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

118 Chapter Five
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based on uniform loads in pounds per square foot (lb per sq ft),
whereas in previous sections the uniform loads were considered as
pounds per linear foot (lb per lin ft).
The Plywood Design Specification [4] recommends three basic
span conditions for computing the uniform load capacity of plywood
panels. The spans may be single-span, two-span, or three-span.
When plywood is placed with the face grain across (perpendicular
to) the supports, the three-span condition is used for support spacings
up to and including 32 in. The two-span condition is used when the
support spacing is greater than 32 in. In designing or checking the
strength of plywood, it should be noted that when the face grain of
plywood is placed across (perpendicular to) supports, the physical
properties under the heading “stress applied parallel to face grain”
should be used (see Table 4-9 for plywood and Table 4-11 for Plyform).
When plywood face grain is placed parallel to the supports, the
three-span condition is assumed for support spacings up to and
including 16 in. The two-span condition is assumed for face grain
parallel to supports when the support spacing is 20 or 24 in. The single-span
condition is used for spans greater than 24 in. It should be
noted that when the face grain of plywood is placed parallel to the
supports, the physical properties under the heading “stress applied
perpendicular to face grain” should be used (see Table 4-10 for plywood
and Table 4-12 for Plyform).
In designing plywood, it is common to assume a 1-ft-wide strip
that spans between the supporting joists of plywood sheathing for
floors or between the studs that support plywood sheathing for wall
forms. The physical properties for plywood are given for 1 ft widths
of plywood.
APA—The Engineered Wood Association recommends using the
center-to-center span length for calculating the bending stress. For
calculating the rolling shear stress and shear deflection, the clear distance
between supports should be used. For bending deflection, the
recommendations are to use the clear span plus ¼ in. for 2 in. nominal
framing, and the clear span plus 5 ⁄8 in. for 4 in. framing. Figure 5-11
shows these various span lengths.
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Allowable Pressure on Plywood Based on Bending Stress
The allowable pressure of plywood based on bending can be calculated
using the conventional engineering equations presented in previous
sections of this book. However, because plywood is fabricated by alternate
layers of wood that are glued together, the previously presented
stress equations must be used with the physical properties of plywood.
For example, in calculating the section modulus for bending, an effective
section modulus S e
is used. Values for the effective section modulus
and other physical properties of plywood per foot of width can be
found in Table 4-9 for plywood and in Table 4-11 for Plyform.
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