GLULAM AND LVL

1.3.4.1 Glulam and LVL under the Eurocodes

EN 1995-1-1 requires glued laminated timber members to comply with EN 14080. This is currently available as EN 14080: 2000 ‘Timber structures – Glued laminated timber – Requirements’. This specifies glulam, whose properties may either be calculated from the timber used to form it, or by testing of an

adequate sample of the manufactured glulam. Timber used to form glulam must conform to EN 14081, the standard for solid timber for structural use. Glulam to EN 14080 may conform to one of the strength classes with characteristic values tabulated in EN 1194: 1999 ‘Timber structures – Glued laminated timber – Strength classes and determination of characteristic values’, except that mean density is omitted. Alternatively, characteristics for the particular type may be evaluated directly, in accordance with EN 1194. Test methods for evaluating the characteristic values of glulam are identical to those for solid timber, but

additional tests are required to check the quality of glued joints within the material. The scope of EN 1194 is limited to horizontally laminated glulam, and so there is no clear route for vertically laminated glulam to be used under EN 1995-1-1.

EN 1995-1-1 requires LVL structural members to comply with EN 14374. This is currently available as EN 14374: 2002 ‘Timber structures – Structural laminated veneer lumber – Requirements’. Characteristic values for types of LVL must be evaluated by the methods used for solid timber, and some further tests devised for wood based panels. EN 14374 also gives additional information on the material, such as embedding strengths, that are given directly in EN 1995-1-1 for solid timber and some other materials.

1.3.4.2 Glulam and LVL under BS 5268-2

Under BS 5268-2, both horizontally and vertically glued laminated timber are included. However, LVL is not covered by the standard. Properties for glulam are determined by calculating the properties of laminates of strength grade timber combined together. A series of conversion factors for various properties are given, depending on the strength class of laminate timber and the number of laminates. Other requirements for the quality of finger joints, etc, also affect the resulting properties calculated for glulam. The net result of these factors seems to be that BS 5268-2 permits a greater increase in bending stress for glulam compared to its constituent solid timber than occurs under European standards if strength class timber is laminated to the formula given in EN 1194.

1.3.5 WOOD-BASED PANELS

1.3.5.1 Wood-based panels under the Eurocodes

EN 1995-1-1 requires wood-based panels to comply with EN 13896 and LVL used as panels to comply with EN 14279. EN 13896: 2002 ‘Wood-based panels for construction – characteristics, evaluation of conformity and marking’ is already published, whilst ‘Laminated Veneer Lumber (LVL) – Specifications, definitions, classification and requirements’ is currently available as EN 14279: 2001. Unlike the standards referenced in EN 1995-1-1 for solid timber, glulam and LVL, these standards cover panels for general use in addition to those suitable for

grades range from those suitable for service class 1 only through to those suitable for all service classes. Extensive information on wood-based panels, their selection and use is given in the BRE Digest 477 series(h-m)_.

The key test methods for determining the mechanical properties of panels are given in EN 789 ‘Timber structures – Test methods – Determination of

mechanical properties of wood-based panels’. The current version of this standard was published in 1996. Revision is under way and EN 789: 2003 will be published in due course; the principal effect of this revision will be on the tests for panel and planar shear, which are currently in Annexes. Following

experience of using these tests, the revision will introduce modified test pieces and the tests will be moved into the main body of the standard. Characteristic values are derived from test results according to EN 1058 ‘Wood-based panels – Determination of characteristic values of mechanical properties and density’. Characteristic values for particular structural grades of panel, other than plywood and LVL, are given in EN 12369-1: 2001 ‘Wood-based panels – Characteristic values for structural design – Part 1: OSB, particleboards and fibreboards’. Alternatively, values for proprietary panels may be derived directly from testing. Also, EN 1995-1-1 requires that the use of softboards to EN 622-4 should be restricted to non-structural uses and should be designed by testing for wind bracing (there are no characteristic values for softboards given in EN 12369-1). One panel product that does not appear to be covered within the EN 1995-1-1 system is cement-bonded particleboard. This product is included in EN 13896, but not in EN 12369-1. It would be possible to evaluate its properties directly to comply with EN 13896, but EN 1995-1-1 does not give the

modification factors for cement-bonded particleboard that it gives for other materials.

