APA Engineered Wodd Handbooks Episode 4 ppt

TABLE 4.3 Design Values for Structural Glued-Laminated Timber Intended Primarily for Members Stressed in Bending Due to Loads Applied Perpendicular to Wide Faces of the Laminations for N

CHAPTER FOUR

STRUCTURAL GLUED

LAMINATED TIMBER (GLULAM)

Borjen Yeh, P.E., PhD

Manager, Research and Development, TSD

of strength, serviceability, and appearance Glued laminated timber beams are ufactured with the strongest laminations on the bottom and top of the beam, wheregreatest tension and compression stresses occur in bending This allows a moreefficient use of the lumber resource by placing higher grade lumber in zones thathave higher stresses and lumber with less structural quality in lower-stressed zones.Glued laminated timber is manufactured from several softwood species, primar-ily Douglas fir-larch, southern pine, hem-fir, spruce-pine-fir, eastern spruce, westernwoods, Alaska cedar, Durango pine, and California redwood Several hardwoodspecies, including red oak, red maple, and yellow poplar, are also used Standardglued laminated timber sizes are given in Section 4.1.2 Any length, subject to themaximum length permitted by transportation and handling restrictions, is available.Glued laminated timber is typically manufactured with kiln-dry lumber having

man-a mman-aximum moisture content man-at the time of fman-abricman-ation of 16% As man-a result, theallowable design stresses for glued laminated timber are higher than dry (moisturecontent of 19% or less) or green lumber The use of kiln-dry laminating lumberalso means that the moisture content of glued laminated timber is relatively uniformthroughout the member, unlike green sawn timbers, which may have widely varyingmoisture contents within a given member This use of uniformly dry lumber givesglued laminated timber excellent dimensional stability Thus, a glued laminatedtimber member will not undergo the dimensional changes normally associated withlarger solid-sawn green timbers, and will remain straight and true in cross-section

A dry glued laminated timber is also less susceptible to the checking and splittingthat is often associated with green timbers

4.2 CHAPTER FOUR

FIGURE 4.2 Disney ICE rink in Anaheim, California, features glulam arches curved to a 75-ft radius to form the ice center’s roof systems.

FIGURE 4.1 Glulam beam supports second floor I-joist construction.

Glued laminated timber is one of the most versatile of the family of gluedengineered wood products and is used in applications ranging from concealedbeams and headers in residential construction to structures with large open spaces(see Figs 4.1 and 4.2) Glued laminated timber has greater strength and stiffnessthan comparable dimensional lumber Pound for pound, it is stronger than steel.Because of their composition, large glued laminated timber members can be manu-factured from smaller trees harvested from second- and third-growth forests andplantations With glued laminated timber, the designer and builder can continue toenjoy the strength and versatility of large wood members without relying on theold growth-dependent solid-sawn timbers

STRUCTURAL GLUED LAMINATED TIMBER (GLULAM) 4.3

In terms of current needs to optimize products from a carefully managed timberresource, glued laminated timber is one of the most resource-efficient approaches

to wood building products It is an engineered product manufactured to meet themost demanding structural requirements But glued laminated timber is not a newproduct The first patents for glued laminated timber were issued in Switzerlandand Germany in the late 1890s A 1906 German patent signaled the true beginning

of glued laminated timber construction One of the first glued laminated timberstructures erected in the United States was a research laboratory at the USDA ForestProducts Laboratory in Madison, Wisconsin The structure was erected in 1934 and

is still in service today

A significant development in the glued laminated timber industry was the duction of fully water-resistant phenol-resorcinol adhesives in 1942 This allowedglued laminated timber to be used in exposed exterior environments without con-cern of glueline degradation The first U.S manufacturing standard for glued lam-inated timber was Commercial Standard CS253-63, which was published by theDepartment of Commerce in 1963 The most recent standard is ANSI / AITCA190.1-92,1which took effect in 1993

intro-4.1.2 Typical Sizes

Individual pieces of laminating lumber used in glued laminated timber turing are typically 13⁄8 in thick for southern pine and 11⁄2in thick for Westernspecies, although other thicknesses may also be used Glued laminated timber prod-ucts typically range in net widths from 21⁄2 to 103⁄4 in., although virtually anymember width can be custom produced

manufac-Glued laminated timber is available in both custom and stock sizes Stock beamsare manufactured in commonly used dimensions and cut to length when the beam

is ordered from a distributor or dealer Typical stock beam widths include 31⁄8, 31⁄2,

51⁄8, 51⁄2, and 63⁄4in., which meet the requirements for most residential constructionapplications Where long spans, unusually heavy loads, or other circumstances con-trol design, custom members are typically specified Custom members are available

in virtually any size and shape that may be required to meet the design conditions.Some of the common custom shapes that are available include curved beams,pitched and curved beams, radial arches and tudor arches (see Figs 4.3 and 4.4)

