Design of Reinforced Concrete Bridge Columns for Near-Fault Earthquakes
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Bridges are key components in the transportation network providing access for emergency response vehicles following major earthquakes. The strong and long period velocity pulse in the fault-normal component of near-fault ground motions exposes structures in near-fault regions to high input energy that could result in high residual displacements in bridge columns. The residual displacement in bridges plays a key role in assessing whether a bridge should be kept open to traffic or closed for repair or replacement. Currently there are no reliable provisions to account for residual displacements caused by near-fault earthquakes in design of reinforced concrete bridge columns. The main objective of the study was to develop a new guideline for the design of reinforced concrete bridge columns subjected to near-fault earthquakes. The goal of the study was achieved through the following tasks: (1) determine the adequacy of existing computer models to estimate residual displacements by comparing the results of the experimental data for six large-scale reinforced concrete bridge columns to those of nonlinear dynamic analyses, (2) determine the residual moment capacity of reinforced concrete columns as a function of maximum displacement ductility, (3) determine critical residual displacement limit with respect to structural performance, (4) develop a simple method to estimate residual displacement, (5) develop residual displacement spectra for different displacement ductilities, soil conditions, and earthquake characteristics, (6) develop a step-by-step design guideline to control the residual drift ratio utilizing the simple method or residual drift spectra with an illustrative example, and (7) evaluate the impact of the proposed design guidelines in terms of cost by redesigning several representative bridges from different parts of the United States. The analysis of residual drift ratio limits indicated that circular bridge columns meeting current seismic codes are able to carry large traffic loads even when the permanent lateral drift is 1.2% or higher, depending on the column strength and geometry. It was found that residual drift ratio is negligible (less than 1%) when one-second spectral acceleration is less than 0.4g. Also, utilizing the proposed design method to control residual drift ratio has negligible effect on the overall cost of the bridge.