For plywood, a second part for EN 12369, EN 12369-2: 2003 ‘Wood-based panels – Characteristic values for structural design – Part 2: Plywood’, is in draft. EN 12369-2 sets out strength and modulus classes, and plywood would be

characterised by four classes, covering bending strength and modulus along and across the panel. This draft also states that some other properties may be

derived from bending strength, modulus and density data as an alternative to direct determination to EN 789 and EN 1058. Hence, publication of EN 12369-2 will formalise the presentation of design data for specific plywoods rather than providing grade data directly. Also, as EN 13896 was published prior to

agreement on EN 12369-2, it does not refer to the latter standard, and so until EN 13896 is revised, there is unlikely to be a direct chain of reference from EN 1995-1-1 to EN 12369-2.

No tabulated characteristic values are currently available for LVL to EN 14279, and so determination from test results for specific products will be required. 1.3.5.2 Wood-based panels under BS 5268-2

BS 5268-2 has two sections covering panel products. Section 4 covers plywood, whilst section 5 covers panel products other than plywood. It also has two

longstanding satisfactory experience with constructions designed to BS 5268-2 and incorporating those grades of plywood. Alternatively, design for plywood or other wood-based panel products may be based on characteristic values for panel materials. The sources for characteristic values are the same product or test standards that are used for design according to the Eurocodes, with formulae to convert these values to permissible stresses or moduli that are similar to the formulae used under the Eurocode. Panel products for roofs, floors and walls, including cement-bonded particleboard (for which standard characteristic values are not tabulated) may also be specified in accordance with EN 12871: ‘Wood-based panels – Performance, specification and requirements for load-bearing boards for use in floors, walls and roofs’ and EN 12872: : ‘Wood- based panels – Guidance on the use of load-bearing boards in floors, walls and roofs.’ Note: BS 7916: 1998 ‘Code of practice for the selection and application of particleboard, oriented strand board (OSB), cement bonded particleboard and wood fibreboards for specific purposes’ is no longer valid and is withdrawn. BS7916 must not be used.

In order to continue using structural plywood from traditional sources when designing to the Eurocodes, characteristic values in accordance with EN 789 and EN 1058 will need to be determined in place of the BS 5268-2 grade stress tables. For other panel products, this procedure is already in place under BS 5268-2: 2002, section 5. Plywood or other board materials will be affected by the changes to the design process, safety factors involved and their production specifications.

1.3.6 ADHESIVES

1.3.6.1 Adhesives under the Eurocodes

Adhesive joints form crucial parts of many timber structures, particularly within components such as glulam and LVL. Unfortunately, specification is limited by the paucity of standards specifying structural adhesives. EN 1995-1-1 sets a general requirement for strength and durability. It then covers the appropriate types of adhesives to EN 301 ‘Adhesives, phenolic and aminoplastic, for load- bearing timber structures: Classification and performance requirements’ for the different service classes. However, EN 1995-1-1 does not refer to EN 12436 ‘Adhesives for load-bearing timber structures – Casein adhesives – Classification and performance requirements’. Other types of adhesive are in commercial use, for instance for finger jointing green timber, but performance specifications or test methods are not yet available within European (or British) standards. A summary of structural wood adhesives is given in STEP 1 ‘Adhesives’(n)_. 1.3.6.2 Adhesives under BS 5268-2

BS 5268-2 gives a table of permitted adhesives. However, these are either to EN 301 or to a British Standard that has now been withdrawn. Hence, the

introduction of EN 1995-1-1 will have little effect on the already poor position for specifying adhesives for use with structural timber. However, BS 5268-2 does have extensive advice on the production of sound adhesive joints that is not replicated in EN 1995-1-1 or any document referenced by it. It has been suggested that similar advice should be added to a UK National Annex for EN 1995-1-1.