Adhesives in Glued-Laminated Timber Construction

Glued-laminated construction was used in Europe prior to World War I, but the first extensive development in the United States was in 1935 when glued-laminated arches were used in a service building at the Forest Products Laboratory, Madison. A shortage of steel during World War II provided the needed stimulus to promote large scale use of glued-laminated structural timbers in the United States. The advantages are: Large timbers from small trees; variable shapes and cross sections; large sizes are possible; a wide variety of species can be used for construction purposes; moisture contents can be easily controlled; there is less variation and strength from one timber to another; and a mixture of species is possible, taking advantage of different strength qualities. The disadvantages are: Adhesives are expensive; large buildings and equipment are required; and cost of labor and supervision is higher due to the required precision. Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla are used in Canada. White oak (Quercus alba) is used for ship keels. The fabrication process consists of seasoning, planing to thickness, scarf-jointing, glue spreading, assembly, clamping, and curing. Adhesives which set at high temperature or have little resistance to moisture are not suitable. Casein, urea-formaldehyde, and resorcinol are most suitable for laminating, but resorcinol is the only commercially acceptable material which will develop a completely waterproof bond by the cold press method. In order to develop adequate glue line strength, lumber surfaces must be finished or machined to close tolerances, moisture content of the lumber must be within a suitable range, and an adequate quantity of the glue must be spread. Assembly time in excess of 30 minutes should not be permitted, pressure ranging from 100 to 200 psi must be applied until the glue squeezes out and hardens, and sufficiently high temperatures for developing high strength in the glue line are required. Specifications are needed to assure that a quality product is produced.

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