Seismic Analysis for Design or Retrofit of Gravity Bridge Abutments
Richards, R. Jr.
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Depending on their seismic resistance, existing bridge abutments may be vulnerable to earthquakes. Seismic resistance may be quantified in terms of a threshold acceleration level beyond which permanent deformation will occur. During a seismic event, whenever the ground acceleration exceeds the threshold, permanent deformation accumulates. However, recent studies demonstrate the possibility of loss of bearing capacity, and subsequent rotation, or mixed sliding/rotation modes of failure. Hence, a revised analytic method is described for determining the most critical threshold acceleration. The method considers the possibility of sliding, bearing capacity, or mixed modes of failure. Load transfer between the bridge abutment and the bridge deck is incorporated. To verify the accuracy of the method, a series of shaking table tests was conducted on model bridge abutments. The results from these tests verify the analytic procedure for predicting critical threshold acceleration based on simple theory. Computation of predicted displacements and rotation are reasonable. Thus it is now possible to evaluate the seismic vulnerability of existing abutments. The theory also relates directly to design recommendations to increase the seismic capacity of both existing bridge abutments and new designs. Not only can the critical threshold acceleration be made higher with passive restraint footing keys, piles or tiebacks but girder seats, knock-off walls and transverse restraint can be designed to reduce damage in extreme events. (Adapted from authors' abstract).