These products, which include glued-laminated timber, prefabricated wood I joists, and structural composite lumber (SCL), are used extensively in residential and nonresidential building construction. They are used instead of nonwood products, such as steel framing elements, or as substitutes for conventional sawn-lumber products.
Characteristics of Glulam Timber
Glued-laminated timber, or glulam as it is often referred to, is the oldest type of the engineered glued wood products. With the employment of wet-use adhesives for laminating, glulam elements may be used in applications where they may be exposed to the elements. These applications include exterior building components, utility structures, marinas and wharfs, and bridge structures, such as pedestrian, highway, and railway bridges.
Preservative Treatment. For any exposed application, glulam members should be pressure impregnated with an improved preservative, such as creosote, pentachlorophenol (penta) in various carriers, and waterborne arsenicals. All treatments, however, are not compatible with all species; a specific treatment may not be available in the job-site region. Some treatments may be pressure impregnated into the laminations before the member is glued, whereas other treatments can be used only in conjunction with the finished product. (See Standard C-18, American Wood Preservers Association, Granbury, TX, and technical notes on pressure treating of glulam of Engineered Wood Systems, Tacoma, WA and AITC, Englewood, CO.)
Versatility and Use. The most versatile of the engineered glued wood products, glulam can be fabricated in a wide variety of shapes, such as those shown in Fig. 10.27. Short-span glulam beams with constant rectangular cross sections, the most commonly used shape, are typically available as stock beams from distribution centers throughout the United States for use in residential and light commercial construction. Inventoried in a variety of sizes and lengths, stock beams are often used for headers and floorbeams as well as for other uses.
Straight or curved beams can be manufactured in lengths of over 100 ft and with large cross-sectional areas. Glulam arches have been erected to span 300 ft or more. For structures requiring very large spans, such as stadiums needing spans of 500 ft or more, glulam timber domes are often the most economical framing system and are esthetically pleasing.
The limitations on length for transporting glulam members from the manufacturing plant to the job site with available rail or truck facilities may control the size of glulam members that can be used for long spans without splicing. Designers and contractors should closely coordinate arrangements for transportation of long glulam components with the manufacturer.
Manufacture of Glulam Members
Structural glued-laminated timber is made by bonding layers of lumber together with adhesive so that the grain direction of all laminations is essentially parallel.
Narrow boards may be edge-glued, and short boards may be end-joined to create greater lengths. The resultant wide and long laminations are then face-glued into large, manufactured timbers. Figure 10.26 shows different types of glulam beams.
Recommended practice calls for lumber of nominal 1- and 2-in thicknesses for laminating. The lumber is dressed to 3â„4-, 13â„8- and 11â„2-in thicknesses, depending on the species before gluing. The thinner laminations are generally used in curved members.
Virtually any species of wood can be used in the laminating process if the design values have been determined. Different species can be intermixed within the depth of a section to achieve optimum resource utilization. Higher-strength species are positioned in a beam in zones that will be subjected to high stresses under design loads. Lower-strength species can be placed in zones with lower in-service stresses.
Similarly, manufacturer of glulam beams can be based on a graded layup concept.
This requires that laminations with a higher lumber grading be used in zones subjected to high design stresses, and lower grades, in lower-stressed areas of the member. As a consequence, glulam members are a resource-efficient wood product, since varying grades and species can be used to achieve desired performance.
Constant-depth members normally are a multiple of the thickness of the lamination stock used. Variable-depth members, because of tapering or special assembly techniques, may not be exact multiples of these lamination thicknesses.
When members wider than the stock available are required, laminations may consist of two boards side by side. These edge joints must be staggered, vertically in horizontally laminated beams (load acting normal to wide faces of laminations), and horizontally in vertically laminated beams (load acting normal to the edge of laminations). In horizontally laminated beams, edge joints need not be edge-glued.
Edge gluing is required in vertically laminated beams.
Edge and face gluings are the simplest to make, end gluings the most difficult.
Although no longer used in the glulam industry, a plane sloping scarf (Fig. 10.27), in which the tapered surfaces of laminations are glued together, can develop 85 to 90% of the strength of an unscarfed, clear, straight-grained control specimen. Finger joints (Fig. 10.28) are less wasteful of lumber and hence are widely used by the glulam industry. Quality can be adequately controlled in machine cutting and in high-frequency gluing.
A
combination of thin tip, flat slope on the side of the individual fingers, and a narrow pitch is desired. The length of fingers should be kept short for savings of lumber, but long for maximum strength. Typical finger joint lengths are 111â„4.
The usefulness of structural glued-laminated timbers is determined by the lumber used and glue joint produced. Certain combinations of adhesive, treatment, and wood species do not produce the same quality of glue bond as other combinations, although the same gluing procedures are used. Thus, a combination must be supported by adequate experience with a laminators gluing procedure.
The only adhesives currently recommended for wet-use and preservative-treated lumber, whether gluing is done before or after treatment, are resorcinol and phenolresorcinol resins. Melamine and melamine-urea blends are also used for highfrequency curing of end joints.
Glued joints may be cured with heat by several methods. Radio-frequency (RF) curing of glue lines is used for end joints and for limited-size members where there are repetitive gluings of the same cross section. Low-voltage resistance heating, where current is passed through a strip of metal to raise the temperature of a glue line, formerly was used for attaching thin facing pieces. The metal could be left in the glue line as an integral part of the completed member. Printed electric circuits, in conjunction with adhesive films, and adhesive films impregnated on paper or on each side of a metal conductor placed in the glue line are other alternatives.
