144 CHAPTER FIVE
In both methods, a flat slab is considered to consist of strips parallel to column
lines in two perpendicular directions. In each direction, a column stripspans
between columns and has a width of one-fourth the shorter of the two perpendic-
ular spans on each side of the column centerline. The portion of a slab between
parallel column strips in each panel is called the middle strip(see Fig. 5.3).
Direct Design Method
This may be used when all the following conditions exist:
The slab has three or more bays in each direction.
Ratio of length to width of panel is 2 or less.
Loads are uniformly distributed over the panel.
Ratio of live to dead load is 3 or less.
Columns form an approximately rectangular grid (10 percent maximum
offset).
Successive spans in each direction do not differ by more than one-third of
the longer span.
When a panel is supported by beams on all sides, the relative stiffness of the
beams satisfies(5.100)
where 1 in direction of l 1
2 in direction of l 2
relative beam stiffness defined in the preceding equation
l 1 span in the direction in which moments are being determined, ctoc
of supports
l 2 span perpendicular to l 1 ,ctocof supports
The basic equation used in direct design is the total static design moment in
a strip bounded laterally by the centerlineof the panel on each side of the center-
line of the supports:
(5.101)
wherewuniform design load per unit of slab area and lnclear span in
direction moments are being determined.
The strip, with width l 2 , should be designed for bending moments for which
the sum in each span of the absolute values of the positive and average negative
moments equals or exceeds Mo.
Mowl 2 l^2 n
80.2
1
2
(^)
l 2
l 1
2
5