Evaluation and upgrading web-gap distortion retrofits in steel girder bridges

Abstract

Distortion-induced fatigue cracking in the unstiffened web-gap of cross-frame diaphragms is the most prevalent type of cracking in steel girder bridges. Multi-steel girder bridges experience differential deflection between adjacent girders when subject to vertical loads resulting in a driving force in cross-frame diaphragms. The driving force developed in the cross-frame legs leads to out-of-plane distortion of the web-gap which results in high stress concentrations at the area, following by fatigue damages. This paper introduces an innovative method to retrofit web-gap distortion cracking, upgrade existing retrofits, and compare its effectiveness with two common retrofit techniques. The method involves cutting the existing connection plate and using angles to attach the disconnected part of the connection plate to the web. The method intends to eliminate the local high stresses at the connection plate end with considering minimal interference in original connection design and load path. Also, two other conventional repair methods were investigated, slot method and top-angle measure, to use as a basis for comparing the methods effectiveness. Laboratory testing was performed on a small-scale steel bridge bay by applying a vertical displacement to the free end of a cross-frame diaphragm to simulate the differential deflection between two adjacent girders in real bridges. The results from the testing were compared to findings from finite element analyses (FEA). Test results as well as FEA results for all investigated retrofit techniques are presented herein. Results showed that the newly developed slot-angle technique has significant potential for effectively reducing the stress concentrations in the web-gap region by removing the location of initial stress concentrations and redistributing those stresses over a wider area.