Fatigue damage to steel bridge diaphragms (FHWA-OK-89-10) 2156

Abstract

The cause of diaphragm cracking on an interstate highway bridge was investigated. The investigation consisted of load-testing the bridge, performing a computeraided analysis of the bridge superstructure and specific components, measuring the chemical and physical properties of the diaphragm steel, and performing fatigue tests on simulated diaphragms. It was determined that the 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. A number of repair techniques were tested. These include attaching an auxiliary flange, smoothing out the cope, reducing restraint in the connection by removing bolts from the connection, and replacing coped diaphragms with uncoped diaphragms. The technique ˆof removing bolts from the connection is recommended for implementation since it is by far the simplest to perform and is effective in extending fatigue life.