHS) is an efficient and cost-effective experimental tool that combines numerical simulation with experimental testing to capture the total structural performance. This paper proposes a framework for real-time hybrid optimization (RTHO); RTHS is used to evaluate the performance of candidate designs within the optimization process. The framework creates a cyber-physical optimization environment using RTHS, a modern experimental technique with roots in earthquake engineering. This paper outlines the framework for RTHO with accompanying proof-of-concept studies. In a preliminary study, the base isolation design of a two-story building was optimized for seismic protection. RTHO was further validated for the optimal selection of multiple semi-active control law parameters for an MR damper installed in the isolation layer of a five-story base-isolated building. Both cases used RTHS to evaluate the candidate designs and particle swarm optimization (PSO) to drive the optimization. RTHO is well-suited to evaluate nonlinear experimental substructures, in particular those that do not undergo permanent damage such as structural control devices. Structural damage, if of interest, can be modeled through the numerical component. This paper proposes and demonstrates the integration of state-of-the-art optimization algorithms with state-of-the-art experimental methods – a cyber-physical approach to structural optimization.



Optimal Design in Wind Engineering Using Cyber-Physical Systems and Non-Stochastic Search Algorithms

Authors:Michael L. Whiteman, Pedro L. Fernández Cabán, Brian M. Phillips, Forrest J. Masters, Jennifer A. Bridge, Justin R. Davis
DOI: 10.1061/9780784481349.007

Abstract:This paper explores a cyber-physical systems (CPS) approach to optimize the design of rigid, low-rise structures subjected to wind loading. The approach combines the accuracy of physical wind tunnel testing with the ability to efficiently explore a solution space using numerical optimization algorithms. The approach is fully automated, with experiments executed in a boundary layer wind tunnel (BLWT), sensor feedback monitored by a computer, and actuators used to generate physical changes to a mechatronic structural model. The approach was demonstrated for a low-rise structure with a parapet wall of variable height. A non-stochastic optimization algorithm was implemented to search along the domain of parapet heights to minimize both positive and negative pressures on the roof a of a 1:18 length scale low-rise building model. Experiments were conducted at the University of Florida Experimental Facility (UFEF) of the National Science Foundation’s (NSF) Natural Hazard Engineering Research Infrastructure (NHERI) program.



Optimal design of structures using cyber-physical wind tunnel experiments with mechatronic models

Authors: Michael L.Whiteman, Brian M.Phillips, Pedro L.Fernández-Cabán, Forrest J.Masters, Jennifer A. Bridge, Justin R.Davis

Abstract:This paper explores the use of a cyber-physical systems (CPS) approach to optimize the design of rigid, low-rise structures subjected to wind loading, with the intent of producing a foundational method to study more complex structures through future research. The CPS approach combines the accuracy of physical wind tunnel testing with the ability to efficiently explore a search space using numerical optimization algorithms. The approach is fully automated, with experiments executed in a boundary layer wind tunnel (BLWT), sensor feedback monitored by a computer, and actuators used to bring about physical changes to a mechatronic structural model. Because the model is undergoing physical change as it approaches the optimal solution, this approach is given the name “loop-in-the-model” optimization. Proof-of-concept was demonstrated for a low-rise structure with a parapet wall of variable height. Parapet walls alter the location of the roof corner vortices, reducing suction loads on the windward facing roof corners and edges and setting up an interesting optimal design problem. In the BLWT, the parapet height was adjusted using servo-motors to achieve a particular design. Experiments were conducted at the University of Florida Experimental Facility (UFEF) of the National Science Foundation's (NSF) Natural Hazard Engineering Research Infrastructure (NHERI) program.



