Civil Engineering Reference
In-Depth Information
The thickness required for a particular one-way slab depends on the bending, the deflec-
tion, and shear requirements. As described in Section 4.2, the ACI Code (9.5.2.1) provides cer-
tain span/depth limitations for concrete flexural members where deflections are not calculated.
Because of the quantities of concrete involved in floor slabs, their depths are rounded
off to closer values than are used for beam depths. Slab thicknesses are usually rounded
off to the nearest in. on the high side for slabs of 6 in. or less in thickness and to the
nearest in. on the high side for slabs thicker than 6 in.
As concrete hardens, it shrinks. In addition, temperature changes occur that cause expan-
sion and contraction of the concrete. When cooling occurs, the shrinkage effect and the short-
ening due to cooling add together. The Code (7.12) states that shrinkage and temperature
reinforcement must be provided in a direction perpendicular to the main reinforcement for
one-way slabs. (For two-way slabs, reinforcement is provided in both directions for bending.)
The Code states that for grades 40 or 50 deformed bars, the minimum percentage of this steel
is 0.002 times the gross cross-sectional area of the slab. Notice that the gross cross-sectional
area is
bh
(where
h
is the slab thickness). The Code (7.12.2.2) states that shrinkage and tem-
perature reinforcement may not be spaced farther apart than five times the slab thickness, or
18 in. When grade 60 deformed bars or welded wire fabric are used, the minimum area is
0.0018
bh
. For slabs with
f
y
1
4
1
2
(0.0018
60,000)
60,000 psi, the minimum value is
0.0014.
f
y
In SI units, the minimum percentages of reinforcing are 0.002 for grades 300 and
350 steels and 0.0018 for grade 420 steel. When
f
y
420 MPa, the minimum percent
equals The reinforcing may not be spaced farther apart than five times the
slab thickness, or 500 mm.
(0.0018
420)
.
f
y
Should structural walls or large columns provide appreciable resistance to shrinkage
and temperature movements, it may very well be necessary to increase the minimum
amounts listed.
Shrinkage and temperature steel serves as mat steel in that it is tied perpendicular to
the main flexural reinforcing and holds it firmly in place as a mat. This steel also helps to
distribute concentrated loads transversely in the slab. (In a similar manner, the AASHTO
gives minimum permissible amounts of reinforcing in slabs transverse to the main flex-
ural reinforcing for lateral distribution of wheel loads.)
Areas of steel are often determined for 1-ft widths of reinforced concrete slabs, foot-
ings, and walls. A table of “areas of bars in slabs” such as Table A.6 in the Appendix is
very useful in such cases for selecting the specific bars to be used. A brief explanation of
the preparation of this table is provided here.
For a 1-ft width of concrete, the total steel area obviously equals the total or average
number of bars in a 1-ft width times the cross-sectional area of one bar. This can be ex-
pressed as (12 in./bar spacing c to c)(area of one bar). Some examples follow, and the
values obtained can be checked in the table. Understanding these calculations enables
one to expand the table as desired.
1.
#9 bars, 6-in. o.c. Total area in 1-ft width
(
1
6
)(1.00)
2.00 in.
2
.
2.40 in.
2
.
Example 4.6 illustrates the design of a one-way slab. It will be noted that the Code
(7.7.1.c) cover requirement for reinforcement in slabs (#11 and smaller bars) is
(
1
5
)(1.00)
2.
#9 bars, 5-in. o.c. Total area in 1-ft width
3
4
in. clear
when the slab is not exposed directly to the ground or the weather.
