The Materials Used in Timber Engineering
1.3 PLYWOOD .1 General
Several countries produce plywood either from softwood or hardwood logs or a mixture of both; however, very few plywoods are considered suitable for structural use. As a general rule only plywood bonded with an exterior quality resin adhe- sive should be specified for structural use. As far as the UK is concerned, the principal structural plywoods are:
Canadian Douglas fir-faced plywood commonly referred to as Canadian Douglas fir plywood.
Canadian softwood plywood
Finnish birch plywood (birch throughout) Finnish birch-faced plywood
Finnish conifer plywood
American construction and industrial plywood Swedish softwood plywood.
Sections 1.3.2–1.3.5 give outline details of the sizes and qualities of these ply- woods and the name of the organizations from which further details can be obtained. Before specifying a particular plywood it is as well to check on its avail- ability. The strength values for various plywoods are tabulated in BS 5268-2. Those values are at a moisture content of 15%.
When designing with plywood, it is necessary to distinguish between the
‘nominal’ thickness and the actual thickness. Designs to BS 5268-2 must use the
‘minimum’ thickness which is based on the assumption that all veneers have their minus tolerance deducted. The designer should also check to see whether sanded or unsanded sheets will be supplied as this can make a difference of several percent to the thickness and strength values.
There are a number of ways of expressing plywood strength values. One is to assume that the whole cross-sectional area of the plywood has equal strength (i.e. the ‘full area’ method). Another is to assume that the veneers which run perpendicular to the direction of stress have no strength at all (i.e. the ‘parallel ply’ method). Yet another method is the ‘layered’ approach in which the per- pendicular veneers are assumed to have some strength but much less than the par- allel veneers. The designer must be careful to use the correct geometrical properties (either full area or parallel ply) with the corresponding strength values. If used correctly in calculations these three methods should give similar results. BS 5268-2 uses the full area method which is also the method used in this manual.
The strength values in the full area method differ with thickness and quality of the plywood.
If appearance is important it is prudent to check whether the description of the face veneer relates to both outer veneers or only to one. In the latter case the better
veneer is described as the ‘front’ face with the other outer veneer being described as the ‘back’ veneer.
When a plywood is described as an exterior grade of plywood, this reference is to the durability of the adhesive and not to the durability of the species used in the plywood construction.
It is common practice when giving the dimensions of a plywood sheet (length and breadth) to state firstly the dimension of the side parallel to the grain of the face veneer. Thus a sheet 2440 ¥1220 will have the face grain of the outer veneer parallel to the 2440 dimension. This is an important description to recognize as structural plywoods are stronger in the direction of the face grain.
If plywood is to be glued and used as part of an engineered component (e.g. a ply web beam or stress skin panel) the designer should satisfy himself as to the suitability of the face quality and integrity of the chosen plywood for gluing. Some plywoods are intended mainly for wall or floor sheathing rather than glued components.
1.3.2 Available sizes and quality of Canadian plywood
The size of plywood sheets usually available is 2440 ¥ 1220 mm (8 ft ¥4 ft) but metric size sheets 2400 ¥1200 mm are available. The face veneer runs parallel to the longer side (Fig. 1.5).
Two basic types of Canadian exterior grade plywood are available in the UK.
These are a plywood with one or both outer veneers of Douglas fir and inner veneers of other species which is known commercially as Douglas fir plywood, and an exterior Canadian softwood plywood which is made from much the same species as Douglas fir plywood but without Douglas fir face veneers. Appendix D of BS 5268-2 lists the species that are used.
The face quality grades available for both Douglas fir and softwood plywoods are:
• ‘Sheathing Grade’ with grade C veneers throughout, unsanded.
• ‘Select Grade’ with grade B face veneer and grade C inner and back veneers, unsanded.
• ‘Select Tight Face Grade’ with grade B ‘filled’ face veneer and grade C inner and back veneers, unsanded.
• ‘Good One Side Grade’ (G1S) with grade A face veneer and grade C inner and back veneers, sanded (strength values for this grade are not given in BS 5268-2).
The Materials Used in Timber Engineering 23
Fig. 1.5 Canadian fir-faced plywood.
• ‘Good Two Sides Grade’ (G2S) with grade A face and back veneers and grade C inner veneers, sanded (strength values for this grade are not given in BS 5268-2).
The Douglas fir grades G1S and G2S are required where a good appearance of the plywood surface is necessary but because the minimum thickness requirements for the face veneers could not be guaranteed, strength values are not given in BS 5268-2.
For further information refer to BS 5268-2.
1.3.3 Available sizes and quality of Finnish construction plywood
The most commonly available size is 1220 ¥2440 mm. The face veneer runs par- allel to the shorter side (Fig. 1.6). Other sizes which are quoted as being generally available are:
1200 ¥1200/2400/2500/3000/3600 1220 ¥1220/2440/2500/3050/3660 1250 ¥1250/2400/2500/3000/3600 1500 ¥1500/2400/2500/3000/3600 1525 ¥1525/2440/2500/3050/3660 The face veneer runs parallel to the first dimension.
