Fatigue Investigation of Slit Rib-to-floor Beam Connections in Steel Orthotropic Bridge Decks

The steel orthotropic bridge deck (SOBD) is a bridge deck system which consists of longitudinal open or closed ribs that pass through transverse floor beams with a continuous steel deck plate on top. All of these components are joined using welded connections which can be costly to fabricate and prone to fatigue cracks. The rib-to-floor beam (RFB) connection in an SOBD is labor intensive and fatigue sensitive. In the past, the RFB connection has either been a fully fitted connection or a connection with an extended cut-out which is larger than the opening in the floor beam web for the rib to pass through. A third RFB detail that includes an additional opening below the rib bottom, which is not an extension of the rib opening, has been proposed and is studied herein. This connection, termed a "slit RFB connection," is similar to the fitted connection with closed, U-shaped ribs passing continuously through matching openings in the floor beam webs, but an additional opening contained entirely within the floor beam web allows for relatively unrestrained out-of-plane rib rotation. The slit RFB connection has not been used or studied previously, and shows potential for efficient, automated fabrication and good fatigue performance.This study investigated the slit RFB connection potential for automated fabrication as well as fatigue performance under various fatigue loading scenarios and SOBD configurations. The slit RFB connection is amenable to automated fabrication since it does not include an extended cut-out with complex welded-joint preparation, weld terminations, or internal stiffening. The slit RFB connection studied uses only fillet welds for the connection which require no labor intensive joint preparation. Since the slit is located entirely within the floor beam web, the RFB fillet weld is uninterrupted around the bottom of the rib and can be made robotically.A finite element analysis study was performed to evaluate the stress response and fatigue performance of the slit RFB connection under multiple fatigue loading scenarios for two different floor beam restraint conditions: independent and restrained/supported. The study showed that the stress response of the connection is highly dependent on the in-plane shear carried by the floor beam. When the slit RFB connection was studied with an independent floor beam (which has no vertical or horizontal restraint) high tension stresses developed along the edges of the slit. These stresses can be lowered slightly by changing the slit geometry, floor beam depth, or floor beam thickness, however, an unreasonably deep floor beam may be needed to ensure good fatigue performance.When an SOBD is used in a bridge deck replacement project, the floor beam bottom flange is usually restrained/supported by a transverse structural member such as a truss, and the shear force carried by the floor beam web is significantly reduced. This reduction in shear force reduces the large tension stresses that develop at the slit edge to levels that may fall below the constant amplitude fatigue limit. In this case, large tension and compression stress concentrations still exist at the edge of the slit due to local deformation of the slit from the reactions on the floor beam web from shear forces (shear flow) in the rib walls. The largest magnitude stresses are compression stresses which are expected to contribute to fatigue damage only if tension stresses develop at the same location under an alternate fatigue load position. The stresses that develop at the weld toes on the floor beam web face and the rib wall are low.The stresses from floor beam web shear and from reacting the shear forces in the rib walls (known as in-plane behavior) are the dominating stresses for the slit RFB connection. The stresses from rib rotation (due to primary bending in the rib when wheel loads are in the span between floor beams, known as out-of-plane behavior) are not significant since the slit in the floor beam web allows the rib to rotate relatively freely. Behavior from in-plane floor beam shear and reaction rib wall shear forces are the main causes of stress at the edges of the slit, even when the loading is out-of-plane.The conclusions of this study are:(1) The slit RFB connection appears to be amenable to automated fabrication.(2) For an independent floor beam (which has no vertical support within its span), the stress response of the slit RFB connection is highly dependent on the magnitude of the in-plane shear force carried by the floor beam; large tension stresses may develop along the slit edge, and the fatigue performance may not be acceptable.(3) When the floor beam is supported by a transverse structural member, the stress response of the slit RFB connection is dominated by reaction forces on the floor beam web from shear forces in the rib walls; the resulting stresses along the slit edge are compressive, and are not excessive, and good fatigue performance is anticipated.(4) The slit in the floor beam web allows the rib to rotate with relative freedom, resulting in small stresses in the slit RFB connection from out-of-plane deformation of the floor beam web; stresses at the fillet weld toes are relatively small.(5) The slit RFB connection has the potential for good fatigue performance when used in deck replacement projects, when the floor beam is restrained/supported by existing transverse structural members of the bridge.