Structural Concrete - Hassoun

672 Chapter 17 Design of Two-Way Slabs
Table 17.11
Design of an Interior Flat-Slab Floor System
M 0l
= 358 K⋅ft Column Strip Middle Strip
Long direction
M factor −0.49M 0 0.21M 0 −0.16M 0 0.14M 0
M u (K ⋅ ft) −175.4 ±75.2 −57.3 ±50.1
Width of strip (in.), b 120 120 120 120
Effective depth (in.), d 8.5 6.5 6.5 6.5
R u = M u
(psi) 243 178 129 119
bd2 Steel ratio ρ (%) 0.48 0.34 0.25 0.23
A s = ρbd (in. 2 ) 4.9 2.65 1.95 1.79
Min. A s = 0.0018bh s (in. 2 ) 2.16 2.16 1.73 1.73
Selected bars 16 no. 5 14 no. 4 10 no. 4 9 no. 4
M 0s = 289.3 K⋅ft Column Strip Middle Strip
Short Direction
M factor −0.49M 0 0.21M 0 −0.16M 0 0.14M 0
M u (K ⋅ ft) −142 ±60.8 −46.3 ±40.5
Width of strip (in.), b 120 120 168 168
Effective depth (in.), d 8.5 6.5 6.5 6.5
R u = M u
(psi) 196 144 78 68
bd2 Steel ratio ρ (%) 0.38 0.28 0.15 0.13
A s = ρbd (in. 2 ) 3.9 2.2 1.64 1.42
Min. A s = 0.0018bh s (in. 2 ) 2.16 2.16 2.42 2.42
Selected bars 13 no. 5 11 no. 4 12 no. 4 12 no. 4
17.11 WAFFLE SLABS
A two-way waffle slab system consists of concrete ribs that normally intersect at right angles.
These slabs might be constructed without beams, in which case a solid column head is made over
the column to prevent any punching due to shear. Wide beams can also be used on the column
centerlines for uniform depth construction. Square metal or fiberglass pans are commonly used to
form these joists. A thin slab of 3 to 5 in. is cast with these joists to form the waffle slab.
Each panel is divided into a column and a middle strip. The column strip includes all joists that
frame into the solid head; the middle strip is located between consecutive column strips. Straight
or bent bars could be used as reinforcement in a waffle slab. The design of a two-way waffle slab
is similar to that of flat slabs by considering the solid head as a drop panel. To prevent any excess
in the diagonal tension in the head, a sufficient size of column must be used or a shear cap must be
provided.
In the design of a waffle slab, the top slabs with each rib form a T-section, with considerable
depth relative to flat plates. Consequently, long spans carrying heavy loads may be designed with
great savings in concrete. Waffle slabs also provide an attractive ceiling, which is achieved by