Diaphragm bolt loosening retrofit for web gap fatigue cracking in steel girder bridges

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Tarries, David
Major Professor
Terry J. Wipf
Lowell F. Greimann
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Civil, Construction, and Environmental Engineering

Many of Iowa's multiple steel girder bridges have shown signs of fatigue cracking due to out-of-plane deflection of the girder web near diaphragm connections. This fatigue prone web gap area is located in the negative moment regions of the girders where the stiffeners with diaphragms are not attached to the top flange of the girders. The Iowa Department of Transportation (Iowa DOT) has attempted to stop fatigue crack propagation but with limited success. For this reason the Iowa DOT has requested research on a new field retrofit to loosen the bolts in the connection between the diaphragm and the girders. The intent of this research is to show that loosening the bolts at the diaphragm/girder connections in steel girder bridges is effective in reducing strain and fatigue in the web gap. Select web gaps in negative moment regions on three heavily traveled bridges were instrumented with strain gages and deflection transducers to measure changes in the web gap due to the retrofit. Each bridge had a different diaphragm type: I-beam, channel, and X-brace. Field tests, using loaded trucks of known weight and configuration, were conducted on the bridge before and after implementing the bolt loosening retrofit. Short-term retrofit testing was conducted in the I-beam and channel diaphragm bridges. A long-term continuous monitoring test was completed on the X-braced bridge. Data was collected over a number of months with ambient truck traffic crossing the bridge. Data of significant load was recorded for analysis to support that the results of loosening the bolts do not change over time. Results indicate that loosening the diaphragm bolts reduces out-of-plane deflection and strain in the web gaps, as well as almost completely eliminating the strain in the diaphragms between the girders. The reduction in strain correlates to a reduction if not an elimination of fatigue cracking in the web gaps and an increase of in-service life of the bridge.

Tue Jan 01 00:00:00 UTC 2002