M. Javad Hashemi

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

AbstractThe 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 th...

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

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.

Development and validation of multi-axis substructure testing system for full-scale experiments

Abstract Structural engineers are engaged in the development, design, construction and maintenance of new generation smart structures that are capable of withstanding multiple catastrophic events such as earthquakes, fire and tsunamis. Accordingly, the prediction of structural performance from initial linear-elastic behaviour to collapse is essential to assess the effectiveness of new design methods and the implementation of new retrofitting strategies. Although there has been much advancement in numerical methods, experimental observations remain critical for better understanding and predicting a structure’s response. The Multi-Axis Substructure Testing (MAST) system has been developed to expand the capabilities of experimental testing for three-dimensional simulation of extreme loads on critical components of large and complex structural systems. The MAST system uses eight high-capacity hydraulic actuators and a sophisticated 6-DOF mixed-mode control system that enables simultaneous application of multi-directional continuously varying states of deformation or load to structural components. An overview of the MAST facility and its components, the actuator assemblies and the details of 6-DOF control system are presented in this paper. Three experiments including quasi-static cyclic, local and distributed hybrid simulation tests were conducted on a seismically excited concrete structure to validate the performance of the MAST system in mixed-mode control by imposing simultaneously the axial load in force control and lateral deformations in displacement control.

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

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 highlynonlinear 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.

Response of Earthquake-Damaged RC Columns Repaired with CFRP Composites Using Hybrid Simulation

International experience from the effects of past earthquakes on the existing reinforced-concrete (RC) structures with limited-ductility has shown that many behave poorly and some possess very low levels of safety, to the extent that they are at risk of collapse. While the seriously damaged RC frames may be demolished and reconstructed, a large number of earthquake-damaged RC frames can be repaired and operative again. The primary objective of this paper is to evaluate the capability of carbon-fibre reinforced polymer (CFRP) repair on rehabilitating the earthquake-damaged columns to their initial collapse resistance capacity. A state-of-the-art hybrid testing facility, referred to as the Multi-Axis Substructure Testing (MAST) system, was used to simulate complex time-varying six-degrees-of-freedom (6-DOF) boundary effects on the physical specimens using mixed load/deformation modes. Based on the experimental results, a comparative collapse risk assessment of the column before and after repair was conducted, which illustrates the effectiveness of using CFRP-repair to restore and improve the collapse resistance of earthquake-damaged RC structures.

A Multi-Peak Evolutionary Model for Stochastic Simulation of Ground Motions Based on Time-Domain Features

ABSTRACT This paper introduces an efficient stochastic method to produce fully nonstationary records having multiple peaks in power spectrum. The zero-crossing characteristics of the acceleration, velocity, and displacement time histories of the output signal are used to estimate the model parameters. This procedure is utilized to resimulate 252 non-pulse-like, horizontal near-field records with rupture distance of less than 10 km and strike–slip mechanism. The model parameters are regressed against moment magnitude, rupture distance, hypo-central depth, and shear wave velocity, enabling the scenario-based simulation of the records. The response spectra of the simulated records are compared with NGA-West2 ground motion prediction equations.