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

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Design of Wood Members for Formwork 125
Two spans,
For ∆ d
= l/360, l d
= [2,220EI/360w d
] 1/3
For ∆ d
= ¹⁄16 in., l d
= [2,220EI/16w d
] 1/4
(5-56a)
(5-56b)
Three or more spans,
For ∆ d
= l/360, l d
= [1,743EI/360w d
] 1/3
For ∆ d
= ¹⁄16 in., l d
= [1,743EI/16w d
] 1/4
(5-57a)
(5-57b)
Example 5-16
Compute the bending deflection of a ¾-in.-thick Plyform Class I panel
that must resist a concrete pressure of 250 lb per sq ft. The plywood
ww.EasyEngineering.n
will be installed across multiple 2-in.-thick joists, spaced at 18 in. on
centers. The physical properties are shown in Table 4-11, and the
allowable stresses are shown in Table 4-12. From Eq. (5-51), calculate
the bending deflection.
Reference Figure 5-10, effective span length is clear span + ¼ in.:
l s
= 18.0 in. – 1.5 in. + ¼ in. = 16.75 in.
∆ d
= w d
l d4
/1,743EI
= (250 lb per sq ft)(16.75 in.) 4 /(1,743)(1,650,000 lb per sq in.)
(0.199 in. 4 )
= 0.035 in.
Allowable Pressure on Plywood Based on Shear Deflection
The shear deflection may be closely approximated for all span conditions
by the following equation. This equation applies to all spans—
single, double, and multiple spans:
∆ s
= w s
Ct 2 l s2
/1,270E e
I (5-58)
where ∆ s
= shear deflection of plywood due to shear, in.
w s
= uniform pressure on plywood, lb per sq ft
C = constant = 120 for face grain of plywood perpendicular to
supports;
= 60 for face grain of the plywood parallel to the
supports
t = effective thickness of plywood for shear, in.
l s
= clear distance between supports, in.
E e
= modulus of elasticity of plywood, unadjusted for shear
deflection, lb per sq in.
I = effective moment of inertia of a 1-ft-wide strip of plywood,
in. 4
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