NCHRP Research Report 906 presents proposed revisions to the AASHTO LRFD Bridge Design Specifications minimum flexural reinforcement provisions with detailed design examples illustrating the application of the proposed revisions. The proposed revisions are based on comprehensive analytical and large-scale testing programs for investigating the minimum flexural reinforcement requirements with consideration to scale effects. The material in this report will be of immediate interest to bridge engineers.
According to the AASHTO LRFD Bridge Design Specifications, minimum reinforcement provisions are intended to reduce the probability of brittle failure by providing flexural capacity greater than the cracking moment. There was a concern with the current AASHTO LRFD minimum flexural reinforcement requirements, especially when applied to pretensioned or post-tensioned concrete flexural members. Increasing the nominal capacity of a member can result in increasing the cracking moment of the same member. This makes the design process iterative, and the current minimum reinforcement requirement for post-tensioned segmentally connected members is difficult to satisfy. This also may lead to less efficient design of pretensioned/post-tensioned concrete flexural members. Provisions for the design of minimum flexural reinforcement must be suitable for all structure and reinforcement types covered by the AASHTO LRFD Bridge Design Specifications and should be consistent with the LRFD design philosophy.
Under NCHRP Project 12-94, Iowa State University was asked to propose revisions to the AASHTO LRFD Bridge Design Specifications minimum flexural reinforcement provisions. The research considered reinforced, pretensioned, and post-tensioned concrete flexural members (e.g., bonded and unbonded tendons for segmentally and other than segmentally constructed bridges).
A number of deliverables, provided as appendices, are not included in the report itself but are available to download from the links below: