Punching shear behavior of flat slabs with post-tensioning in one direction
Authors: Ismail Omnia S. , Khalil Ayman H. H. , El-Kateb Mahmoud 
Published: December 2024

Post-tensioned flat slab (PT) system has become widely used in the construction field in the recent time. The main advantages of the post-tensioned slab system are strength improvement and resource savings. Additionally, the conventional post-tension flat slab is an economical system, especially for long-span structures. However, it may not be suitable for structures with significant variations in spans in both directions of the slab. The unidirectional post-tensioned slab, which involves a post-tensioning system in one direction of the slab and a normal reinforced concrete flat slab system in the perpendicular direction. This offers an effective solution for the specific issues mentioned above. This system is also recommended for structures with expansion joints that do not have enough space to install the post-tension system and tensioning machine. In the current research, the punching shear behavior of conventional bidirectional PT slab was compared to unidirectional PT slab counterparts. Five post-tensioned specimens were tested to investigate the punching shear behavior for unidirectional PT slabs at different parameters. Variation of precompression stress level using different numbers of tendons and distribution of tendons were the variable parameters studied in unidirectional PT slabs. All slabs were exposed to static loading until failure using a 2200 kN hydraulic jack with a pump capacity. Deflection behavior of all slabs was measured using a linear variable differential transformer device (LVDT). Additionally, strain gauges were fixed on concrete, steel bars, and tendons to evaluate the strain of each item. The results obtained that the five tendons unidirectional PT specimens had a 1.4% reduction of punching shear resistance compared to bidirectional PT specimen counterparts. Increasing precompression stress improved punching shear capacity. Specimens with banding tendons at the punching shear zone enhanced punching shear capacity by 4.5% more than those with distributed tendons. The study also indicated that the ultimate punching shear failure load was the highest in conventional post-tensioned specimen. Additionally, the unidirectional post-tensioned slabs with five tendons and a precompression stress of around 1.5 MPa exhibited a 1.2% slightly higher ultimate failure load than the unidirectional counterpart with seven tendons and a precompression stress of around 2.5 MPa, However the seven tendons specimen had the best deflection behavior compared to all specimens. The three tendons specimen with a precompression stress around 1 MPa, the lowest precompression stress, exhibited the lowest ultimate failure load. The calculated punching shear capacities of all specimens from conventional post-tensioned flat slabs equations of various codes, such as the American Building Code (ACI-214), Euro Code (EC) and Egyptian Practice Code ECP-203, are compared to the existing experimental results. For the studied specimens, the cost of unidirectional PT slabs was more economical by around 29% compared to a conventional PT slab.