|ASTM Standards for Engineered Wood Products
Focus on Glulam, I-Joists, and Structural Composite Lumber
by Borjen Yeh, Ph.D.
In the forest products industry, there are diverse definitions for what comprisesengineered wood products. However, structural glued laminated timber (glulam), prefabricated wood I-joists, and structural composite lumber (SCL) products, as shown in Figure 1, are unequivocally part of the engineered wood products family. These products have been successfully used in a variety of applications worldwide. ASTM standards developed under the jurisdiction of Subcommittee D07.02 on Lumber and Engineered Wood Products have contributed significantly to the growth of these industries, as shown in Figure 2, through product acceptance by the model building codes, regulatory agencies, and design professionals.
Structural Glued Laminated Timber (Glulam)
Glulam is a structural composite product glued up from selected grades of kiln-dry laminating lumber, or laminations, with the grain of all pieces parallel to the longitudinal axis of the member. In manufacturing, each short-length lumber is end-jointed by end (finger or scarf) joints to form a long-length lamination. These laminations are then face-bonded with a wet-use adhesive meeting the requirements of ASTM D 2559, Standard Specification for Adhesives for Structural Laminated Wood Products for Use Under Exterior (Wet Use) Exposure Conditions, to make a deeper member.
Glulam beams are produced with the strongest laminations on the bottom and top of the beam where the greatest strength is required. Conversely, laminations with less structural quality are placed in the core zone where the stresses are low. This provides an efficient use of the lumber resource and large glulam members can be manufactured from smaller trees harvested from second- and third-growth forests and plantations. Because of their composition, glulam products with a wide range of sizes, profiles, and lengths can be produced in manufacturing plants for intended strength and appearance.
The first patents for glulam were issued in Switzerland and Germany in the late 1890s. However, the true beginning of glulam construction did not occur until 1906 in Europe. One of the first glulam structures built in the United States was a research laboratory at the USDA Forest Products Laboratory in Madison, Wis., where ASTM Committee D07 on Wood usually meets. This building was erected in 1934 and is still in service today.
The first consensus-based manufacturing standard for glulam in the U.S., Commercial Standard CS253-63, was published by the U.S. Department of Commerce in 1963. Through the years, this glulam manufacturing standard has evolved into the current American National Standard for Wood Products - Structural Glued Laminated Timber, ANSI A190.1-2002, published by the American National Standards Institute. This American consensus standard provides details of manufacturing requirements for glulam and is recognized by the model building codes in the United States.
Acknowledging the need for a consensus approach in deriving design properties for glulam manufactured in accordance with the consensus manufacturing standard, the glulam industry, USDA Forest Products Laboratory, and other interested parties worked together to develop and publish ASTM D 3737, Standard Practice for Establishing Stresses for Structural Glued Laminated Timber, in 1978. For glulam products manufactured in accordance with ANSI A190.1, this ASTM standard gives a consistent and efficient approach in establishing the glulam design properties, and is also recognized by the model building codes in the United States.
While having been in use for 25 years, ASTM D 3737 has been in a state of constant revision to reflect the state-of-the-art in glulam. A typical example was the adoption of E-rated lamination requirements into ASTM D 3737, which enhances the structural reliability of glulam. In the last few years, fiber-reinforced technology has become more widely available. As a result, the ASTM D07.02.02 Section on Glulam is developing a standard for fiber-reinforced glulam. This ASTM standard, when published, will be vital to the success of this innovative product and the future growth of the glulam industry.
Prefabricated Wood I-Joists
Prefabricated wood I-joists are I-shaped structural joists made of top and bottom flanges, with a web in between. The flanges are typically made with selected grades of end-jointed solid-sawn lumber or full-length SCL and can be of any practical size with the predominant net sizes ranging from 1 5/16 by 1 1/2 in. (33 by 38 mm) to 1 1/2 by 3 1/2 in. (38 by 89 mm). The web is typically manufactured with structural-use wood panels, such as plywood and oriented strand board (OSB), and generally 3/8 to 7/16 in. (9.5 to 11 mm) in thickness. Wood I-joists are primarily used as roof and floor joists that are covered by sheathing (plywood or OSB) materials. For residential construction, the typical I-joist depth is in the range of 9 1/2 to 16 in. (241 to 406 mm). Larger depths are available for commercial construction with larger flanges and a thicker web.
The manufacturing processes for wood I-joists are proprietary and, unlike glulam, there is no consensus manufacturing standard in existence. However, in a typical I-joist manufacturing sequence, a wedge-shaped groove is first machined into the flange material. The web materials are then fitted into the groove in the top and bottom flanges and jointed along the I-joist length with other web materials under pressure. The geometries of the web-to-flange and web-to-web joints are proprietary to each manufacturer with the intent to provide adequate clamping pressure for the adhesive to cure. The adhesives used are wet-use adhesives meeting the requirements of ASTM D 2559.
