Instability of composite beams in hogging bending
This work is concerned with local buckling and lateral distortional buckling, two aspects of instability that govern the design of composite beams in hogging regions. Local buckling in hogging regions of a continuous composite beam was modelled by moment curvature characteristics of a cantilever, modified by two curvature ratios, K1 and K2. Test based expressions for K1 and K2, in terms of a combined slenderness λc, were developed, and subsequently used in numerical analyses of 50 two-span composite beams to assess moment redistribution allowed for Class 2 beams by draft Eurocode 4. The analyses include effects of non-linear material properties, residual stresses and local buckling. The parametrical studies include adverse values, in relation to practice, of relative length of adjacent spans, span-to-depth ratio, and ratio of hogging to sagging moment of resistances. It is concluded that the redistribution of elastic bending moments allowed by the draft Eurocode 4 is safe and economical. Distortional lateral buckling of composite beams with both continuous and discrete U-frame actions was studied experimentally. Distortional lateral buckling was found in the tests of two composite beams with inverted U-frame actions. Web distortion was effectively reduced by vertical web stiffeners, which form a part of discrete U-frames together with the slab and the connection of U-frame. The work provides background to assess lateral buckling strength for composite beams with both continuous and discrete U-frame actions. A further theoretical approach on the topic of discrete inverted U-frame action was presented. Strength and stiffness of discrete U-frame connections were also studied. The strength of a discrete U-frame connection was found to be influenced by both the shear failure of concrete, and the yielding of steel top flange in the connection. A simple rule to assure strength of U-frame connections is proposed by checking these two failures separately. The prediction of shear failure of a U-frame connection is based on a truss model, and the prediction of failure in the steel top flange is based on a rigid plastic mechanism. A semi-empirical formula for flexibility of a U-frame connection was derived. They were all checked against test results. Interactive U-frame force and U-frame stiffness were also studied. A tentative design method for discrete U-frame composite beams was proposed.