GLULAM DESIGN PROPERTIES AND LAYUP COMBINATIONS ENGINEERED WOOD SYSTEMS WOOD The Miracle Material™ Wood is the right choice for a host of construction applications. It is the earth’s natural, energy efficient and renewable building material. Engineered wood is a better use of wood. The miracle in today’s wood products is that they make more efficient use of the wood fiber resource to make stronger plywood, oriented strand board, I-joists, glued laminated timbers, and laminated veneer lumber. That’s good for the environment, and good for designers seeking strong, efficient, and striking building design. A few facts about wood. I We’re not running out of trees. One-third of the United States land base – 731 million acres – is covered by forests. About two-thirds of that 731 million acres is suitable for repeated planting and harvesting of timber. But only about half of the land suitable for growing timber is open to logging. Most of that harvestable acreage also is open to other uses, such as camping, hiking, and hunting. Forests fully cover one-half of Canada’s land mass. Of this forestland, nearly half is considered productive, or capable of producing timber on a sustained yield basis. Canada has the highest per capita accumulation of protected natural areas in the world – areas including national and provincial parks. I We’re growing more wood every day. American landowners plant more than two billion trees every year. In addition, millions of trees seed naturally. The forest products industry, which comprises about 15 percent of forestland ownership, is responsible for 41 percent of replanted forest acreage. That works out to more than one billion trees a year, or about three million trees planted every day. This high rate of replanting accounts for the fact that each year, 27 percent more timber is grown than is harvested. Canada’s replanting record shows a fourfold increase in the number of trees planted between 1975 and 1990. I Manufacturing wood is energy efficient. Percent of Percent of Wood products made up 47 percent of all Material Production Energy Use industrial raw materials manufactured in the Wood 47 4 United States, yet consumed only 4 percent of the energy needed to manufacture all Steel 23 48 industrial raw materials, according to Aluminum 2 8 a 1987 study. I Good news for a healthy planet. For every ton of wood grown, a young forest produces 1.07 tons of oxygen and absorbs 1.47 tons of carbon dioxide. NOTICE: Wood, the miracle material for the environment, The recommendations in this for design, and for strong, lasting construction. report apply only to glulam that bears the APA EWS trademark. Only glulam bearing the APA EWS trademark is subject EWS Y117 to the Association’s quality auditing program. B IND EWS 24F-1.8EANSI A190.1-2002 SP MILL 0000 GLULAM DESIGN As indicated in Table 1, bending mem- or continuous, multiple-span beams PROPERTIES AND bers can be further divided into balanced which may have either the top or bottom LAYUP COMBINATIONS and unbalanced layups as shown in of the member stressed in tension. Figure 1. Unbalanced beams are asym- In addition to stamping the beam with metrical in their layups with the highest the APA EWS trademark signifying that Introduction quality laminations, referred to as tension the member has been manufactured in Glued laminated timbers (glulam) are laminations, used only on the bottom of accordance with the provisions of ANSI manufactured by end joining individual the member. These are intended for use Standard A190.1 for Structural Glued pieces of dimension lumber or boards in simple-span applications or short, Laminated Timber, unbalanced beams also together with structural adhesives to cre- cantilevered conditions where only the have the word TOP prominently stamped ate long length laminations. These long bottom of the beam is subjected to maxi- on the top of the member as shown in length laminations are then face bonded mum tension stresses. Results of a large Figure 2. This is provided as guidance to together with adhesives to create the number of full-scale beam tests con- the contractor to ensure that the member desired glulam shape. Through the lami- ducted or sponsored by the glued lami- is installed with the proper orientation. If nating process, a variety of shapes can be nated timber industry over the past 30 members are inadvertently installed with created ranging from straight rectangular years have shown that the quality of the an improper orientation, i.e., “upside laminations used in the outer tension cross-sections to complex curved shapes down,” the allowable Fb value for the with varying cross-sections. Thus, glulam zone controls the overall bending compression zone stressed in tension is is one of the most versatile of the family of strength of the member. applicable. The controlling bending glued engineered wood