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Carol Shield, Principal Investigator and Jerome F. Hajjar, Co-Principal Investigator

Repair of Fatigued Steel Bridge Girders with Carbon Fiber Strips

In the construction of older steel girder bridges, it was common that bridge girders were hot-rolled steel wide flange beams. In order to increase the cross section of these members in high moment regions, steel plates were welded to the outsides of flanges of the beams. Unlike present-day practice, in older bridge designs it was not required to terminate these cover plates in the positive moment region of the bridge. This was because it was not well understood that the welds at the ends of cover plates in negative moment regions could cause stress concentrations resulting in fatigue cracking. Now, decades later, many of the tension flanges in the negative moment regions are cracking due to fatigue at the ends of the cover plates in regions of the welds, which serve as stress risers. When cracks occur in a girder tension flange, it is common to repair the bridge by bolting steel cover plates to the flange. When these cracks occur in negative moment regions of continuous girders with concrete slabs as a bridge deck, this repair technique requires the removal of part of the bridge deck, adversely affecting traffic on the bridge.

Recent research has shown that it may be possible to use carbon fiber reinforced polymer (CFRP) strips on the bottom side of the top tension flange of these girders. Carbon fiber composites have demonstrated excellent fatigue life in aerospace applications; hence, they have great promise for repair of fatigue cracks in steel bridge girders. Adhering CFRP strips to the inside faces of the tension flange in the negative moment region of steel bridge girders potentially offers a viable and economical long term solution to repairing girders that have cracked due to fatigue. This research is conducting experimental testing to determine the validity of using CFRP strips to repair fatigued steel bridge girders, particularly within the tension flange in the negative moment region of the girder.

These researchers' usage of supercomputing resources, particularly the finite element analysis program, abaqus, is focusing on justification of the test specimens and test set-ups compared to the actual application of CFRP strips. Through this research, a series of experiments are being conducted to define the development length of the adhesive between CFRP strips and the steel tension flange so that a sufficient load in the tension flange can be transferred to CFRP strips. Due to the scaled and simplified specimens compared to the actual application, it is necessary that these experimental set-ups and specimens represent the actual application of CFRP strips and adhesive applied to the tension flange. Particularly, the stress distribution in the adhesive of the specimens must compare well to the stress distribution in the adhesive applied to the tension flange of the steel girders. This justification of the experimental specimens and test set-ups is achieved by conducting finite element analyses both on test specimens and on CFRP strips bonded to the tension flange. These results are then compared to the stress distributions in the adhesives. The postprocessor for abaqus enables a visual comparison of the results. It is well known that there is a stress singularity at the crack tip, and this requires finer mesh around the crack tip.

This information is available in alternative formats upon request by individuals with disabilities. Please send email to alt-format@msi.umn.edu or call 612-624-0528.

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