Glued laminated timber has a reputation for being used in striking applications such

as vaulted ceilings and other designs with soaring open spaces In churches, schools,restaurants, and other commercial buildings, glued laminated timber is often spec-ified for its beauty as well as its strength for good reason

Glued laminated timber has the classic natural wood appearance that holds atimeless appeal Aesthetics aside, there are many other applications where thestrength and durability of glued laminated timber beams make them the ideal struc-tural choice Typical uses range from simple purlins, ridge beams, floor beams, andcantilevered beams to complete commercial roof systems In some instances, ware-house and distribution centers with roof areas exceeding 1 million ft2have beenconstructed using glued laminated timber framing In large open spaces, glued lam-inated timber beams can span more than 100 ft

4.4 CHAPTER FOUR

FIGURE 4.3 Two-lane highway bridge in Colorado using glulam radial arches.

One of the greatest advantages of glued laminated timber is that it can be ufactured in a wide range of shapes, sizes, and configurations In addition to straightprismatic sections, beams can also be produced in a variety of tapered configura-tions, such as single-tapered, double-tapered, and off-centered ridges Curvedshapes range from a simple curved beam to a pitched and tapered curved beam to

man-a complex man-arch configurman-ation Spman-ans using glued lman-aminman-ated timber man-arches man-are tually unlimited For example, in reticulated glued laminated timber framed domestructures, arches can span more than 500 ft

vir-Glued laminated timber trusses also take many shapes including simple pitchedtrusses, complicated scissors configurations, and long-span bowstring trusses withcurved upper chords When designed as space frames, glued laminated timber trusssystems can create great clear spans for auditoriums, gymnasiums, and other ap-plications requiring large open floor areas When manufactured with waterproofphenol resorcinol adhesives, glued laminated timber products can be fully exposed

to the environment, provided they are properly pressure-preservative treated posed applications include utility poles and cross-arms, marinas, docks, and otherwaterfront structures and bridges

Ex-Bridges represent a growing market for glued laminated timber in pedestrianand light vehicular applications for stream and roadway crossings Glued laminatedtimber is also used in secondary highway bridge designs ranging from straightgirders to soaring arches And the railroads are finding glued laminated timber to

be a viable structural product for use in their heavily loaded bridge structures

In all of these uses, the strength and stiffness of glued laminated timber givebuilders and designers more design versatility than they have with other structuralproducts And, these advantages come at a cost that is competitive with other struc-tural systems Table 4.1 lists economical spans for selected timber framing systems

STRUCTURAL GLUED LAMINATED TIMBER (GLULAM) 4.5

FIGURE 4.4 This 236,000 ft 2 potash storage building in Portland, Oregon, features glulam arches.

using glued laminated timber members in buildings This table may be used forpreliminary design purposes to determine the economical span ranges for the se-lected framing systems However, all systems require a more extensive analysis forfinal design

4.1.4 Availability

Glued laminated timber members are available in both custom and stock sizes.Custom beams are manufactured to the specifications of a specific project, whilestock beams are made in common dimensions, shipped to distribution yards, andcut to length when the beam is ordered Stock beams are available in virtually everymajor metropolitan area Although glued laminated timber members can be customfabricated to provide a nearly infinite variety of forms and sizes, the best economy

is generally realized by using standard-size members When in doubt, the designer

is advised to check with the glued laminated timber suppliers or manufacturersconcerning the availability of a specific size glued laminated timber members prior

to specification The following trade associations are available for technical tance:

assis-4.6 CHAPTER FOUR

TABLE 4.1 Economical Spans for Glulam Framing Systems

Roof

Simple-span beams

Tapered, double tapered-pitched, or curved 25–105

Trusses (four- or more ply chords)

Trusses (two- or three-ply chords)

Floor

Headers

APA—The Engineered Wood Association and

Engineered Wood Systems (EWS), a related corporation of APA

7011 South 19th Street

Tacoma, WA 98466

Phone: (253) 565-6600

Fax: (253) 565-7265

American Institute of Timber Construction

7012 South Revere Parkway, Suite 140

Englewood, CO 80112

Phone: (303) 792-9559

Fax: (303) 792-0669

STRUCTURAL GLUED LAMINATED TIMBER (GLULAM) 4.7

TABLE 4.2 U.S and Canada Glued Laminated Timber Production

(Million board ft)

aNA, not available; data collection started in 1995.

