CROSS FRAME DIAPHRAGM FATIGUE AND LOAD DISTRIBUTION BEHAVIOR IN STEEL HIGHWAY BRIDGES

An investigation into the behavior of cross frame diaphragms in steel highway bridges was conducted. The investigation involved both field testing of three bridges as well as analytical studies using finite element analysis. Diaphragms were removed from the structure and less than half replaced by a new diaphragm design. The remaining diaphragms were permanently removed from the structure. The removal of the diaphragms provided a unique opportunity to study the effects of diaphragms on the load distribution characteristics of the structure. Strain measurements, under controlled loading conditions, were obtained with diaphragms and with the new diaphragm arrangement in place. Combined with the previous measurements for the as-built condition, a direct comparison was made. The research team found that the girder flange stresses doubled when the diaphragms were removed. With the new diaphragm pattern, the increase in stresses was limited to 20%. The increase without the diaphragms was still significant below the stress levels predicted by the American Association of State Highway and Transportation Officials (AASHTO) design methods. Two additional bridges were instrumented in their as-built condition and the data were compared with the results obtained from the finite element analysis. This allowed for the prediction in the change of load distribution due to diaphragm removal or modification. Researchers developed a method to predict diaphragm member forces under vehicle live load for both the strength and fatigue limit states. Other types of fatigue damage in steel highway bridges were studied and generalized repair procedures are provided.