In specifying Canadian glulam products, it is necessary to indicate both the stress grade and the appearance grade required.
The specification of the appropriate stress grade depends on whether the intended end use of a member is for a beam, a column, or a tension member as shown in Table 2.
|Table 2: Canadian Glulam - Stress Grades|
|Bending Grades||20f-E and
|Used for members stressed principally in bending (beams) or in combined bending and axial load.|
|Specify EX when members are subject to positive and negative moments or when members are subject to combined bending and axial load such as arches and truss top chords.|
|Used for members stressed principally in axial compression, such as columns.|
|Used for members stressed principally in axial tension, such as bottom chords of trusses.|
For the bending grades of 20f-E, 20f-EX, 24f-E and 24f-EX, the numbers 20 and 24 indicate allowable bending stress for bending in Imperial units (2000 and 2400 pounds per square inch). The f refers to flexure and E indicates that most laminations must be tested for stiffness by machine.
Stress grades with EX designation (20f-EX and 24f-EX) are specifically designed for cases where bending members are subjected to stress reversals. In these members the lamination requirements in the tension side are the mirror image of those in the compression side.
Similarly the descriptions for compression grades,16c-E and 12c-E, and tension grades,18t-E and 14t-E indicate the allowable compression and tension stresses.
Glulam is manufactured in three appearance grades: Industrial, Commercial, and Quality as shown in Table 3.
|Table 3: Canadian Glulam - Commercial Species|
|Commercial Species Group Designation||Species in Combination||Wood Characteristics|
|Douglas fir, western larch||Woods similar in strength and weight. High degree of hardness and good resistance to decay. Good nail holding, gluing and painting qualities. Colour ranges from reddish-brown to yellowish-white.|
|Hem-Fir||Western hemlock, amabilis fir, Douglas fir||Lightwoods that work easily, take paint well and hold nails well. Good gluing characteristics. Colour range is yellow-brown to white.|
|Spruce-Pine||Spruce (all species except coast sitka spruce), lodgepole pine, jack pine||Woods of similar characteristics, they work easily, take paint easily and hold nails well. Generally white to pale yellow in colour.|
Unlike visually graded sawn timbers where there is a correlation between appearance and strength, there is no relationship between the stress grades and the appearance grades of glulam since the exposed surface can be altered or repaired without affecting the strength characteristics.
The appearance of glulam is determined by the degree of finish work done after laminating and not by the appearance of the individual lamination pieces.