The AASHTO Guide Specifications for LRFD Seismic Bridge Design is established in accordance with the NCHRP 20-07/Task 193 Task 6 Report. Task 6 contains five (5) Sections corresponding to Tasks 1 to 5 as follows:
SECTION 1 includes a review of the pertinent documents and information that were available.
SECTION 2 presents the justification for the 1000- year return period (which is approximately equivalent to a 7% probability of exceedance in 75 years) as
recommended for the seismic design of highway bridges.
SECTION 3 includes a description of how the βno analysisβ zone is expanded and how this expansion is incorporated into the displacement based approach.
SECTION 4 describes the two alternative approaches available for the design of highway bridges with steel superstructures and concludes with a recommendation to use a force based approach for steel superstructures.
SECTION 5 describes the recommended procedure for liquefaction design to be used for highway bridges. This aspect of the design is influenced by the recommended design event and the no analysis zone covered in Tasks 2 and 3, respectively. The recommendations proposed are made taking into account the outcome of these two tasks for Seismic Design
Category D. The following recommendations are documented:
Task 2
1. Adopt the 7% in 75 years design event for development of a design spectrum.
2. Ensure sufficient conservatism (1.5 safety factor) for minimum support length requirement. This conservatism is needed to enable to use the reserve capacity of hinging mechanism of the bridge system. This conservatism shall be embedded in the specifications to address unseating vulnerability. At a minimum it is recommended to embed this safety factor for sites outside of California.
3. Partition Seismic Design Categories (SDCβs) into four categories and proceed with the development of analytical bounds using the 7% in 75 years design event.