Preheating the wood to ensure reactivity of the applied adhesive has limited application in structural laminating. The method requires adhesive application as a wet or dry film simultaneously to all laminations, and then rapid handling of multiple laminations.
Curing the adhesive at room temperature has many advantages. Since wood is an excellent insulator, a long time is required for elevated ambient temperatures to reach inner glue lines of a large assembly. With room-temperature curing, equipment needed to heat the glue line is not required, and the possibility of injury to the wood from high temperature is avoided.
Prefabricated Wood I Joists
Flanges of prefabricated wood I-shaped joists are either sawn lumber, visually graded or machine stress rated, or some type of structural composite lumber product, such as laminated veneer lumber (LVL) (Art. 10.30.4). The web members can be either plywood or oriented strand board (OSB) although OSB is most often used (Art. 10.12).
Numerous manufacturers produce wood I joists, and the flange and web materials used depend on manufacturer preference. Inasmuch as these joists are proprietary products, the manufacturer provides design information, usually in the form of load/span tables, as well as installation and handling guidelines.
Manufacturers establish design values for these products in accordance with the provisions of the Specification for Establishing and Monitoring Structural Capacities of Prefabricated Wood I-Joists, ASTM D5055. Each manufacturer uses the design values thus determined to obtain National Evaluation Service (NER) building- code approval from model code sponsors and to generate proprietary load/span tables. To establish consistent performance levels for I-joists, APAThe Engineered Wood Association promulgated APA Standard PRI-400 for I-joists used in residential floor construction. This standard has been recognized by all of the model code agencies.
Wood I joists are available with a wide range of depths and load/span capabilities.
Although joist sizes vary with manufacturer preference, most manufacturers produce joists with depths of 91â„2 in or 117â„8 in for direct substitution for 2 X 10 and 2 12 dimension lumber. Other depths 14 and 16 in and deeper also are available. The deeper products are typically used in longer-span applications, such as for light commercial construction.
Prefabricated wood I joists have many advantages that make them an economical construction material. Some of these advantages are:
Manufactured product. They are shipped to the job site precut to length, thereby eliminating waste. Consistent product quality is assured by the manufacturing process so that all material arriving at the job site is usable. Wood I joists are manufactured from dry components, thus eliminating shrinkage, warping, and twisting.
Long lengths available. I joists can be manufactured and shipped to the job site in long lengths. This minimizes labor costs due to handling and allows the joists to be used in multispan applications.
Lightweight. Their low weight makes it very easy for construction workers to easily handle long lengths on the job site, whether for long clear spans or multiple spans. A typical residential I-joist only weighs 2 pounds per foot.
Ease of fabrication. The structural panel webs are easily cut to permit passage of wiring, conduit, plumbing, and mechanical ductwork. Manufacturing provide charts that indicate the maximum permissible size of round or rectangular openings that can be cut in the web without adversely affecting structural performance of the joists. APA also publishes hole charts consistent with PRI 400.
Structural Composite Lumber
Structural composite lumber (SCL) comprises a family of secondary manufactured engineered glued wood products. The most widely available type of SCL, laminated veneer lumber (LVL), is similar to plywood in that thin sheets of veneer are structurally bonded together to create large panels, usually in widths of 2 or 4 ft. These panels are typically produced in thicknesses of 11â„2 or 13â„4 in, and in long lengths.
(Lengths vary, depending on the manufacturer). However, unlike plywood, which has the veneers cross-banded, all veneers in LVL products are oriented with grain approximately parallel, much like the positioning of sawn lumber in glulam timber.
Another type of SCL, parallel-strand lumber (PSL), is manufactured from long strands of veneer rather than veneer sheets as used in LVL manufacturing. PSL is manufactured in a variety of widths ranging from 13â„4 in up to 7 in. Various depths and lengths are also produced. Designers should consult the manufacturers for information on size available.
The large PSL sections can be resawn into a variety of smaller dimension lumber or timber products. An extensive use of this product is for beams and headers. Two pieces of 13â„4-in-wide LVL can be nailed together to create a 31â„2-in-wide beam for use in conventional 2 X 4 framing. Wider beams of LVL can be created by nailing three or more pieces together or by cutting larger sections from PSL billets. SCL is used for scaffold plank, truss chords, flanges for wood I joists, ridge beams in mobile homes, and a myriad of other building and industrial uses.
Structural design properties for SCL are generally much higher than comparable values for sawn lumber. SCL is available with allowable bending stresses up to 3000 psi with corresponding modulus of elasticity up to 2,100,000 psi. ASTM Standard D5456 sets forth the procedures for determining the design properties for SCL products and published values vary among the various manufacturers. APA has promulgated a performance standard for LVL, PRL 501, which establishes uniform design properties among manufacturers.
In addition to exhibiting higher strength characteristics than other wood products, design properties for SCL products have less variability. This is largely due to the control in manufacturing of natural strength-reducing characteristics of wood, such as slope of grain, knots, and density. Also, the random dispersal of these strength-reducing characteristics throughout the finished member tends to offset the individual effects of these defects on the overall strength of the end product, much like the use of varying grades of sawn lumber in the manufacture of glulam timber.
This combination of higher strengths and reduced variability makes SCL an economical wood structural material.
Environmental Considerations
With the increasing emphasis on more efficient uses of the available wood-fiber resource, engineered glued wood products are becoming more attractive. Each of the glued products described in the preceding articles makes optimum use of the base wood products in creating high-end, high-quality engineered products. Innovations in the engineered wood products industry are ongoing and it is these innovative engineered wood products that will allow wood to continue to be a viable construction material for building applications in the future.
