2001 ASD Supplements - unprotected PDF - American Wood Council

GL-4INTRODUCTION TO STRUCTURAL GLUED LAMINATED TIMBER2.1 Products DescriptionStructural glued laminated timber (glulam) is a structuralmember glued up from suitably selected and preparedpieces of wood either in a straight or curved form with thegrain of all pieces parallel to the longitudinal axis of themember. The allowable stresses and capacities given inthis Supplement are applicable only to glued laminatedtimber members produced in accordance with AmericanNational Standard for Wood Products — Structural GluedLaminated Timber, ANSI/AITC A190.1.Glued laminated timber members are produced inlaminating plants by gluing together dry lumber, normallyof 2-inch or 1-inch nominal thickness, under controlledconditions of temperature and pressure. Members with awide variety of sizes, profiles, and lengths can be producedhaving superior characteristics of strength,serviceability, and appearance. Glued laminated timberbeams are manufactured with the strongest laminationson the bottom and top of the beam, where greatest tensionand compression stresses occur in bending. This allows amore efficient use of the lumber resource by placing highergrade lumber in zones that have higher stresses and lumberwith less structural quality in lower stressed zones.Glued laminated timber members are manufacturedfrom several softwood species, primarily Douglas Fir-Larch and Southern Pine, Hem-Fir, Spruce-Pine-Fir,Eastern Spruce, Western Woods, Alaska Cedar, DurangoPine, and California Redwood. In addition, several hardwoodspecies, including Red Oak, Red Maple, and YellowPoplar, are also used. Standard glued laminated timbersizes are given in Section 9 of this Supplement. Any length,up to the maximum length permitted by transportation andhandling restrictions, is available.A glued laminated timber member can be manufacturedusing a single grade or multiple grades of lumber,depending on the intended use. In addition, a mixed-speciesglued laminated timber member is also possible. Whenthe member is intended to be primarily loaded either axiallyor in bending with the loads acting parallel to thewide faces of the laminations, a single grade combinationis recommended. On the other hand, a multiple grade combinationprovides better cost-effectiveness when themember is primarily loaded in bending due to loads appliedperpendicular to the wide faces of the laminations.On a multiple grade combination, a glued laminatedtimber member can be produced as either a balanced orunbalanced combination, depending on the geometricalarrangement of the laminations about the mid-depth ofthe member. As shown in Figure 2.1, a balanced combinationis symmetrical about the mid-depth, so both faceshave the same allowable bending stress. Unbalanced combinationsare asymmetrical and when used as a beam, theface with a lower allowable bending stress is stamped asTOP. The balanced combination is intended for use incontinuous or cantilevered over supports to provide equalcapacity in both positive and negative bending. Whereasthe unbalanced combination is primarily for use in simplespan applications, they can also be used for short cantileverapplications (cantilever less than 20% of the back span)or for continuous span applications when the design iscontrolled by shear or deflection.Figure 2.1No. 2DNo. 2No. 2No. 3No. 3No. 3No. 2No. 1Tension LamUnbalancedUnbalanced and BalancedLayup CombinationsTension LamNo. 1No. 2No. 3No. 3No. 3No. 2No. 1Tension LamBalanced2.2 Common UsesGlued laminated timber members can be used as primaryor secondary load-carrying components in structures.Table 2.1 lists economical spans for selected timber framingsystems using glued laminated timber members inbuildings. Other common uses of glued laminated timbermembers are for utility structures, pedestrian bridges, highwaybridges, railroad bridges, marine structures, noisebarriers, and towers. Table 2.1 may be used for preliminarydesign purposes to determine the economical spanranges for the selected framing systems. However, all systemsrequire a more extensive analysis for final design.APA – The Engineered Wood Association