Application of model-based compensation methods to real-time hybrid simulation benchmark

Authors: Gaston A. Fermandois
DOI: 10.1016/j.ymssp.2019.05.041

Abstract:Real-time hybrid simulation (RTHS) is an experimental testing technique widely used for performance evaluation of structural systems such as large buildings and bridges subjected to earthquake loading. While RTHS testing has demonstrated over the last 20?years to be an efficient and cost-effective alternative to shaking table tests, especially for large structural systems with rate-dependent behavior, accurate and stable results from this methodology are highly dependent on the test specimen, loading equipment, and controller design for dynamic compensation. This paper presents a study on the accuracy and stability of model-based compensation (MBC) approaches for the implementation of a real-time hybrid simulation benchmark problem. The controller architecture is based on feedforward compensator, designed for reference tracking, while a feedback regulator provides improved robustness for undesired disturbance and sensor noise. The results provide evidence of the improved performance of MBC controllers compared to benchmark results. Moreover, the MBC controllers surpass the benchmark controller in terms of robustness, when multiple partitioning cases and control plant uncertainty are considered in the numerical simulations.



A computational framework for fast?time hybrid simulation based on partitioned time integration and state?space modeling

Authors: Giuseppe Abbiati, Igor Lanese, Enrico Cazzador, Oreste S. Bursi, Alberto Pavese

Abstract: Hybrid simulation reproduces the experimental response of large? or even full?scale structures subjected to a realistic excitation with reduced costs compared with shake table testing. A real?time control system emulates the interaction between numerical substructures, which replace subparts having well?established computational models, and physical substructures tested in the laboratory. In this context, state?space modeling, which is quite popular in the community of automatic control, offers a computationally cheaper alternative to the finite?element method for implementing nonlinear numerical substructures for fast?time hybrid simulation, that is, with testing timescale close to one. This standpoint motivated the development of a computational framework based on partitioned time integration, which is well suited for hard real?time implementations. Partitioned time integration, which relies on a dual assembly of substructures, enables coupling of state?space equations discretized with heterogeneous time step sizes. In order to avoid actuators stopping at each simulation step, the physical substructure response is integrated with the same rate of control system, whereas a larger time step size is allowed on the numerical substructure compatibly with available computational resources. Fast?time hybrid simulations of a two?pier reinforced concrete bridge tested at the EUCENTRE Experimental Laboratory of Pavia, Italy, are presented as verification example.



Design of Experimental Apparatus for Real-Time Wind-Tunnel Hybrid Simulation of Bridge Decks and Buildings

Authors: Oh-Sung Kwon, Ho-Kyung Kim, Un Yong Jeong, You-Chan Hwang
DOI: 10.1061/9780784482247.022

Abstract: Due to the challenges in numerical simulation of wind-structure interaction, the dynamic response of long-span bridges or high-rise buildings subjected to wind loads has been primarily evaluated through wind tunnel tests. The wind-tunnel tests, especially aeroelastic tests, require calibration of springs, masses, and the damping properties of an experimental specimen which takes considerable time and efforts. In hybrid simulation, where a numerical model and a physical specimen are tightly integrated, a component that is difficult to be represented with a numerical model is represented experimentally, while the rest of the structural system is represented numerically. In this paper, designs of two configurations of experimental apparatus for real-time wind-tunnel hybrid simulation are presented: one for section model tests of bridge decks and another one for high-rise buildings. The experimental apparatus for section model tests, which consists of four linear motors, is for aeroelastic tests of section model of a long-span bridge. The experimental apparatus for buildings consists of two linear motors to test aeroelastic response of scaled high-rise building model. The rational on the selection of the design configurations is discussed which is followed by configuration of the experimental setup and a potential strategy for running real-time hybrid simulation.



Computational Challenges in Real-Time Hybrid Simulation of Tall Buildings under Multiple Natural Hazards

Authors: Chinmoy Kolay, James M. Ricles, Thomas M. Marullo, Safwan Al-Subaihawi, Spencer E. Quiel
DOI: 10.4028/

Abstract: The essence of real-time hybrid simulation (RTHS) is its ability to combine the benefits of physical testing with those of computational simulations. Therefore, an understanding of the real-time computational issues and challenges is important, especially for RTHS of large systems, in advancing the state of the art. To this end, RTHS of a 40-story (plus 4 basement stories) tall building having nonlinear energy dissipation devices for mitigation of multiple natural hazards, including earthquake and wind events, were conducted at the NHERI Lehigh Experimental Facility. An efficient implementation procedure of the recently proposed explicit modified KR-a (MKR-a) method was developed for performing the RTHS. This paper discusses this implementation procedure and the real-time computational issues and challenges with regard to this implementation procedure. Some results from the RTHS involving earthquake loading are presented to highlight the need for and application of RTHS in performance based design of tall buildings under earthquake hazard.



