ABSTRACT
Many steel bridges in the United States designed before the mid-1980s are highly susceptible to distortion-induced fatigue cracking (Jajich & Schultz, 2003). This vulnerability is substantially increased if the out-of-plane driving force caused by differential girder displacement is increased for any reason. This research examines one such case where a multigirder steel bridge, originally built in the 1960s, was retrofitted to improve seismic performance. As part of the retrofit, single angle K-type cross frames were replaced with stiffer double-angle cross-type frames. This retrofit led to an increase in the driving force, and after only a few years following the retrofit, numerous fatigue cracks were identified in the web-gap region. A previous study confirmed the significant role of the new cross-bracing in the development of fatigue cracks and recommended a few repair measures (Lindquist et al., 2015). This paper presents the results of a detailed parametric study to further evaluate only those retrofit strategies which are able to reduce web-gap stresses below the constant amplitude fatigue threshold defined by AASHTO. The retrofit techniques evaluated include creating a slot in the stiffener to soften the connection, attaching angle-sections on either side of the stiffeners to stiffen the web near the web-gap, and providing a positive connection between the connection plate and the girder flange. A dual-level Finite Element (FE) modelling is developed to model the macroand micro-model.
The highest stress reduction was observed when a positive connection is provided between the connection plate and the girder flange. In this study, both edges of the angle legs are welded to sub-members. The results indicate smaller angle section has better performance than larger angle section. Also it is shown that increasing the thickness of the angles results in reducing the web-gap stress. An alternative retrofit option to reduce the web-gap stress and shift down the location of the stress concentration in the web is to create a slot in the connection plate resulting in a softening of the connection. This method was the only method evaluated to soften the web-gap among the methods considered in this paper. Seismic retrofitted bridge plan-view. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig240_1.tif"/>
The other method evaluated to stiffen the web-gap region is to attach angle sections on either sides of the connection plate. The distance of the attached angle section from the connection plate varied between 1 in. and 5 in., and the shortest distance from the stiffener for an attached angle section results in the minimum stress in the web-gap region. In this case study, attaching the angle section at a distance shorter than 2 in. provides enough reduction in the web-gap stress to prevent the area from cracking. Also this study shows that the most efficient configuration for stiffening the web-gap for this method, is attaching a single angle so that the free flange locates in the connection plate side.