Three basic types of Finnish exterior grade construction plywood are available in the UK as well as a flooring plywood. The three construction plywoods are:
• Birch plywood made entirely of birch veneers (commonly described as birch
‘thro and thro’).
• Birch-faced plywood which has the outer veneers on both faces made from birch and the inner veneers of conifer and birch. There are three variations of this type:
– Combi, having two birch veneers on each face with the inner veneers alter- nately conifer and birch;
– Combi Mirror with one birch veneer on each face and the inner veneers alternately conifer and birch;
– Twin, with one birch veneer on each face and conifer veneers internally.
• Conifer plywood with outer veneers of spruce or pine and the inner veneers from conifers.
Fig. 1.6 Finnish birch, birch-faced or conifer plywood.
The thickness of birch veneers is usually 1.4 mm and the conifer veneers range from 1.4 to 2.8 mm.
The face qualities available for birch face veneers, based on practice nearly a century old, are A (practically without defect), through B, S, BB to WG (sound knots up to 65 mm diameter, open splits and checks up to 4 mm wide, etc.). Today the grading follows ISO standards for the qualities of both birch and conifer face veneers being E (practically without defect), through I, II, III to IV in a similar manner to the older birch grading (WG equivalent to IV). The reader may well find reference to either grading system for birch face veneers in technical litera- ture and the market place. BS 5268-2 refers only to the number grading system.
It is normal practice to give the grades of the face veneer and then the back veneer, e.g. I/III. Because the veneers are thin the quality of the face veneer does not have any significant effect on the strength properties of the plywood.
For this reason Conifer plywoods I/I and IV/IV are given the same values in BS 5268-2.
For further information refer to BS 5268-2 or contact Finnish Plywood International.
1.3.4 Available sizes and quality of American construction plywood
The volume of American Construction and Industrial plywood exported to the UK has increased in recent years, mainly in the C–D grade for use as sheathing. The most commonly available sheet size is 2440 ¥1220 mm. The face veneer runs par- allel to the longer side (as shown in Fig. 1.5).
The type of American exterior grade construction plywood covered in BS 5268- 2 is a plywood with both outer veneers of Group l species (usually Douglas fir for export to the UK) and inner veneers of either Group 1 or Group 2 species. Annex D of BS 5268-2 lists the species that are used in Groups 1 and 2. The acceptable species listed in Annex D are more restrictive than those given in the American standard PS 1-95.
The grading of the face veneers is similar to the Finnish approach with the best quality A through to D. The quality of the plywood sheet is described by the grading of the two outer veneers. The following are the grades listed in BS 5268-2:
• ‘C–D grade’ unsanded, with the face veneer grade C of species Group 1, back veneer grade D of species Group 1 and the inner veneers grade D of species Groups 1 or 2.
• ‘C–C grade’ unsanded, which has veneers throughout of grade C of species Group 1.
• ‘B–C grade’ sanded, which has a face veneer of grade B and back and inner veneers of grade C, all veneers being of species Group 1.
• ‘A–C grade’ sanded, which has a face veneer of grade A and back and inner veneers of grade C, all veneers being of species Group 1.
• ‘Underlayment C–D plugged’, with the face veneer grade C or grade C plugged of species Group 1, back veneer grade D of species Group 1 and the inner veneers of grade D and species Group 1 or 2.
The Materials Used in Timber Engineering 25
• ‘Underlayment C–C plugged’, with the face veneer grade C or grade C plugged of species Group 1, back veneer grade C or C plugged of species Group 1 and the inner veneers of grade D and species Group 1 or 2.
BS 5268-2 requires that these plywoods must be marked to show compli- ance not only with the American standard but with the specific restrictions of the UK code, viz. the limitation on the timber species acceptable for Groups 1 and 2.
BS 5268 states that the C–D grade is not suitable for use either in service class 3 conditions or as gussets for trussed rafters. For all the plywood grades listed above there is a reduction in bending, tension and compression strengths for panel dimensions less than 600 mm varying linearly from zero reduction at 600 mm to 50% at 200 mm and less.
For further information refer to BS 5268-2 or contact the American Plywood Association.
1.3.5 Available sizes and quality of Swedish construction plywood
Sheet sizes of 2400 ¥1200 mm and 2440 ¥1220 mm are available. The face veneer is parallel to the longer side (Fig. 1.7). Spruce (whitewood) is mainly used in manufacture, but fir (redwood) is permitted.
There are several face qualities, the most common for structural uses being C/C.
However, the important reference for structural purposes in the UK is ‘P30 grade’.
The figure 30 is the bending strength in N/mm2at the lower 5% exclusion limit.
P30 can be obtained either unsanded or sanded but only unsanded values are given in BS 5268-2.
For further information refer to BS 5268-2 or the Swedish Forest Products Laboratory.
1.4 PARTICLEBOARD, ORIENTED STRAND BOARD, CEMENT-BONDED