Seismic Performance of Steel MRF Structures with Nonlinear Viscous Dampers from Real-Time Hybrid Simulations

Authors: Bai Ping Dong, Richard Sause, James M. Ricles
DOI: 10.4028/

Abstract: Real-time hybrid earthquake simulations (RTHS) were performed on steel moment-resisting frame (MRF) structures with nonlinear viscous dampers. The test structures for the RTHS contain a moment-resisting frame (MRF), a frame with nonlinear viscous dampers (DBF), and a gravity load system with associated seismic mass and gravity loads. The MRFs have reduced beam section beam-to-column connections and are designed for 100%, 75%, and 60%, respectively, of the base shear strength required by ASCE 7-10. RTHS were performed to evaluate the seismic performance of these MRF structures. Two phases of RTHS were conducted: (Phase-1) the DBF is the experimental substructure in the laboratory; and (Phase-2) the DBF with the MRF is the experimental substructure. Results from the two phases of RTHS are evaluated. The evaluation shows that the RTHS provide a realistic and accurate simulation of the seismic response of the test structures. The evaluation also shows that steel MRF structures designed with reduced strength and with nonlinear viscous dampers can have excellent seismic performance.



Accurate real-time hybrid earthquake simulations on large-scale MDOF steel structure with nonlinear viscous dampers

Authors: Baiping Dong, Richard Sause, James M. Ricles
DOI: 10.1002/eqe.2572

Abstract: This paper presents real-time hybrid earthquake simulation (RTHS) on a large-scale steel structure with nonlinear viscous dampers. The test structure includes a three-story, single-bay moment-resisting frame (MRF), a three-story, single-bay frame with a nonlinear viscous damper and associated bracing in each story (called damped braced frame (DBF)), and gravity load system with associated seismic mass and gravity loads. To achieve the accurate RTHS results presented in this paper, several factors were considered comprehensively: (1) different arrangements of substructures for the RTHS; (2) dynamic characteristics of the test setup; (3) accurate integration of the equations of motion; (4) continuous movement of the servo-controlled hydraulic actuators; (5) appropriate feedback signals to control the RTHS; and (6) adaptive compensation for potential control errors. Unlike most previous RTHS studies, where the actuator stroke was used as the feedback to control the RTHS, the present study uses the measured displacements of the experimental substructure as the feedback for the RTHS, to enable accurate displacements to be imposed on the experimental substructure. This improvement in approach was needed because of compliance and other dynamic characteristics of the test setup, which will be present in most large-scale RTHS. RTHS with ground motions at the design basis earthquake and maximum considered earthquake levels were successfully performed, resulting in significant nonlinear response of the test structure, which makes accurate RTHS more challenging. Two phases of RTHS were conducted: in the first phase, the DBF is the experimental substructure, and in the second phase, the DBF together with the MRF is the experimental substructure. The results from the two phases of RTHS are presented and compared with numerical simulation results. An evaluation of the results shows that the RTHS approach used in this study provides a realistic and accurate simulation of the seismic response of a large-scale structure with rate-dependent energy dissipating devices.