Source: APA—The Engineered Wood Association (April 2000).

is beginning to make market inroads In addition, fiber-reinforced technology should

be more widely available in the near future to help glued laminated timber, andwood construction in general, penetrate the commercial building market All ofthese product innovations will be important to the future growth of this industry.Approximately one-half of the glued laminated timber produced in the UnitedStates goes to new residential and remodeling uses The next largest segment is thenonresidential market, as shown in Fig 4.6

ANSI / AITC A190.1, the American National Standard for Structural Glued nated Timber,1is a national consensus standard for glued laminated timber manu-facturing Detailed manufacturing requirements for glued laminated timber are doc-umented in ANSI / AITC A190.1 This standard is recognized in the U.S modelbuilding codes, and a construction specification for glued laminated timber shouldinclude reference to this standard

ASTM D 3737, Standard Practice for Establishing Stresses for Structural Glued Laminated Timber,2provides a consensus approach in deriving allowable propertiesfor glued laminated timber manufactured in accordance with ANSI A190.1 In theU.S glued laminated timber industry, two computer programs developed by themajor trade associations, APA—The Engineered Wood Association and AmericanInstitute for Timber Construction (AITC) are recognized by the model buildingcodes as an alternative to the procedures given in ASTM D3737 for establishing

FIGURE 4.5 Glued laminated timber production in North ica. (Forecast for 2001 and beyond by APA.)

Amer-Industrial/Other 2%

Residential/R&R 52%

International 8%

Nonresidential 38%

FIGURE 4.6 Glued laminated timber end use in the United States. (2000

production volume per APA.)

design properties for glued laminated timber These associations share the databaserequired for their computer programs on generic laminating lumber grades

4.4.1 Lay-up Principles

The laminating process used in glued laminated timber manufacturing results in arandom dispersion of strength-reducing growth characteristics, such as knots andslope of grain, of lumber throughout the glued laminated timber member Conse-quently, glued laminated timber has higher mechanical properties with a lowervariability than sawn lumber products of comparable sizes For example, the co-

efficient of variation for the modulus of elasticity (E ) of glued laminated timber is

published as 10%, which is equal to or lower than any other wood product

STRUCTURAL GLUED LAMINATED TIMBER (GLULAM) 4.9

Tension Lam UNBALANCED

FIGURE 4.7 Unbalanced and balanced lay-up combinations.

manufac-tured using a single grade or multiple grades of lumber, depending on the intendeduse A mixed-species glued laminated timber member is also possible When themember is intended to be primarily loaded either axially or in bending with theloads acting parallel to the wide faces of the laminations, a single-grade combi-nation is recommended On the other hand, a multiple-grade combination providesbetter cost-effectiveness when the member is primarily loaded in bending due toloads applied perpendicular to the wide faces of the laminations

On a multiple-grade combination, a glued laminated timber member can beproduced as either a balanced or unbalanced combination, depending on the geo-metrical arrangement of the laminations about the middepth of the member This

is further explained below

manufac-tured as unbalanced or balanced members, as shown in Fig 4.7 The most criticalzone of a glued laminated timber bending member with respect to controllingstrength is the outermost tension zone In unbalanced beams, the quality of lumberused on the tension side of the beam is higher than the lumber used on the corre-sponding compression side, allowing a more efficient use of the timber resource.Therefore, unbalanced beams have different bending stresses assigned to the com-pression and tension zones and must be installed accordingly To ensure properinstallation of unbalanced beams, the top of the beam is clearly stamped with theword ‘‘TOP’’ (see Fig 4.8)

While the unbalanced combination is primarily for use in simple-span tions, it could also be used for short-cantilever applications (cantilever less thanapproximately 20% of the back span) or for continuous-span applications when thedesign is controlled by shear or deflection If members are inadvertently installed

applica-in an improper orientation, i.e., upside down, the allowable bendapplica-ing stress for thecompression zone stressed in tension should be used In this case, the controllingbending stress and the capacity of the beam in this orientation shall be checked todetermine if they are still adequate to carry the design loads

Balanced members are symmetrical in lumber quality about the midheight anced beams are used in applications such as cantilevers or continuous spans, where

Bal-4.10 CHAPTER FOUR

FIGURE 4.8 Glued laminated timber with a ‘‘TOP’’ stamp.

either the top or bottom of the member may be stressed in tension due to serviceloads They can also be used in single-span applications, although an unbalancedbeam is more efficient for this use

factor in designing glued laminated timber Bending members are typically specified

on the basis of the maximum allowable bending stress of the member For example,

a 24F designation indicates a member with an allowable bending stress of 2400psi Similarly, a 20F designation refers to a member with an allowable bendingstress of 2000 psi These different stress levels are achieved by varying the per-centages and grade of higher-quality lumber in the beam lay-up Use of differentspecies may also result in different stress designations

To identify whether the lumber used in the beam is visually or mechanicallygraded, the stress combination also includes a second set of designations For ex-ample, for an unbalanced 24F lay-up using visually graded lumber, the lay-updesignation may be identified as a 24F-V4 The ‘‘V’’ indicates that the lay-up usesvisually graded lumber (‘‘E’’ is used for mechanically graded lumber, which issorted by the modulus of elasticity (MOE) of the laminating lumber.) The number

‘‘4’’ further identifies a specific combination of lumber used to which a full set ofdesign stresses such as horizontal shear, MOE, etc are assigned Figure 4.9 shows

a typical trademark for a glued laminated timber beam

are typically installed with the wide face of the laminations perpendicular to theapplied load, as shown in Fig 4.10 These are commonly referred to as horizontally

STRUCTURAL GLUED LAMINATED TIMBER (GLULAM) 4.11

B IND

EWS Y117 EWS 24F-1.8E WW

(1) Indicates structural use: B-Simple span bending member C-Compression member T-Tension member CB-Continuous or cantilevered span bending member.

