A2.2—WRI thickness selection for single-axle
wheel load
This procedure selects the concrete slab thickness for a
single axle with wheels at each end of the axle, using
Fig. A2.1
,
A2.2
, and
A2.3
. The procedure starts with
Fig. A2.1
, where
a concrete modulus of elasticity E, slab thickness H, and
modulus of subgrade reaction k are assumed or known. For
example, taking
E = 3000 ksi
Thickness = 8 in. (trial value)
Subgrade modulus k = 400 lb/in.
3
Figure A2.1
gives the relative stiffness parameter D/k =
3.4 × 10
5
in.
4
;
the procedure then uses
Fig. A2.2
.
Wheel contact area = 28 in.
2
Diameter of equivalent circle = = 6 in.
Wheel spacing = 45 in.
This gives the basic bending moment of 265 in.-lb/in. of
width/kip of wheel load for the wheel load using the larger
design chart in
Fig. A2.2
. The smaller chart in the figure
gives the additional moment due to the other wheel as 16 in.-
lb/in. of width kip of wheel load.
Moment = 265 + 16 = 281 in.-lb/in./kip
28 4
×
(
) π
⁄
Fig. A1.6—PCA design chart for post loads where subgrade
modulus is 50 pci.
Fig. A1.7—PCA design chart for post loads where subgrade
modulus is 200 pci.
Table A1.1—Allowable distributed loads for
unjointed aisle with nonuniform loading and
variable layout (Packard 1976)
Slab
thickness,
in.
Subgrade
k,
*
lb/in.
3
Allowable load, lb/ft
2†
Concrete flexural strength, psi
550
600
650
700
5
50
535
585
635
685
100
760
830
900
965
200
1075
1175
1270
1370
6
50
585
640
695
750
100
830
905
980
1055
200
1175
1280
1390
1495
8
50
680
740
800
865
100
960
1045
1135
1220
200
1355
1480
1603
1725
10
50
760
830
895
965
100
1070
1170
1265
1365
200
1515
1655
1790
1930
12
50
830
905
980
1055
100
1175
1280
1390
1495
200
1660
1810
1965
2115
14
50
895
980
1060
1140
100
1270
1385
1500
1615
200
1795
1960
2120
2285
*
k of subgrade; disregard increase in k due to subbase.
†
For allowable stress equal to 1/2 flexural strength.
Note: Based on aisle and load widths giving maximum stress.
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