Large-scale real-time hybrid simulation of a three-story steel frame building with magneto-rheological dampers

Authors: Yunbyeong Chae1, James M. Ricles, Richard Sause

Abstract: A series of large-scale real-time hybrid simulations (RTHSs) are conducted on a 0.6-scale 3-story steel frame building with magneto-rheological (MR) dampers. The lateral force resisting system of the prototype building for the study consists of moment resisting frames and damped brace frames (DBFs). The experimental substructure for the RTHS is the DBF with the MR dampers, whereas the remaining structural components of the building including the moment resisting frame and gravity frames are modeled via a nonlinear analytical substructure. Performing RTHS with an experimental substructure that consists of the complete DBF enables the effects of member and connection component deformations on system and damper performance to be accurately accounted for. Data from these tests enable numerical simulation models to be calibrated, provide an understanding and validation of the in-situ performance of MR dampers, and a means of experimentally validating performance-based seismic design procedures for real structures. The details of the RTHS procedure are given, including the test setup, the integration algorithm, and actuator control. The results from a series of RTHS are presented that includes actuator control, damper behavior, and the structural response for different MR control laws. The use of the MR dampers is experimentally demonstrated to reduce the response of the structure to strong ground motions. Comparisons of the RTHS results are made with numerical simulations. Based on the results of the study, it is concluded that RTHS can be conducted on realistic structural systems with dampers to enable advancements in resilient earthquake resistant design to be achieved.



Hybrid fire testing: Discussion on stability and implementation of a new method in a virtual environment

Authors: Ana Sauca, Thomas Gernay, Fabienne Robert, Nicola Tondini, Jean-Marc Franssen
DOI: 10.1108/JSFE-01-2017-0017

Abstract: Purpose – The purpose of this paper is to propose a method for hybrid fire testing (HFT) which is unconditionally stable, ensures equilibrium and compatibility at the interface and captures the global behavior of the analyzed structure. HFT is a technique that allows assessing experimentally the fire performance of a structural element under real boundary conditions that capture the effect of the surrounding structure. Design/methodology/approach – The paper starts with the analysis of the method used in the few previous HFT. Based on the analytical study of a simple one degree-of-freedom elastic system, it is shown that this previous method is fundamentally unstable in certain configurations that cannot be easily predicted in advance. Therefore, a new method is introduced to overcome the stability problem. The method is applied in a virtual hybrid test on a 2D reinforced concrete beam part of a moment-resisting frame. Findings – It is shown through analytical developments and applicative examples that the stability of the method used in previous HFT depends on the stiffness ratio between the two substructures. The method is unstable when implemented in force control on a physical substructure that is less stiff than the surrounding structure. Conversely, the method is unstable when implemented in displacement control on a physical substructure stiffer than the remainder. In multi-degrees-of-freedom tests where the temperature will affect the stiffness of the elements, it is generally not possible to ensure continuous stability throughout the test using this former method. Therefore, a new method is proposed where the stability is not dependent on the stiffness ratio between the two substructures. Application of the new method in a virtual HFT proved to be stable, to ensure compatibility and equilibrium at the interface and to reproduce accurately the global structural behavior. Originality/value – The paper provides a method to perform hybrid fire tests which overcomes the stability problem lying in the former method. The efficiency of the new method is demonstrated in a virtual HFT with three degrees-of-freedom at the interface, the next step being its implementation in a real (laboratory) hybrid test.



Mixed Force and Displacement Control for Testing Base-Isolated Bearings in Real-Time Hybrid Simulation

Authors: Narutoshi Nakata, Richard Erb, Matthew Stehman

Abstract: This paper presents a robust mixed force and displacement control strategy for testing of base isolation bearings in real-time hybrid simulation. The mixed-mode control is a critical experimental technique to impose accurate loading conditions on the base isolation bearings. The proposed mixed-mode control strategy consists of loop-shaping and proportional-integral-differential controllers. Following experimental validation, the mixed-mode control was demonstrated through a series of real-time hybrid simulation. The experimental results showed that the developed mixed-mode control enables accurate control of dynamic vertical force on the base isolation bearings during real-time hybrid simulation.



