The Materials Used in Timber Engineering
1.4 PARTICLEBOARD, ORIENTED STRAND BOARD, CEMENT-BONDED PARTICLEBOARD AND WOOD FIBREBOARDS
• ‘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
historical comparison between, say, the new materials and those that were described some years previously.
These European standards give little if any advice on the use or application of the new materials. As a consequence, the user friendly advice given previously in BS 5669 ‘Particleboards’ and BS 1142 ‘Specification for fibre building boards’ has been redrafted and published in BS 7916 ‘Code of practice for the selection and application of particleboard, oriented strand board (OSB), cement-bonded parti- cleboard and wood fibreboards for specific purposes’. Annex A of BS 7916 also gives the ‘nearest fit’ between the former BS board designations and the European standards designations. Further information on the effect of these changes is given in Wood-Based Panels – Transition from British Standards to European Standards published by the Wood Panels Industry Federation.
1.4.2 Environmental conditions
The environmental conditions in which the various types and grades of boards can be used are similar to solid timber but with a different terminology, i.e.
timber service class 1 is equivalent to dry conditions service class 2 is equivalent to humid conditions
service class 3 is equivalent to exterior conditions (Note that none of the products covered in section 1.4 except cement- bonded particleboard meet the requirements for this condition.)
There are additionally ratings for biological hazard (risk of decay), viz.
class 1 undercover, fully protected and not exposed to wetting
class 2 undercover, fully protected where high environmental humidity can lead to occasional but not persistent wetting
class 3 not covered and not in contact with the ground; either continuously exposed to the weather or protected but subject to frequent wetting class 4 in contact with the ground or fresh water and exposed to permanent
wetting.
Formaldehyde-based bonding agents are frequently employed in the manufac- ture of these products. There are stringent rules in the European standards limit- ing the rate and quantity of formaldehyde emission from the products.
1.4.3 Structural usage
BS 5268-2: 1996 refers to one particleboard grade, C5, and one tempered hard- board grade, TE. The C5 grade particleboard has never been produced in com- mercial quantities and the hardboard found only limited use as wall sheathing and the webs of I beams.
Eurocode 5 admits certain particleboards, oriented strand boards, high-density medium fibreboards, tempered hardboards and medium-density fibreboards.
The Materials Used in Timber Engineering 27
Reliable strength data values are not currently available for many of these prod- ucts and likewise the present information on creep movements has to be treated with caution.
As the use of many of these products in the UK is for floor or roof decking and wall sheathing it is possible to use them quite satisfactorily with limited design data based on experience in use and proof by testing. For example, for domestic flooring a P5 or P7 grade particleboard should be 18/19 mm thickness where the supporting joists are spaced at 450 mm centres.
1.4.4 Particleboard
Particleboard is defined as a panel material manufactured under pressure and heat from particles of wood (wood flakes, chips, shavings, saw dust, wafers, strands and similar) and/or other lignocellulosic material in particle form (flax shives, hemp shives, bagasse fragments and similar) with the addition of an adhesive. Wood chipboard is a particular derivative of this family, being made from small particles and a binder.
The history of the use of particleboards in the UK for structural purposes and floor/roof decking in particular has not been good. Unless a degree of inherent moisture resistance is provided by the board there are potential problems in use from water spillage or condensation. Boards without moisture resistance proper- ties will suffer irrecoverable swelling and loss of strength when wetted – a situa- tion that may be described as ‘the soggy Weetabix syndrome’. This condition is unknown in Scandinavia where fully moisture-resistant grades are available capable of total immersion for months without significant change of dimension or strength. This problem was overcome to some extent with the introduction in BS 5669 of moisture-resistant boards such as C3(M) and C4(M). The C5 grade board was intended to have moisture-resistant characteristics but in commercial terms the closest a designer could get to the BS 5268-2 requirements was C4(M).
The correlation between BS grades and the European grades for structural applications is given in Table 1.2.
Chipboard finds many applications as decking ranging from domestic applica- tions to heavy duty decking in mezzanine floor construction in thicknesses from 18 mm to 38 mm. In all cases it is advisable to use a board suitable for humid conditions, i.e. the P5 or P7 type.
Table 1.2 Correlation of types of particleboard
British standards European standards
Use BS type BS EN type Use
Load bearing C4(M) P5 Load bearing
(moisture resistant) (humid conditions)
No equivalent – P6 Heavy duty load bearing
(dry conditions) Heavy duty load bearing C5 P7 Heavy duty load bearing
(moisture resistant) (humid conditions)
1.4.5 Oriented strand board
Oriented strand board (OSB) is a multilayer board made from strands of wood of a predetermined shape together with a binder. The strands in the external layers are generally aligned with the long edge of the board although alignment with the board width is also allowed. The strands in the centre layer or layers may be randomly oriented, generally at right angles to the external layers. The layup has a similarity to plywoods and the use for decking follows similar rules, i.e. the ‘face grain’ should lie perpendicular to the deck supports.
The equivalent grades of OSB are given in Table 1.3. Very little, if any, dif- ferentiation of the type of OSB was made in the application of BS 5669. The European classifications require more care in the specification of the material for a particular application.
