Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

120 Chapter Five
Example 5-12
Class I Plyform with a thickness of 1 in. is proposed as sheathing for
a concrete wall form. The lateral pressure of the freshly placed concrete
against the Plyform is 900 lb per sq ft. The Plyform will be placed
across grain, so the bending stress is applied parallel to face grain.
Determine the maximum spacing of the studs based on bending.
From Table 4-11, the value of the effective section modulus for
stress applied parallel to face grain is S e
= 0.737 in. 3 /ft, and from
Table 4-12 the indicated allowable bending stress for Class I Plyform
is 1,930 lb per sq in. Substituting these values into Eq. (5-41), the
permissible spacing based on bending can be calculated as follows:
l b
= [120F b
S e
/w b
] 1/2
= [120(1,930 lb per sq in.)(0.737 in. 3 /ft)/(900 lb per sq ft)] 1/2
= 13.8 in.
For constructability, a 12-in. stud spacing may be selected that
provides easier fabrication of the forms to match the 8 ft lengths of
the Plyform. The shear stress and the deflection due to bending and
shear should also be checked for the 1-in.-thick Plyform to ensure
adequate shear strength and rigidity for the deflection criteria. Shear
and deflection of plywood are presented in the following sections.
Example 5-13
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Standard ¾-in.-thick plywood panels of the type shown in Table 4-9
are to be used as sheathing for a concrete form. The plywood will be
installed across 2-in. nominal-thickness joists, spaced at 16 in. on centers.
The plywood will be installed across grain, so the bending stress
will be applied parallel to face grain. Assume a Group II species with
an S-1 stress level rating in dry conditions and determine the permissible
pressure, based on bending, that can be applied to the plywood
panels.
From Table 4-9, the effective section modulus S e
= 0.412 in. 3 /ft
and from Table 4-10 the indicated allowable bending stress F b
= 1,400
lb per sq in. The allowable pressure can be calculated from Eq. (5-39)
as follows:
w b
= 120F b
S e
/(l b
) 2
= 120(1,400 lb per sq in.)(0.412 in. 3 /ft)/(16.0 in.) 2
= 270 lb per sq ft
The allowable uniformly distributed load of 270 lb per sq ft is
based on bending stress. The plywood should also be checked for
shear and deflection to ensure that it has adequate strength and rigidity
to sustain this pressure.
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