Torsional behaviour of precast concrete edge beams and connections

This Paper examines the effect of connections on the torsional behaviour of precast concrete edge beams in temporary and permanent condition in precast concrete buildings. Two types of connection are considered: the end connection to columns, and the connection between the beam and the precast floor units. In both cases the site-placed concrete gives some measure of composite action. 600 mm deep and 300 mm wide L-shaped edge beams with the rigid end condition failed in torsion where there was twisting of the beam, accompanied by large rotations and yielding of shear reinforcement. Torsional capacities exceeded the AC 1318 and BS 8110 ultimate design values by factors of l·43 and 2·51, respectively. When edge beams with real column connections were subjected to the same loading, failure occurred in the joints unless there was a top connection. When the same beams were tested compositely with hollow-cored slab there was no torsional failure in the beam. This represented the permanent condition in a precast frame. There are two factors which contribute to the elimination of torsion in precast edge beams: a self-equilibrating horizontal propping force provided by the slab, and a reduction in the eccentricity of slab loading. the design recommendations are that edge beams need not be reinforced agains torsion, provided that the beam contains the minimum area of stirrups, that hollow-cored flooring with a bearing of 75 mm is used, and that tie reinforcement is provided between the edge beam and slab. The in situ concrete should be at least grade 25 N/mm².