OSB finds many applications as a substitute for plywood for decking or wall sheathing. As the number of logs available for peeling veneers for plywood falls with the passage of time, so OSB will become more widely used particularly as it is possible to obtain it in very large sheets, e.g. 4.8 ¥2.4 m for wall sheathing.
The Materials Used in Timber Engineering 29
Table 1.3 Correlation of types of oriented strand board
British standards European standards
Use BS type BS EN type Use
General purpose F1 OSB/2 Load bearing (dry
(unconditioned) conditions)
Load bearing (humid F2 OSB/3 Load bearing (humid
conditions) (conditioned) conditions)
No equivalent – OSB/4 Heavy duty load bearing
(humid conditions)
1.4.6 Cement-bonded particleboard
Cement-bonded particleboard is defined as a sheet material manufactured under pressure, based on wood or other vegetable particles bound with hydraulic cement and possibly containing additives.
Cement-bonded particleboard is specified in a single grade in the European stan- dards. It is suitable as a decking and cladding in a humid environment and is finding application in external situations where the board is not immersed in water but subject to intermittent wetting. The drawback to the use of this type of board is its self-weight: at 1000 kg/m3it is much heavier than plywood, particleboard and OSB (commonly 700 kg/m3).
The comparable grades are given in Table 1.4.
1.4.7 Wood fibreboards
Wood fibreboards are a generic family of products manufactured from lignocellu- sosic fibres with the application of heat and/or pressure. Synthetic resin adhesives and other additives may be included in the process. The classification of the fibre- boards is into two groups:
• wet processfibreboards, which have a moisture content of at least 20% at the time of manufacture;
• dry processfibreboards, where the moisture content is below 20% at manu- facture and the density is greater than 450 kg/m3.
Wet process fibreboards are further classified by density:
• hardboards, coded HB and having density greater than 900 kg/m3, can be given additional properties such as fire retardancy, moisture resistance, resist- ance against biological attack, etc., by specific treatments such as ‘tempering’
or ‘oil tempering’ or the addition of adhesives or additives;
• high-density medium boards, coded MBH and density >560 kg/m3 but
£900 kg/m3;
• low-density medium boards, coded MBL and density ≥400 kg/m3 but
£560 kg/m3.
The medium boards have found application as a breathable wall sheathing mater- ial having a degree of structural strength that gives racking resistance and also allows use as the web member of a composite beam. As with the hardboards their properties can be improved during manufacture.
• softboards, coded SB and density ≥230 kg/m3but <400 kg/m3, find applica- tion as thermal and sound insulation materials and, in addition to the improve- ments previously described for wet process boards, impregnation with a petrochemical yields the well-known bitumen impregnated fibreboard.
Dry process fibreboards, generally known as medium-density fibreboards (MDF), are also classified by density:
• HDF, an MDF board with density ≥800 kg/m3
• light MDF, with density £650 kg/m3
• ultra-light MDF, with density £550 kg/m3
Table 1.4 Correlation of types of cement-bonded particleboard British standards European standards
Use BS type BS EN type Use
Internal use (dry T1 – No equivalent
conditions)
Internal and T2 OPC bonded Cement-bonded
external use particleboard (ordinary)
Portland cement (dry, humid and exterior conditions)
The properties of these boards can be modified during manufacture in a similar manner to the other fibreboards.
There is further classification and marking:
• for conditions of use (see section 1.4.2): dry no symbol
humid H
exterior E
• for application: general no symbol
load bearing L
load bearing for all durations of load A load bearing for instantaneous or
short-term durations of load S
The load-bearing capacity is further classified as 1 for normal loading and 2 for heavy duty loading. The various symbols described are then combined to give a particular specification as shown below:
Fibreboard type: condition of use + application purpose + load duration + load-bearing category
(the last two are optional codes) so, for example, a heavy duty load-bearing hard- board for use in humid conditions for all types of loading is HB.HLA2.
1.4.7.1 Tempered hardboard
Sometimes known as ‘oil tempered hardboard’ the strength and durability charac- teristics are excellent. There are three drawbacks to its use. The movement with change in moisture content is very high so boards coming directly from manufac- ture at a low moisture content will pick up atmospheric moisture until the equilib- rium moisture content is reached with a consequent expansion. The magnitude of this movement can cause buckling of the hardboard in wall panels of about 50 mm when fastened to studs spaced at 600 mm. It is often a user recommendation that boards should be conditioned by applying water and allowing the boards to reach equilibrium. The boards then tend to shrink ‘drum tight’ onto the supporting frame.
The second problem is in hand nailing the boards. The board surfaces are so hard that starting a nail is difficult. Tempered hardboard has a smooth surface on one face and the other is ‘rough’ – technically described as the mesh face. It is somewhat easier to nail into the mesh face but mesh surface appearance is usually unaccept- able. The third limitation is in board thickness. The maximum thickness is around 8 mm, thus limiting its application to wall sheathing or the webs of built-up beams.
The BS 1142 grade THE has been replaced by the BS EN 316 grade HB.HLA2, the code that was described in the previous paragraph. For further information refer to the Wood Panel Industries Federation.
1.5 ENGINEERED WOOD PRODUCTS