Structural plywood and structural LVL Bending Strength and Bending Stiffness for Loading Normal to the Face

When loaded normal to the face of the plywood sheet, parallel ply theory assumes veneers with

face grain direction parallel to the span are the sole contributors to bending strength and the major

contributors to bending stiffness. Veneers with grain direction perpendicular to the span direction

contribute nothing to bending strength and only 3% to bending stiffness. The outermost veneers

furtherest from the panel neutral axis and orientated in the span direction carry the maximum tension and

compression flexure forces and are the major contributors to the second moment of area (I) and section

modulus (Z) and therefore bending capacity.

In typical applications where the plywood is loaded normal to the face, such as flooring, bending stiffness will

often be the governing criteria that determines the plywood specification. When setting deflection limits for

applications in which clearance limits are critical, allowance should be made for the modulus of elasticity

given in AS1720.1-1997 (and reprinted in TABLE 5.1), being an average modulus of elasticity. However,

it should also be noted that the process control applied to EWPAA/JAS-ANZ branded products minimises

the variability of the E value from the published average value.

For evaluation of bending strength TABLE 5.4 provides comparative bending strength (f’b.Z) values for a

range of standard plywood constructions and stress grades.

For evaluation of bending stiffness TABLE 5.5 gives comparative values of (EI) for structural plywood

loaded normal to the face, for a range of stress grades and standard plywood constructions. The table

provides indicative stiffness values for both plywood supported with face grain orientated parallel to the

span and for plywood supported with face grain orientated perpendicular to span.

Shear Strength (interlamina shear) for Loading Normal to the Face

The interlamina shear strength of structural plywood loaded normal to the panel face is calculated based on

a shear area of:

As

where: b

= load distribution width (refer

= 2/3 bt (derived from the basic beam shear equation)

TABLE 5.2);

and: t

= full thickness of the plywood sheet.

For applications where high concentrated loads are present, the plywood capacity for punching or local

shear may also need to be checked. The relevant shear area is then:

As

= perimeter of loaded area x full thickness of the panel

It should be noted that the shear capacity of structural plywood loaded normal to the face is governed by

the “rolling” shear tendency of the plywood cross-bands. Rolling shear is a term used to describe shearing

forces which tend to roll the wood fibres across the grain. The reduced shear capacity of plywood loaded

normal to the face, due to rolling shear, is accounted for in AS1720.1-1997, by the use of an assembly

factor g19 in the calculation of both interlamina and punching shear capacity

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