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

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Forms for Walls 235
Allowable Pressure Based on Shear Stress
For pressure controlled by shear stress, use Eqs. (9-5) and (9-6),
w s
= 19.2F s
(Ib/Q)/l s
for two spans (9-5)
w s
= 20F s
(Ib/Q)/l s
for three spans (9-6)
where w s
= uniform load on the plywood, lb per sq ft
F s
= allowable rolling shear stress, lb per sq in.
Ib/Q = rolling shear constant, in. 2 per ft of width
l s
= clear span between supports, in.
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Allowable Pressure Based on Shear Deflection
For pressure controlled by shear deflection, use Eq. (9-7),
∆ s
= w s
Ct 2 (l s
) 2 /1,270E e
I (9-7)
where ∆ s
= permissible deflection of the plywood, in.
w s
= uniform load, lb per sq ft
C = a constant, equal to 120 for the face grain of plywood
perpendicular to the supports and equal to 60 for the face
grain of the plywood parallel to the supports
t = thickness of plywood, in.
E e
= modulus of elasticity of the plywood, unadjusted for
shear deflection, lb per sq in.
I = moment of inertia of a plywood strip 12 in. wide, in. 4 /ft
Maximum Spans for Lumber Framing Used to
Support Plywood
Tables 9-11 and 9-12 give the maximum spans for lumber framing
members, such as studs and joists that are used to support plywood
subjected to pressure from concrete. The spans listed in Table 9-11 are
based on using grade No. 2 Douglas Fir or grade No. 2 Southern Pine.
The spans listed in Table 9-12 are based on using grade No. 2 Hem-Fir.
The allowable stresses are based on a load-duration less than 7 days
and moisture content less than 19%. The deflections are limited to
l/360 with maxima not to exceed 1/4 in.
Although Table 9-11 and 9-12 are for single members; these tables
can be adapted for use with multiple member, such as double wales.
For example, suppose the total load on double 2 × 4 wales is 1,200 lb
per lin ft. Because the wales are doubled, each 2 × 4 wale carries
1,200/2 = 600 lb per lin ft. If the wales are No. 2 Southern Pine over
more than four supports, Table 9-11 shows a 32-in. span for 600 lb per
lin ft.
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