THERMO-MECHANICAL FATIGUE OF STEEL PILES IN INTEGRAL ABUTMENT BRIDGES
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The issue of fatigue in steel piles of Integral Abutment Bridges (IABs) is investigated. A three-dimensional, non-liner finite element (FE) model is constructed for a bridge located in a harsh climate. Historic temperature data for the region is obtained and a sinusoidal model was developed to represent the daily and seasonal temperature changes. The FE is parametrically run for 5 cases with bridge lengths varying between 400 and 1800 feet under the cyclic load of daily and seasonal temperature variations. The pile behavior and stresses in the piles are evaluated and a fatigue model is used to determine the fatigue life of the piles. The Palmgren-Miner rule is used to evaluate the combined effects and contribution of both types of temperature cycles. The critical location of the pile is modeled locally utilizing a global-local modeling approach. "Successive initiation" in conjunction with a strain-based fatigue damage model is implemented in the local model to determine the thermo-mechanical fatigue crack initiation site, propagation path, and rate in the piles. The results show that maximum stress occurs in the pile furthest from the center of the bridge in its flange right below the concrete abutment. Plastic deformation is observed in all the piles and in all the cases studied indicating the possibility of low cycle fatigue. Lateral displacement and maximum plastic deformation in the piles increases as the length of the bridge increases. A linear relationship was found between the length of the bridge and the lateral displacement for both seasonal and daily temperature variations. The longer the bridge is the shorter the fatigue life. The crack modeling results indicate that the crack initiates in the tip of the flange. Multiple cracks form in the flange, which causes an increased propagation rate. The propagation rate decreases when the crack reaches the web. The crack could initiate in the pile in the first decade, but it will take several decades to reach the web. The final failure of the pile may not occur for several decades.