Development of criteria for fatigue repairs in bridge girders damaged by out-of-plane distortion

Abstract

Numerous highway bridge girders across the United States have experienced fatigue cracking due to repetitive out-of-plane distortion at unstiffened web gaps caused by poorly designed diaphragm connection details. Due to the widespread nature of the problem, effective and economical means of repair and retrofit need to be developed.-Once the source of out-of-plane distortion is removed, such repair and retrofit techniques involve the arrest of fatigue crack growth, either by drilling holes at crack tips, or by completely removing a portion of densely cracked web plate material by flame-cutting. In the process of repairing fatigue cracks, stress concentrations result, affecting the fatigue life of the structure. Currently, no standardized criteria exist for the use of hole-drilling and flame-cutting as repair techniques. The first portion of the study analyzes the effects of spacing and orientation of multiple drilled hole spacing and orientation on stress concentrations using finite element analyses. The findings are then used to derive general guidelines for the use of drilled holes. In addition, safer alternatives to drilling holes are developed and recommended. A study of flame-cut holes is also conducted. Experimental data indicates that flame-cut holes are at worst Category D details based on AASHTO's fatigue criteria. Results of laboratory testing and finite element analyses are used in the development of flame-cut hole guidelines. A finite element model predicts the behavior of both repaired and unrepaired cracks along the longitudinal web-to-flange connection of a plate girder subjected to a linear bending stress to determine if repair by drilling holes is really necessary. The results of the finite element analyses and their correlation with experimental data indicate that arrest of a longitudinal web-to-flange by hole-drilling is necessary.