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

122 Chapter Five
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the allowable rolling shear stress that can be obtained from Table 4-10
for plywood and from Table 4-12 for Plyform.
The value of shear V depends on the loads that are applied to the
member and the number of supports for the member. The load on
plywood is generally a uniformly distributed load. The spans may be
single or multiple. As presented earlier, the traditional engineering
equation for calculating the shear in a single-span beam with a uniformly
distributed load is as follows:
For a single span, V = W/2
= wl/24 lb
ww.EasyEngineering.n
When plywood is used as sheathing, the Plywood Design Specification
recommends the following equations for calculating the
allowable uniformly distributed pressure:
For a single span, w s
= 24F s
(Ib/Q)/l s
(5-43)
For two spans, w s
= 19.2F s
(Ib/Q)/l s
(5-44)
For three or more spans, w s
= 20F s
(Ib/Q)/l s
(5-45)
where w s
= allowable pressure of concrete on plywood, lb per sq ft
F s
= allowable stress in rolling shear of plywood, lb per sq in.
Ib/Q = rolling shear constant of plywood, in. 2 per ft of width
l s
= clear distance between supports, in.
Equations (5-43) through (5-45) can be rewritten to calculate the
allowable span length in terms of the uniformly distributed load, the
allowable shear stress, and the rolling shear constant of plywood.
Rearranging terms in the equations, the allowable span length due to
rolling shear stress for plywood can be calculated as follows:
For a single span, l s
= 24F s
(Ib/Q)/w s
(5-46)
For two spans, l s
= 19.2F s
(Ib/Q)/w s
(5-47)
For three or more spans, l s
= 20F s
(Ib/Q)/w s
(5-48)
Example 5-15
Consider the ¾-in.-thick plywood panel in Example 5-13 that is to be
supported by 2-in. joists, spaced at 16 in. on centers. Calculate the
allowable pressure based on rolling shear stress.
For rolling resistance shear, the clear span is calculated as the net
distance between supports, reference Figure 5-11. Thus, for 2-in.
nominal joists spaced at 16 in. on centers, the clear span will be:
l s
= 16.0 in. – 1.5 in.
= 14.5 in.
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