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<br />96 <br /> <br />11.6.2 Slab <br /> <br />The design considered here for basement floors will be for a reinforced concrete slab. <br />Ultimate strength design theory is used. First, the loads must be calculated or assumed. Let <br />the effective load per square foot on the slab be called w. The determinants for ware <br />water height, partial superstructure load transferred to the slab, and slab dead load. <br /> <br />w = 62.4 H (( M - t/12 (150 <br /> <br />H = height of water above bottom of slab in "feet" <br /> <br />t = slab thickness in "inches" <br /> <br />l>F = shift in superstructure load from footing to slab due to decrease in bearing capacity <br />and increase in settlement of footing due to pore pressure from flood water <br /> <br />l>F = a (Superstructure load to footings) <br />Area of slab <br /> <br />a = 1/4 <br /> <br />Now, using ACI318-63, method 3, table 2, case 6: <br />Mu = 1.4M (actual) = (1.4)(0.061wA') for AfB 0.5 <br /> <br />For ultimate strength design, the capacity moment equation becomes: <br /> <br />Mu = <D[bd'f ,(1 - 0.59q)] (capacity) <br /> <br />b ~ 12" f', = 2,500 psi fy ~ 40,000 psi <br /> <br />q Asfy = d = t - 3f4' - 1(, (%)" = t = 1 'Is" <br />bdf'c <br /> <br />As @ bar As x U = Sp = bar spacing in "inches" <br />Sp <br />When actual Mu equals capacity Mu, the equation becomes: <br /> <br />1.4(0.061)(62.4b+ 23 - 12.5t)N O.'L [12(t-1.125)'. <br />12 <br /> <br />2,500 <br /> <br />bar As (12)(40) ( =->:- ( ) ( ) <br />... 1 - 0.59 x uar "s 12 40 ] <br />Sp(2.5)(12)(t - 1.125) (Sp)(2.5)(12)(t-1.125) <br /> <br />The variables A, slab span; t, slab thickness; b, height of water loading; and bar As, single bar <br />cross-sectional area may all be assumed, and the equation may be solved for bar spacing. <br />This has been done for 12-ft, 15-ft, and 18-ft spans using #4 or #6 reinforcement, and the <br />results are plotted and shown in Figures 11.14 through 11.19 (Reference No. 26). <br /> <br />The design figures may be used as follows: <br /> <br />1. A maximum loading height has already been determined for soil/water loading on the <br />basement wall. Use the water surface elevation associated with that loading condition as <br />the starting point for slab design. <br /> <br />2. Enter the figure with the appropriate span length and reinforcement size. <br /> <br />3. For the depth of water, move horizontally to the right to intersect the curve with the <br />desired slab thickness. Then look straight down to find the required spacing of the <br />reinforcement. <br />