ALBERT

Description: 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. © 2010 John Wiley & Sons, Ltd.

Description: Real-time testing with dynamic substructuring is a novel experimental technique capable of assessing the behaviour of structures subjected to dynamic loadings including earthquakes. The technique involves recreating the dynamics of the entire structure by combining an experimental test piece consisting of part of the structure with a numerical model simulating the remainder of the structure. These substructures interact in real time to emulate the behaviour of the entire structure. Time integration is the most versatile method for analysing the general case of linear and non-linear semi-discretized equations of motion. In this paper we propose for substructure testing, L-stable real-time (LSRT) compatible integrators with two and three stages derived from the Rosenbrock methods. These algorithms are unconditionally stable for uncoupled problems and entail a moderate computational cost for real-time performance. They can also effectively deal with stiff problems, i.e. complex emulated structures for which solutions can change on a time scale that is very short compared with the interval of time integration, but where the solution of interest changes on a much longer time scale. Stability conditions of the coupled substructures are analysed by means of the zero-stability approach, and the accuracy of the novel algorithms in the coupled case is assessed in both the unforced and forced conditions. LSRT algorithms are shown to be more competitive than popular Runge-Kutta methods in terms of stability, accuracy and ease of implementation. Numerical simulations and real-time substructure tests are used to demonstrate the favourable properties of the proposed algorithms. Copyright © 2007 John Wiley & Sons, Ltd.

Description: A multi-level pseudo-dynamic (PSD) seismic test programme was performed on a full-scale three-bay two-storey steel-concrete composite moment-resisting frame built with partially encased composite columns and partial-strength connections. The system was designed to provide strength and ductility for earthquake resistance with energy dissipation located in ductile components of beam-to-column joints including flexural yielding of beam end-plates and shear yielding of the column web panel zone. In addition, the response of the frame depending on the column base yielding was analysed. Firstly, the design of the test structure is presented in the paper, with particular emphasis on the ductile detailing of beam-to-column joints. Details of the construction of the test structure and the test set-up are also given. The paper then provides a description of the non-linear static and dynamic analytical studies that were carried out to preliminary assess the seismic performance of the test structure and establish a comprehensive multi-level PSD seismic test programme. The resulting test protocol included the application of a spectrum-compatible earthquake ground motion scaled to four different peak ground acceleration levels to reproduce an elastic response as well as service ability, ultimate, and collapse limit state conditions, respectively. Severe damage to the building was finally induced by a cyclic test with stepwise increasing displacement amplitudes. Copyright © 2008 John Wiley & Sons, Ltd.

Description: This paper presents the results of a multi-level pseudo-dynamic seismic test program that was performed to assess the performance of a full-scale three-bay, two-storey steel-concrete composite moment-resisting frame built with partially encased composite columns and partial-strength beam-to-column joints. The system was designed to develop a ductile response in the joint components of beam-to-column joints including flexural yielding of beam end plates and shear yielding of the column web panel zone. The ground motion producing the damageability limit state interstorey drift caused minor damage while the ultimate limit state ground motion level entailed column web panel yielding, connection yielding and plastic hinging at the column base connections. The earthquake level chosen to approach the collapse limit state induced more damage and was accompanied by further column web panel yielding, connection yielding and inelastic phenomena at column base connections without local buckling. During the final quasi-static cyclic test with stepwise increasing displacement-amplitudes up to an interstorey drift angle of 4.6%, the behaviour was ductile although cracking of beam-to-end-plate welds was observed. Correlations with numerical simulations taking into account the inelastic cyclic response of beam-to-column and column base joints are also presented in the paper together. Inelastic static pushover and time history analysis procedures are used to estimate the structural behaviour and overstrength factors of the structural system under study. Copyright © 2008 John Wiley & Sons, Ltd.

Description: Partial-strength composite steel-concrete moment-resisting (MR) frame structures represent an open research field in seismic design from both a theoretical and an experimental standpoint. Among experimental techniques, vibration testing is a well-known and powerful technique for damage detection, localization and quantification, where actual modal parameters of a structure at different states can be determined from test data by using system identification methods. However, the identification of semi-rigid connections in framed structures is limited, and hence this paper focuses on a series of vibration experiments that were carried out on a realistic MR frame structure, following the application of pseudo-dynamic and quasi-static cyclic loadings at the European laboratory for structural assessment of the Joint Research Centre at Ispra, Italy, with the scope of understanding the structural behaviour and identifying changes in the dynamic response. From the forced vibration response, natural frequencies, damping ratios, modal displacements and rotations were extracted using the circle fitting technique. These modal parameters were used for local and global damage identification by updating a 3D finite element model of the intact structure. The identified results were then correlated with observations performed on the structure to understand further the underlying damage mechanisms. Finally, the latin hypercube sampling technique, a variant of the Monte Carlo method, was employed in order to study the sensitivity of the updated parameters of the 3D model to noise on the modal inputs. Copyright © 2009 John Wiley & Sons, Ltd.

Description: A variant of the Rosenbrock-W integration method is proposed for real-time dynamic substructuring and pseudo-dynamic testing. In this variant, an approximation of the Jacobian matrix that accounts for the properties of both the physical and numerical substructures is used throughout the analysis process. Only an initial estimate of the stiffness and damping properties of the physical components is required. It is demonstrated that the method is unconditionally stable provided that specific conditions are fulfilled and that the order accuracy can be maintained in the nonlinear regime without involving any matrix inversion while testing. The method also features controllable numerical energy dissipation characteristics and explicit expression of the target displacement and velocity vectors. The stability and accuracy of the proposed integration scheme are examined in the paper. The method has also been verified through hybrid testing performed of SDOF and MDOF structures with linear and highly nonlinear physical substructures. The results are compared with those obtained from the operator splitting method. An approach based on the modal decomposition principle is presented to predict the potential effect of experimental errors on the overall response during testing. Copyright © 2009 John Wiley & Sons, Ltd.

Description: This paper presents novel predictor-corrector time-integration algorithms based on the Generalized-α method to perform pseudo-dynamic tests with substructuring. The implicit Generalized-α algorithm was implemented in a predictor-one corrector form giving rise to the implicit IPC-ρ∞ method, able to avoid expensive iterative corrections in view of high-speed applications. Moreover, the scheme embodies a secant stiffness formula that can closely approximate the actual stiffness of a structure. Also an explicit algorithm endowed with user-controlled dissipation properties, the EPC-ρb method, was implemented. The resulting schemes were tested experimentally both on a two- and on a six-degrees-of-freedom system, using substructuring. The tests indicated that the numerical strategies enhance the fidelity of the pseudo-dynamic test results even in an environment characterized by considerable experimental errors. Moreover, the schemes were tested numerically on severe non-linear substructured multiple-degrees-of-freedom systems reproduced with the Bouc-Wen model, showing the reliability of the seismic tests under these conditions. © 2004 John Wiley and Sons, Ltd.

Description: 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. © 2010 John Wiley & Sons, Ltd.