Seismic Performance of Skewed Bridges under Orthogonal Ground Motion Components

Open Access
Bhatnagar, Unmukt Rajeev
Graduate Program:
Civil Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 05, 2013
Committee Members:
  • Dr Swagata Banerjee, Thesis Advisor
  • Orthogonal effects
  • Fragility Curves
  • Directionality Effects
  • Seismic Analysis
  • Skewed Bridge
Earthquakes are a natural calamity, feared by most and cause great destruction in and around the seismic zone where they occur. Bridges, being an important component of the transportation system, are therefore required to be designed such that they can withstand these large impacts and remain functional post-earthquake event. The present study is focused on conducting a parametric study and evaluating the performance of skewed bridges under seismic activities by considering the orthogonal effects in nonlinear time history analysis, wherein, two seismic waves, in the form of time histories, act simultaneously at the structure at right angles to each other. Such a set of waves is known as the orthogonal set of time histories. Any one of the two seismic waves can be assumed to be acting in a principal direction, i.e. along the direction of the ground motion. The other seismic wave, thus, acts in a direction perpendicular to it. Responses of the bridge are obtained in the directions along the length of the bridge (longitudinal) and perpendicular to it (transverse). These responses are statistically independent of each other. For conducting the analysis, Painter Street Overpass located in Rio Dell, CA is modeled as a dual-beam stick bridge using OpenSees. After validating the analytical model, the bridge is subjected to a suite of orthogonal set of time histories developed for the California region. The angle formed by the seismic wave acting along the principal direction, with the central axis of the bridge, in the longitudinal is called as the angle of incidence. This angle is made to rotate from 00 to 1800 with a step of 150. Skew angle of the bridge is defined as the angle made by the bents or abutments with the axis along the transverse direction of the bridge. The above procedure is repeated for different skew angles of the bridge varying from 00 to 500 with a step of 100. The effect of the variation of these two parameters on the response of the structure is analyzed to determine the critical angle of incidence for a ground motion on the bridge. It is concluded that angle of incidence, by itself cannot determine the critical response of the structure, as critical angle of incidence, which generated the critical response in the structure, varied for different ground motions used for analysis. It is suggested to include a seismic parameter like peak ground acceleration (PGA) to the parametric study, to determine the combination that produces a critical response. Fragility curves are developed in order to provide insight into the vulnerability of the bridge to different seismic loading conditions. The effect of change in skew angle and angle of incidence is studied on the fragility plots for different damage states of the bridge, as described by HAZUS-MH, thus linking the variation in PGA to the parametric study. It is derived that inclusion of PGA in probabilistic terms results in obtaining a range of critical incident angle and skew angle, for which the critical response can be generated in the structure. A study is conducted, in which variation of the median value of the PGA is observed along with variation in angle of incidence and skew angle of the bridge. It is inferred that the peak response in the structure occurs for incident angles ranging from 300 to 800, and for skew angles of 400 and 500.