(2) Mill number.

(3) Identification of ANSI Standard A190.1, Structural Glued Laminated Timber ANSI A190.1 is the American National Standard for glulam beams.

(4) Applicable laminating specification.

(5) Western woods (see note 6).

(6) Structural grade designation The APA EWS 24F-1.8E designation is a glulam grade commonly used in residential applications Combining a group of six layup combinations made with Douglas-fir- Larch, Spruce-Pine-Fir, southern pine, and/or Hem fir, this grade provides strength (allowable bending stress of 2,400 psi and allowable shear stress of 195 psi) and stiffness (modulus of elasticity of 1.8 x

10 6 psi) needed for typical residential applications, while greatly simplifying the design specification (7) Designation of appearance grade INDUSTRIAL, ARCHITECTURAL, PREMIUM, or FRAMING.

4.12 CHAPTER FOUR

laminated members If this same member is rotated 90⬚such that the load is appliedparallel to the wide face of the laminations, it is considered to be a verticallylaminated member Glued laminated timber members have different tabulated stressproperties, depending on whether the member is used in a horizontal or verticalorientation

Many different species of lumber can be used to produce glued laminated timber

In addition, a wide range of grades of both visually graded and mechanically gradedlumber can be used in the manufacture of glued laminated timber This wide variety

of available species and grades results in numerous options for the producers tocombine species and grades to create a wide array of glued laminated timber lay-

up combinations

For some lay-up combinations, the use of different species within the samemember is permitted This is done when it is desirable to use a lower-strengthspecies in the core of a glued laminated timber and a higher-strength species in theouter zones However, it should be cautioned that when mixed species are used,they may result in an appearance that may not be suitable for an exposed application

as the species will typically have different coloration and visual characteristics.Glued laminated timber lay-up combinations can be developed based on thedesired allowable properties and available lumber resources Once the lumber re-sources are identified, the allowable stresses for glued laminated timber are deter-mined in accordance with the principles of ASTM D3737 or using recognizedcomputer software

vir-tually any size or shape of structural member, the laminating process used in gluedlaminated timber manufacturing also permits the manufacturer to optimize the use

of the available wood fiber resource by selecting and positioning the lumber based

on the stresses it will be subjected to in-service For example, for members stressedprimarily in bending, a graded lay-up of lumber is used throughout the depth ofthe beam with the highest-quality laminations used in the outer zones of the beamwhere the bending stresses are highest with lower-quality laminations being used

in zones subjected to lower bending stresses Lay-up combinations for members

stressed primarily in bending about the X–X axis (see Fig 4.10) are provided in

Table 4.3 These members may range in cross-section from straight rectangularbeams to pitched and tapered curved beams

Although permitted, for members primarily stressed in axial loading or in

bend-ing about the Y–Y axis (also see Fig 4.10), the use of glued laminated timber

combinations given in Table 4.3 is not efficient In such cases, the designer shouldselect glued laminated timber combinations from Table 4.4 Similarly, glued lami-nated timber combinations in Table 4.3 are inefficiently utilized if the primary use

is not bending about the X–X axis.

It should be noted that Tables 4.3 and 4.4 tabulate the lay-up combinations based

on species, whether the combination is for a balanced or unbalanced layup andwhether the lumber used is visually or mechanically graded as signified by a V(visual) or E (E-rated or mechanically graded) The allowable properties given inTables 4.3 and 4.4 should be used in conjunction with the dimensions provided inSection 4.4.4

The values for allowable properties given in Tables 4.3 and 4.4 are based onuse under normal duration of load (10 years) and dry conditions (less than 16%

TABLE 4.3 Design Values for Structural Glued-Laminated Timber Intended Primarily for Members Stressed in Bending Due to Loads Applied Perpendicular to Wide Faces of the Laminations for Normal Duration of Load and Dry Conditions of Use 1,2,3

TABLE 4.3 Design Values for Structural Glued-Laminated Timber Intended Primarily for Members Stressed in Bending Due to Loads Applied Perpendicular to Wide Faces of the Laminations for Normal Duration of Load and Dry Conditions of Use 1,2,3(Continued )

TABLE 4.3 Design Values for Structural Glued-Laminated Timber Intended Primarily for Members Stressed in Bending Due to Loads Applied Perpendicular to Wide Faces of the Laminations for Normal Duration of Load and Dry Conditions of Use 1,2,3(Continued )

TABLE 4.3 Design Values for Structural Glued-Laminated Timber Intended Primarily for Members Stressed in Bending Due to Loads Applied Perpendicular to Wide Faces of the Laminations for Normal Duration of Load and Dry Conditions of Use 1,2,3(Continued )

