For slabs supported by columns having a ratio of long to short sides not greater than 2, tests indicate that the nominal shear strength may be taken equal to V. = 4AVFb,d (13.11a) according to ACI Code 22.6.5, where b, = the perimeter along the critical section, and A is the lightweight concrete factor (see Section 5.5a). However, for slabs supported by very rectangular columns, the shear strength predicted by Eq. (13.11a) has been found to be unconservative. According to tests reported in Ref. 13.15, the value of V, approaches 2Ab,d as 3, the ratio of long to short sides of the column, becomes very large. Reflecting these test data, ACI Code 22.6.5 states further that V_ in punching shear shall not be taken greater than V. = (2 +AVEba (13.11b) The variation of the shear strength coefficient, as governed by Eqs. (13.11a) and
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A multistory commercial building is to be designed as a flat plate system with floors of uniform thickness having no beams or drop panels. Columns are laid out on a uniform 20 ft spacing in each direction and have a 16 in. square section and a vertical dimension 10 ft from floor to floor. Specified service live load is 100 psf including partition allowance. Using the direct design method, design a typical interior panel, determining the required floor thickness, size and spacing of reinforcing bars, and bar details including cutoff points. To simplify construction, the reinforcement in each direction will be the same; use an average effective depth in the calculations. Use all straight bars. For moderate spans such as this, it has been determined that supplementary shear reinforcement would not be economical, although column capitals may be used if needed. Thus, slab thickness may be based on Eqs. (13.11 a ), (13.11 b ), and (13.11 c ); or column capital dimensions can be selected using those equations if slab thickness is based on the equations in Section 13.5. Material strengths are fy = 60,000 psi and fc= 4000 psi.
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