Analysis of distortion-induced fatigue cracking of a trapezoidal steel box girder bridge including retrofit investigation.

摘要

Distortion induced fatigue is a common problem for aging steel bridges. The distortion creates secondary bending stresses at web gap locations that are not accounted for in design, thereby initiating fatigue cracks. A large number of such distortion-induced fatigue cracks have recently been found on Delaware Bridge 1-501 in Newport, Delaware. This multi box steel tub girder bridge has cracking that has initiated in the weld which connects the web to the internal bracing via connection plates.;To investigate this problem, global finite element models of two of the bridge's spans were created. In order to calibrate the models for use in analysis, a diagnostic load test was performed using strain transducers to measure strains associated with bending of the girders. Once the models were found to reasonably represent the actual response of the structure, the model was then refined for local modeling of the web gap region. As expected, stresses within the gap were found to be large. Reduction of the stresses within these gap locations is required to increase the remaining life of the bridge.;Several retrofit methods were designed to accomplish this stress reduction, based on effective retrofits used to correct similar problems in the past. These retrofit methods include drilling holes at the crack tips, positively attaching the connection plates to flanges, increasing the length of the web gaps, and removing the diagonal elements in the diaphragms. The retrofits were modeled and analyzed under fatigue loading and the resulting stresses were compared to the original structure. Of these methods, the positive attachment was found to be most effective in reducing the web gap stresses; however, all of the approaches provided significant reductions in the stress range. Recommendations for applying this type of retrofit detail along with the drilling of holes at the end of crack tips are provided.