REPAIR OF BRIDGE STEEL GIRDERS DAMAGED BY DISTORTION-INDUCED FATIGUE

Abstract

This study investigates the repair of steel bridge girders damaged by distortion-induced fatigue. The study is presented in three parts. The first part describes finite element modeling techniques used to evaluate the potential for fatigue cracks in steel girders subjected to distortion-induced fatigue. The modeling techniques employed in this study were intended to identify areas near welded connections in bridge girders with the highest potential for developing fatigue cracks, and to evaluate the effectiveness of retrofit measures in reducing the potential for crack propagation near welded connections with existing fatigue cracks. Computer simulations correlated well with experimentally observed crack patterns and were useful in providing an indication of the effectiveness of various retrofit measures. The second part of the study investigates the use of composite materials to repair fatigue damage in steel girders. A total of 17 specimens with pre-existing fatigue cracks were tested under cyclic loading to evaluate the performance of composite overlays when used to repair fatigue damage in steel structures. Two control specimens were tested without overlays and the remaining 15 were repaired with Carbon Fiber Reinforced Polymer (CFRP) overlays of various thicknesses. Results indicate that this method was very effective, and that proper implementation of this type of repair can reduce the crack propagation rate to negligible values. The third part of the study consisted of physical and computer simulations of 914-mm (36-in.) deep girder-cross frame subassemblies subjected to cyclic loading. The simulations were carried out to investigate the stress demands caused by distortion-induced fatigue and to evaluate the effectiveness of various retrofit measures. Previously used repair methods for distortion-induced fatigue damage have attempted to reduce the stress demand in the web gap region by increasing the flexibility of cross-frame to girder connection or by restraining the lateral motion of the cross-frame by attaching the connection plate to the slab. A new retrofit approach was investigated in this study designed to reduce the stress demand in the web gap region by distributing the force at the girder-cross frame connection over a larger area. A new retrofit detail is proposed based on this approach, which consists of adding steel angles connecting the girder web and the connection plate, and a steel bar on the back side of the girder web to distribute the lateral force over a wider region of the web. Experimental and computer simulation results are presented showing that this repair method is very effective in preventing the growth of horseshoe-shaped cracks around the web-cross frame connection and of straight cracks near the junction between the flange and web