System and damage identification studies of a seven-story reinforced concrete building structure subjected to shake table tests

Abstract eng:
A full-scale seven-story reinforced concrete shear wall building slice was tested on the UCSD-NEES shake table in the period Oct. 2005 – Jan. 2006. The objective of this test program was to verify the seismic performance of a reinforced concrete wall system designed for lateral forces obtained from a displacement-based design methodology, which are significantly smaller than those dictated by current seismic design provisions. The shake table tests were designed so as to damage the building progressively through several historical seismic motions reproduced on the shake table. At various levels of damage, several low amplitude white noise base excitations were applied through the shake table to the building. In addition to white noise base excitation tests, ambient vibration tests were also performed on the building specimen at different damage levels. In this study, measured response data from 28 longitudinal accelerometers (three on each floor slab and one on the web wall at mid-height of each story) were used to identify the modal parameters of the test structure. Six different state-of- the-art system identification methods, consisting of three input-output and three output-only methods, were applied to dynamic response measurements obtained using accelerometers in order to estimate modal parameters (natural frequencies, damping ratios and mode shapes) of the building in its undamaged and various damage states. The identified modal parameters obtained using different methods are compared to study the performance of these system identification methods and also to investigate the sensitivity of the estimated modal parameters to actual structural damage. The modal parameters identified using different methods are found in reasonable agreement for each damage state. Based on the identified modal parameters, a sensitivity-based finite element model updating strategy is applied for damage identification of the test structure. In the FE model updating procedure, an objective function is defined as a combination of residuals in natural frequencies and mode shape components. The first step to identify damage in the test structure using the FE model updating algorithm is to obtain a reference (calibrated) FE model based on the modal parameters identified at the undamaged (or baseline) state of the building. In this step, effective Young’s moduli of the main wall at all seven stories are taken as updating (or calibrating) parameters. Once the reference model is determined, the same updating parameters are updated at the three different damage states of the structure. In all model updatings performed, the natural frequencies and mode shapes of the first three longitudinal modes identified using SSI-DATA are used in the objective function. The damage identification results are consistent with the actual damage observed (visually) in the building and inferred from LVDT and strain gages data. In the last part of this study, the performance of the three output-only system identification methods used is systematically investigated as a function of the uncertainty/variability in various input factors, based on the finite element simulated response of the building. The response of the structure is simulated with a three-dimensional nonlinear finite element model developed in the structural analysis framework OpenSees. Two methods are employed to quantify the variability of the identified modal parameters due to variation of the four input factors.

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