APA Engineered Wodd Handbooks Episode 3 pot

In wood structural panel box beams, the lumber flanges carrymost of the bending, and one or more panel webs carry the shear.. The equation is: AEC G 12⫻Elumber for tension and compressio

CHAPTER THREE WOOD STRUCTURAL PANELS

Components made of wood structural panels and lumber are often major tural members, which depend on the glued or mechanically fastened joints to com-bine the separate pieces into an efficient structural unit capable of carrying thedesign loads Materials in these components may be stressed to an appreciablyhigher level than in nonengineered construction

struc-Since improperly designed or fabricated components could constitute a hazard

to life safety and property damage, it is strongly recommended that they be designed

by qualified design professionals, using recognized design and fabrication methods,and that adequate quality control be maintained during manufacture

To ensure that such quality control has been carefully maintained, it is mended that the services of an independent third-party testing agency be employed

recom-A requirement that each unit bear the trademark of an approved agency will ensureadequate independent inspection

Working design capacities for wood structural panels are given in Chapter 2.References are also made to the American Forest and Paper Association publication

National Design Specification for Wood Construction (NDS)1for other wood ucts

prod-Presentation of these specific design methods is not intended to preclude furtherinnovation Therefore, where adequate test data are available, the design provisions

*Caution on the use of equations: Metric equivalents are frequently given in this chapter Many of the

equations contain constants or variables that are intended to permit the use of mixed units and may make these equations incapable of being used by directly substituting equivalent metric units.

may be appropriately modified If they are modified, any such change should benoted when cross-referencing the design procedure to those presented in this Hand-book.

Quality of workmanship and the conditions under which wood structural panelsare used vary widely Because the authors and publisher of this Handbook have nocontrol over those elements, they cannot accept responsibility for wood structuralpanel or lumber performance or designs as actually constructed

3.1.1 Growth of Industry and History

There is little if any actual box beam industry in the United States These ponents are often built on-site, for residential construction, using nails Nailed boxbeams are most-often used by do-it-yourselfers since the time spent in fabricationhas less value to the builder than the cost of buying a ready-made alternative, such

com-as a glulam or LVL beam

Factory-built beams may be fastened together mechanically with nails or staples

or with a structural adhesive such as resorcinol Best estimates put the size of theindustry at about 2,000,000 ft2(3⁄8in [9.5 mm] basis) Because of the availability

of alternative engineered wood products, the use of wood structural panels in boxbeams is not expected to grow

Of the different fabricated-component industries utilizing wood structural panels,the structural insulated panel (SIP) industry is the major user Here the usage is inthe neighborhood of 100,000,000 ft2(3⁄8in [9.5 mm] basis) and is growing steadily

as designers and users recognize the benefits of using SIPs SIPs are used in bothresidential and nonresidential construction They are built to specification in a fac-tory for rapid installation at the job site

Stressed-skin panels are assemblies that have wood structural panel faces andbacks with framing lumber or ribs in between No statistics on utilization of woodstructural panels for use in stressed-skin panels are available Their use has declined

in recent years, but they are still occasionally used in floors and roofs in the ufactured housing industry

BEAMS

3.2.1 General

This design method applies only to box beams with joints glued with structuraladhesive Design of mechanically fastened box beams is covered in Section 3.3.The primary difference in analysis between the two methods of fastening box beamcomponents together is in the analysis of rolling shear stresses With glued beamcomponents, rolling shear must be considered in the design With mechanicallyfastened box-beam components, planar (rolling) shear is seldom a consideration

Beam Behavior. In wood structural panel box beams, the lumber flanges carrymost of the bending, and one or more panel webs carry the shear Joints betweenthem transfer stresses between components

Vertical stiffeners set between flanges distribute concentrated loads and resistweb buckling Deflection resulting from shear is usually significant and must be

added to the bending deflection Lateral restraint is often required to maintain bility End joints in flange laminations and webs may require splicing.

sta-Shape. Loads, spans, and allowable stresses, as well as desired appearance, termine the beam proportions The depth and cross section may be varied alongthe length of the beam to fit design requirements, provided the resisting momentsand shears at all sections are adequate Typical cross sections are shown inFig 3.1

de-3.2.2 Design Considerations

Design Loads. Live loads are typically those that are caused by objects movedinto the structure after it is completed, including the occupants and their equipmentand possessions Snow, wind, and earthquake are special cases of live loads Thedesign live loads should not be less than required by the governing building code.Dead loads are those that will remain in place for 10 years or more Dead load isthe actual weight of the members and the permanent elements it supports Allow-ance should be made for any temporary erection (construction) loads, or movingconcentrated loads, such as cranes

