Precast concrete hollow-core floor unit support and continuity

Technical Abstract

Three types of special tie reinforcement at the end supports of hollow-core precast concrete floor units with a cast-in-place topping slab were investigated. This tie reinforcement is intended to prevent collapse of the floors in the event of inadequate seating lengths or imposed movements due to volume changes or earthquake excitations. The tie reinforcement passed over the supporting beam and was anchored in two filled voids in the ends of the precast units. Two types of test were conducted.

In one test the vertical load was applied when the support seating was zero. In this case it was found that the shear capacity at the support can be calculated from the shear friction across the cracks in the cast-in-place concrete topping slab and in the two filled voids used to anchor the ties plus any vertical component of force in the tie bars, providing anchorage failure of the tie bars did not occur. A cautious value for the shear-friction coefficient µ to be used in calculating the available shear friction capacity is recommended.

In the other test the vertical load was applied after the precast floor unit had been pulled horizontally off the supporting beam. The maximum shear force which can be transferred by kinking of the tie reinforcement at the connection if the floor unit suddenly slides off or loses its support was evaluated using condition of energy equilibrium. The tie reinforcement was shown to be capable of supporting at least the service gravity loads in these extreme conditions if it is well designed