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INTRODUCTION

The primary objective of all current U.S. seismic codes is to prevent collapse of the structure under the design earthquake. The codes recognize that it is uneconomical to design a bridge to resist a large earthquake elastically, and therefore some degree of damage is permitted and expected (Figure 1). It is intended that this damage be limited primarily to ductile behavior (flexural yielding) of the columns or pier walls, to nominal abutment damage, and to shear key breakage. These bridge elements lend themselves to relatively easy inspection and repair should acceptable damage be sustained during a seismic event. Unacceptable damage includes loss of girder support, column failure, foundation failure, and connection failure.

These performance requirements indicate why proper modeling of the bridge system is important. The calculated internal distribution of forces, expected deformations, and prediction of collapse mechanisms are directly related to the adequacy of the overall system model. Yielding of a single element in a structure is acceptable in a particular mode providing it does not lead to collapse. The formation of a local failure mechanism must occur before overall collapse can take place. The distribution (or redistribution) of loads in the structure, their relation to the formation of plastic hinges, and the prediction of the eventual failure mechanism, are the central goals of bridge systems analysis.

Structural evaluation of an overall bridge system is a challenging undertaking. Evaluations are typically performed at ultimate conditions, and limit analysis is used where progressive yielding is permitted until the structure collapses. Traditional code-based analysis procedures generally do not lend themselves to accurate determination of overall bridge system behavior. The internal force distributions (or redistributions) are different for each structure and will require careful evaluation and engineering insight. In many instances a single model does not provide sufficient insight into the overall system behavior. A series of incremental models providing progressive yielding, a bracketing of likely behaviors, or a sophisticated nonlinear model may often be necessary to provide an appropriate indication of force distributions within the structure, and of overall bridge behavior.

The purpose of this document is to provide practicing engineers general guidelines for overall bridge modeling. Although the discussions presented here are in general applicable to all bridges, the intent was to address short- and medium-span bridges (those with span lengths less than 500ft, or 150 m). Most recommendations in this document are extracted from the cited codes and references. These recommendations should not be construed as absolute rules, and should in no way limit the creativity and responsibility of the Engineer in analyzing the structure with the best and most appropriate available tools. However, if followed, the recommendations should provide a good indication of the seismic behavior for a broad class of bridge types encountered in current practice.