TABLE 4.3 Design Values for Structural Glued-Laminated Timber Intended Primarily for Members Stressed in Bending Due to Loads Applied Perpendicular to Wide Faces of the Laminations for Normal Duration of Load and Dry Conditions of Use 1,2,3(Continued )

TABLE 4.4 Design Values for Structural Glued-Laminated Timber for Normal Duration of Load and Dry Conditions of Use 1,2,3

(Continued )

TABLE 4.4 Design Values for Structural Glued-Laminated Timber for Normal Duration of Load and Dry Conditions of Use 1,2,3

4.20 CHAPTER FOUR

moisture content) When used under other conditions, see Section 4.4.3 for ment factors It is important to note that the allowable bending stresses given inTable 4.3 are based on members loaded as simple beams When glued laminatedtimber is used in continuous or cantilevered beams, the allowable bending stressesgiven in column 4 of Table 4.3 should be used for the design of stress reversal(when compression zone is stressed in tension)

adjust-Tables 4.5 and 4.6 provide the grade requirements for the laminations used inmanufacturing the glued laminated timber listed in Tables 4.3 and 4.4, respectively

In addition to the lay-up combinations tabulated in Tables 4.3–4.6, the glulamindustry periodically evaluates the use of new layup combinations and stressesbased on the use of a computer program such as the one identified as GlulamAllowable Properties (GAP) The GAP program is based on the provisions ofASTM D3737 and has been verified by extensive laboratory testing of full-sizedglued laminated timber beams at the APA Research Center in Tacoma, Washington,

and at other laboratories throughout North America As these new special lay-ups

are evaluated and approved, they are added to the code evaluation reports as part

of the periodic reexamination process

bend-ing stresses as well as axial stresses such as occur in arches or beam-columns, abending member combination as tabulated in Table 4.3 is typically the most effi-cient Tapered beams or pitched and tapered curved beams are special configurationsthat are also specified using Table 4.3 bending member combinations

on the top of a beam are sometimes used to improve drainage, to provide extrahead for downspouts and scuppers, to facilitate discharge of water, and to reducethe height of the wall Table 4.7 provides allowable stresses and mean moduli ofelasticity for glued laminated timber with straight-tapered end cuts on the com-

pression face The allowable stresses are provided for bending, F b, and compression

perpendicular to grain, F c⬜, and replace the allowable values provided in Table 4.3when tapered end cut members are used

radial stresses are induced When the bending moment is in the direction that tends

to decrease the curvature or increase the radius, the radial stress is in radial tension,

Frt On the other hand, when the bending moment is in the direction that tends toincrease the curvature or decrease the radius, the radial stress is in radial compres-

sion, F rc Table 4.8 provides allowable radial tensile stresses for glued laminatedtimber These values are subject to adjustments for duration of load and wet con-ditions of use (16% moisture content or higher) If the adjusted value is exceeded,appropriate mechanical reinforcements shall be used to resist all applied radialtensile stresses The maximum moisture content of the laminations shall not exceed12% at the time of the reinforcement manufacturing

The allowable radial compressive stress has been traditionally limited to the

design value in compression perpendicular to grain, F c⬜, of the grade and speciesbeing used Also given in Table 4.8 are allowable radial compressive stresses forglued laminated timber These allowable radial compressive stresses are not subject

to the adjustments for the duration of load, but shall be adjusted for wet conditions

of use when appropriate

TABLE 4.5 Grade Requirements for Members Stressed Principally in Bending and Loaded Perpendicular to the Wide Faces of Laminations 1,2

TABLE 4.5 Grade Requirements for Members Stressed Principally in Bending and Loaded Perpendicular to the Wide Faces of Laminations 1,2(Continued )

TABLE 4.5 Grade Requirements for Members Stressed Principally in Bending and Loaded Perpendicular to the Wide Faces of Laminations 1,2(Continued )

TABLE 4.5 Grade Requirements for Members Stressed Principally in Bending and Loaded Perpendicular to the Wide Faces of Laminations 1,2(Continued )

TABLE 4.5 Grade Requirements for Members Stressed Principally in Bending and Loaded Perpendicular to the Wide Faces of Laminations 1,2(Continued )

TABLE 4.5 Grade Requirements for Members Stressed Principally in Bending and Loaded Perpendicular to the Wide Faces of Laminations 1,2(Continued )

TABLE 4.5 Grade Requirements for Members Stressed Principally in Bending and Loaded Perpendicular to the Wide Faces of Laminations 1,2(Continued )

TABLE 4.5 Grade Requirements for Members Stressed Principally in Bending and Loaded Perpendicular to the Wide Faces of Laminations 1,2(Continued )

4.29

TABLE 4.5 Grade Requirements for Members Stressed Principally in Bending and Loaded Perpendicular to the Wide Faces of Laminations 1,2(Continued )

TABLE 4.6 Grade Requirements for Members with Two or More Laminations Stressed Principally in Bending Parallel to the Wide Faces of the Laminations 1,2,3,4

aValue is applicable to members that have up to one-half the depth on the compression side removed

by taper cutting Value is for dry conditions of use and 12 in or less in depth.

bDesign value in compression perpendicular to grain for the core laminations of the combination.

