Structural Concrete - Hassoun

9.7 One-Way Joist Floor System 335
The weight of the web of the beam shall be added to this value. Span of the beam is 24 ft.
Estimated total depth = L ( ) 24
20 × 0.8 = 20 × 0.8 × 12 = 11.5in. say, 12 in.
Slab thickness is 5 in. and height of the web is 12 – 5 = 7in.
( ) 7
Factored weight of beam web =
12 × 150 × 1.2 = 105 lb∕ft
2. Axial load on an interior column:
Total uniform load on beam = 4.5 + 0.105 = 4.605 K∕ft
P u = 4.605 × 24 ft = 110.5 K
9.7 ONE-WAY JOIST FLOOR SYSTEM
A one-way joist floor system consists of hollow slabs with a total depth greater than that of solid
slabs. The system is most economical for buildings where superimposed loads are small and spans
are relatively large, such as schools, hospitals, and hotels. The concrete in the tension zone is ineffective;
therefore, this area is left open between ribs or filled with lightweight material to reduce
the self-weight of the slab.
The design procedure and requirements of ribbed slabs follow the same steps as those for
rectangular and T-sections explained in Chapter 3. The following points apply to design of one-way
ribbed slabs:
1. Ribs are usually tapered and uniformly spaced at about 16 to 30 in. (400 to 750 mm). Voids are
usually formed by using pans (molds) 20 in. (500 mm) wide and 6 to 20 in. (150 to 500 mm)
deep, depending on the design requirement. The standard increment in depth is 2 in. (50 mm).
2. The ribs shall not be less than 4 in. (100 mm) wide and must have a depth of not more than
3.5 times the width. Clear spacing between ribs shall not exceed 30 in. (750 mm) (ACI Code,
Section 9.8.1).
3. Shear strength, V c , provided by concrete for the ribs may be taken 10% greater than that for
beams. This is mainly due to the interaction between the slab and the closely spaced ribs (ACI
Code, Section 9.8.1.5).
4. The thickness of the slab on top of the ribs is usually 2 to 4 in. (50 to 100 mm) and contains
minimum reinforcement (shrinkage reinforcement). This thickness shall not be less than 1
12
of the clear span between ribs or 1.5 in. (38 mm) (ACI Code, Section 9.8.2.1.1).
5. The ACI coefficients for calculating moments in continuous slabs can be used for continuous
ribbed slab design.
6. There are additional practice limitations, which can be summarized as follows:
• The minimum width of the rib is one-third of the total depth or 4 in. (100 mm), whichever
is greater.
• Secondary reinforcement in the slab in the transverse directions of ribs should not be less
than the shrinkage reinforcement or one-fifth of the area of the main reinforcement in
the ribs.
• Secondary reinforcement parallel to the ribs shall be placed in the slab and spaced at
distances not more than half of the spacings between ribs.