The objective of this study was to extend the findings of NCHRP Project 12-15(4) (see NCHRP Report 267) by providing additional information on fatigue crack growth behavior of steel bridge members under randomly applied, variable-amplitude loadings in the fatigue limit, extreme life region. Fatigue tests were carried out on a series of full-depth plate girders in order to augment available information on the effects of infrequently exceeding the constant amplitude fatigue limit (CAFL) during the service life of steel bridges. AASHTO Category E;pr web attachments and cover-plated flange details and Category C transverse stiffeners and diaphragm connection plates were tested during the program.

The variable amplitude test results were found to supplement and verify the preliminary observations reported in Report 267. They demonstrated that low levels of exceedance of the CAFL will result in fatigue cracking even when the root-mean-cube effective stress range is below the CAFL.

The final report has been published as NCHRP Report 354.

In addition to the test program directed at the primary objective, a small portion of the total effort was expended on a reassessment of the fatigue specifications in the AASHTO Standard Specifications for Highway Bridges. Minor revisions to the fatigue design provisions were recommended to, and adopted by, the AASHTO Subcommittee on Bridges and Structures. The evaluation and recommended specifications were published in: NCHRP Report 286, "Evaluation of Fatigue Test Data and Design Criteria on Welded Details."