Nonlinear Performance and Damage Potential of Degraded Structures Under Long Duration Earthquake

Abstract eng:
Occurrences of mega earthquakes have been reported more frequently in recent years. Due to their long duration and large number of cycles, the degradation of both stiffness and strength have significant effects on structural performance and damage accumulation. On May 12, 2008, a large Ms 8.0 earthquake occurred in the Longmenshan Fault zone in Wenchuan County, Sichuan Province, China. This earthquake lasted for over 350 seconds and caused more than 69,000 casualties and about 124 billion USD in financial losses. Just five years later, on April 20, 2013, a large Ms 7.0 earthquake occurred at the same Longmenshan Fault zone in Lushan County (about 90km from the Wenchuan epicenter) with a typical duration of 80 seconds. Both earthquakes hit rural countries in western China where low-rise buildings and self-built masonry in-filled reinforced concrete structures are prevalent. Significant damage and collapse were reported in many reconnaissance reports. For both earthquakes, a significant number of ground motion records have been obtained by the National Strong Motion Observation Network System of China. This invaluable data can be used to conduct a comparison study of the nonlinear dynamic response of structural systems at stations that recorded the two earthquakes. In this study the duration effects where decoupled from other ground motion characteristics by using a spectrally equivalent procedure applied to the Lushan records to be directly comparable with the corresponding ones from Wenchuan. Nonlinear time history analysis of various models was carried out. These models included, elastic, elastic-perfectly-plastic with hardening, and a degraded peakoriented hysteretic model that accounts for strain hardening, post-capping, residual strength, cyclic stiffness and strength degradation. Both, elastic and inelastic response spectra were generated and compared in order to better understand the demands from each earthquake. The ductility demands, hysteretic energy demands, inelastic displacement ratio and residual displacement ratio were also compared for the two earthquakes. Sensitivity analysis of the inelastic response were studied in terms of the ductility, strength reduction factors, and the cyclic degradation level. Incremental dynamic analysis and fragility evaluation were also investigated on a typical wooden braced frame with masonry infill. The results indicated that long duration shaking has a significant impact on the structural inelastic response and the collapse capacity of structures with high degradation and pinching characteristics. The current design practice to estimate potential collapse of structures, which is based on the spectral demands, amplitude of ground motion and peak values of drift and forces may not be adequate to characterize the potential collapse of certain systems that are sensitive to ground motion duration effects. Accumulate inelastic response caused by long duration shaking should be included as part of the evaluation of these systems in order to have a more adequate way to estimate collapse potential.

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