Effects of Removing Diaphragms from Steel Girder Bridge

Many multigirder steel bridges built before the 1980s are experiencing distortion-induced fatigue cracking at diaphragm-girder connections. A means of eliminating the cyclic stresses that cause those fatigue problems is to remove the interior diaphragms (diaphragms not at the girder supports). The effects of removing all interior channel diaphragms from an existing simple-span bridge with rolled-steel girders were evaluated based on field measurements of girder stresses and deflections made before and after the diaphragms were taken out. Results from tests with trucks of known weight indicate that removing the diaphragms resulted in increases between 6 and 15 percent in the maximum bottom flange stresses experienced by the most heavily loaded girder. Results for loading by trucks of unknown weight in normal traffic indicate that the most heavily loaded girder may carry up to 17 percent more load as a result of removing the diaphragms.