What Are Glued-Laminated Structural Members?

The gluing together of multiple laminations of standard 2-in.-nominal-thickness lumber has been used for many years to produce large beams and girders. This is really the only option for using sawn wood for large members that are beyond the feasible range of size for single sawn pieces.

However, there are other reasons for using the laminated beam that include the following:

Higher Strength. Lumber used for laminating consists of a moisture content described as kiln dried. This is the opposite end of the quality range from the green wood condition ordinarily assumed for solid-sawn members.

This, plus the minimizing effect of flaws due to lamination, permits use of stresses for flexure and shear that are much higher than those allowed for single-piece members. The result is that much smaller sections can often be used, which helps to offset the usually higher cost of the laminated products.

Better Dimensional Stability. This refers to the tendency for wood to warp, split, shrink, and so on. Both the use of the kiln-dried materials and the laminating process itself tend to create a very stable product. This is often a major consideration where shape change can adversely affect the building construction.

Shape Variability. Lamination permits the production of curved, tapered, and other special profile forms for beams. Cambering as compensation for service load deflection, sloping for roof drainage, and other useful custom profiling can be done with relative ease. This is otherwise possible only with a truss or a built-up section.

Laminated beams have seen wide use for many years and industry wide standards are well established. Cross-sectional sizes are derived from the number of laminations and the size of the individual pieces used. Thus depths are multiples of 1.5 in. and widths are slightly less than the lumber size as a result of the finishing of the product.

Minor misalignments and the unavoidable sloppiness of the gluing process result in an unattractive surface. Finishing of the sides of beams may simply consist of smoothing them off, although various special surface textures can also be created.

Investigation and design of glued-laminated timber members are done primarily with the procedures explained for solid-sawn beams as described in Section 4.4. Criteria for design is provided in most building codes, in the National Design Specification (NDS) (Ref. 3), and in the literature provided by manufacturers and suppliers of the products.

These elements are manufactured products and are mostly not able to be transported great distances, so information about them should be obtained from local suppliers. Individual elements of glued-laminated timber can be custom profiled to produce a wide variety of shapes for structures.

For very large elements this is not a problem, but for smaller structures the curvature limits of 2-in.-nominal-thickness lumber may be critical. Manufacturers of laminated products usually produce the arch and gabled elements as standard forms.

Structural design of the products is usually done by the manufacturer’s engineers. Form limits, size range, connection details, and other considerations for these products should be investigated with individual manufacturers.

Custom shapes can be produced, such as those with double curvature. Many imaginative structures have been designed using the form variation potential of this process.

Columns may be produced with 1.5-in. laminations, presenting the same advantages as those described for beams: higher strength and dimensional stability being most critical. It is also possible to produce glued-laminated columns of greater length than that obtainable with solid-sawn members.

In general, laminated columns are used less frequently than beams or girders and are mostly chosen only when a special shape is desired or when some of the inherent limitations of other options are restrictive.

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