TABLE 4.8 Allowable Radial Stressesa

STRUCTURAL GLUED LAMINATED TIMBER (GLULAM) 4.33

TABLE 4.9 Load-Duration Factor for Glued

Laminated Timber, C D

Load duration C D Typical design loads

Seven days 1.25 Construction load Ten minutes 1.6b Wind / earthquake load

aThe impact load duration factor shall not apply to glued laminated timber members treated with waterborne preservatives to a heavy retention required for marine ex- posure, nor to members pressured treated with fire retar- dant chemicals.

bCheck applicable building code for the appropriate load duration factor subject to earthquake loading.

The adjustment factors provided in this section are for nonreference end-use ditions and material modification effects These factors shall be used to modify theallowable properties when one or more of the specific end-use or material modifi-cation conditions fall outside the limits of the reference conditions given in thissection

apply to normal load duration, which means a structural member is subject to fulldesign loads for a cumulative duration of approximately 10 years For other cu-mulative duration of the full design loads, the allowable properties, except for mod-ulus of elasticity and compression perpendicular to grain, shall be adjusted by theload duration factor given in Table 4.9

Wet Service Factor, C M Design values provided in this section are applicable todry use conditions of glued laminated timber (moisture content in service is lessthan 16%, as in most covered structures) and its connections When glued laminatedtimber is exposed to wet service conditions, the adjustment factors given in Tables4.3 and 4.4 apply

Temperature Factor, C t The temperature factor, C t, shall be applied when gluedlaminated timber is exposed to a sustained elevated temperature ranging from 100–

150⬚F When the equilibrium moisture content of a glued laminated timber memberexceeds the reference condition limitation during sustained elevated temperatureexposure, both the temperature and wet service (moisture) factors shall be applied.When the equilibrium moisture content of glued laminated timber falls within thelimits of the reference conditions during sustained exposure to elevated tempera-tures, only the temperature factor shall be applied The temperature factors are given

in Table 4.10

structural wood members should be kept below 20% If this is not feasible, thenpreservative treatment may be required unless the heartwood of a naturally decay-

aIn-service conditions are defined in Section 4.4.3.

TABLE 4.11 Preservative Treatment Effect on Glued Laminated Timber

No adjustment is required when glued laminated timber is preservative-treated using the following American Wood Preservers’ Association Standards

C1-88 All Timber Products—Preservative Treatment by Pressure Processes

C14-89 Wood for Highway Construction—Preservative Treatment by Pressure

Processes

C15-88 Wood for Commercial—Residential Construction—Preservative Treatment by

Pressure Processes

C28-93 Standard for Preservative Treatment of Structural Glued Laminated Members

and Laminations before Gluing of Southern Pine, Pacific Coast Douglas-fir, Hem Fir, and Western Hemlock by Pressure Processes

resistant species such as redwood, Port Orford cedar, or Alaska yellow cedar isused

Most preservative chemicals used today do not significantly alter the strengthproperties of structural wood products However, the method of pre- and postcon-ditioning, as well as the treatment method itself, may weaken the wood For pre-servative treatment methods with AWPA accepted manufacturing control, as listed

in Table 4.11, the effect on strength degradation of glued laminated timber is ligible For more information concerning preservative treatment of glued laminatedtimber, refer to Chapter 9 of this handbook and to APA EWS Technical Note S580,

neg-Preservative Treatment of Glulam Beams.3

design stresses shall be considered for wood products treated with impregnated fire retardants The glued laminated timber industry does not recom-mend the use of fire-retardant treatments with glued laminated timber Specificadjustment factors for fire retardants used in conjunction with glued laminated tim-ber shall be obtained from the company providing the treatment services, and theglued laminated timber manufacturer accepts no responsibility for any structuralglued laminated timber that is fire retardant treated

pressure-Beam Stability Factor, C L Allowable bending stresses of glued laminated timber

shall be adjusted by the beam stability factor, C L, whenever applicable Refer to the

National Design Specification for Wood Construction6(NDS), published by ican Forest and Paper Association (AF&PA), for the determination of an appropriate

Amer-STRUCTURAL GLUED LAMINATED TIMBER (GLULAM) 4.35

TABLE 4.12 Exponents for Volume Factor Equation

Exponent symbol

Exponent Western species Southern pine Hardwoods

beam stability factor It is important to note that C Lis not accumulative with the

volume factor, C v, given below under Volume Factor, for the design of glued inated timber

lam-Column Stability Factor, C P Allowable values for compression parallel to grain

of glued laminated timber are affected by the dimensions and modulus of elasticity.Refer to the NDS or Section 4.7.2 for the determination of an appropriate columnstability factor

affected by geometry and size Generally, larger sizes have a correspondingly lowerallowable bending stress than smaller members To account for this behavior, a

volume factor, C v, which is the product of beam width, depth, and length shall be

applied C vshall not exceed 1.0 and is computed as follows:

where b⫽width of bending member being checked (in.) For multiple-piece width

lay-ups, b⫽width of widest piece in the lay-up For practical purposes,

b is assumed to be ⱕ10.75 in

d⫽depth of bending member being checked (in.)

