Fatigue Performance and Structural Detail Optimization of Arc-Shape Cutouts in Diaphragm of Steel Box Girder

The fatigue cracks at the arc-shape cutouts in the diaphragm of the steel box girder are one of the major fatigue deteriorations of the girder. To improve the fatigue performance of the structural details and determine the optimal structures of the cutouts, a suspension bridge (the stiffening girder of the bridge being the steel box girder) was taken as the research object, the finite element model for the segment of the steel box girder was established and the fatigue stress analysis was carried out. Based on the Palmgren-Miner linear accumulated damage rule and the S~N curves as provided in the Eurocode 3, the fatigue lives of the cutouts were calculated, the fatigue performance of the 6 kinds of the common arc-shape cutouts at home and abroad was compared and the influences of the cutout radii and diaphragm thickness on the fatigue stresses were analyzed. The results demonstrate that under the action of the wheel loads, the obvious tensile and compressive stress concentration zones exist at the arc-shape cutouts in the diaphragm. The fatigue cracks initiate from the compressive stress concentration zone and the direction of the crack propagation is basically perpendicular to that of the principal compressive stress. The shapes of the cutouts have significant influences on the fatigue performance of the cutouts. Among the typical shapes of the cutouts commonly used at home and abroad, the round arc+straight line cutout (cutout 4) is the best shape of the cutout for the rigid diaphragm. The appropriate increase of the radius of the cutout 4 and the diaphragm thickness is helpful to the improvement of the fatigue life of the cutout and the effect of increase of the cutout radius is better than that of increase of the diaphragm thickness.