Analysis of Bridge Performance under the Combined Effect of Earthquake and Flood-induced Scour

Open Access
Ganesh Prasad, Gautham
Graduate Program:
Civil Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 16, 2011
Committee Members:
  • Dr Swagata Banerjee, Thesis Advisor
  • bridge scour
  • reinforce concrete bridges
  • Regional multihazard
  • earthquake
Earthquake in the presence of flood-induced scour is a critical multihazard scenario for bridges located in seismically-active, flood-prone regions of the United States. Bridge scour causes loss of lateral support at bridge foundations and results in increased seismic vulnerability of bridges. The present study evaluates the combined effect of earthquake and flood-induced scour on the performance of four example reinforced concrete bridges with different number of spans. For the analysis, Sacramento County in California is considered as the study region where the annual probabilities of occurrence of earthquakes and floods are reasonably high. The seismic hazard of the study region is considered through a suite of ground motion time histories which were generated for this region. Regional flood hazard is expressed in the form of a flood hazard curve that provides the annual peak discharges corresponding to flood events with various annual exceedance probabilities. Bridge scour, an outcome of the flood events, are calculated and used in the development of finite element models of these bridges. Nonlinear time-history analyses are preformed to evaluate the seismic performance of the example bridges having scour at bridge piers. In parallel, analyses are performed to evaluate bridge seismic performance in the absence of scour. Fragility curves are developed to represent the performance of example bridges under this multihazard scenario. Comparison of bridge fragility characteristics obtained in the presence and absence of scour presents the increased seismic vulnerability of the bridges in the presence of flood-induced scour. The study identifies the most sensitive range of bridge scour for which the rate of degradation of bridge seismic performance is significant. A sensitivity study is performed to investigate the influence of various input parameters on the overall performance of the example bridges. For this, the 5-span example bridge is chosen to analyze under a 100-year flood event. Risk curves of this example bridge are developed that determine annual probability of exceeding different levels of societal loss arising from bridge seismic damage in the presence and absence of flood-induced scour. This societal loss is measured in terms of post-event bridge repair/restoration cost. Results show that the seismic risk of the example bridge may increase significantly in the presence of scour.