Regardless of the
intended purpose of the reinforcement, it must be structurally stiff (strong enough to
support working loads) and/or widely spaced (individual elements spaced wide enough so
that workers can walk between them). Further, it must be supported at the proper position,
so as to provide its intended advantages. All requirements for a quality slab-on-grade
should be detailed and clearly shown on the design drawings.
DESIGN EXAMPLES
These design examples illustrate the selection of
reinforcing steel for the purposes of controlling cracks caused by shrinkage. The subgrade
drag equation is used for calculation of the steel areas, which are determined here for
both a common joint spacing as well as for a wide joint spacing. The reinforcing steel is
not to be continuous through any of the contraction or construction joints. This holds for
both wires and bars.
For these design
examples, assume that an 8-in. slab is required. Column spacings are 48 ft
center-to-center and construction joints will have this spacing for strip placement of the
concrete.
1. 24-ft joint spacings:
This joint spacing, commonly encountered in the field, of 24 ft, using
a saw-cut contraction joint, may be reinforced with bars (ASTM A 615, A 616, A 617, or A
706) or welded wire fabric (ASTM A 185 or A 497). The appropriate areas are selected using
the subgrade drag equation as follows:
• For Grade
60 reinforcing bar:
As = F L w / 2 fs where F (friction factor) = 1.5 (commonly used value), L = 24 ft,
w=100 psf and fs = 2/3 fy where fy is 60,000 psi.
As = 0.045 sq.in./ft of slab width,
required each way.
Use #3 bars at 29 in. c.-c. in each
direction, As = 0.046 sq in./ft. (If the designer follows ACI's structural slab
limitation of 5h or 18 in. (whichever is less), then #3 bars at 18 in. are selected, As = 0.073 sq.in./ft).
• For ASTM A 185 plain
welded wire fabric:
The subgrade drag equation is the same except for the use
of fy which is 65,000 psi.
As = 0.042 sq.in./ft of slab width, required each way.
Use W4.5 wire at 12-in. spacings in each direction,
designated as: 12 x 12 - W4.5 x W4.5.
2. 48-ft joint spacings:
This
is a wide joint spacing and could be considered as a "no joint" slab design. No
saw-cut contraction joints are used longitudinally; however, if strip placement of the
slab is used, then a contraction joint would be cut at 48 ft spacings transversely along
the pour strip. In the subgrade drag equation, the length L is now 48 ft and the yield
strength, fy, is either 60,000 psi (bars) or 70,000 psi (deformed wire), as follows:
• For Grade 60 reinforcing
bars:
As = F L w / 2 fs where F = 1.5, L = 48 ft, w = 100 psf, and fs is 2/3 fy = 2/3 x 60,000 psi =
40,000 psi.
As= 0.090 sq.in./ft of slab width each way.
Use #4 bars at 25 in. spacings c.-c., each
way (or #4 bars at 18 in. c.-c.)
• For ASTM A 497 deformed
welded wire fabric:
The subgrade drag equation is the same except for the use
of fy which is 70,000 psi.
As = 0.077 sq.in./ft of slab width,
required each way.
Use D8 wire at 12-in. spacings in each
direction, designated as: 12 X 12 - D8 x D8
The steel areas selected
using the subgrade drag equation are for shrinkage (and temperature) effects. If the
reinforcement is intended to be structurally active and to resist bending stresses
produced by loading to the slab, then the subgrade drag equation is not appropriate.
(Refer to CRSI's EDR No. 33).
