On some Fatigue Problems Related to Steel Bridges

Doctoral thesis, 2001

This thesis concerns three specific fatigue topics related to steel members in bridges, namely: (1) Modular Bridge Expansion Joints (MBEJs). Several premature fatigue failures of MBEJs have occurred in Europe and USA during the past decades. The principal cause of such failures has been an insufficient knowledge of the magnitude of the acting forces including dynamic effects. This has led to an inadequate design with respect to fatigue. In this thesis, a proposal for vertical and horizontal design loads acting on MBEJs is obtained by evaluating a large amount of field test data put at the author's disposal during his visit at Lehigh University, USA. A relatively simple model to describe the dynamic behaviour of MBEJs is also proposed. Reasonable load ranges based on traffic considerations as well as an adequate fatigue detail category based on laboratory tests is proposed for fatigue analysis of MBEJs. (2) Constant Amplitude Fatigue Limit (CAFL) for Riveted Connections. For old riveted steel bridge members an adequate estimate of the CAFL, i.e. the stress range level below which there will be no fatigue crack growing during cyclic loading, is essential, especially to determine whether previous traffic loading has produced fatigue damage accumulation or not. However, there is a lack of information concerning fatigue test data in the high-cycle region, i.e. for stress ranges close to the CAFL. Based on full-scale tests an adequate estimate of the CAFL for riveted connections is made by a proposed original method. (3) Web Breathing in Girders with Slender Webs. When a girder with slender web is subjected to in-plane cyclic loading, repeated web buckling deflections (so called: web breathing) may lead to fatigue cracks. Such cracks generally commence at the weld toe of the fillet welds that connect a web to its boundary elements. Four full-scale I-girders with slender webs were laboratory tested. The investigation mainly focused on gaining a better knowledge of the fatigue behaviour of slender girders with relatively large panel aspect ratios and subjected to combined bending and shear. Nonlinear FE-analyses of the test girders were performed to investigate the stress state along the boundary of the web during loading and good agreement with the results of laboratory measurements was found. The risk for fatigue cracking due to web breathing seems to increase with increasing panel aspect ratio. Moreover, web panels subjected to combined bending and shear are more prone to fatigue cracking due to web breathing than similar panels subjected to predominant shear. An analysis of a box section arch railway bridge, where a breathing tendency of some web panels was observed, is also presented. Finally, based on evaluations of experiments a new fatigue design approach for avoiding cracking due to web breathing is proposed.