Band A. By the formula in the code the negative moment is, in which the value of j is taken from Table 1, page 20, corresponding to the stresses = Accept fifteen 5%-in rods, each way, which give the required sectional area for the positive moment. Ten of these rods are raised over the supports and furnish 20 X 0.307 6.14 sq in to resist the negative bending moment. The straight bars should extend twenty rod-diameters beyond the adjacent lines of inflection, and the bent bars to the quarter-points of adjoining spans. Band B. By the formula in the code both the negative and positive moments Accept twelve 2-in round rods, each way, which give the required sectional area for the positive moment. Six of these rods, raised over the supports and carried to the quarter-points of adjoining spans, furnish the same sectional area across the section of negative moment. The straight rods, as in the case of the A bands, should extend twenty rod-diameters beyond the lines of inflection. 19. Typical Design of Interior Panel, Four-Way System. New York City Code. Fig. 2, illustrating these computations, was furnished through the courtesy of the Concrete Steel Company of New York. v = 650 lb per sq in 60 lb per sq in (shearing unit stress around periphery of drop) "2 = 120 lb per sq in (shearing unit stress around periph ery of column capital) Panel-data: Dimensions, center-to-center of columns, 20 ft by 20 ft. Drops are to be used. This code uses the designation of the American Concrete in which (4 X 80) is the periphery of the drop, in inches, and 6.5 the effective depth of the slab, in inches. As 60 lb is permitted, accept a square drop with side equal to 6 ft 8 in. (4) Size of column capital: Assume a round capital of diameter equal to 0.225L in which M the bending moment in inch-pounds, at column-head section, computed as in Paragraph (6) following, and based upon an assumed depth of 114 in. The values of k and j are taken from Table I, page 20, and correspond to (c) Punching-shear around periphery of column capital, in which (3.14 × 54) is the periphery of the column capital and 9.25 the effective depth of the combined slab and drop, both in inches. As 120 lb per sq in is permitted, an offset of 34 in, making the total depth 114 in, is chosen. (6) Moments: W = (20 X 20 X 296) + (6.66 × 6.66 × 39) : = 120 130 lb The negative moment at the column-head section is Accept nine 2-in square, deformed bars of sufficient length to give nine 11⁄2-in square bars over the column-head as well as in the center of the span. Since the cross-sectional area already available from the diagonal bands is 2.25 X 2 X sine 45° 3.18 sq in, the additional amount required is = Accept sixteen 2-in square, deformed bars. They must be supplied from the direct bands by raising eight, at each end, over the column-heads. The other five bars of the thirteen in the direct bands lie in the center of the span. For the length and position of the bars the illustration on page 261 may be referred to. 20. Tables for the Design of Girderless Floors, Two-Way System. New York City Code. The following tables, III, IV, and V, based upon the New York City Code, as amended to 1926, give the proper dimensions of concrete and reinforcements for INTERIOR SQUARE PANELS designed according to the TWO-WAY SYSTEM. These tables can be used for estimating purposes and as guides in making the necessary assumptions as to slab-thickness, depth of drop, etc., for designs based upon the New York, Chicago, or American Concrete Institute requirements. |