Neglecting the weight of the drop panel, the service dead load is (150/12) 5)=94 psf; thus



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Neglecting the weight of the drop panel, the service dead load is (150/12)(7.5)=94 psf; thus

  • Neglecting the weight of the drop panel, the service dead load is (150/12)(7.5)=94 psf; thus

  • wu=1.2wD +1.6wL

  • =1.2(94) +1.6(120)

  • =132 + 204

  • =336 psf



















For fy=40 ksi, a flat slab with drop panel, and α = smaller of 4.34 and 5.42, Table-1 gives

  • For fy=40 ksi, a flat slab with drop panel, and α = smaller of 4.34 and 5.42, Table-1 gives

  • For both exterior and interior panels.



In order that the full 3-in. projection of the drop below the 7.5 in. slab is usable in computing reinforcement, the 6 ft 8 in. side of the drop is revised to 7 ft so that one-fourth of the distance between the edges of the 5-ft column capital and the 7-ft drop is just equal to (10.5 - 7.5) = 3 in.

  • In order that the full 3-in. projection of the drop below the 7.5 in. slab is usable in computing reinforcement, the 6 ft 8 in. side of the drop is revised to 7 ft so that one-fourth of the distance between the edges of the 5-ft column capital and the 7-ft drop is just equal to (10.5 - 7.5) = 3 in.











Check the five limitations (the sixth limitation does not apply here) for the direct design method. These five limitations are all satisfied.

  • Check the five limitations (the sixth limitation does not apply here) for the direct design method. These five limitations are all satisfied.

  • Total factored static moment M0.

  • Referring to the equivalent rigid frames A, B, C, and D in Fig. 4&5, the total factored static moment may be taken from the results previously found; thus







For the short or long edge beam Fig.6(a), the torsional constant C is computed on the basis of the cross-section shown in Fig.6(a).

  • For the short or long edge beam Fig.6(a), the torsional constant C is computed on the basis of the cross-section shown in Fig.6(a).







For the short or long interior beam [Fig.6(b)], a weighted slab thickness of 8.5 in. is used, on the assumption that one-third of the span has a 10.5 in. thickness and the remainder has a 7.5 in. thickness.

  • For the short or long interior beam [Fig.6(b)], a weighted slab thickness of 8.5 in. is used, on the assumption that one-third of the span has a 10.5 in. thickness and the remainder has a 7.5 in. thickness.





The percentages of the longitudinal moments going into the column strip width are shown in lines 10 to 12 of Table-2. The column strip width shown in line 2 is one-half of the shorter panel dimension for both frames A and C, and one-fourth of this value for frames B and D. The sum of the values on lines 2 and 3 should be equal to that on line 1, for each respective frame.

  • The percentages of the longitudinal moments going into the column strip width are shown in lines 10 to 12 of Table-2. The column strip width shown in line 2 is one-half of the shorter panel dimension for both frames A and C, and one-fourth of this value for frames B and D. The sum of the values on lines 2 and 3 should be equal to that on line 1, for each respective frame.



These values are shown in line 5 of Table-2.

  • These values are shown in line 5 of Table-2.

  • The percentages shown in lines 10 to 12 are obtained from Table-2A, by interpolation if necessary.









(a) Moments in column and middle strips

  • (a) Moments in column and middle strips

  • The typical column strip is the column strip of equivalent rigid frame C of Fig.5; but the typical middle strip is the sum of two half middle strips, taken from each of the two adjacent equivalent rigid frames C. The factored moments in the typical column and middle strips are shown in Table-3.





Slab thickness for flexure

  • Slab thickness for flexure







Design of Reinforcement

  • Design of Reinforcement

  • The design of reinforcement for the typical column strip is shown in Table-4; for the typical middle strip , it is shown in Table-5. Because the moments in the long direction are larger than those in the short direction, the larger effective depth is assigned to the long direction wherever the two layers of steel are in contact. This contact at crossing occurs in the top steel at intersection of middle strips and in the bottom steel at the intersection of middle strips. Assuming #5 bars and ¾ in. clear cover, the effective depths provided at various critical sections of the long and short directions are shown in Fig.9







Wide beam action. Investigation for wide beam action is made for sections 1-1 and 2-2 in the long direction, as shown in fig.10. The shot short direction has a wider critical section and short span; thus it does not control. For section 1-1, if the entire width of 20 ft is conservatively assumed to have an effective depth of 6.12 in.

  • Wide beam action. Investigation for wide beam action is made for sections 1-1 and 2-2 in the long direction, as shown in fig.10. The shot short direction has a wider critical section and short span; thus it does not control. For section 1-1, if the entire width of 20 ft is conservatively assumed to have an effective depth of 6.12 in.



Wide beam action.

  • Wide beam action.

  • if however bw is taken as 84 in. and d as 9.12 in. on the contention that the increased depth d is only over a width of 84 in.











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