Lower strength class

Higher strength class > (1/6)d

(a) Homogeneous glued-laminated timber (b) Combined glued-laminated timber

Fig. 6.3. Example of a homogeneous and a combined glued laminated timber member.

The strength classes are defined as follows:

Homogeneous glued laminated timber: where all of the laminations in the section are of the same grade (strength class) and species, or species combinations.

Combined glued laminated timber: where the section is made from inner and outer laminations of different grades (strength classes) and species, or species combinations. The outer laminations will have the higher strength class and must equate to at least one-sixth of the member depth on both sides.

In BS EN 1194:1999 the characteristic strength, stiffness and density values of glued laminated members are given for four strength classes, defined as GL 24, GL 28, GL 32 and GL 38, each followed by the suffix h, where the glulam is homogeneous, or c, if it is combined.

As in the case of the reference system used for timber in BS EN 338, the numbers used in the glulam system represent the characteristic bending strength of the glulam in N/mm2 and the properties of some of the strength classes given in BS EN 1194 are reproduced in Table 6.2.

Mechanical properties of glued laminated timber can be derived from the equivalent properties of the softwood laminations being used. The relationships between the materials are given in Table A.1 of BS EN 1194 and summarised in Table 6.3.

The properties of the laminations are those given in BS EN 338:2003 for softwood species, or they can be derived as described in BS EN 1194:1999. Particular strength requirements must also be met for the end joints in all laminations and alternative options for achieving this are given in BS EN 1194:1999, Clause 6.3.2.

The equations in Table 6.3 are valid for homogeneous glulam made from softwood laminations but can also be used to calculate the properties of glued laminated timber that does not conform to any of the four strength classes given in the standard.

Where the mechanical properties of a combined glued laminated member are required, on the basis that the zones of different laminations equate to at least one-sixth of the beam depth (as shown in Figure 6.3b) or two laminations, whichever is greater, the relationships given in Table 6.3 will apply to the properties of the individual parts of the cross-section of the member.

Table 6.2 The strength, stiffness and density properties of homogeneous glulam strength classes (GL 24h, GL 28h and GL 32h) and combined glulam strength classes (GL 24c, GL 28c and GL 32c)*
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