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

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Design of Wood Members for Formwork 127
From Eq. (5-58) the shear deflection can be calculated as:
∆ s
= w s
Ct 2 l s2
/1,270E e
I
= 250(120)(3/4) 2 (16.5) 2 /1,270(1,500,000)(0.199)
= 0.012 in.
The calculated shear deflection of 0.012 in. is small compared to the
bending deflection of 0.035 in. that was calculated in Example 5-14. As
previously discussed, shear deflection often accounts to a small percentage
of the total deflection. For this Plyform panel, the total deflection is:
Total deflection = bending deflection + shear deflection
= 0.035 in. + 0.012 in.
= 0.047 in.
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Assume the maximum permissible deflection is l/360 = 0.05. The
calculated deflection is 0.047, which less than 0.05; therefore, the ¾ in.
Plyform is satisfactory for bending and shear deflection.
Tables of Equations for Calculating Allowable Span
Lengths for Wood Beams and Plywood Sheathing
The previous sections have provided numerous equations and examples
of calculating bending stress, shear stress, and deflection of wood
beams and plywood sheathing. As noted previously, for form design
it is often desirable to calculate the allowable span length of a member
based on bending, shear, and deflection.
Table 5-3 summarizes the equations that are most often used for
calculating the allowable span lengths for wood beams with uniformly
distributed loads. Table 5-4 summarizes the equations for calculating
the allowable span lengths of plywood sheathing for uniformly distributed
concrete pressure. The symbols and units in the equations
have been presented in preceding sections of this chapter.
Compression Stresses and Loads on Vertical Shores
Shores must safely support all dead and live loads from the formwork
system above the shores. Special care must be taken in calculating
stresses and allowable loads on columns, because many failures in
formwork have been due to inadequate shoring and bracing of forms.
The maximum load that a vertical shore can safely support varies
with the following factors:
1. Allowable unit stress in compression parallel to grain
2. Net area of shore cross section
3. Slenderness ratio of shore
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