IIR Compensation in Real-Time Hybrid Simulation using Shake Tables with Complex Control-Structure-Interaction

Authors: Matthew Stehman, Narutoshi Nakata
DOI: 10.1080/13632469.2015.1104745

Abstract: This article considers the use of actuator compensation in real-time hybrid simulation (RTHS) containing experimental substructures with complex control-structure-interaction (CSI). The existence of CSI in shake table testing is derived using theoretical relations. An infinite-impulse-response (IIR) compensator is developed to compensate for the shake table time delay as well as the effects of CSI. The efficacy of the IIR compensator is verified through numerical and experimental investigations of substructure shake table testing completed at Johns Hopkins University. IIR compensation is not limited to substructure shake table testing, and the concept is applicable to any RTHS that suffers from complex CSI.



Compensation techniques for experimental errors in real-time hybrid simulation using shake tables

Authors: Narutoshi Nakata, Matthew Stehman
DOI: 10.12989/sss.2014.14.6.1055

Abstract: Substructure shake table testing is a class of real-time hybrid simulation (RTHS). It combines shake table tests of substructures with real-time computational simulation of the remaining part of the structure to assess dynamic response of the entire structure. Unlike in the conventional hybrid simulation, substructure shake table testing imposes acceleration compatibilities at substructure boundaries. However, acceleration tracking of shake tables is extremely challenging, and it is not possible to produce perfect acceleration tracking without time delay. If responses of the experimental substructure have high correlation with ground accelerations, response errors are inevitably induced by the erroneous input acceleration. Feeding the erroneous responses into the RTHS procedure will deteriorate the simulation results. This study presents a set of techniques to enable reliable substructure shake table testing. The developed techniques include compensation techniques for errors induced by imperfect input acceleration of shake tables, model-based actuator delay compensation with state observer, and force correction to eliminate process and measurement noises. These techniques are experimentally investigated through RTHS using a uni-axial shake table and three-story steel frame structure at the Johns Hopkins University. The simulation results showed that substructure shake table testing with the developed compensation techniques provides an accurate and reliable means to simulate the dynamic responses of the entire structure under earthquake excitations.



Intercontinental Hybrid Simulation for the Assessment of a Three-Span R/C Highway Overpass

Authors: Stathis Bousias, Anastasios Sextos, Oh-Sung Kwon, Olympia Taskari, Amr Elnashai, Nikos Evangeliou, Luigi Di Sarno

Abstract: This paper presents hybrid simulations of a three-span R/C bridge among EU, US, and Canada. The tests involved partners located on both sides of the Atlantic with each one assigned a numerical or a physical module of the substructured bridge. Despite the network latency in linking remote sites located on the two sides of the Atlantic the intercontinental hybrid simulation was accomplished and repeated successfully, highlighting the efficiency, and repetitiveness of the approach. Adaptations, challenges, and limitations are discussed, focusing on the implications of network communication latency, the insensitivity of the sub-structuring arrangement, and the accuracy of the results obtained.



Real-time hybrid simulation of smart base-isolated raised floor systems for high-tech industry

Authors: Pei-Ching Chen, Shiau-Ching Hsu, You-Jin Zhongand Shiang-Jung Wang
DOI: 10.12989/sss.2019.23.1.091

Abstract: Adopting sloped rolling-type isolation devices underneath a raised floor system has been proved as one of the most effective approaches to mitigate seismic responses of the protected equipment installed above. However, pounding against surrounding walls or other obstructions may occur if such a base-isolated raised floor system is subjected to long-period excitation, leading to adverse effects or even more severe damage. In this study, real-time hybrid simulation (RTHS) is adopted to assess the control performance of a smart base-isolated raised floor system as it is an ef?cient and cost-effective experimental method. It is composed of multiple sloped rolling-type isolation devices, a rigid steel platen, four magnetorheological (MR) dampers, and protected high-tech equipment. One of the MR dampers is physically tested in the laboratory while the remainders are numerically simulated. In order to consider the effect of input excitation characteristics on the isolation performance, the smart base-isolated raised floor system is assumed to be located at the roof of a building and the ground level. Four control algorithms are designed for the MR dampers including passive-on, switching, modified switching, and fuzzy logic control. Six artificial spectrum-compatible input excitations and three slope angles of the isolation devices are considered in the RTHS. Experimental results demonstrate that the incorporation of semi-active control into a base-isolated raised floor system is effective and feasible in practice for high-tech industry.



