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Many investigations have been carried out to date into the behaviour of transversely stiffened web panels in bending and shear and many different theories have been proposed. Different code rules have been developed based on these theories. The UK's steel bridge code, BS 5400 Part 3, based its design rules for transverse stiffeners on the work of Rockey et al., while early drafts of Eurocode prEN 1993-1-5 were based on the work of Hoglund. The former's tension field theory places a much greater demand on stiffener strength than does the latter's rotated stress field theory. Due to a lack of European agreement, EN 1993-1-5 was modified late on its drafting to include a stiffener force criterion more closely aligned to that in BS 5400 Part 3. The rules for stiffener design in EN 1993-1-5 are thus no longer consistent with the rotated stress field theory and lead to a greater axial force acting in the stiffener. The rules for the design of the web panels themselves in shear however remain based on Hoglund's rotated stress field theory, creating an inconsistency. Recent investigations by the authors have suggested that the rules in BS 5400 Part 3 and, to a lesser extent, in the current version of EN 1993-1-5 can be unduly pessimistic. This paper investigates the behaviour of transversely stiffened plate girders in bending and shear using non-linear finite element analyses. It considers slender symmetrical steel girders with and without axial force and also steel-concrete composite plate girders, which are therefore asymmetric. It discusses the observed web post-buckling behaviour, compares it with the predictions of other current theories and recommends modified design rules. It includes investigation into whether a stiffness-only approach to stiffener design can be justified, rather than a combined stiffness and force approach. The shear-moment interaction behaviour of the girders as a whole is also investigated and compared to the codified predictions of BS 5400 Part 3 and EN 1993-1-5. Francesco Presta PhD, CEng, MIStructE Senior Engineer, WS Atkins and Partners Overseas Chris R. Hendy MA(Cantab), CEng, FICE Head of Bridge Design and Technology, Atkins Highways and Transportation Emilio Turco Professor of Structural Engineering, DAP, University of Sassari