Effects of Near-Fault Ground Motion and Fault-Rupture on the Seismic Response of Reinforced Concrete Bridges
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Recorded fault normal near-fault earthquakes typically exhibit an intense velocity pulse due to the forward directivity effects and a static permanent ground displacement caused by the relative movement of the two sides of faults. However, there are no established provisions to account for these effects in the design of reinforced concrete bridges. The purpose of this study was to investigate near-fault ground motion effects on typical reinforced concrete bridge columns and the fault-rupture effects on seismic response of a bridge system that crosses an active fault. The ultimate objective of this study was to develop practical and proven bridge design guidelines that incorporate the effects of near-fault ground motions. One task of this research was to test and analyze the performance of four large-scale bridge columns subjected to typical near-fault ground motions. In addition, a large-scale two-span bridge model supported on three piers, one on each of the University of Nevada, Reno (UNR) shake tables, was tested under a near-fault ground motion with incoherent motions that included the fault-rupture effect. Four large-scale reinforced concrete circular columns with different initial periods were tested on a shake table under simulated earthquakes. The models were divided into two groups. The design of the first two was based on the current California Department of Transportation (Caltrans) Seismic Design Criteria (SDC) near-fault earthquake provisions. In the course of this study a new design spectrum was developed and was used for the second group of two columns. The most distinct measured column response was the relatively high magnitude of residual displacements even under moderate levels of motion. This was true for all four column models. The shake table test results revealed the necessity of control of residual displacement at the design stage. The data also showed that the plastic hinge length in sufficiently confined columns subjected to near-fault earthquakes is comparable to that of columns experiencing far-field motion. Based on the trends seen from shake table testing and further dynamic analysis, a guideline for the design of single pier reinforced concrete bridge columns was developed that incorporate the control of residual displacement. As part of this study a hysteresis model was further refined to better estimate the residual displacement. The study the effect of fault-rupture, a quarter-scale reinforced concrete bridge model was subjected to series of incoherent ground motions that simulated fault-rupture. An identical bridge model had been tested under uniform excitations in a previous study. The data from that study provided a bench mark based on which the effect of fault- rupture could be evaluated. The bridge model was subjected to a total of six runs until one of the shake tables reached its displacement limit. During this run some of the bridge columns approached failure but there was no steel bar rupture. The measured data in the fault-rupture study showed a major shift in the location of the most critical pier compared to the bridge that was subjected to uniform motion. The pier that was the least damaged under uniform ground motion suffered the largest damage in the case of shaking simulating fault-rupture.
Report No. CCEER-07-06