A control framework for uniaxial shaking tables considering tracking performance and system robustness

Authors: Pei?Ching Chen, Chin?Ta Lai, Keh?Chyuan Tsai
DOI: 10.1002/stc.2015

Abstract: Shaking table testing has been regarded as one of the most straightforward experimental approaches to evaluate the seismic response of structures subjected to earthquake ground motions. Therefore, reproducing an acceleration time history accurately becomes crucial for shaking table testing. In this study, a control framework for uniaxial shaking tables is proposed which incorporates a feedback controller into a weighted command shaping controller. It implements through outer?loop control in addition to the conventional existing proportional?integral inner?loop control. The model?based command shaping controller which considers the control? structure interaction can be designed to shape either displacement or acceleration references. The weightings for the shaped displacement and acceleration can be calculated by a linear interpolation algorithm which considers the dominant frequency of the desired acceleration time history as well as the correlation between the displacement and acceleration responses of the shaking table. Accordingly, the weighted combination of the shaped displacement and acceleration generates the control command to the shaking table. On the other hand, the feedback controller deals with the system uncertainty and modeling error. Loop?shaping design method is adopted to synthesize the feedback controller. Finally, the control framework is verified by several shaking table tests with and without a flexible specimen. Experimental results demonstrate the performance and robustness of the proposed control framework for shaking table test systems.



Adaptive model-based tracking control for real-time hybrid simulation

Authors: Pei-Ching Chen, Chia-Ming Chang, Billie F. Spencer Jr., Keh-Chyuan Tsai
DOI: 10.1007/s10518-014-9681-2

Abstract: Model-based feedforward–feedback tracking control has been shown as one of the most effective methods for real-time hybrid simulation (RTHS). This approach assumes that the servo-hydraulic system is a linear time-invariant model. However, the servo-control closed-loop is intrinsically nonlinear and time-variant, particularly when one considers the nonlinear nature of typical experimental components (e.g., magnetorheological dampers). In this paper, an adaptive control scheme applying on a model-based feedforward–feedback controller is proposed to accommodate specimen nonlinearity and improve the tracking performance of the actuator, and thus, the accuracy of RTHS. This adaptive strategy is used to estimate the system parameters for the feedforward controller online during a test. The robust stability of this adaptive controller is provided by introducing Routh’s stability criteria and applying a parameter projection algorithm. The tracking performance of the proposed control scheme is analytically evaluated and experimentally investigated using a broadband displacement command, and the results indicates better tracking performance for the servo-hydraulic system can be attained. Subsequently, RTHS of a nine-story shear building controlled by a full-scale magnetorheological damper is conducted to verify the efficacy of the proposed control method. Experimental results are presented for the semi-actively controlled building subjected to two historical earthquakes. RTHS using the adaptive feedforward–feedback control scheme is demonstrated to be effective for structural performance assessment.



Hybrid Model for Railroad Bridge Dynamics

Authors: Robin E. Kim, Fernando Moreu, Billie F. Spencer Jr.
DOI: 10.1061/(ASCE)ST.1943-541X.0001530

Abstract: Railroads carry approximately 40% of the ton-miles of the freight in the United States. On the average, a bridge occurs every 2.25 km (1.4 mi) of track, making them critical elements. The primary load on the railroad bridges is the train, resulting in numerous models being developed to understand the dynamic response of bridges under train loads. However, because the problem is time-dependent and coupled, developing adequate models is challenging. Most of the proposed models fail to provide a simple yet flexible representation of the train, bridge, and track. This paper proposes a new hybrid model that is effective for solving the track–bridge interaction problem under moving trains. The main approach is to couple the finite-element model of the bridge with a continuous beam model of the track using the assumed modes method. Both single-track and multitrack bridges are considered. The hybrid model is validated against field measurements for a double-track bridge. This model is then used to predict critical train speeds. The results demonstrate that the hybrid model provides an effective and fundamental tool for predicting bridge dynamics subject to moving trains. The flexible feature of the model will allow accommodating more sophisticated vehicle models and track irregularities.



