Seismic Performance Of Abutment Shear Keys In Bridge Structures

Professor P. Benson Shing
Professor Jose Restrepo

Shear keys are used in bridge abutments to provide lateral restraints to bridge superstructures under normal service loads and moderate earthquake forces. In the event of a severe earthquake, shear keys should function as structural fuses to prevent the transmission of large seismic forces to the abutment piles. A comprehensive study was carried out in a project supported by Caltrans to acquire a good understanding of the behavior and lateral load resisting mechanisms of external shear keys in bridge abutments. Six 40%-scale shear key-stem wall specimens were tested. One specimen had two shear keys isolated from the stem wall with a construction joint, and four had shear keys monolithic with the stem wall. One had post-tensioned shear keys, designed with an innovative concept to allow rocking. The tests have shown that post-tensioned shear keys can develop very high ductility through rocking.

To complement the experimental study and acquire a better understanding of the loading resisting mechanismĀ of shear keys, nonlinear nite element models have been developed. The models account for the cohesive force and shear-friction resistance in concrete as well as the dowel action of reinforcing bars crossing cracks and construction joints including geometric nonlinearity. A parametric study has been performed with nonlinear nite element models to investigate the in uence of the angle of skew on the lateral resistance of the shear key. Results of the experimental and numerical studies have been used to develop reliable simpli ed analytical methods for calculating the lateral resistance of shear keys considering the shear key geometry, the concrete strength, the amount of the vertical dowel reinforcement connecting the shear key to the stem wall, the surface condition of the construction joint if any, and the angle of skew of the abutment. These methods can be used for the design of shear keys and stem walls to achieve desired performance, and are being considered by Caltrans for possible adoption into the Caltrans Design Criteria.

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