Structural Concrete - Hassoun

9.6 Distribution of Loads from One-Way Slabs to Supporting Beams 329
only. The ACI Code, Sections 7.6.1, 8.6.1, and 24.4.3, specifies the following minimum steel ratios:
For slabs in which grade 40 or 50 deformed bars are used, ρ = 0.2%, and for slabs in which grade
60 deformed bars or welded bars or welded wire fabric are used, ρ = 0.18%. In no case shall such
reinforcement be placed farther apart than five times the slab thickness or more than 18 in.
For temperature and shrinkage reinforcement, the whole concrete depth h exposed to shrinkage
shall be used to calculate the steel area. For example, if a slab has a total depth of h = 6 in. and
f y = 60 ksi, then the area of steel required per 1-ft width of slab is A s = 6(12)(0.0018) = 0.129 in. 2 .
The spacings of the bars, S, can be determined as follows:
S = 12A b
(9.2)
A s
where A b is the area of the bar chosen and A s the calculated area of steel.
For example, if no. 3 bars are used (A b = 0.11 in. 2 ), then S = 12(0.11)∕0.129 = 10.33 in.,
say, 10 in. If no. 4 bars are chosen (A b = 0.2 in. 2 ), then S = 12(0.2)∕0.129 = 18.6 in., say, 18 in.
Maximum spacing is the smaller of five times slab thickness (30 in.) or 18 in. Then no. 4 bars
spaced at 18 in. are adequate (or no. 3 bars at 10 in.). These bars act as secondary reinforcement
and are placed normal to the main reinforcement calculated by flexural analysis. Note that areas of
bars in slabs are given in Table A.14.
9.5 REINFORCEMENT DETAILS
In continuous one-way slabs, the steel area of the main reinforcement is calculated for all critical
sections, at midspans, and at supports. The choice of bar diameter and detailing depends mainly on
the steel areas, spacing requirements, and development length. Two bar systems may be adopted.
In the straight-bar system (Fig. 9.4), straight bars are used for top and bottom reinforcement
in all spans. The time and cost to produce straight bars is less than that required to produce bent
bars; thus, the straight-bar system is widely used in construction.
In the bent-bar, or trussed, system, straight and bent bars are placed alternately in the floor slab.
The location of bent points should be checked for flexural, shear, and development length requirements.
For normal loading in buildings, the bar details at the end and interior spans of one-way
solid slabs may be adopted as shown in Fig. 9.4.
9.6 DISTRIBUTION OF LOADS FROM ONE-WAY SLABS TO SUPPORTING BEAMS
In one-way floor slab systems, the loads from slabs are transferred to the supporting beams along
the long ends of the slabs. The beams transfer their loads in turn to the supporting columns.
From Fig. 9.5 it can be seen that beam B 2 carries loads from two adjacent slabs. Considering
a 1-ft length of beam, the load transferred to the beam is equal to the area of a strip 1 ft wide and S
feet in length multiplied by the intensity of load on the slab.
This load produces a uniformly distributed load on the beam:
U B = U S S
The uniform load on the end beam, B 1 , is half the load on B 2 because it supports a slab from one
side only.
The load on column C 4 is equal to the reactions from two adjacent B 2 beams:
Load on column C 4 = U B L = U S LS