Minimum‐Weight Design of Continuous Composite Girders

Abstract

Minimum‐weight plastic design of continuous composite girders with braced compact sections, as allowed in the autostress design procedure, was investigated according to American Association of State Highway and Transportation Officials (AASHTO) specifications. The required plastic‐moment distribution of continuous symmetrical two‐span composite girders is determined for AASHTO HS20–44 truck and dead loading. The required ratio of positive‐ to negative‐plastic‐moment capacity leading to girders with minimum structural weight is developed. The optimum I‐shaped sections that lead to minimum structural weight for the given required plastic moments are presented for both the positive‐ and negative‐moment regions of the composite girders. An elastic‐plastic‐softening type of moment‐curvature relationship is proposed for the negative‐moment region of continuous composite girders with compact sections. This curve is compared with experimental results of composite girders with various web and flange slenderness ratios. A numerical example is worked out for a given design condition. The structural weights were compared with composite girders consisting of uniform sections.