Category:756 Seismic Design

PRIMARY GUIDANCE

· Seismic design of bridges is guided by the AASHTO design specifications, which delineate seismic zones in Missouri. · New bridges on major roads and Earthquake Emergency Routes are modeled (comprehensively analyzed) and designed to resist earthquakes according to Seismic Performance Categories (SPC) B, C and D. · New bridges on minor routes in SPC B, C and D, include limited seismic details to improve their resistance to earthquakes; however, they are not modeled (comprehensively analyzed) and specifically designed to resist earthquakes. · A decision is made on each bridge rehabilitation project as to the necessity and extent of seismic retrofitting.

DISCUSSION

All new bridges on the state system include small details and design checks to resist earthquakes. In Seismic Performance Categories B, C and D, additional design and detailing work is needed because of the increased probability of damaging earthquakes.

When existing bridges in SPC B, C and D are identified as needing repairs or maintenance, a decision on whether to include seismic retrofitting in the scope of the project should be made based on the location of the bridge, the extent of the rehabilitation work and the expected life of the of the bridge after the work. For example, if the bridge needs painting or deck patching, no retrofitting is recommended. However, redecking or widening the bridge indicates that MoDOT is planning to keep the bridge in the state system with an expected life of at least 30 more years. In these instances, the project core team should consider cost effective methods of retrofitting the existing bridge.