Unseating of Simply Supported Spans During Earthquakes

The Washington State Department of Transportation (WSDOT) is currently retrofitting many older bridges to prevent their superstructures form unseating during earthquakes. In bridges whose simply supported spans have inadequate bearing lengths, WSDOT most frequently connects adjacent spans with high-strength rod restrainers. The study described in this report was undertaken to determine whether restrainers installed in this manner are effective in preventing span unseating and to develop a method for identifying vulnerable simply supported spans. A companion report considered the design of seismic restrainers for in-span hinges.

The researchers developed a nonlinear analytical model of a four-span, simply supported, prestressed concrete bridge. Variations of this model were subjected to four ground motions to determine the maximum relative displacements between the simply supported spans and their supports. The maximum relative displacements at the piers depended most on the bearing friction resistance, the earthquake motion and the size of the joints in the deck. The maximum relative displacements at the abutments depended most on the bearing resistance and the earthquake motion.

Based on the results of the parametric study, the researchers developed a new method to estimate the susceptibility of bridges to unseating of simply supported spans. The researchers also found that restrainers connecting adjacent spans are ineffective in reducing the relative displacements between the superstructure spans and their supports.


Publication Date: 
Wednesday, May 1, 1996
Publication Number: 
WA-RD 387.2
Last modified: 
10/12/2016 - 15:42
Panos Trochalakis, Marc O. Eberhard, John F. Stanton
Washington State Transportation Center (TRAC)
Number of Pages: 
Bridge abutments, Bridge design, Bridge piers, Bridge superstructures, Bridges, Dislocation (Geology), Earthquake resistant design, Earthquake resistant structures, Evaluation, Mathematical models, Measures of effectiveness, Nonlinear systems, Prestressed concrete bridges, Prevention, Retrofitting,