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Two way slab

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Two way slab

1. 1. Direct Method   Reinforcement in Two-way Slabs  Footing Classification  Footing Design
2. 2. The reinforcement is for slabs without beams.
3. 3. Look at Example 13-7 Design of a flat plate floor without spandrel beams - Direct Design Method Look at Example 13-9 Design of a Two-way slab with beams in both directions.
4. 4. Shear failure occurs when inclined cracks form due to flexural and shearing stresses. Two types of shear failure mechanism in 2-way slabs. (a) One-way shear (b) Two-way shear (Punching shear).
5. 5. One-way shear Two-way shear
6. 6. Inclined cracking Slab slides down column Top (negative reinforcement pulls out of slab leaving no connection to the column Inclined cracking
7. 7. Critical perimeter is located d / 2 from column face, where d = effective depth of slab
8. 8. Critical perimeter is located d / 2 from column face, where d = effective depth of slab Slabs with Drop Panels
9. 9. Two-way Shear with Negligible Moment Transfer ACI Eqn. 11-1 Vu Vn where, Vu =Factored shear force (tributary area bounded by lines of zero shear) Vn = Nominal shear resistance of slab 0.85
10. 10. Two-way Shear with Negligible Moment Transfer Vn Vc Vs where, Vc =Shear resistance of concrete Vs = Shear resistance of steel (in most slabs, Vs = 0)
11. 11. (a) Two-way Shear with Negligible Moment Transfer For two-way shear in slabs (& footings) Vc is smallest of 4 ACI 11-35 Vc 2 f c b0 d c where, = long side/short side of c column concentrated load or reaction area b0 = length of critical perimeter around the column
12. 12. (b) Two-way Shear with Negligible Moment Transfer d s ACI 11-36 Vc 2 f c b0 d b0 where, = 40 for interior columns s 30 for edge columns 20 for corner columns
13. 13. (c) Two-way Shear with Negligible Moment Transfer ACI 11-37 Vc 4 f c b0 d Take smaller of (a), (b) and (c)
14. 14. Tributary area for Vu calculations Figure 13-41 MacGregor
15. 15. If V Vn can be increased by: , Vc u (1) Increase slab thickness (2) Use drop panel (3) Increase b0 by increasing column size or adding a fillet or capital (4) Add shear reinforcement