Glulam Beam and Header Applications

EWP Training Series: Module F

This presentation is a detailed introduction to the uses and specification of glulam for beams, headers and columns. Topics include glulam construction and anatomy, typical applications, appearance classifications, preservative treatments and finishing, notching, drilling and connection details, specifying beams and headers, and understanding the glulam trademark (grade stamp).

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Training objectives: Upon completing this course, students will be able to identify and describe:

  • Basic glulam characteristics
  • Balanced and unbalanced beams, when to specify each
  • Appearance classifications
  • Selecting and sizing stock beams
  • Reading the grade stamp
  • Checking
  • Recommended connection, notching and drilling practices
  • Treated glulam and typical applications
  • Factory and field applied stains/finishes
 
Approximate length: 73 minutes.

Participant Questions and Answers

Does painting a cut area of a glulam member really provide adequate penetration compared to the pressure treated areas?

In some cases where drying of the member is slow, the member should be treated in-place with a wood preservative and also covered. In-place preservative treatment is usually not as effective as pressure preservative treatment meeting AWPA standards. However, when properly applied, treatment in place offers greater protection than untreated or painted surfaces.

Preservatives for in-place treatment are similar to those used in the pressure impregnation method of wood in retorts and usually require a licensed applicator to handle them. Different solvents are often used to facilitate dispersion of the chemical or chemicals used. The selection of the type depends on the use of the member and availability of the treatments.

SURFACE TREATMENTS
Surface treatments can be applied to protect newly exposed, untreated wood from decay. They are commonly applied to checks, mechanical damage, and field-fabricated areas to act as a barrier against the entry of decay organisms. However, the penetration of surface treatments is very shallow and, therefore, ineffective against established internal decay

INJECTION OF PRESERVATIVE
Researchers at the University of Washington have reported using a specially modified lag screw with a grease gun to inject preservative chemicals. Other methods of in-place treatment have also been used successfully. Some applicators have special equipment for injecting the preservatives under pressure. Typical treatment methods require injection of preservative into holes drilled fairly close in order to increase the retention of the preservatives. Usually this is done in small areas more subject to decay than the remainder of the piece. In all cases, the drilled holes should be filled with pressure-treated wood plugs inserted tightly in the hole with a drive fit. The reduction in capacity due to the holes must be evaluated in all cases.

FUMIGANTS
Fumigants are specialized preservative chemicals that volatilize into toxic gases that migrate through the wood, killing decay fungi. They can diffuse for several feet from the point of application along the wood grain [5,8]. Fumigants in both solid and liquid form have been used successfully as in-place treatments for wood subject to decay conditions [4,5], however, they should not be used if the member extends into an occupied enclosed or un-vented space because of the possibility that toxic fumes might accumulate there. Because fumigants dissipate with time, fumigant-treated members exposed to weather or moisture should be re-inspected and retreated at regular intervals based on the exposure. 5 to 10 year intervals have been recommended for bridge applications. The fumigant method is ideally suited for those situations where a member is to be treated and later covered and kept dry. Applicators are generally required to be licensed prior to using fumigants.

Does treated Glulam require special connectors?

The preservative treating and metal connector industries recommend that metal connectors in contact with ACQ treated wood be hot dip galvanized in accordance with ASTM A653 Class G185 sheet with 1.85 ounces of zinc per square foot minimum. We recommend that you consult further with the steel component producer or American Iron and Steel Institute.

What are the shrinkage characteristics of glulam relative to I-joists?

For more than 15 years, the glulam industry in North America has been offering glulam products with I-joist compatible (so called “IJC”) depths to the construction industry with wide acceptance by framers, contractors, designer, distributors, and building regulators. The IJC depths are typically 9-1/2, 11-7/8, 14, 16, 18, 20, 22, and 24 inches, which matches the standard depths of prefabricated wood I-joists. While the moisture content of different wood products in the same engineered wood system varies, the lumber flanges used in I-joists are also subject to dimension changes, just like IJC glulam. Unless the end-use environment is extremely dry, the IJC glulam depth is not expected to change substantially. Therefore, there have not had serviceability or structural issues that have been reported in the field for IJC glulam.

APA Technical Note Y260 explains dimensional changes in structural glued laminated timber due to changes in moisture content as glulam stabilizes in the end-use environment. These changes tend to be minimal in comparison to sawn timbers and lumber.

What reference resources are available to identify GLB “Structural Grade Designations”? e.g “24f-V4 vs. 24f-V3”, etc...

See ANSI 117-2015, Section 1.3 Combination Symbols: Each structural glued laminated timber lay-up is assigned a combination symbol. The combination symbol is used to identify a combination and to distinguish one combination from another. Each combination is assigned design values based on ASTM D3737 or full scale tests in accordance with ASTM D7341 (13). Design values for combinations are tabulated in Annex A. Lay-up requirements for combinations are tabulated in Annex B.