Glulam (glued-laminated timber) is an engineered structural wood product that consists of multiple individual layers of dimension lumber that are glued together under controlled conditions. All Canadian glulam is manufactured using waterproof adhesives for end jointing and for face bonding and is therefore suitable for both exterior and interior applications. Glulam has high structural capacity and is also an attractive architectural building material.
Glulam is commonly used in post and beam, heavy timber and mass timber structures, as well as wood bridges. Glulam is a structural engineered wood product used for headers, beams, girders, purlins, columns, and heavy trusses. Glulam is also manufactured as curved members, which are typically loaded in combined bending and compression. It can also be shaped to create pitched tapered beams and a variety of load bearing arch and trusses configurations. Glulam is often employed where the structural members are left exposed as an architectural feature.
Available sizes of glulam
Standard sizes have been developed for Canadian glued-laminated timber to allow optimum utilization of lumber which are multiples of the dimensions of the lamstock used for glulam manufacture. Suitable for most applications, standard sizes offer the designer economy and fast delivery. Other non-standard dimensions may be specially ordered at additional cost because of the extra trimming required to produce non-standard sizes. The standard widths and depths of glulam are shown in Table 6.7, below. The depth of glulam is a function of the number of laminations multiplied by the lamination thickness. For economy, 38 mm laminations are used wherever possible, and 19 mm laminations are used where greater degrees of curvature are required.
Standard widths of glulam
Standard finished widths of glulam members and common widths of the laminating stock they are made from are given in Table 4 below. Single widths of stock are used for the complete width dimension for members less than 275 mm (10-7/8″) wide. However, members wider than 175 mm (6-7/8″) may consist of two boards laid side by side. All members wider than 275 mm (10-7/8″) are made from two pieces of lumber placed side by side, with edge joints staggered within the depth of the member. Members wider than 365 mm (14-1/4″) are manufactured in 50 mm (2″) width increments, but will be more expensive than standard widths. Manufacturers should be consulted for advice.
| Initial width of glulam stock | Finished width of glulam stock | ||
|---|---|---|---|
| mm. | in. | mm. | in. |
| 89 | 3-1/2 | 80 | 3 |
| 140 | 5-1/2 | 130 | 5 |
| 184 | 7-1/4 | 175 | 6-7/8 |
| 235 (or 89 + 140) | 9-1/4 (or 3-1/2 + 5-1/2) | 225 (or 215) | 8-7/8 (or 8-1/2) |
| 286 (or 89 + 184) | 11-1/4 (or 3-1/2 + 7-1/4) | 275 (or 265) | 10-7/8 (or 10-1/4) |
| 140 + 184 | 5-1/2 + 7-1/4 | 315 | 12-1/4 |
| 140 + 235 | 5-1/2 + 9-1/4 | 365 | 14-1/4 |
Notes:
- Members wider than 365 mm (14-1/4″) are available in 50 mm (2″) increments but require a special order.
- Members wider than 175 mm (6-7/8″) may consist of two boards laid side by side with logitudinal joints staggered in adjacent laminations.
Standard depths of glulam
Standard depths for glulam members range from 114 mm (4-1/2″) to 2128 mm (7′) or more in increments of 38 mm (1-1/2″) and l9 mm (3/4″). A member made from 38 mm (1-1/2″) laminations costs significantly less than an equivalent member made from l9 mm (3/4″) laminations. However, the l9 mm (3/4″) laminations allow for a greater amount of curvature than do the 38 mm (1-1/2″) laminations.
| Width | in. | Depth range | |
|---|---|---|---|
| mm | in. | ||
| 80 | 3 | 114 to 570 | 4-1/2 to 22-1/2 |
| 130 | 5 | 152 to 950 | 6 to 37-1/2 |
| 175 | 6-7/8 | 190 to 1254 | 7-1/2 to 49-1/2 |
| 215 | 8-1/2 | 266 to 1596 | 10-1/2 to 62-3/4 |
| 265 | 10-1/4 | 342 to 1976 | 13-1/2 to 77-3/4 |
| 315 | 12-1/4 | 380 to 2128 | 15 to 83-3/4 |
| 365 | 14-1/4 | 380 to 2128 | 15 to 83-3/4 |
Note:
1. Intermediate depths are multiples of the lamination thickness, which is 38 mm (1-1/2″ nom.) except for some curved members that require 19 mm (3/4″ nom.) laminations.
