The AASHTO Standard Specifications for Highway Bridges requires consideration of redundancy when designing steel bridge members. There is limited guidance in the specifications concerning when a structure is, or is not, redundant. The specification states that a structure is nonredundant " ... when the failure of a single element could cause collapse." The specification does not, however, define collapse, which has led to variation in specification interpretation and bridge analysis. In addition, the specification does not require consideration of redundancy in the design of bridges other than steel.

Redundancy can be present in the superstructure, substructure, or in the entire bridge. In addition, according to the 1986 FHWA report, Inspection of Fracture Critical Bridge Members, there can be load path, structural, or internal redundancy. Guidance is lacking for each of these types of redundancy.

A new bridge design specification and bridge evaluation manual are under development for consideration by AASHTO; both are based on Load and Resistance Factor Design (LRFD) methodology. The purpose of using LRFD is to achieve more uniform reliability for all materials and structural systems used in bridge design and load capacity rating. To achieve this, consideration must be given to the varying degrees of redundancy inherent in different bridges. For example, bridges can be classified, at a minimum, as either nonredundant, dual-load-path redundant, or multiple-load-path redundant.

Research is needed to develop a framework for consideration of redundancy in bridge design and load capacity evaluation. Proposed criteria should be based on levels of serviceability rather than collapse. For example, serviceability could be considered at the following levels: a structure that will remain undamaged under all traffic loads and conditions; a damaged structure that can continue to carry normal traffic but that can be repaired; and a severely damaged structure that will allow existing traffic to safely leave the bridge but will require closure.

The objective of this study was to develop a framework for considering redundancy in design and load capacity evaluation of highway bridge superstructures, possibly in the form of a matrix of resistance factor modifiers for varying types of redundancy and levels of serviceability. This framework was limited to common steel and concrete bridges.

NCHRP Project 12-36(2), Redundancy in Highway Bridge Superstructures, Phase 2, continued the work started in this project and proposed additions to the AASHTO LRFD Bridge Design Specifications for the consideration of redundancy in design and evaluation.