ᐉ ⫽length of bending member being checked between points of zero ment (ft)

mo-p⫽as defined in Table 4.12

Table 4.12 provides the exponent values for use with the volume effect factor.Separate exponent values are given for western species, hardwoods, and southernpine No volume adjustment is required for properties other than allowable bendingstresses

Curvature Factor, C c The curvature factor, C c, is used to adjust the allowablebending stresses of curved glued laminated timber members only It takes intoaccount the difference in extreme outer fiber stress between a curved member and

a straight prismatic member, as well as any residual stresses that may remain in alamination that has been bent to the stated curvature However, the curvature factor,

Cc, shall not be applied to the allowable bending stress in the straight portion of amember, regardless of curvature in other portions Also, this factor is not applicable

to cambered glued laminated timber members or in the design of pitched and

ta-pered curved glued laminated timber members The curvature factor, C c, shall becalculated in accordance with the following equation:

2

t

R

4.36 CHAPTER FOUR

TABLE 4.13 Flat Use Factor,a C ƒu

Member dimensions parallel to wide faces of laminations C ƒu

a Values for Cƒu are rounded values from the equation (12 / d )1 / 9where d

is the dimension of the wide faces of the laminations in inches.

where t⫽thickness of lamination (in.)

R⫽radius of curvature of inside face of lamination (in.)

t / Rⱕ1⁄100for hardwoods and southern pine

t / Rⱕ1⁄125for other species

Flat Use Factor, C ƒu Allowable bending stresses of glued laminated timber shall

be adjusted by the flat use factor, C ƒu, when loaded in bending parallel to wide

faces of the laminations (the y–y axis) The allowable bending stresses for loads applied parallel to the wide faces of the laminations, F by, as given in Tables 4.3and 4.4 of this chapter, are based on members with laminations 12 in wide For

members with laminations less than 12 in wide, the tabulated F byvalues shall be

adjusted by a flat use factor, C ƒu, as listed in Table 4.13 When the width of thelaminations is greater than 12 in., as may occur in members with multiple-piece

laminations, C ƒu shall be obtained by use of the equation given in footnote (a) to

Table 4.13

Net section properties for both western species and southern pine glued laminated

timber are given in Section 4.7.1 Note that the plane of the glueline is in the X–

X direction Further, the width of glued laminated timber is in the X–X direction and its depth is in the Y–Y direction The thickness of each lamination for western

species and southern pine glued laminated timber members is based on 11⁄2 and

13⁄8in., respectively, which are typical for these species However, other laminationthicknesses may be used in glued laminated timber manufacturing, and the avail-ability should be verified prior to design

4.5 PHYSICAL PROPERTIES

This section contains information concerning physical properties of glued laminatedtimber members

STRUCTURAL GLUED LAMINATED TIMBER (GLULAM) 4.37

TABLE 4.14 Average Specific Gravity and Weight Factor

Species combination

Specific gravitya

aSpecific gravity is based on weight and volume when oven-dry.

bWeight factor shall be multiplied by net cross-sectional area in in 2 to obtain weight in pounds per lineal foot.

4.5.2 Specific Gravity

Table 4.14 provides specific gravity values for some of the most common woodspecies used to manufacture glued laminated timber These values are used in de-termining various physical and connection properties Further, weight factors areprovided at four moisture contents When the net cross-sectional area (in.2) is mul-tiplied by the appropriate weight factor, it provides the weight of the glued lami-nated timber member per linear foot of length For other moisture contents, thetabulated weight factors can be interpolated or extrapolated

Glued laminated timber members may be manufactured using different species

in different portions of the cross section In this case the weight of the gluedlaminated timber may be computed by the sum of the products of the cross-sectionalarea and the weight factor for each species

Due to the hygroscopic nature of wood, it changes dimensions as its moisturecontent is altered below the fiber saturation point For most species the longitudinalshrinkage of normal wood drying from fiber saturation point to oven-dry condition

is approximately 0.1–2% However, certain atypical types of wood may exhibitexcessive longitudinal shrinkage, and these types should be avoided in use wherelongitudinal stability is important

The change in radial (R), tangential (T ), and volumetric (V ) dimensions is

com-puted as:

where X0⫽initial dimension or volume

X⫽new dimension or volume

eME ⫽coefficient of moisture expansion (in / in / %MC for linear expansion,

in.3/ in.3/ %MC for volumetric expansion), as given in Table 4.15, and

Tangential (in / in / %)

