Fatigue life analysis of a steel trapezoidal box girder bridge using measured strains
Date
2011
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Publisher
University of Delaware
Abstract
The Newport Viaduct is a 1,984 feet long bridge owned by the Delaware Department of Transportation located in Newport, DE. During an in-depth inspection in 2006, approximately 665 cracks were discovered near the internal cross frame diaphragm connection plates and the girder webs. A 2.5 inch gap exists between the termination of the diaphragm connection plate and the flanges of the girder. This is a known fatigue prone detail subject to out of plane deformations which lead to the formation of distortion induced fatigue cracks. Previous research done on the Newport Viaduct used global finite element models to estimate the fatigue life of the web gap details and analyze potential retrofit options. The purpose of this project is to measure site specific strains caused by variable traffic loads and use this data to carry out a fatigue life analysis of the web gap details. In order to assist in the fatigue life analysis, localized finite element models of the web gap details were created. Given geometric differences, separate models were created for the top and bottom web gap details. The finite element models were created using FEMAP and solved using ABAQUS. The analysis results showed a high stress gradient in the web gap region with a maximum near the weld toe. Moreover, the results showed that the bottom web gap detail experienced higher stresses than the top web gap detail given the same loading. In-service monitoring of the Newport Viaduct was carried out for 23 days. Resistive foil type strain gages were installed directly in the top and bottom web gap regions and data was collected and processed at 100Hz using the Rainflow algorithm to count the number of load cycles and their magnitude. The recorded data was used in conjunction with the bottom web gap finite element model and Miner’s Rule to determine an effective stress at the weld toe. The results predict a mean fatigue life of 22 years for the bottom web gap detail. Furthermore, current uncracked web gap details are expected to crack in the future.