The Impact of the AASHTO LRFD Design Code on Bridge Stiffness and Strength: Part 1: Methods and Design Comparisons

About the report:

The Commonwealth of Virginia is currently transitioning from the long-used AASHTO Allowable Stress Design (ASD) specification to AASHTO's Load and Resistance Factor Design (LRFD) specification. The new specification features revised live loads, more conservative impact factors, a new load distribution method for analysis, and a probabilistically based limit state design approach. Traditional deflection limits have been made optional in the LRFD format. The extensive changes in the specification make it unclear as to whether bridges designed using the LRFD specification will be more or less flexible than those designed using the ASD specification. A series of bridges were designed using both ASD and LRFD formats to investigate what, if any, significant changes in bridge flexibility might be encountered and whether deflection limits are more or less likely to be violated with the LRFD format. Based upon the design of six single-span bridges with varying spans and numbers of girders, and a partial design of a three-span continuous bridge, it was concluded that the LRFD designs tend to be lighter and more economical in general and also tend to be more flexible than ASD designs of the same bridge. The weight savings appears to be more pronounced on single-span bridges than on multiple-span bridges because of the greater significance of local and lateral stability concerns in the finished bridges for multi-span bridges.

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