Simulated Traffic Fatigue Loading of Steel Bridges

This paper discusses new fatigue design provisions for steel bridges, recently adopted by AASHTO, based on constant-amplitude fatigue data obtained in NCHRP Project 12-7, Effects of Weldments on Fatigue Strength of Steel Beams. According to these provisions, bridges are designed to withstand a certain number of constant-amplitude cycles of stress equal to the design live-load plus impact stress. Required constant-stress cycles are developed using Miner's law for cumulative damage to reflect the estimated volume of truck traffic causing variable-amplitude stress cycles that are usually well below the design live-load stresses. The new provisions are expected to result in conservative designs. However, there are still gaps in the available information, including areas such as: 1) the magnitude and frequency of traffic loadings on bridges; 2) the actual stress caused by these traffic loadings, and 3) the fatigue life of various types of bridge members under variable-amplitude loadings. Studies initiated to obtain information on these topics are discussed.