Allowable Working Capacities. Working capacities are determined as described

in Chapter 2, with due regard for duration of loading For symmetrical sections,the design should be based on the allowable stress in axial tension or compression,whichever is less When butt joints occur in the tension flange, the design should

be based on 0.8 of the allowable tensile stress

Values for compression and tension parallel with lumber grain depend on cies, grade, number of laminations, slope of scarf joints, and moisture condition.Values are applied as outlined in Chapter 2

spe-Allowable stresses for stress-grade lumber flanges shall not exceed those given

in the latest edition of the NDS.1Allowable stress level at any point in the flangesmust be determined based on the number of laminations continuous at that point.Any lamination with a butt joint within 10 times the lamination thickness of thepoint under investigation is considered discontinuous

Allowable Deflection. Deflection should not exceed that allowed by the applicablebuilding code Maximum deflections recommended, shown in Table 3.1, are the

proportions of the span, L, in inches.

More severe limitations may be required for special conditions, such as forsupporting vibrating machinery, long spans, or beams over large glass windows orsliding doors

Camber. Camber may be provided opposite to the direction of anticipated tion for purposes of appearance or utility It will have no effect on strength or actualstiffness

deflec-Where roof and floor beams are cambered, a recommended amount is 1.5 timesthe deflection due to dead load only This will provide a nearly level beam underconditions of long-term dead load application

Additional camber may be introduced as desired to provide for drainage orappearance Members used in low-slope roof applications must be designed to pre-vent ponding of water This may be done either by cambering or by providing slope

or stiffness such that ponding will not occur

(If extra webs are outside)

(If extra webs are inside)

Typical Section in Center Portion of Span

FIGURE 3.1 Typical beam sections.

TABLE 3.1 Standard Allowable Deflections for Beams

Floor beams Live load only L / 360

Dead plus live load L / 240

Roof beams Live load only L / 240

Dead plus live load L / 180

3.2.3 Trial Section

The first step in the actual design of a wood-structural-panel box beam is theselection of a trial section Suitable beam depths vary somewhat, ranging generallyfrom1⁄8to1⁄12of the span (although ratios up to1⁄22have been successfully used).The depth should ordinarily be equal to an available width of wood structural panel,such that waste is minimized As a rule, the flange depth should be equal to at leastfour times the adjoining wood-structural-panel web thickness in order to have suf-ficient contact area between the flange and web for gluing

Selection from Table. Table 3.2 lists preliminary bending and shear capacities fortypical glued box beams with two webs

First, determine the design requirements in terms of maximum moment andshear A cross section that meets the design requirements may then be selecteddirectly from the table Tabular maximums, however, may also be subject to anumber of adjustments based on duration of load, allowable flange tension stress(grade of lumber), and web thickness and grade Note that further adjustment will

be necessary when butt joints are allowed in lumber flanges For example, it may

be necessary to add a lamination to those shown in the table The final design mustthen take into account provisions of Section 3.6.6

Lumber Flanges

Symmetrical Sections Symmetrical cross sections are generally used in

wood-structural-panel beams for several practical reasons These practical considerationsusually outweigh the savings in material that theoretically can be achieved withunsymmetrical sections

The design stresses for flanges are those for allowable stress in axial tensionand axial compression With symmetrical sections, the lower of these allowablestresses will limit the flange design The equations in this section assume a sym-metrical section

Allowance for Surfacing To allow for resurfacing of flange laminations for

gluing, each lamination should be considered 1⁄8in (3 mm) smaller in dimensionperpendicular to gluing surfaces (1⁄16in per surface) (2 mm) than its standard, netlumber size

Beams should be designed for an actual depth, h, slightly less than their nominal

depth, to allow for resurfacing, which may occur for the sake of appearance oruniformity of depth

Actual depth of beams under 24 in (610 mm) deep should be considered3⁄8in.(10 mm) less than nominal; for beams 24 in (610 mm) and deeper, actual depth

TABLE 3.2 Preliminary Maximum Moments and Shears for Glued Box Beams

Preliminary selection estimates a

Table basis

DOL ⫽ 1.00

Panel webs

Thickness ⫽ 15 ⁄ 32 in Span rating ⫽ 32/16

Grade ⫽ Rated Sheathing—Structural I

EA⫽ 4,150,000 lb/ft of width, each web

F v t v⫽ 62 lb/in., each web

F s (Ib / Q)⫽ 210 lb/in., each web

Lumber flanges

Species and Grade: Douglas fir-larch Select Structural

F t⫽ 1,000 psi, unadjusted for size factor

TABLE 3.2 Preliminary Maximum Moments and Shears for Glued Box Beams (Continued )

TABLE 3.2 Preliminary Maximum Moments and Shears for Glued Box Beams (Continued )

aBases and adjustments:

1 Basis: normal duration of load (C D): 1.00

Adjustments:

0.90 for permanent load (over 50 years)

1.15 for 2 months, as for snow

(C Fin numerator and denominator cancel when flanges are same width.)

