EWPAA Structural Plywood and LVL Design Manual - Engineered ...

4 Plywood & LVL Physical And Mechanical Properties4.1 IntroductionStructural plywood and structural LVL are composed of individual timber veneers which can be selected,positioned and orientated to optimise the finished product properties for the intended end application.Structural LVL is typically manufactured with all veneer grain directions parallel with the member length.This maximises strength and stiffness in the spanned direction.Structural plywood, being a panel product, is manufactured with veneer grain orientation alternating in thepanel length and width directions to give engineered strength and stiffness properties in both paneldirections. Veneers can be selected and orientated to either maximize strength and stiffness in one paneldirection or alternatively provide more equal properties in both directions.4.2 Cross-LaminationThe alternating change in grain direction of theveneers in plywood is referred to as crosslamination,and in addition to enhanced strengthand stiffness properties, a number of other usefulcharacteristics are imparted, as discussed below.Where required, these characteristics can also beincorporated into LVL, by the inclusion of crosslaminatedveneers in the LVL member.Resistance to SplittingCross-lamination of the veneers means there is nonatural cleavage plane and therefore plywood willnot readily split either lengthwise or crosswise.This allows plywood to be nailed at closerspacings and with reduced distances to the paneledges, than could be achieved with sawn timberand some other engineered wood based panelproducts.Cross-lamination in plywood as a result of alternating theveneer grain direction of adjacent veneersImpact Resistance and Resistance to PuncturePlywood performs well under heavy concentrated loads and impact loads as the cross-laminations inplywood distribute the stresses over a wide area of the panel. This can be important in many structuralapplications including structural flooring in commercial or industrial situations, wall claddings, materialshandling applications and barriers against airborne missiles in cyclones.Panel Shear StrengthThe cross-lamination of veneers in plywood results in high shear strength within the plane of the panel.This is one of the characteristics that results in plywoods superior performance in a number of criticalstructural applications including plywood webs in beams, plywood gussets in portal frames and as abracing material.4.3 Dimensional Stability under Changes in Moisture ContentPlywood's cross-laminated construction improves its dimensional stability in the plane of the panel incomparison to solid wood. Solid wood undergoes little expansion or contraction along the wood grainunder moisture content changes, however, across the grain, it may undergo considerable movementdue to changes in moisture content. In plywood, the veneer movement due to moisture changes isrestricted across the grain relative to that along the grain due to the cross-laminations. As a result,structural plywood has superior dimensional stability to other timber and wood based panels. TABLE 4.1 details14