Two-Level Design Spectrum for Highway Bridges

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Title: Two-Level Design Spectrum for Highway Bridges

Author(s): R. Mathur, H. Lee, and G. Orsolini

Publication: Special Publication

Volume: 187

Issue:

Appears on pages(s): 345-360

Keywords: bridges; columns; displacement; ductility; hinges; reinforcement.

Date: 6/1/1999

Abstract:
The damage to highway bridges during the recent California earthquakes has highlighted the need for improved seismic analysis and design procedures. This paper is a study f one such approach that aims to enhance the seismic performance of bridges. Chosen as an application of this approach is a 12-span viaduct that is part of the San Joaquin Hills Toll Roads project in Orange County, CA. Current California Department of Transportation (Caltrans) and AASHTO design criteria require bridges to be designed for a maximum credible earthquake to safeguard against collapse. Damage prevention under moderate earthquakes is not ensured. Thus, to provide a higher and more uniform level of safety and reliability, the design criteria for the toll bridges involved a two level design approach: one corresponding to a 72-year return period event and the other corresponding to a maximum credible level design event. Design forces and displacements resulting from the lower level event were used to ensure that no damage occurs as a result of this event. The forces and displacements resulting from the maximum credible analysis were used to ensure that the structure will not collapse. In the paper this two level approach is used to design the column steel based on strength criteria. Displacement ductility checks are then performed on the columns to ensure that they can undergo the inelastic deformations produced by the upper level spectrum loads. In 1992 when the bridges on the toll roads were designed, the two level approach was a relatively new design method. However, since the recommendations put forward by ATC-32(1) in 1996, the two level spectrum design approach is finding increased use in the design of bridges.