Reliability of Highway Bridge Columns Under Earthquake Loadingby Michel Ghosn, City Coll of New York, New York, United States,
Ge Chen, City Coll of New York, New York, United States,
Abstract: Seismic design procedure for highway bridges normally specify that any inelastic action be confined to the columns. Concrete bridge columns are usually designed to resist an elastic force equivalent to the force level implied by the inelastic earthquake spectra. The columns are also detailed to insure that they withstand the expected inelastic displacements without significant strength degradation. The safety of bridges subjected to seismic loads thus entails two conditions: 1) That the plastic hinge regions be able to withstand the required plastic moments; and 2) That they provide enough ductility to avoid the column's collapse when the plastic moment is reached. Traditional risk assessment models emphasized the strength aspects of the design requirements but neglected the interaction between strength and ductility or the ductility capacity imparted by the detailing of the columns. This paper develops a new model for the analysis of the reliability of bridge columns subjected to earthquake loads. The model is based on the experimental and analytical work of Park and Priestly at the University of Canterbury. Probability of failure is defined as the probability that the displacement ductility capacity of the bridge column is less than the ductility demand at the assumed earthquake level. Ductility capacity is formulated as a function of the column geometry and the volumetric ratio of confining steel. Ductility demand can be calculated by time history analysis of a bridge structure under simulated earthquakes or using the results of an equivalent static analysis using inelastic earthquake spectra. Results of this model expressed in terms of reliability indices are presented for a bridge example designed according to AASHTO's guidelines. The model is also tested on two actual bridges subjected to the San Fernando earthquake. The results indicate that the model is capable of predicting the failure risk of bridges under seismic loads.
Subject Headings: Bridge columns | Highway bridges | Earthquakes | Bridge design | Seismic loads | Ductility | Structural models | Inelasticity
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