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

60 Chapter Four
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The panels are sanded on both sides and oiled at the mill unless
otherwise specified. The face oiling reduces the penetration of moisture
and also acts as a release agent when the forms are removed from
the concrete. Although the edges are sealed at the mill, any exposed
edges resulting from fabrication at a job should be sealed with appropriate
oil before the plywood is used for formwork.
The section properties of Plyform Classes I and II, and Structural
I Plyform are given in Table 4-11. The allowable stresses are shown
Table 4-12.
High-Density Overlaid Plyform
Both classes of Plyform are available with a surface treatment of hard,
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smooth, semiopaque, thermosetting resin-impregnated materials that
form a durable bond with the plywood. The plywood industry uses
the name HDO (High-Density Overlay) to describe the overlaid surface.
Overlaid Plyform panels have a tolerance of plus or minus ¹⁄32 in.
for all thicknesses through ¹³⁄16 in. Thicker panels have a tolerance of
5% over or under the specified thickness. The treated surface is highly
resistant to abrasion, which has permitted in excess of 100 uses under
favorable conditions and with good care. The abrasion-resistant surface
does not require oiling. However, it may be desirable to lightly
wipe the surface of the panels with oil or other release agents before
each pour to ensure easy stripping of the panels.
Equations for Determining the Allowable
Pressure on Plyform
Equations (4-1) through (4-7) can be used to calculate the maximum
pressures of concrete on Plyform used for decking or sheathing.
When computing the allowable pressure of concrete on Plyform, use
the center-to-center distances between supports to determine the
pressure based on the allowable bending stress in the fibers.
When computing the allowable pressure of concrete on Plyform
as limited by the permissible deflection of the plywood, use the clear
span between the supports plus ¼ in. for supports whose nominal
thicknesses are 2 in. and the clear span between the supports plus 5 ⁄8 in.
for the supports whose nominal thicknesses are 4 in.
When computing the allowable pressure of concrete on Plyform
as limited by the allowable unit shearing stress and shearing deflection
of the plywood, use the clear span between the supports.
The recommended concrete pressures are influenced by the number
of continuous spans. For face grain across supports, assume three
continuous spans up to a 32-in. support spacing and two spans for
greater spacing. For face grain parallel to supports, assume three
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