۱- Introduction

۲- Bridge description

۳- Bridge instrumentation

۴- Test procedure

۵- Field test results

۶- Finite element analysis

۷- Summary and conclusions

References

Abstract

Many steel bridges in the United States designed before the mid-1980s are highly susceptible to distortion-induced fatigue cracking. 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 double-deck bridge complex, originally built in the 1960s, was retrofitted to improve seismic performance. As part of the retrofit, single angle K-type diaphragms were replaced with stiffer double-angle cross-type diaphragms. This seismic retrofit led to an increase in web-gap stresses, and within approximately one year following the retrofit, inspectors identified numerous fatigue cracks in the web of longitudinal girders where connection plates terminate near top flanges. A repair measure was implemented to provide a positive connection between the connection plate and the girder flange. The purpose was to reduce the high stress concentrations in the web-gap region by restricting the out-of-plane distortion in the web-gap region. Field tests were carried out in addition to developing finite element (FE) models to investigate the efficacy of the repair technique. The results confirmed a significant decrease in the web-gap stress after implementation of the repair, and a subsequent FE analysis showed that the new load path through the repair angle section did not introduce a new fatigue sensitive area. In fact, the repair resulted in stresses well below the constant amplitude fatigue threshold (CAFT) for this type of detail.

Introduction

Most continuous-span double-deck viaducts built in the San Francisco Bay Area during the 1950s and 1960s were damaged during the 1989 Loma Prieta earthquake [1]. The deficiencies in these viaducts led to an immediate review of all double-deck bridge structures in the United States. One particular seismic retrofit project (and the focus of this work) involved a Midwestern double-deck bridge complex with substructure and superstructure elements without adequate capacity based on current seismic criteria [2,3]. A seismic retrofit strategy was adopted based on the criteria described in the FHWA Seismic Retrofitting Manual for Highway Bridges [4]. Almost immediately following completion of the seismic retrofit, horseshoe-shaped cracks were identified in the unstiffened regions (frequently called the web gap) of the longitudinal girders where the original K-type diaphragms were replaced with new, stiffer cross-diaphragms. This problem mostly occurs in bridges built in the United States prior to the mid-1980s when the design specifications required connection plates to have a tight-fit detail at the girder flanges, but not welded [5–۷]. Normally, part of the connection plate, attached to plate girders, must be clipped to clear the web-to-flange weld resulting in a soft and short section that is more flexible in the transverse direction than the rest of the girder’s height. This area in the web is known as the web-gap region. Cracks originating in this region are likely the result of secondary stresses resulting from out-of-plane distortion in the unstiffened web-gap region and account for approximately 90% of all fatigue cracking [23]. As a result, a significant number of older bridges have been affected by distortion-induced fatigue cracking in the web-gap region.