Both the AASHTO `Standard Specifications for Highway Bridges' and the AASHTO `LRFD Bridge Design Specifications' require consideration of redundancy when designing highway bridges. These specifications provide limited guidance for determining when a structure is, or is not, redundant. The lack of guidance has led to a large variation in the interpretation of the specification. This report is the culmination of NCHRP Project 12-36, which consisted of two phases. In Phase I, redundancy was defined and procedures were developed for quantifying redundancy in highway bridges. A direct calculation method and a simplified method using tabulated "system factors" were developed, and a framework for explicitly including measures of redundancy in design and evaluation was proposed. There was no report published on the Phase I effort. In Phase II, the simplified method developed in Phase I was expanded to provide system factors for continuous span superstructures. The complete system factors tables are applicable to steel and prestressed concrete bridges comprised of 1-beams or box girders. Although the system factors were developed using reliability analysis, they apply to bridges designed by AASHTO working stress, load factor, or load and resistance factor design criteria. These procedures for quantifying redundancy are significant because they are based on an assessment of the reliability of the bridge system rather than simply the individual bridge members. The introduction of system reliability may prove to be particularly advantageous for evaluating the load capacity of older bridges. (A)
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