Multiple Degrees of Freedom Positioning Correction for Hybrid Simulation

Authors: Chia-Ming Chang, Thomas M. Frankie, Billie F. Spencer Jr., Daniel A. Kuchma
DOI: 10.1080/13632469.2014.962670

Abstract: This study proposes a high-precision positioning correction method for multiple degree-of-freedom loading units in hybrid simulation. These loading units can impose inaccurate displacements to the specimens due to the elastic deformation at the reaction wall or connections. To compensate for these displacement errors, an online correction method adjusts the displacement command by the difference between the target and achieved displacement. This correction method also accompanies an accurate 6DOF monitoring system to detect the displacement errors. Two examples of hybrid simulation tests are provided to demonstrate the precise displacements attained on the specimens through this control method.



Real-Time Hybrid Simulation of a Smart Base-Isolated Building

Authors: Takehiko Asai, Chia-Ming Chang, B. F. Spencer Jr.

Abstract: Traditional passive base-isolation systems provide an effective means to mitigate the responses of seismically excited structures. A challenge for these systems can be found in accommodating the large base displacements during severe earthquakes. Recently, active baseisolation systems, combining actively controlled actuators with passive isolation bearings, have been shown experimentally to produce reduced base displacements, while maintaining similar responses of the superstructure obtained by the passive base-isolation systems. The active control devices used in hybrid isolation systems are typically driven by an external power source, which may not be available during severe seismic events. Another class of isolation systems is smart base isolation, in which semiactive control devices are used in place of their active counterparts. This control strategy has been proven effective against a wide range of seismic excitation; however, there has been limited effort to experimentally validate smart base-isolation systems. In this study, the focus is on experimentally investigating and verifying a smart baseisolation system using real-time hybrid simulation (RTHS), which provides a cost-effective means to conduct such experiments because only the portion of the structure that is poorly understood needs to be represented experimentally, while the reminder of the structure can be modeled using a computer. In this paper, a prototype magnetorheological damper is physically tested, while the isolated building concurrently is simulated numerically. A model-based compensation strategy is used to carry out high-precision RTHS. The performance of the semiactive control strategies is evaluated using RTHS, and the efficacy of the smart base-isolation system is demonstrated. This smart base-isolation system is found to reduce base displacements and floor accelerations in a manner comparable with the active isolation system without the need for large external power sources.



Substructure Hybrid Simulation Boundary Technique Based on Beam/Column Inflection Points

Authors: Zaixian Chen, Xueyuan Yan, Hao Wang, Xingji Zhu, Billie F. Spencer
DOI: 10.3390/su10082655

Abstract:Compatibility among substructures is an issue for hybrid simulation. Traditionally, the structure model is regarded as the idealized shear model. The equilibrium and compatibility of the axial and rotational direction at the substructure boundary are neglected. To improve the traditional boundary technique, this paper presents a novel substructure hybrid simulation boundary technique based on beam/column inflection points, which can effectively avoid the complex operation for realizing the bending moment at the boundary by using the features of the inflection point where the bending moment need not be simulated in the physical substructure. An axial displacement prediction technique and the equivalent force control method are used to realize the proposed method. The numerical simulation test scheme for the different boundary techniques was designed to consider three factors: (i) the different structural layers; (ii) the line stiffness ratio of the beam to column; and (iii) the peak acceleration. The simulation results for a variety of numerical tests show that the proposed technique shows better performance than the traditional technique, demonstrating its potential in improving HS test accuracy. Finally, the accuracy and feasibility of the proposed boundary technique is verified experimentally through the substructure hybrid simulation tests of a six-story steel frame model.



Adaptive model reference control method for real-time hybrid simulation

Authors: Amirali Najafi, Billie F. SpencerJr.