Laminating stock may be end jointed into lengths of up to 40 m (130′) but the practical limitation may depend on transportation clearance restrictions. Therefore, shipping restrictions for a given region should be determined before specifying length, width or shipping height.
Glulam appearance grades
In specifying Canadian glulam products, it is necessary to indicate both the stress grade and the appearance grade required. 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.
Glulam is available in the following appearance grades:
- Industrial
- Commercial
- Quality
The appearance grade defines the amount of patching and finishing work done to the exposed surfaces after laminating (Table 6.8) and has no strength implications. Quality grade provides the greatest degree of finishing and is intended for applications where appearance is important. Industrial grade has the least amount of finishing.
| Grade | Description |
|---|---|
| Industrial Grade | Intended for use where appearance is not a primary concern such as in industrial buildings; laminating stock may contain natural characteristics allowed for specified stress grade; sides planed to specified dimensions but occasional misses and rough spots allowed; may have broken knots, knot holes, torn grain, checks, wane and other irregularities on surface. |
| Commercial Grade | Intended for painted or flat-gloss varnished surfaces; laminating stock may contain natural characteristics allowed for specified stress grade; sides planed to specified dimensions and all squeezed-out glue removed from surface; knot holes, loose knots, voids, wane or pitch pockets are not replaced by wood inserts or filler on exposed surface. |
| Quality Grade | Intended for high-gloss transparent or polished surfaces, displays natural beauty of wood for best aesthetic appeal; laminating stock may contain natural characteristics allowed for specified stress grade; sides planed to specified dimensions and all squeezed-out glue removed from surface; may have tight knots, firm heart stain and medium sap stain on sides; slightly broken or split knots, slivers, torn grain or checks on surface filled; loose knots, knot holes, wane and pitch pockets removed and replaced with non-shrinking filler or with wood inserts matching wood grain and colour; face laminations free of natural characteristics requiring replacement; faces and sides sanded smooth. |
Glulam camber
For long straight members, glulam is usually manufactured with a built in camber to ensure positive drainage by negating deflection. This ability to provide positive camber is a major advantage of glulam. Recommended cambers are shown in Table 5 below.
| Type of Structure | Recommendation |
|---|---|
| Simple Glulam Roof Beams | Camber equal to deflection due to dead load plus half of live load or 30 mm per 10 m (1″ per 30′) of span; where ponding may occur, additional camber is usually provided for roof drainage. |
| Simple Glulam Floor Beams | Camber equal to dead load plus one quarter live load deflection or no camber. |
| Bowstring and Pitched Trusses | Only the bottom chord is cambered. For a continuous glulam bottom chord; camber in bottom chord equal to 20 mm per 10 m (3/4″ in 30′) of span. |
| Flat Roof Trusses (Howe and Pratt Roof Trusses) | Camber in top and bottom glulam chords equal to 30 mm per 10 m (1″ in 30′) of span. |
Glulam manufacture
The dimension lumber pieces that make up glulam are end jointed and arranged in horizontal layers or laminations. The lumber used for the manufacture of glulam is a special grade (lamstock) that is purchased directly from lumber mills. The lamstock is dried to a maximum moisture content of 15 percent and planed to a closer tolerance than that required for visually graded lumber. Laminating multiple pieces together is an effective way of using high strength dimension lumber of limited length to manufacture glulam members in many cross sectional shapes and lengths. The special grade of lumber used for glulam, lamstock, is received and stored at the laminating plant under controlled conditions. The lamstock must be dried to a moisture content of between 7 and 15% before laminating to maximize adhesion and minimize shrinkage in service. The lumber laminations (lamstock) are visually and mechanically sorted for strength and stiffness into lamstock grades. The assessments of strength and stiffness are used to determine where a given piece will be situated in a beam or column. For example, high strength pieces are placed in the outermost laminations of a beam where the bending stresses are the greatest and for columns and tension members, the stronger laminations are more equally distributed. This blending of strength characteristics is known as grade combination and ensures consistent performance of the finished product. The laminations are glued under pressure using a waterproof adhesive. See Figure 3.7, below, for a schematic representation of glulam manufacture. Glulam beams may also be cambered, which means that they may be produced with a slight upward bow so that the amount of deflection under service loads is reduced. A typical camber is 2 to 4 mm per metre of length. Glulam is manufactured to meet the requirements outlined in CSA O122 Structural GluedLaminated Timber.
