Chapter 2 Review of Literature
2.4 Experimental evaluation of seismic performance of structural components
2.4.1 Hybrid simulation
Compared to the above two testing methods, hybrid simulation is the most advanced seismic testing. In this testing method, a simulation of responses of the sample structure under seismic loading is based on a step-by-step numerical integration of governing equations of motion considering both the numerical and physical components of a structural system.
Contrary to purely numerical simulation, the hybrid simulation method combines the physical testing of a part of the structure i.e. substructures with numerical models of the remainder of the structure. It provides complete picture of how earthquake events can affect large structures such as buildings and bridges without having to physically test the entire structure. This enables structural engineers to accurately and efficiently capture the effects that a substructure has on the overall structure, while subjecting the substructure to the same forces and motions it would experience as part of the prototype structure. During the simulation, the physical model of the overall hybrid model are tested in one or more laboratories using computer controlled actuators, while the numerical model are simultaneously analysed on one or more analytical platforms. Considerable research is being carried out worldwide to extend hybrid simulation to applications where there are complexities in non-linear modelling in numerical techniques. Efforts are in progress to improve potentialities for testing portions of the structure in different laboratories of the world, including techniques to overcome the deficiencies of network quality (Schellenberg and Mahin 2006a). Fundamental studies for hybrid simulation are still in process related to improving approaches used for formulation and solution. Characterization and correction for errors related to numerical modelling and simulation, experimental control, scaling, strain rate effects and latency were also studied by many researchers.
Schellenberg and Mahin (2006b) demonstrated how geometric nonlinearities could be accounted for in the numerical module of hybrid simulation on a simple one-story one- bay portal frame with two ductile columns, wherein large deformation (second-order) effects due to gravity loads were consistently taken into account till collapse state.
Schellenberg et al. (2008) presented extension of the hybrid simulation testing method to model structural collapse. Hybrid simulation test was performed on a one-storey
2.4 Experimental evaluation of seismic performance of structural components
portal frame with two ductile columns until collapse to demonstrate and validate the newly proposed approach based on considering OpenSees and OpenFresco. The physical specimens of the columns were not axially loaded, as an alternative, second-order analytical geometric transformations involving the degrees-of-freedom of the experimental subassemblies were used to affect the actions of the axial load. This eliminated the need for complex active or passive gravity load setups. Comparison of results of the two hybrid simulations- with and without accounting for gravity load effects, showed that the hybrid model with P-Delta effects developed negative post-peak stiffness and thus increasing lateral displacements until collapse occured.
Whyte et al. (2008) evaluated the quality of hybrid simulation using a control-theory concept of reachability analysis of a dynamic system, which predicts the sets of states the systems can attain under uncertain dynamic excitation starting from uncertain initial conditions evaluated using uncertain measurements. This method was used to compute the size of reachable tubes (an ellipsoidal tube of the trajectory envelope of states of the system at any instant during a hybrid simulation) for response of a linear single degree of freedom (SDOF) system responding to dynamic excitation such as free vibration and under earthquake excitations. They investigated propagation of disturbances representing the uncertainty of the initial condition of the system and the uncertainty associated with the application of the dynamic excitation to the system response by computing the size of the ellipsoid approximation of the state of the system at each instant of its dynamic response.
They also presented a comparison of trajectory envelopes of the system responding to the same excitation under different magnitudes of disturbance. The results established the success of the proposed methods for evaluating the quality of hybrid simulation.
Yang et al. (2008) proposed an on-line error monitoring method to assess the severity of experimental errors during a hybrid simulation. This method predicts the accuracy of the simulation results during the simulation. Hybrid simulation of seismic response of an innovative lateral load resisting system is used to demonstrate the effectiveness of the proposed on-line error monitoring indicators.
Hung (2010) investigated the seismic behaviour of RC Coupled wall systems (CWSs) in which high performance fibre reinforced concrete is used to replace regular concrete in vulnerable regions of the structure through computational and hybrid simulation techniques. A strategy for estimating the tangent stiffness of structures during hybrid simulation was proposed. It was shown that when the strategy was combined with the widely used Operator Splitting Method (OSM) for hybrid simulation, the simulation accuracy got enhanced compared to the traditional OSM. They developed a new conditionally stable algorithm, called Full Operator Method (FOM). They showed that FOM has enhanced accuracy compared to OSM and that it is possible to modify FOM into an unconditionally stable algorithm for cases where the estimated tangent stiffness is larger than the real tangent stiffness. Hybrid simulation of an 18-story prototype based on FOM indicated that the new technique was able to model seismic behaviour of CWSs accurately.
Yang et al. (2012) proposed an online optimization method to improve the accuracy of numerical–experimental hybrid simulations of bridges subjected to earthquake forces.
The online optimization method aimed to solve the problem induced by the inconsistency of the multiple identical bridge piers, where only one or a few of them were simulated experimentally by physical specimens, while the others were numerically simulated within a hybrid framework.
2.4 Experimental evaluation of seismic performance of structural components
The number of researchers working on hybrid simulation have considerably increased worldwide over the last few years, that resulted increase in numbers of publications as well. However, in India the status of hybrid simulation is still at nascent stage.