Ultimate strength of composite beams
The study of composite beams is characterised by the connection between the two components: the concrete slab and the steel girder. In this thesis, two different problems, related to this connection were studied: the problem of partial interaction in composite beams of long spans with low degrees of shear connection, and the problem of transverse flexibility of the stud connection 'joint' between the concrete slab and the steel beam as part of a discrete inverted U-frame. This thesis is therefore divided into two separate parts. The study of beams with partial shear connection and solid slabs or slabs with metal decking is considered in the first part. Such beams, with a uniform stud spacing over each shear span, with spans longer than 10 m - generally propped during construction, and with low degrees of shear connection, could fail prematurely and suddenly in shear rather than gradually in bending, due to the limited slip deformation capacity of the studs in shear. In order to investigate the behaviour of simply supported and continuous composite beams with different degrees of shear connection, different geometry and different shear spans under design ultimate loading conditions, a numerical computer simulation program was written. The program takes account of the relative displacement between the slab and the beam and the non-linear behaviour of steel, concrete and stud connectors. A data bank of maximum slip results for different beams is obtained for ultimate beam loads designed to the interpolation method in Eurocode 4. The computer simulation gives a conservative but safe assessment of the suitability of the degree of interaction for a specific design ultimate load . The results were used to formulate a tentative design method for composite beams with solid slabs and partial shear connection. Discrete inverted U-frame action exists between composite bridge beams with intermittent vertical web stiffeners which provide lateral restraint to the bottom flange in the hogging bending region near the internal supports. The design method in BS 5400:Part 3 for discrete U-frame action gives values for the transverse flexibility of a number of standard structural steel connections which are used in the calculation of the effective buckling length and the lateral deflection of the compression flange. To obtain similar values for steel-concrete joints, tests were done to scale 1:1 on six flange-slab connections with different stud configurations, but constant conservative dimensions for the steel flange and the concrete slab. Their crack patterns can be predicted by using a truss analogy. All test specimens either failed in shear or by puffing out of the studs, and shear cracking and shear failure criteria for concrete beams can be used to predict the cracking and failure loads. The transverse elastic flexibiities of the joints in these tests only represent the behaviour of the same joints in a complete structure up to the point where the shear cracks propagated over the full width of the specimens. Based on these few test results, a limited tentative design equation is proposed for the transverse flexibility of these type of stud connections, although further research is required into the influence of the increased flexibility on the buckling mode and the variation of the flexibility with variables other than the stud configurations.