products and is For a balanced beam, the grade of stress and the capacity of the beam in used in applications ranging from con- laminations used is symmetrical through- this orientation must be checked to cealed beams and headers in residential out the depth of the member. This type determine if they are adequate to meet construction to soaring domed stadiums. of member is typically used for cantilever the design conditions. Glulam Layup Principles Bending Members In addition to being able to produce FIGURE 1 virtually any size or shape of structural BALANCED VERSUS UNBALANCED LAYUP EXAMPLE member, the laminating process also permits the manufacturer to optimize the T.L . No. 2D use of the available wood fiber resource No. 1 No. 2 by selecting and positioning the lumber No. 2 No. 2 based on the stresses it will be subjected to in-service. For example, for members stressed primarily in bending, a graded No. 3 No. 3 layup of lumber is used throughout the depth of the beam with the highest quality laminations used in the outer zones of the beam where the bending No. 2 No. 2 stresses are highest. Lower quality lamina- No. 1 No. 1 tions are used in zones subjected to lower T.L . T.L . bending stresses. Layup combinations for Balanced Unbalanced T.L. = Tension Lamination members stressed primarily in bending are provided in Table 1. These members may range in cross-section from straight rectangular beams to pitched and tapered curved beams. 3 Axially Loaded Members For members stressed primarily in axial FIGURE 2 tension or axial compression, where the TOP IDENTIFICATION FOR AN UNBALANCED LAYUP stresses are uniform over the cross- section of the member, single-grade lamination layups, such as those given in Table 2 are recommended since there is no benefit to using a graded layup. Combined Stress Members If the member is to be subjected to high bending stresses as well as axial stresses, such as occur in arches or beam- columns, a bending member combina- tion as tabulated in Table 1 is typically the most efficient. Tapered beams or pitched and tapered curved beams are special configurations that are also specified using Table 1 bending member combinations. The use of kiln-dried laminating lumber is greatly minimized. Other strength Development of Allowable Stresses also means that the moisture content of a considerations accounted for in this glulam is relatively uniform throughout standard are those associated with using The laminating process used in the the member unlike green sawn timbers dry lumber and characteristics unique to manufacture of glulam results in a ran- which may have widely varying moisture the glued laminated timber manufactur- dom dispersal of naturally occurring contents within a given member. This use ing process such as being able to vary the lumber strength-reducing characteristics of uniformly dry lumber gives glulam grade of lumber throughout the depth of throughout the glulam member. This excellent dimensional stability. Thus, a the member. results in mechanical properties for glu- glulam member will not undergo the lam having higher values and lower vari- Many different species of lumber can be dimensional changes normally associated ability as compared to sawn lumber used to produce glued laminated timber. with larger solid-sawn green timbers, and products. For example, the coefficient of In addition, a wide range of grades of will remain straight and true in cross- variation for the modulus of elasticity (E) both visually graded and mechanically section. A “dry” glulam is also less suscep- of glulam is published as 10% which is graded lumber can be used in the manu- tible to the checking and splitting which is equal to or lower than any other wood facture of glulam. This wide variety of often associated with “green” timbers. product. available species and grades results in Allowable stresses for glulam are numerous options for the producers to Since glulam is manufactured with kiln- determined in accordance with the prin- combine species and grades to create a dried lumber having a maximum moisture ciples of ASTM D 3737, Standard Practice wide array of glulam layup combinations. content at the time of fabrication of 16%, for Establishing Stresses for Structural Glued this results in higher allowable design For some layup combinations, the use of Laminated Timber. A key strength consid- stresses as compared to dry (moisture different species within the same mem- eration accounted for in this standard is content of 19% or less) or green lumber. ber is permitted. This is done when it is the random dispersal of strength reduc- desirable to use a lower strength species ing characteristics previously discussed. in the core of a glued laminated timber By randomly distributing the strength- and a higher strength species in the reducing characteristics found in dimen- outer zones.