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

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Design of Wood Members for Formwork 121
Example 5-14
Class I Plyform sheathing, ¾ in. thick, is to be used for forming a concrete
floor. The combined total dead and live load pressure on the
form is 175 lb per sq ft. The Plyform will be placed with the face grain
across multiple supporting joists, three or more spans. The spacing of
joists will be 24 in. center to center. Is this ¾-in.-thick Plyform adequate
to sustain the 175 lb per sq ft uniformly distributed pressure?
From Table 4-11, the value of the effective section modulus for
stress applied parallel to face grain is S e
= 0.455 in. 3 /ft, and from Table
4-12 the allowable bending stress for Class I Plyform is 1,930 lb per sq
in. The allowable uniformly distributed pressure can be calculated
from Eq. (5-39) as follows:
ww.EasyEngineering.n
w b
= 120F b
S e
/(l b
) 2
= 120(1,930 lb per sq in.)(0.455 in. 3 /ft)/(24 in.) 2
= 183 lb per sq ft
Because the allowable pressure of 183 lb per sq ft is greater than
the applied 175 lb per sq ft pressure, the ¾-in.-thick Class I Plyform is
satisfactory for bending stress. The margin of safety for bending can
be calculated as follows:
Margin of safety = 183/175
= 1.05, or 5%
Although the ¾-in. Class I Plyform is adequate for bending, it
must also be checked for shear and deflection.
Allowable Pressure on Plywood Based on
Rolling Shear Stress
The general engineering equation for calculating the shear stress in a
member due to flexure is
f v
= VQ/Ib
For plywood the equation for rolling shear resistance is written in
the form:
F s
= V/[Ib/Q] (5-42)
When plywood is subjected to flexure, shear stresses are induced
in the plies and layers of glue of the plywood. Because some of the
plies in plywood are at right angles to others, certain types of load
subject them to stresses that tend to make them roll, which is defined
as rolling shear. The term “Ib/Q (in. 2 /ft)” is called the rolling shear
constant. It can be obtained from tables of the physical properties (see
Table 4-9 for plywood and Table 4-11 for Plyform). The value of F s
is
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