DISPLACEMENT-BASED APPROACH FOR SEISMIC DESIGN OF HIGHWAY BRIDGES ACCORDING TO NEW AASHTO GUIDE SPECIFICATIONS

Following the San Fernando earthquake (1971), significant effort was expended to develop comprehensive design guidelines for the seismic design of bridges in the United States. That effort led to updates of both the AASHTO and Caltrans design provisions and ultimately resulted in the development of ATC-6, Seismic Design Guidelines for Highway Bridges, which was published in 1981. That document was subsequently adopted by AASHTO as a Guide Specification in 1983; the guidelines were formally adopted into the Standard Specifications for Highway Bridges in 1991, then revised and reformatted as Division I-A. Later, Division I-A became the basis for the seismic provisions included in the AASHTO LRFD Bridge Design Specifications. Iranian Road and Railway Bridges Seismic Resistant Design Code (Publication No. 463) has been published according to AASHTO Division I-A and most of modern bridges in Iran, has been designed according to that provisions with 475-yr return period earthquake ground motion and 50-year design life. The new AASHTO guideline was founded on displacement-based design principles, recommended a 1000-year return period earthquake, and comprised a new set of guidelines for seismic design of bridges. The title of the guideline reflects the fact that the Guide Specifications are approved as an alternate to the seismic provisions in the AASHTO LRFD Bridge Design Specifications which its 8th edition has been published in 2017. New Guide Specifications differ from the current procedures in the LRFD Specifications in the use of displacement-based design procedures, instead of the traditional, force-based “R-Factor” method. This new approach is split into a simplified implicit displacement check procedure and a more rigorous pushover assessment of displacement capacity. The selection of which procedure to use is based on Seismic Design Categories (SDC), similar to the seismic zone approach used in the AASHTO LRFD Bridge Design Specifications, Also included is detailed guidance and commentary on earthquake-resisting elements (ERE) and systems (ERS), global design strategies (GDS), demand modeling, capacity calculation, and liquefaction effects. Similar to theLRFD force-based method, capacity design procedures underpin the Guide Specifications’ methodology, and these procedures include prescriptive detailing for plastic hinging regions and design requirements for capacity protection of those elements that should not experience damage. The stages for seismic design of highway bridges with this new approach are discribed in flowchart of Figure 1. The first edition of the new Guide Specifications was published in 2009 and the second edition has been released in 2011. Some of computer programs such as SAP2000 and CSi Bridge uses the provisions of the new Guide Specifications for automatic seismic design of bridge structures.

Keywords:

Highway bridge , Vulnerability , FVD , Seismic retrofit , Near fault site