Quality Control
Glulam is an engineered wood product requiring exacting quality control at all stages of manufacture. Certified manufacturing plants adhere to quality control standards that govern lumber grading, finger joining, gluing and finishing. Canadian manufacturers of glulam are required to be qualified and certified under CSA O177 Qualification Code for Manufacturers of Structural Glued- Laminated Timber. This standard sets mandatory guidelines for equipment, manufacturing, testing and record keeping procedures. As a mandatory manufacturing procedure, tests must be routinely performed on several critical manufacturing steps, and recording of test results must be done. For example, representative samples are tested for adequacy of glue bond and all end joints are stress tested to ensure that each joint exceeds the design requirements. Each member fabricated has a quality assurance record indicating glue bond test results, lumber grading, end joint test and laminating conditions for each member fabricated, including glue spread rate, assembly time, curing conditions and curing time. In addition, mandatory quality audits are performed by independent certification agencies to ensure that in-plant procedures meet the requirements of the manufacturing standard. A certificate of conformance to manufacturing standards for a given glulam order is available upon request.
Glulam species
Glulam is primarily produced in Canada from two species groups; Douglas fir-Larch and SprucePine. Hem-Fir species are also used occasionally.
| Canadian Glulam – Commercial Species | ||
|---|---|---|
| Commercial Species Group Designation | Species in Combination | Wood Characteristics |
| Douglas Fir-Larch (D.Fir-L) | 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. |
Glulam strength grades
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 | |||
|---|---|---|---|
| Stress Grade | Species | Description | |
| Bending Grades | 20f-E and 20f-EX | D.Fir-L or Spruce Pine | Used for members stressed principally in bending (beams) or in combined bending and axial load. |
| 24f-E and 24f-EX | D.Fir-L or Hem-Fir | 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. | |
| Compression Grades | 16c-E 12c-E | D.Fir-L Spruce Pine | Used for members stressed principally in axial compression, such as columns. |
| Tension Grades | 18t-E 14t-E | D.Fir-L Spruce Pine | 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). 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. The “E” indicates that most laminations must be tested for stiffness by machine. The lower case letters indicate the use of the grade as follows: “f” is for flexural (bending) members, “c” is for compression members, and “t” is for tension members. 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. 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.
Moisture Control of Glulam
The checking of wood is due to differential shrinkage of the wood fibres in the inner and outer portions of a wood member. Glulam is manufactured from lamstock having a moisture content of 7 to 15 percent. Because this range approximates the moisture conditions for most end uses, checking is minimal in glulam members. Proper transit, storage and construction methods help to avoid rapid changes in the moisture content of laminated members. Severe moisture content changes can result from the sudden application of heat to buildings under construction in cold weather, or from exposure of unprotected members to alternate wet and dry conditions as might occur during transit and storage. Canadian glulam routinely receives a coat of protective sealer before shipping and is wrapped for protection during shipping and erection. The wrapping should be left in place as long as possible and ideally until permanent protection from the weather is in place. During on-site storage, glulam should be stored off the ground with spacer blocks placed between members. If construction delays occur, the wrapping should be cut on the underside to prevent the accumulation of condensation.
Treatment and sealant for glulam
Preservative treatment is not often required but should be specified for any application where ground contact is likely. Advice on suitable preservative treatment should be sought from the manufacturer. Untreated glulam can be used in humid environments such as swimming pools, curling rinks or in industrial buildings which use water in their manufacturing process. Where the ends of glulam members will be subject to wetting, protective overhangs or flashings should be provided. In applications where direct water contact is not a factor, a factory applied sealer will prevent large swings in moisture content. The alkyd sealer applied to glulam members in the factory provides adequate protection for most high-humidity applications. Since wood is corrosion-resistant, glulam is used in many corrosive environments such as salt storage domes and potash warehousing.
For more information on individual glulam manufacturers in Canada, refer to the following links:
Western Archrib
Mercer Mass Timber
Nordic Structures
Goodfellow
Kalesnikoff Mass timber
Element5
