Tests on Unbonded Post-Tensioned Concrete Flat Plate with T-shaped Column Connections

Abstract

Experiments on six full-scale concrete flat plates with T-shaped column connections subjected to discrete gravity loads are reported. One of the slab specimens had ordinary bonded reinforcement only, while the remaining five were post-tensioned by unbonded tendons with various effective prestressing stresses. The effects of other parameters such as slab thickness and top flexural reinforcement ratio were also investigated. All specimens were loaded to failure, so that failure modes and the ultimate load capacity of the connections were obtained. The strain characteristics of reinforcement and the development of the stresses in the unbonded tendons during testing were recorded.

All experimental results showed a combined failure mode, but the flexural failure mode of the slab dominated in all specimens. At final stages, punching shear failure surfaces emerged in the slabs suddenly adjacent to the T-shaped columns. Such a failure surface was local and not continuous around the column as is the case for a square column connection. The test results showed that the prestressed concrete specimens keep significant ductility and have a higher resistance to punching shear failure. In general, the punching shear resistance of the slabs increased with increase in the prestressing force. The punching shear strength test results were compared with the practical predictions stated in worldwide codes such as ACI 318–02, Eurocode 2 and GB 50010. Because of the conservative approaches adopted in these codes, the calculated load capacities of the slabs are significantly underestimated when taking into account the effect of shear reinforcement. However, in good agreement the whole concrete capacity is taken into account, the experimental loads and calculated loads were achieved when the shear reinforcement was ignored. It is recommended that the formulae proposed by the three codes are suitable for adaptation for practical connection design.

Keywords

flat plate T-shaped column crack pattern failure mode ultimate strength critical section punching bending

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