Lateral-torsional buckling of beams with continuous bracing

Continuous multispan steel bridges are common along the state and interstate highway systems. The structural system is tied together by a reinforced concrete deck slab and transverse steel members, or diaphragms, which are connected to the girders. Many of these bridges were designed as noncomposite systems and constructed with no shear stud connectors between the top flange of the girders and the deck slab. Although there are no shear connectors, continuous lateral support of the top flange is provided by embedment in the slab. Buckling of the girders can only result from negative bending moments near the interior supports of these continuous span bridges. Often, the flange thickness and width at the interior supports in the continuous girders is increased to resist high negative moments so that the girder is a nonprismatic beam.; Design equations for lateral-torsional buckling (LTB) resistance in the American Institute of Steel Construction (AISC) LRFD Specifications (1998) account for only prismatic and web-tapered beams. A study of LTB capacity of stepped beams has been performed, and simple design equations are proposed. Finite element method (FEM) buckling analysis results for stepped beams under uniform moment have been used to develop new design equations that account for the change in cross section of stepped beams. Traditional moment gradient factors for prismatic beams were reviewed and were found to be sufficiently accurate for some stepped beam cases. A new moment gradient factor equation has been developed from the finite element analysis results and is proposed for the determination of the LTB resistance of stepped beams under general loading conditions.; Furthermore, FEM buckling analyses for a number of different beams with prismatic or stepped cross sections having continuous lateral top flange bracing have been conducted. The beams have been subjected to a concentrated load or a uniformly distributed load applied at top flange and end moments. New moment gradient factors for prismatic or stepped beams with continuous lateral top flange bracing have been developed using the results of the FEM investigation. In all applicable areas, a significant review and comparison between existing design specifications and proposed equation is presented.