Abstract: The real-time hybrid simulation (RTHS) methodology is an experimental technique involving substructuring of a full-scale experiment into numerical and experimental partitions. It offers a cost-effective solution and is highly practical in confined laboratory settings. Successful implementation of RTHS is dependent on successful tracking control and robustness of the hybrid simulation loop. This paper addresses the benchmark problem in RTHS, which intends to assess available actuator tracking controllers and other advanced computational frameworks for successful RTHS implementation. Most existing control algorithms tend to instability when faced with challenges of plant uncertainty and nonlinearity. Stability has been at odds with excellent tracking, where controllers with rigorous tracking have had poor stability performance and robust controllers have had poor tracking performance. This paper introduces an Adaptive Model Reference Control (aMRC) method for displacement tracking of actuators, which offers an excellent tracking ability and maintains robustness under unmodeled dynamics and uncertainties. The proposed controller is composed of feedforward and feedback links, a reference model, and an adaptation law. The tracking and robustness performance of the proposed algorithm are evaluated through a numerical RTHS of the three-story steel frame building described in the benchmark problem statement. The benchmark problem defines different mass and damping configurations while partitioning the structure. Additionally, the experimental substructure is made uncertain by modeling several actuator and stiffness parameters probabilistically, per the benchmark problem. The performance of the proposed controller is compared to several commonly employed control techniques and assessed using the evaluation criteria described in the benchmark problem statement. The results show that the proposed aMRC algorithm tracks the desired reference signal well while maintaining robustness.



Backstepping adaptive control for real-time hybrid simulation including servo-hydraulic dynamics

Authors: Yuting Ouyang, Weixing Shia, Jiazeng Shana, Billie F. Spencer
DOI: 10.1016/j.ymssp.2019.05.042

Abstract: A backstepping adaptive control method is proposed for on-line estimation of unknown servo-hydraulic dynamics and the compensation of time-varying lags in real-time hybrid simulation tests. The response tracking problem becomes a critical challenge when realistic experimental conditions are taken into consideration, such as control-structure interaction effects and sensor measurement noise. Unlike a conventional time-lag compensator, the proposed adaptive controller generates a command trajectory for the actuated system according to adaptive laws. Besides bringing response tracking error to zeros, the estimation of a first-principle actuator dynamic model is also facilitated in the proposed approach. Lyapunov stability analysis is systematically presented for designing the adaptive control law. Illustratively, a three-story seismically excited structure with different control strategies is utilized to demonstrate the efficiency and robustness of the proposed controller. A benchmark problem is then utilized for the verification of controller’s advancement. Four simulation cases with different damping/mass conditions and four ground excitation scenarios are selected for the application. As stated, favorable tracking performance has been observed with a remarkable improvement in performance evaluation.



Continuous Real-Time Hybrid Simulation Method for Structures Subject to Fire

Authors: Xuguang Wang, Robin E. Kim, Oh-Sung Kwon, In-Hwan Yeo
DOI: : 10.1061/(ASCE)ST.1943-541X.0002436

Abstract: The continuous hybrid fire-simulation method proposed in this paper is a robust method that allows numerical models with a certain level of complexity to be used in a real-time hybrid fire simulation. Extrapolation and interpolation are used for continuously generating displacement commands during the simulation. The elastic deformation of the loading frame is compensated for during the continuous command generation. The stability issues relating to the stiffness of the loading system and the proposed error-compensation scheme are discussed in depth. A large-scale hybrid fire simulation was carried out to validate the proposed method. A steel moment-resisting frame with reduced beam section connections was selected for the validation test. One column of the selected structure was physically represented in the lab, and the rest of the structure was modeled numerically. The physical specimen was heated with a standard fire curve, with the temperature in the numerical model increasing following the numerical heat-transfer analysis result. A multiresolution numerical model was used as the numerical substructure. The test results confirmed the proposed method can accurately simulate the behavior of a structure subjected to high temperature and subsequent failure.