SPECIAL SESSIONS

1 INTRODUCTION

Orthotropic welded steel-deck bridge rib systems, developed by German engineers in the 1950s for the structural systems of bridges (1, 2), were first adopted for ship and naval structures. The word orthotropic is from “orthogonal” for “ortho” and “anisotropic” for “tropic”, so an orthotropic deck has anisotropic structural properties along the two orthogonal directions of its plane. The open (torsionally soft) and closed (torsionally stiff) rib-framing system for orthotropic deck bridges was developed by the Germans: these alternatives are the two basic types of ribs that are parallel to the main span of the bridge. These ribs are also used to stiffen other plate components of the bridge. Flat plates, angles, or half beams are types of open ribs that are always welded to the deck plate at only one location, where a bent or rolled piece of steel plate is welded to the deck plate to form a closed space. The common steel angle can either be used as an open or closed rib depending on how it is welded to the steel deck plate. If the angle is welded at only one leg, then it is an open rib; if the angle is rotated to 45° and both legs are welded to the deck plate forming a triangular space or rib, it is a closed rib. Engineers have experimented with a variety of concepts to shape, roll, or bend a flat plate of steel into the optimum closed rib. The trapezoidal rib has been found to be the most practicable by engineers and the worldwide steel industry. Recently, the Japanese built the record span suspension bridge plus the record span cable-stayed bridge with trapezoidal rib construction (Chen and Lian 2000). The ribs are normally con-

nected by welding to transverse floor beams, which can be a steel hot-rolled shape, small plate girder, box girder, or full-depth diaphragm plate. When fulldepth diaphragms are used, access openings are needed for bridge maintenance purposes. The holes also reduce dead weight and provide a passageway for mechanical or electrical utilities. Since the deck plate is welded to every component, the deck plate is the top flange for the ribs, the transverse floor beam, and the longitudinal plate girders or box girders. All these various choices for the ribs, floor beam, and main girders can be interchanged, resulting in a great variety of orthotropic deck bridge superstructures (Chen and Lian 2000).