3 Basis: one web effective in bending because web joints are assumed to be unspliced.

Adjustment: 2.0 for web splices

should be considered1⁄2in (13 mm) less than nominal This resurfacing also results

in reduced flange dimensions

Bending Moment—Symmetrical Sections In a symmetrical section allowable

bending moment may be calculated by the formula

I T⫽total moment of inertia of beam cross section

c⫽distance from beam neutral axis to outermost fiber

Unsymmetrical Sections When the cross section is not symmetrical about its

center, the resisting moment may be calculated as above, except that the distancefrom the neutral axis to the extreme fiber of each flange is used in place of the

value 0.5h and the moment of inertia is calculated with due regard for the location

of the neutral axis The location of the neutral axis is computed based on the totalcross section, without reduction for butt joints

Net Moment of Inertia (I n). The net moment of inertia, I n, is the sum of I of the

flanges plus the sum of all effective web material I n is used in determining thebeam’s allowable bending moment and deflections

Wood Structural Panel Webs When calculating moment of inertia of the wood

structural panel webs, consider only effective material parallel to the span The

effective thickness, t⬘, is 1⁄12 of the appropriate area, A The effective area, A, is derived from the axial stiffness, AEC G , of the panel AEC Gis obtained from Chapter

2, where it is in units of lb / ft of panel width Dividing by E gives the designer the

TABLE 3.3 Effective Area of Flange Laminations

Butt joint Spacing

(t⫽ lamination thickness) Effective factor

this by 12 in / ft of width gives the effective thickness, t⬘, of panel transformed by

Elumber The equation is:

AEC G

12⫻Elumber

for tension and compression

where AE⫽panel stiffness capacity, from Chapter 2

C G⫽adjustment to stiffness capacity, from Chapter 2

Elumber⫽modulus of elasticity for lumber flanges

Butt joints in wood structural panel webs are usually spliced to transmit shearonly, with a splice plate only as deep as the clear distance between flanges If suchbutt joints in webs are staggered 24 in (610 mm) or more, only one web need bedisregarded in computing moment of inertia for bending stress When unequal web

thicknesses are used, use the most critical condition for computing I n, unless thelocation of butt joints is specified in the design For joints closer than 24 in (610

mm), the contribution of the webs should be neglected in computing I n

When webs are spliced full-depth to carry direct flange stresses, they may all

be included in computing the moment of inertia from allowable section capacities

as given in Chapter 2

Flange Lumber Butt joints in the lumber flanges are required by the

Fabrica-tion SpecificaFabrica-tion (SecFabrica-tion 3.2.4) to be spaced at least 30 times the laminaFabrica-tionthickness in adjoining laminations Adjoining laminations refers to multiple-plylumber flanges that are in direct face-to-face contact such as shown in Fig 3.1.Butt joints in the lumber flanges are required to be spaced at least 10 times thelamination thickness in nonadjoining laminations (where web material separatesmultiple flanges), if not otherwise stipulated in the design Ignore any panel materialbetween laminations

If butt-joint location is not otherwise stipulated by the designer, the net moment

of inertia of flanges in which butt joints occur may be calculated by ignoring onelamination and 10% of the two adjoining laminations The effective area of suchadjoining laminations shall be computed by multiplying their gross area by the

percentages in Table 3.3 Butt joints spaced closer than 10t (t⫽lamination ness) shall be considered as occurring in the same section

thick-Wood Structural Panel Webs. Webs are primarily stressed in shear through theirthickness, although they may also carry bending moment, if individual panels areproperly spliced to transmit both types of stresses In addition, sufficient contact

area with the flanges must be provided to transmit the stresses between web andflange.

The number and thickness of the webs may be varied along the beam length inproportion to the shear requirements considering both shear through the panel thick-ness at the neutral axis and planar / rolling shear between flange and web Whereweb joints result in a change of beam thickness, wood structural panel or lumbershims may be glued to the flanges to maintain beam width as required for appear-ance or for gluing pressure

When the depth of a beam is tapered, the net vertical component of the directforces in the flanges should be considered in determining the net shear to be resisted

by the webs and the flange-web joints This vertical component may add to or

subtract from the external shear It is equal to M / L1, where M is the bending moment acting on the section and L1is the horizontal distance from the section tothe intersection of the flange centerlines

Horizontal Shear The allowable horizontal shear on a section can be calculated

by the following formula:

F t v v⬘ allowable shear capacity through the panel thickness (lb / in.), as given

in Chapter 2, with adjustments such as duration of load, if applicable.Note that per Plywood Design Specification, Section 3.9.1,3F v, and

by extension F v t v, can commonly be increased by 19% for plywoodand 33% for marine-grade plywood

