Fatigue specifications for the design of new, and evaluation of existing, highway bridges are provided by AASHTO in the LRFD Bridge Design Specifications (LRFD) and the Manual for Bridge Evaluation (MBE), respectively. Recently published NCHRP Report 721 Fatigue Evaluation of Steel Bridges contains the latest developments in fatigue evaluation of existing bridges.

Steel fatigue refers to localized damages caused by cyclic stresses of nominal magnitudes well below the static yield strength of the steel. Fatigue damage on steel bridges has been categorized as either load-induced or distortion-induced. Load-induced fatigue is due to the primary in-plane stresses in the steel plates that comprise bridge member cross-sections. …