Composite beam behaviour in braced frames

A numerical study has been conducted to investigate the influence of compos ite beam-to-steel column joints on the behaviour of composite beams, for a number of subframes representing the spans and lending arrangements of curr ent commercial buildings in the UK. A two-stage analysis has been performed on each of these subframes, varying the steelwork connection, the percenta ge of reinforcement over the support and the degree of shear connection bet ween the steel beam and the concrete slab. The studies use a computer model which has been developed to simulate the behaviour of steel-framed buildin gs with composite floor decks. The program is capable of simulating the beh aviour of two-dimensional subframes, and considers their two-stage behaviou r, both during unpropped construction and as fully composite beams. It is a lso capable of taking into account the partial interaction between the stee l beam and the composite slab, the orientation of the profiled metal deck, the effect of additional reinforcement over supports and the semi-rigid nat ure of the joint between the composite beam and steel column. In contrast t o the usual observations made in isolated joint tests, the study indicates very low values of strains in reinforcing bars at the composite beam-to-ste el column joint at the ultimate limit state. The results also indicate that the common types of composite joints available are capable of providing th e rotation capacity required to sustain the ultimate load with about 1% of reinforcement over the support, without the use of expensive column web sti ffeners. (C) 1999 Elsevier Science Ltd. All rights reserved.