Prefabricated wood I-joists were initially commercialized in the 1960s and have been widely accepted by the construction industry due in part to their engineered efficiency (I-shape) and consistent product quality. The first consensus standard for wood I-joists, ASTM D 5055, Specification for Establishing and Monitoring Structural Capacities of Prefabricated Wood I-Joists, was published in 1990 after years of development efforts by Subcommittee D07.02. This consensus standard provides detailed requirements for the evaluation of mechanical and physical properties of wood I-joists, and has been recognized by the model building codes in the United States and Canada. The code evaluation agencies have also adopted it as the minimum requirement for the code evaluation report. This is significant since ASTM D 5055 does not specify required levels of performance and individual wood I-joist manufacturers generally have their proprietary products recognized through a code evaluation report as alternative building materials meeting the code requirements. As a result of the adoption by the code evaluation agencies, ASTM D 5055 has become the uniform standard that is enforceable for the design property evaluation of all proprietary wood I-joists for use in North America.
Since its initial publication, ASTM D 5055 has been revised numerous times. A recent change in the analytical moment calculation procedures reflects the dynamics of the industry and the extensive knowledge and experience gained through the production and end-use applications in the last 40 years. It is anticipated that the revisions of ASTM D 5055 will remain active for years to come.
Structural Composite Lumber
Structural composite lumber is a generic term used to describe a family of wood composite products, including laminated veneer lumber (LVL) and parallel strand lumber (PSL). As more new wood composite products are entering the market, the Subcommittee D07.02 is expanding the definition to include laminated strand lumber (LSL) and oriented strand lumber (OSL). The difference between these products of SCL lies mostly in the type and size of the substrates, and the manufacturing processes used to manufacture the finished products. For example, LVL is made of thin layers of veneers, while PSL is made of wood strand elements, bonded with wet-use adhesives meeting the requirements of ASTM D 2559. All SCL products are manufactured with wood fibers oriented primarily along the length of the structural member. Due to its composition, SCL is typically used as headers and beams, truss chords, ridge beams in mobile homes, I-joist flanges, and scaffold planks. It is also used as columns, studs, and even as structural members in upholstered furniture frames.
The manufacturing processes for SCL are proprietary and vary widely among product types (LVL, PSL, LSL, and OSL) and from manufacturer to manufacturer. Just like wood I-joists, there is no consensus manufacturing standard in existence for SCL. The typical thickness of SCL, except for PSL, is in the range of 3/4 to 3 1/2 in. (19 to 89 mm) with 1 3/4 in. (44 mm) as the predominant thickness for beam and header application in residential construction. The depth of SCL varies, depending on the end-use applications.
LVL was first produced in the early 1970s, followed by PSL in the 1980s and LSL in the 1990s. OSL is a relatively new product just made available in the last couple of years. As a result of the reduced lumber supplies in national forests in the late 1960s, all of these products were developed in response to the need for efficient use of the forest resource and consistent product quality. As more and more SCL products were entering the market, the need for a consensus standard in evaluating the mechanical and physical properties of SCL became apparent.
After years of development efforts by the Subcommittee D07.02, the first ASTM SCL standard, ASTM D 5456, Specification for Evaluation of Structural Composite Lumber Products, was published in 1993. Since its publication, ASTM D 5456 has been recognized by the model building codes in the United States and Canada, and also been adopted as the minimum requirement for the code evaluation report. As they do for wood I-joists, individual SCL manufacturers generally rely on the code evaluation report for recognition of their products as alternative building materials meeting the code requirements. The adoption of ASTM D 5456 by the code evaluation agencies makes it the uniform standard that is enforceable for the design property evaluation of all proprietary SCL products for use in the United States and Canada.
In the last 10 years, ASTM D 5456 has evolved through numerous revisions. The most recent changes include the adoption of ASTM D 6815, Specification for Evaluation of Duration of Load and Creep Effect of Wood and Wood-Based Products, as a requirement for evaluating the long-term performance of SCL, and the development of full-scale shear test methods as an alternative to the traditional small-scale block shear tests. As more innovative SCL products within the scope of ASTM D 5456 are introduced into the construction market for the years to come, this standard will continue to grow and provide the needed guidance for the evaluation of SCL products.
Structural glued laminated timber, prefabricated wood I-joists, and structural composite lumber products are part of the engineered wood products family. ASTM standards developed under the jurisdiction of the Subcommittee D07.02 have contributed significantly to the success of the affected industries. As the demand for the efficient use of forest resource and consistent product quality increases, engineered wood products are expected to play an even more significant role in the North American economy. The ASTM standards developed for these engineered wood products will continue to be updated and to provide the needed provisions for the product evaluation on a consensus basis. //
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