Influence of Cutout Geometry on Stresses at Welded Rib-to-Diaphragm Connections in Steel Orthotropic Bridge Decks

The geometry of the cutout at the intersection of longitudinal ribs and transverse diaphragm plates in steel orthotropic bridge decks can have a significant impact on the stress in the region of the welded rib-to-diaphragm connections. Many different approaches to detailing this connection have been tried, with mixed success. To provide additional guidance to designers, a finite element parametric study that systematically investigated the effect of altering the geometry of certain variables was performed. Parameters considered included overall cutout shape, cutout depth, diaphragm plate thickness, and deck plate thickness. Results indicate that the suggested cutout geometry shown in the current (2002) AASHTO LRFD Bridge Design Specifications for closed-rib orthotropic bridge decks produces increased stresses on this fatigue-sensitive connection. An improved geometry is proposed to replace that currently presented in the specification. Results of this study indicate that larger cutout geometries offer less resistance to out-of-plane displacements induced by longitudinal rib rotations. As a result, out-of-plane stresses are decreased at the connection. However, cutouts that are excessively deep will increase in-plane stresses at the welded rib-to-diaphragm connection. If the cutout is too shallow, out-of-plane stresses may become prohibitively high owing to restraint of longitudinal rib rotations. Cutouts that are between 1/3 and 1/2 the depth of the rib with transition radii at the top of the cutout of 50 to 75 mm and vertical cuts are most favorable. The cutout geometry shown in the AASHTO LRFD specification results in an overstiff cutout detail. A proposed improved geometry is presented.