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EASy Reference Guide - S38SX
Service Bulletin
How to calculate cribbing needs
Cribbing requirements for setting up on Asphalt with a
43,800 pound outrigger load.
We have already calculated the pressure being exerted
by the dunnage pad alone is 76 PSI (dividing 43,800 by
576 the size of the dunnage pad in square inches.)
76 PSI exceeds the capacity of the asphalt, which is 29
PSI, the outrigger would sink.
Cribbing must be used to increase the surface area
over which the force is applied. To determine the di-
mensions “A” of the cribbing required, divide the Total
Force (43,800) by the soil capacity (29).
43,800 (lbs)
=
1510 (sq. in.)
29 (PSI)
The result (1510) is the surface area, expressed in
square inches, required to support the outrigger. Con-
sult the table below and select a cribbing size slightly
larger than required. In this example, we recommend a
Cribbing size of 40” X 40” (1600 sq. in.).
Square inch surface required
Dimension “A”
400
20 x 20
625
25 x 25
900
30 x 30
1225
35 x 35
1600
40 x 40
2025
45 x 45
2500
50 x 50
You can also take the square root of 1510, which is
38.8. Rounding up to 40, we would need 40” x 40” of
cribbing to support the weight of the outrigger. If we use
10 pieces of 4 x 4 timbers (actual width of 3.5” square)
measuring 40” long we get the following:
4” square timber X 10 pieces = 40 sq. in.
40 sq. in. X 40” long timbers = 1600 sq. in.
1600 sq. in. would be just enough cribbing to support
the outrigger on asphalt.
The same procedure can be used for any soil surface,
just be sure to use the correct soil capacity pressure
capacity.
A
A
Figure 17
Cribbing under the Outrigger Pad
