Bozorgzadeh et al. 2008
In this study, the authors assess ten shear keys at bridge abutments with the goal to reduce the input force to the abutment piles by inducing sliding shear failure, making the shear keys sacrificial members. Models were constructed at 1:2.5 scale and excited under a cyclic lateral load. An analytical model was then developed to provide guidelines for the capacity design of shear keys at bridge abutments.
System Concept
Large diagonal cracks in abutments demonstrated a diagonal shear failure after the Northridge Earthquake in 1994. These shear keys are intended to minimize the force that is transferred to the abutments. Using smooth construction joints at the shear keys and minimizing the reinforcement between the abutment and shear key is intended to allow for motion of the shear key, dissipating energy through the sliding shear failure of the key, and limiting the input force to the abutments.
Experimental Study, Results, and Discussion
Shear keys at 1:2.5 scale were tested in five series. Variables included were the incorporation of a construction joint between the shear key and abutment stem wall, the placement of vertical reinforcement bridging between the abutment and shear key, and the amount of horizontal tie reinforcement. The test mimicked superstructure performance during an earthquake. The abutment wall was posttensioned to a strong floor and a hold down frame prevented a lateral load applied to the shear key from moving upward. Strain gauges recorded movement of the reinforcement and shear key.
It was observed that large cracks occurred in the cases of rough construction joints, indicating a diagonal shear failure. The presence of wing walls was also found to increase the shear capacity. In the test with limited vertical reinforcement, sliding shear failure was observed, with the rest of the tests exhibiting diagonal shear failure. This indicates that smooth construction joints with limited vertical reinforcement will induce sliding shear failure that dissipates energy.
Analytical Study
A simple analytical model was created to assist in designing the appropriate vertical reinforcement to induce sliding shear failure. The model was calibrated using data backed out of successful tests yielding sliding shear failure through the use of a smooth construction joint and accounting for kinks in the reinforcement resulting from experimentally measured sliding shear deformation.
Reference
Bozorgzadeh, A., Megally, A., Restrepo, J., and Ashford, S.A. (2006). "Capacity Evaluation of Exterior Sacrificial Shear Keys of Bridge Abutments," Journal of Bridge Engineering, 11.5, pp. 555-565.