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

Downloaded From : www.EasyEngineering.net
Properties of Form Material 55
Plywood is graded by the veneers of the plies. Veneer Grade N or
A is the highest grade level. Grade N is intended for natural finish,
whereas Grade A is intended for a plane-table surface. Plywood
graded as N or A has no knots or restricted patches. Veneer Grade B
has a solid surface but may have small round knots, patches, and
round plugs. Grade B is commonly used for formwork. The lowest
grade of exterior-glue plywood is a veneer Grade C, which has small
knots, knotholes, and patches.
Table 4-9 gives the section properties of plywood per foot of width.
The effective section properties are computed taking into account the
species of wood used for the inner and outer plies and the variables
involved for each grade. Because this table represents a wide variety
of grades, the section properties presented are generally the minima
that can be expected.
ww.EasyEngineering.n
Allowable Stresses for Plywood
The allowable stresses for plywood are shown in Table 4-10. When the
allowable stresses are used from this table, the plywood must be manufactured
in accordance with Voluntary Product Standard PS 1-95 and
must be identified by the APA—The Engineered Wood Association.
The allowable stresses are divided into three levels, which are related
to grade. Plywood with exterior glue, and with face and back plies containing
only N, A, or C veneers, must use level 1 (S-1) stresses. Plywood
with exterior glue, and with B, C plugged, or D veneers in either face or
back, must use level 2 (S-2) stresses. All grades with interior or intermediate
glue (IMG or exposure 2) shall use level 3 (S-3) stresses.
The woods that may be used to manufacture plywood under Voluntary
Product Standard PS 1-95 are classified into five groups based
on the elastic modulus in bending and important strength properties.
Group 1 is the highest grade, and Group 5 is the lowest grade. Groups
1 and 2 are the common classifications used for formwork. A list of the
species that are in each group classification is provided in ref. [2].
Plyform
The plywood industry produces a special product, designated as Plyform,
for use in forming concrete. Plyform is exterior-type plywood
limited to certain species of wood and grades of veneer to ensure
high performance as a form material. The term is proprietary and may
be applied only to specific products that bear the trademark of APA—
The Engineered Wood Association. Products bearing this identification
are available in two classes: Plyform Class I and Plyform Class II.
Class I is stronger and stiffer than Class II. For special applications, an
APA Structural I Plyform is available that is stronger and stiffer than
Plyform Classes I and II. It is recommended for high pressures where
the face grain is parallel to supports.
Downloaded From : www.EasyEngineering.net