Volumetric (in 3 / in 3 / %) FSP (%)

where M0⫽initial moisture content % (M0ⱕFSP)

M⫽new moisture content % (MⱕFSP)

FSP⫽fiber saturation point, also given in Table 4.15 for selected speciesFor more information concerning the effects of moisture changes on glued lam-

inated timber, refer to APA EWS Technical Note Y260, Dimensional Changes in Structural Glued laminated Timber.4

The thermal expansion of solid wood, including glued laminated timber, is puted by the relationship:

where X0⫽reference dimension at T0

X⫽computed dimension at T

eTE⫽coefficient of thermal expansion (in / in /⬚F), see Table 4.16

⌬T⫽temperature change (⬚F), defined as follows:

where T0⫽reference temperature (⬚F),⫺60⬚FⱕT0ⱕ130⬚F

T⫽new temperature (⬚F), ⫺60⬚FⱕT0ⱕ 130⬚F

The coefficient of thermal expansion of oven-dry wood parallel to grain appears

to be independent of specific gravity and species In tests of both hardwoods andsoftwoods, the parallel-to-grain values have ranged from about 1.7⫻ 10⫺6to 2.5

⫻10⫺6per⬚F

The linear expansion coefficients across the grain (radial and tangential) areproportional to the density of wood These coefficients are about 5–10 times greater

STRUCTURAL GLUED LAMINATED TIMBER (GLULAM) 4.39 TABLE 4.16 Coefficient of Thermal Expansion, eTE, for Solid Woods

aAlso applies when species name includes the designation ‘‘North.’’

than the parallel-to-grain coefficients The radial and tangential thermal expansioncoefficients for oven-dry wood in the oven-dry specific gravity (SG) range of about0.1–0.8 can be approximated by the following equations

where SG⫽specific gravity, provided in Table 4.14

Table 4.16 provides the numerical values for eTEfor the most commonly usedcommercial species or species groups

Wood that contains moisture reacts to varying temperature differently than doesdry wood When moist wood is heated, it tends to expand because of normal ther-mal expansion and to shrink because of loss in moisture content Unless the wood

is very dry initially (perhaps 3 or 4% MC or less), the shrinkage due to moistureloss on heating will be greater than the thermal expansion As a result, the netdimensional change on heating will be negative Wood at intermediate moisturelevels (about 8–20%) will expand when first heated, then gradually shrink to avolume smaller than the initial volume, as the wood gradually loses water while inthe heated condition

Even in the longitudinal (grain) direction, where dimensional change due tomoisture change is very small, such changes will still predominate over correspond-ing dimensional changes due to thermal expansion unless the wood is very dryinitially For wood at usual moisture levels, net dimensional changes will generally

be negative after prolonged heating

Computation of actual changes in dimensions can be accomplished by mining the equilibrium moisture content of wood at the temperature value andrelative humidity of interest Then the relative dimensional changes due to temper-ature change alone and moisture content change alone are computed By combiningthese two changes the final dimension of lumber and timber can be established

Industry recommendations for the finished appearance of glued laminated timberhave traditionally identified three grades: premium, architectural, and industrial All

of these appearance grades permit the individual laminations used in the gluedlaminated timber layup to possess the natural growth characteristics of the lami-nating lumber grades specified In order to account for the visual impact of thesenatural growth characteristics, cosmetic repairs may be made to the finished member

as required to meet the desired finished appearance

However, the use of the term appearance grades associated with these

nonstruc-tural cosmetic classifications often leads to confusion in the marketplace Designersand users often think of appearance grades in terms of structural distinctions, when

in fact structural properties for a given glued laminated timber layup are dent of appearance considerations As an example, a premium appearance classi-fication may be specified based on the presumption that the member will have betterstructural characteristics than an architectural or industrial classification when, infact, all three classifications will have identical strength characteristics if manufac-tured to the same layup combination To minimize this confusion, these differentappearance characteristics are more appropriate to be identified as appearance clas-

indepen-sifications than grades APA EWS Technical Note Y110, Glued Laminated Timber Appearance Classifications for Construction Applications,5provides detailed infor-mation on these appearance classifications

resilience under load Wood’s unique stiffness makes wood floors comfortable towalk, work, and stand on These qualities also contribute greatly to wood’s excellentdamping characteristics, thus giving wood-framed structures the ability to absorbimpact loadings

The size and span capabilities of sawn lumber beams are generally limited bythe physical characteristics of the timber supply Consequently, stiffness, or deflec-tion under load, at the relatively short spans commonly used with sawn lumberbeams is seldom a governing factor in the design of these wood structural elements.With the availability of glued laminated timber (glued laminated timber), it is pos-sible for architects and engineers to design wood members in large sizes and longlengths

Design professionals recognize that for long spans, design is often controlled bydeflection limits rather than by beam strength One way to reduce the adverse