Structural Behaviour of Post Tensioned Concrete Structures: Flat Slab. Slabs on Ground
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In this investigation strength and structural behaviour of prestressed concrete is studied with one full scale test of one flat slab, 16000 mm x 19000 mm, and three slabs on ground each 4000 mm x 4000 mm with thickness 150 mm. The flat slab was constructed and tested in Aalesund. This slab has nine circular columns as support, each with diameter 450 mm. Thickness of this test slab was 230 mm and there were two spans in each direction, 2 x 9000 mm in x-direction and 2 x 7500 mm in y-direction from centre to centre column. The slab was reinforced with twenty tendons in the middle column strip in y-direction and eight tendons in both outer column strips. In x-direction tendons were distributed with 340 mm distance. There were also ordinary reinforcement bars in the slab. Strain gauges were welded to this reinforcement, which together with the deflection measurements gives a good indication of deformation and strains in the structure. At a live load of 6.5 kN/m2 shear failure around the central column occurred: The shear capacity calculated after NS 3473 and EuroCode2 was passed with 58 and 69 %, respectively. Time dependent and non-linear FE analyses were performed with the program system DIANA. Although calculated and measured results partly agree well, the test show that this type of structure is complicated to analyse by non-linear FEM. Prestressed slabs on ground have no tradition in Norway. In this test one reinforced and two prestressed slabs on ground were tested and compared to give a basis for a better solution for slabs on ground. This test was done in the laboratory at Norwegian University of Science and Technology in Trondheim. The first slab is reinforced with 8 mm bars in both directions distributed at a distance of 150 mm in top and bottom. Slab two and three are prestressed with 100 mm2 tendons located in the middle of slab thickness, and distributed at a distance of 630 mm in slab two and 930 mm in slab three. Strain gauges were glued to the reinforcement in slab one and at top and bottom surface of all three slabs. In slab two and three there were four load cells on the tendons. Each slab were loaded with three different load cases, in the centre of slab, at the edge and finally in the corner. This test shows that stiffness of sub-base is one of the most important parameters when calculating slabs on ground. Deflection and crack load level depends of this parameter. Since the finish of slabs on ground is important, it can be more interesting to find the load level when cracks start, than deflection for the slab. It is shown in this test that crack load level was higher in prestressed slabs than in reinforced slab. There was no crack in the top surface with load in the centre, but strain gauges in the bottom surface indicate that crack starts at a load of 28 kN in the reinforced slab, and 45 kN in the prestressed slabs. Load at the edge give a crack load of 30 kN in reinforced slab, 45 kN and 60 kN in prestressed slabs. The last load case gives crack load of 30 kN in reinforced slab, 107 kN and 75 kN in prestressed slabs. As for the flat slab, FE analyses were performed for all of the three slabs on ground, and analyses shows that a good understanding of parameters like stiffness of sub-base and tension softening model, is needed for correct result of the analyses.