Glulam

Glulam or glue-laminated timber refers to large, structural members made by gluing together pieces of dimension lumber. It’s a unique structural material because it can be formed into many curved shapes and the sizes are limited only by transportation restrictions. It is generally used for columns and beams, and frequently for curved members loaded in combined bending and compression. Glulam also is used as an exposed architectural product or it can be hidden or left unfinished to only serve a structural role.

Manufacturing

The lumber used for the manufacture of glulam is a special grade purchased directly from lumber mills. It is dried to a maximum moisture content of 15 percent, and planed to a closer tolerance than generally required for dimension lumber. The lumber is graded or machine rated to determine strength. The pieces are end jointed and then arranged in horizontal layers or laminations with the location of the pieces determined by the strength requirements. For example, a beam made for a single span will have the highest quality lumber situated at the ends where tension is greatest.

Glulam is manufactured at certified plants where standards governing lumber grading, end joining, gluing and finishing are used to control quality. Qualified manufacturers can supply a certificate of conformance for their products upon request.

Uses

Glulam is a structural product used for headers, beams, girders, columns and heavy trusses. It is often used where the structure of a building is left exposed as an architectural feature.

Glulam can be manufactured in a vast array of straight and curved configurations. It offers the architect artistic freedom without sacrificing structural requirements.

Grades

The specification of the appropriate stress grade depends on whether the intended use of a member is for a beam, a column or a tension member.

Some stress grades are specifically for bending members or multiple spans where the member will be subjected to both tension and compression. For these members, the lamination requirements in the tension side are the mirror image of those in the compression side.

Glulam is manufactured in several appearance grades. In the United States, these are Premium, Architectural, Industrial and Framing appearance grades. In Canada, the grades are Quality, Commercial, and Industrial.

Sizes

Standard sizes have been developed for Canadian glued-laminated timber to allow optimum utilization of lumber. 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.

Standard finished widths of glulam members range from 2-1/2 inches to 10-3/4 inches. Wider sections also are available to order.

For United States glulam made from western species and for Canadian glulam, standard depths are multiples of 1-1/2", and for southern yellow pine glulam, multiples of 1-3/8". Standard depths for glulam members range from 4-1/2" (114 mm) to 7" (2128 mm) or more in increments of 1-1/2" (38 mm) and 3/4" (l9 mm).

While the cost is greater a tighter radius of curvature is possible if 3/4" (19 mm) laminations are used.

For long straight members, glulam is usually manufactured with a built-in camber to ensure positive drainage by negating deflection. The ability to provide positive camber is a major advantage of glulam.

Moisture Control

Glulam is manufactured from lamstock with 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.

Glulam routinely receives a coat of protective sealer before shipping and is further 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 condensation.

Fire Safety

Because glued-laminated timber is manufactured in large sizes, it is often used in heavy timber construction to meet minimum size and fire-resistance rating requirements of North American building codes.

Tips for Ordering Glulam

For the best economy and efficiency when ordering glulam members, specifiers should:

• Select a product with the smallest cross-sectional area or the least weight required for the job.
  
• Specify 1-1/2" (38 mm) laminations and standard depths whenever possible.
  
• Use 1-1/2" (38 mm) laminations in straight members and in all curved members with radius of curvature of 27 feet (8400 mm) or more.
  
• Limit the size of glulam members to those that can be shipped economically and legally. This applies to both lengths and heights, since local overall shipping height limitations, usually about 14 feet to 20 feet (4 to 6m), may restrict arch sizes.
  
• Use the proper appearance grade for the project by matching the appearance and the cost premium to the requirements for appearance and visibility.
  
• In some instances, using larger than necessary members may simplify overall economy by simplifying connection details. Consult a glulam manufacturer.
  
• Minimize the number of concealed or semi-concealed connections that require costly shop fabrication.
  
• Outline protection measures to be taken during erection to protect the members from damage, including provision for temporary bracing.
  
• Paint steel connections to prevent rust from staining the wood. Connections for high humidity service conditions should be galvanized.

Further information about Glulam:

APA - The Engineered Wood Association

Provides product guides, builder tips, storage suggestions, design concepts and much more for engineered wood products.

American Institute of Timber Construction (AITC )

Provides information on products and species and on the technical capabilities of engineered timber.