Seismic vulnerability assessment of skewed reinforced concrete bridges

Abstract

Any change in the structure occupancy or use can significantly affect mass distributions in plan and elevation. The mass-eccentric structures having in-plan or vertical irregularities can suffer from non-uniform deformation demands due to the torsional effect. The existing design practices recommend special design requirements for such irregular structures to reduce additional seismic demands caused by torsion since they can result in significant live and economic losses during earthquake events. Many previous studies have conducted seismic vulnerability analyses of various irregular structures to understand their seismic behavior and quantity the failure likelihood under earthquake shaking. Because of the prevailing challenge of extreme computational expense in the frailty analysis, many researchers have utilized simplified 2D analytical models for their studied structures. However, overly idealized models often cannot reflect the true structural behavior under seismic loads, especially when any structural irregularity exits in the system. Yet, the use of more detailed models in earthquake simulations may not be practically possible with existing methods. Also, almost all previous studies have adopted three or four failure limit states in the vulnerability analysis and have derived corresponding seismic fragility curves; however limited numbers of fragility curves could not deliver the complete seismic vulnerability information of the structures. This research investigates seismic performance of reinforced concrete frame structures that have vertical or in-plan irregularities from the change in the structure use. Fourteen prototype models are developed by assuming different live load distributions in elevation or plan. Unlike other studies, 3D detailed analytical models are employed for numerical simulations and a total of eight limit states are utilized based on the allowable drift ratio. A new fragility curve derivation method is introduced which couples the structural analysis and reliability analysis to derive efficient yet accurate vulnerability curves with the first-order reliability method. A series of nonlinear dynamic response history analysis is conducted with ten earthquake ground motions; all seismic fragility curves are successfully obtained using personal computers. The functional representations of the derived curves are reported for designers and engineers. The term “seismic fragility surface” is firstly introduced in this study. The fragility surface is constructed from the fragility curves at all limit states, and it offers the thoroughgoing vulnerability relationship of the structure to the earthquake loading. The direct effects of vertical and in-plan irregularities on the structural performance of the studied reinforced concrete frames are clearly shown from the obtained fragility curves and surfaces.