Fatigue Assessment of Repaired Highway Bridge Welds Using Local Approaches

Abstract

This thesis aims to address a number of unanswered questions regarding repair and fatigue design of welded joints in bridges, including developing and evaluating repair methods for enhancing the fatigue behaviour of web stiffeners in steel bridge girders, using local stress-based methods for evaluating the effectiveness of various repair methods and predicting the fatigue life of welded joints, and studying the effectiveness of high frequency mechanical impact (HFMI) treatments under actual in-service loading conditions in the long fatigue life regime. Along with extensive fatigue testing programs and finite element (FE) analyse studies, a strain based fracture mechanics (SBFM) model is used to predict the fatigue behaviour of repaired welds under realistic loading conditions. Through this research, a methodology is developed for generating structural stress design curves for retrofitted highway bridge welds based on small-scale fatigue experiments, relatively simple and inexpensive fatigue tests of smooth specimens, conventional static materials tests, and laboratory measurements. The idea of retrofitting web stiffener ends in steel bridge girders susceptible to distortion-induced fatigue using adhesively-bonded fibre reinforced polymer (FRP) angles is introduced through this research. The proposed retrofit method is relatively cheap and easy to use and does not require deck removal or any other severe modification to the steel girder. Fatigue tests were conducted on specimens designed to model the conditions in the region between a web stiffener and a flange in a steel girder bridge. Fatigue life increases on the order of several hundred percent were achieved by implementing the proposed retrofit. A coarse FE analysis is used to predict the effectiveness of the proposed retrofit methods in terms of the reduction in the structural stress value. A comprehensive variable amplitude (VA) fatigue testing program and analysis was performed to address a number of concerns raised regarding the use of impact treatments for the fatigue enhancement of welds in the high cycle (> 10 million cycles) domain. The test results are then used to evaluate a number of available recommendations for the fatigue design of impact treated welds. The nominal, structural, and effective notch stress approaches are considered. An SBFM model was lastly used to predict the effectiveness of an HFMI treatment applied to welded details. The model is evaluated using the experimental results and found to be capable of predicting the fatigue lives for both the as-welded and impact treated specimens for all of the studied loading conditions. The idea of using the analytical structural stress S-N curves to predict the fatigue behaviour of welded joints with a similar load carrying condition welds was then explored.