Effect of floor joint design on catenary actions of precast floor slab system

To prevent progressive collapse of building structures, the establishment of the catenary action mechanism of beams or slabs is crucial so that adequate post-collapse resistance can be attained. It is believed that the joint design of precast floor system, e.g. the tie design, plays a key role in facilitating such a mechanism. An experimental study has been undertaken to investigate the catenary behaviour of the precast concrete slab system following the removal of the intermediate wall supports. To this end, five large-scale concrete floor assembly tests have been devised and carried out. Each test consisted of two standard hollow core floor slab units with various tie arrangements at the joints, which resembled a single storey floor structure supported by cross walls. The floor joint ties were placed on the pre-existing keyways where the grout was cast after the test assemblies were set up. The grout strength was specified to be 20-30 MPa and 10 or 12 mm tie bars were used with an embedment length ranging from 250 mm to 350 mm. Test results indicated that specimens experiencing bar fracture failure patterns collapsed prior to the formation of the catenary action, but those specimens with the pull-out failure pattern showed clear evidence of catenary behaviour. Furthermore, the difference in the post-collapse behaviour and the failure patterns indicated the characteristics of the catenary action. Test results reveal that for the ties designed with inadequate embedment length, the slip and the resulting large deflection will effectively trigger the catenary action. However, the full bond will limit the development deflection and lead to the fracture of tie bars before the catenary action is trigged.