## U30

ln/33

ln/33

ln/33

ln/36

ln/36

75,000

ln/28

ln/31

ln/31

ln/31

ln/34

1. ln is length of clear in long direction, face-to-face of support.

2. Minimum thickness if slabs without drop panels should not be less than 5 in.

3. Minimum thickness of slabs with drop panel should not be less than 4 in.

Notes:

1. ln is length of clear in long direction, face-to-face of support.

2. Minimum thickness if slabs without drop panels should not be less than 5 in.

3. Minimum thickness of slabs with drop panel should not be less than 4 in.

The minimum thickness of two-way slabs is dependent on the relative stiffness of the beams am. If 0.2 < am < 2.0, the slab thickness should not be less than 5 in. or

If am > 2.0, the denominator in the above equation should be replaced with (36 + 9b), but the thickness should not be less than 3.5 in. When am < 0.2, the minimum thickness is given by Table 7.8.

7.13.1.1 Column Strips, Middle Strips, and Equivalent Frames

For the Direct Design Method, to take into account the change of the moment across the panel, the floor system is divided into column and middle strips in each direction. The column strip has a width on each side of a column centerline equal to 0.25l2 or 0.25(j, whichever is less (see Figure 7.12). A middle strip is bounded by two column strips. The moment diagram across each strip is assumed to be constant and the reinforcement is designed for each strip accordingly.

In the next step of the Direct Design Method, equivalent frames are set up. Each equivalent frame consists of the columns and beams that share a common column or grid line. Beams are attached to the slabs that extend to the half-panel division on each side of the grid line, so the width of each equivalent frame consists of one column strip and two half middle strips (see Figure 7.12). Equivalent frames are set up for all the grid lines in both directions of the floor system.

7.13.1.2 Total Factored Static Moment

The first analysis step of the Direct Design Method is determining the total static moment in each span of the equivalent frame

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