Experimental and Analytical Study of Girder Unseating in Skew Bridges During Earthquakes

Abstract eng:
Skew bridges are known to be more vulnerable to unseating during earthquakes than straight bridges of the same span length. Various reasons for this vulnerability have been proposed based on simplified modeling and empirical evidence and many design codes increase the minimum support length requirements for skewed bridges by factors based on engineering judgment. In this paper, an unseating mechanism is proposed based on observed unseating of skew bridges in recent earthquakes. It is hypothesized that under earthquake action, a skewed bridge superstructure first closes the expansion gap, then impacts the abutment back wall, and finally rotates about the obtuse corner, leading to excessive in-plane displacements at the acute corner at the opposite end of the span. In addition, shake table experiments of single-span simply supported skew bridges with seattype abutments are introduced, which have recently been conducted in the Earthquake Engineering Laboratory at the University of Nevada, Reno. The objectives of the experiment were to: (1) test the proposed unseating mechanism; (2) validate a detailed 3D analytical model developed in OpenSees that considers pounding and friction effects at the abutment; and (3) confirm the applicability of the simplified method for estimating the additional support length required due to skew. Details of the design, instrumentation, and setup of the experiments are described. Impact forces between bridge deck and abutment were measured as well as the displacements, accelerations, and rotations of the superstructure. It is shown that the proposed unseating mechanism matches well with the observed behavior. The experimental results indicate that the combination of forced vibration in translational modes and free vibration in rotational mode around the center of stiffness of substructure leads to the unseating of symmetrical skew bridge at acute corners. Furthermore, a rigorous OpenSees model which accounts for the impact and sliding effects between the bridge deck and abutment was developed and details of the model are introduced. Good correlation is obtained between the experiment and the OpenSees model. The implications for revising the minimum support length requirements have yet to be studied.

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