Fatigue damage to steel bridge diaphragms

The cause of diaphragm cracking on an interstate highway bridge is investigated. The investigation consists of load-testing the bridge, performing a finite element analysis of the diaphragm, and conducting fatigue tests on simulated diaphragms. It is determined that a high degree of restraint at the diaphragm-to-girder connection causes individual diaphragms at each transverse location to act as continuous members reaching from one side of the bridge to the other. Differential deflections of longitudinal members induce moments in the diaphragms, resulting in tensile stresses along the bottom of the diaphragms. These tensile stresses are amplified by the presence of a bottom-flange cope at the diaphragm-to-girder connections. High tensile stresses lead to initiation of fatigue cracks in diaphragms at copes. Repair techniques tested include smoothing the cope, reducing restraint in the connection by removing bolts from the connection, and replacing coped diaphragms with uncoped diaphragms. The technique of removing bolts from the connection is recommended for implementation.