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the growing importance of the material. nish Birch (Finply) and British made
Definition
Structural plywood is a sheet form of timber produced from an odd num- ber of veneer layers glued together with the grain direction of alternate layers at right angles. Timber being anisotropic, has a high tensile strength along the grain but negligible strength across the grain (analogous to reinfor- ced concrete with all reinforcing rods parallel). Plywood is formed by the rotary cutting of logs to produce veneers which are then glued together.
The alternate layers, being glued at right angles, provide adequate cross grain strength (analogous to distribu- tion steel in reinforced concrete slabs) and eliminates the splitting tendency inherent in natural timber.
The moisture movement of timber of predominantly across the grain, hence the alternate crossed layers of veneer restrain movement with change of moisture content and provide di- mensional stability. Design data and properties for the three example speci- fications of structural plywood -Canadian Fir (PMBC Exterior), Fin-
(ABPVM specification BP 101/65)- are incorporated in British Standard CP112/1967; structural use of timber. Advantages and limitations
Plywood possesses the inherent ad- vantages of wood as a structural mater- ial-availability, low cost, high streng- th, is easily cut and fastened with simple and cheap tools, durable for external exposure on site, freedom from corrosion, lightness, thermal qualities and aesthetic appeal.
It has the additional advantages of being a material in sheet form with a greater resistance to splitting, uniform- ity of strength and dimensional stabili- ty than the constituent timber. Nail holding and impact resistance are also improved when compared with natural wood.
Plywood has consequently been widely accepted as a lightweight structural material available in a large panel size, convenient to handle and easy to work.
From the purely engineering aspect the most significant feature is the high strength/weight ratio of structural
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52ft span portal frames for school assembly hall in Leicestershire, UK. Plywood was found to be the ideal material for forming the internal skin of this industrial wind tunnel, con- structed at the Central Electricity Gen- erating Board's Marchwood Engineer- ing Laboratories, where it was vital that the design dimensions were rigidly adhered to and that no buckling, roughness or discontinuities marred the interior surfaces.
and 4. 44ft folded plates at Knares- borough Grammar School assembly hall, Architects: Frank Chippendale and Associates, UK.
Panel arches of 14ft span for a super- market in Richmond, British Columbia, fabricated and erected in a week.
plywood which exceeds that of struc- tural steel; and the greater thickness of the equivalent weight of plywood sheet gives structural properties which greatly increase the strength, rigidity and buckling resistance.
Whenever lightness or handling convenience is a significant factor in addition to structural strength, ply- wood is worthy of consideration, hence its use in concrete for work, floor and roof sheating and prefabri- cated components for roofs, walls and floors.
Like the natural timber from which it is composed, plywood is combu- stible but has built-in fire resistance. The phenolic resin bonding of struc- tural plywood is not affected, so im- mediate failure of a plywood com- ponent in fire will not take place. However, for reasons of economy, plywood components are made up of relatively thin sections and where ade- quate fire resistance is required to meet certain bye-laws additional fire resistant cladding to the exposed plywood faces may be necessary.
Untreated plywood falls within Class three of the 'spread of flame' test
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Far East BUILDER, November 1969
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