C G⫽adjustment factor, depending on panel type, as given in Chapter 2

I T ⫽total moment of inertia of all flanges and webs about the neutral axisregardless of any butt joints (in.4)

N⫽number of webs effective in shear (typically the same as the number

Scarf Joints and Finger Joints Scarf joints 1 in 8 or flatter shall be considered

as transmitting full allowable stress in tension or flexure Scarf joints 1 in 5 shall

be considered as transmitting 75% of the allowable stress Scarf joints steeper than

1 in 5 shall not be used Finger joints are acceptable, at design levels supported byadequate test data

Butt Joints When backed with a glued wood-structural-panel splice plate on

one side having its strength axis perpendicular to the joint, the same width as thepanels spliced, of a grade and span rating the same as the panel itself, joints may

be considered capable of transmitting tensile or flexural stresses as in Table 3.4(normal duration of loading) Splices are to be at least 14 in long on each side ofthe joint With adequately supported test data, it may be possible to make splicesshorter Mated faces of glued joints must be clean and free of oils and waxes prior

to application of the adhesive

TABLE 3.4 Panel Butt Joints—Tension or Flexure (Minimum splice length ⫽ 16 in on

each side of joint)

3 ply 4 ply 5 ply OSB

End Joints for Compression. End joints across the face grain may be considered

as transmitting 100% of the compressive strength of the panels joined when forming to the requirements of this section (normal duration of load)

con-End Joints for Shear

Scarf Joints and Finger Joints Scarf joints along or across the face grain, with

slope of 1 in 8 or flatter, may be designed for 100% of the shear strength of thepanels joined Finger joints are acceptable, at design levels supported by adequatetest data

Butt Joints Butt joints, along or across the face grain, may be designed for

100% of the strength of the panels joined when backed with a glued plywood spliceplate on one side, no thinner than the panel itself, of a grade and species groupequal to the plywood spliced, and of a length equal to at least 12 times the panelthickness

Shear strength may be taken proportionately for shorter splice-plate lengths

Combination of Stresses. Joints subject to more than one type of stress (for ample, tension and shear), or to a stress reversal (for example, tension and com-pression), shall be designed for the most severe case

ex-Permissible Alternative Joints. Other types of glued joints, such as groove joints, or those backed with lumber framing, may be used at stress levelsdemonstrated by acceptable tests

tongue-and-A 2 in wide (50 mm) nominal stiffener alone may be used as a shear-spliceplate when the web is 24 in (610 mm) deep or less and is no thicker than3⁄8in.(10 mm) or carries no more shear than would be allowed on a 3⁄8 in (10 mm)panel

Holes in Webs Holes in webs should be avoided if possible If they are

re-quired, they should be located in areas of low shear, with proper consideration forthe shear capacity of the remaining section It is good practice to avoid sharpcorners, and to use a wood-structural-panel ‘‘doubler’’ in the area of the hole (i.e.laminate another layer of wood structural panel around the hole)

Flange-Web Joints Joints between flanges and webs at any section must be

designed to transfer the shear acting along that section Stresses are transferredwholly by glue, not by a combination of glue with mechanical fasteners

Beams with One or Two Webs The allowable flange-web shear on a glued,

symmetrical two-web section in which only one face of each web contacts theflange, or on an I section, may be calculated by Eq (3.5)

d⫽flange depth (in.)

I T⫽total moment of inertia about the neutral axis of alleffective material, regardless of any butt joints (in.4)

C g⫽Ib

F s

Qweb Panel shear in the plane capacity (lb / ft of width)(N / m), with

⫽Ib

Qweb shear constant from Chapter 2 (in.

2/ ft)

Qflange⫽statical moment about the neutral axis of all effective

material in the upper (or lower) flanges, regardless

of any butt joints (in.3)

Beams with More than Two Webs For purposes of designing the flange-to-web

glued joint, maximum flange-web shear on beams with more than two webs may

be computed using the assumption that the horizontal shear stress is equal in allwebs For calculations, flanges are then broken down into areas tributary to eachweb, and flange-web shear figured separately for each contact surface Tributary

areas are generally assigned such that the first moment (Q) of the area tributary to

each web is proportional to the thickness of the webs

For a beam in which the center web is less than twice the thickness of an outerweb, the maximum stress occurs on the outside web, and allowable shear is given

by the following formula

Other notations are as previously described in this section

Deflection The deflection of wood-structural-panel beams may be taken as the

sum of the calculated deflections due to bending and to shear It should generallynot exceed the values given in Table 3.1

The bending deflection (⌬b) may be calculated by conventional engineering mulas, with due regard to loading conditions and fixity of supports Deflection due

for-TABLE 3.5 Shear Factor S F

Span / depth ratio Factor S F

be made using the refined method

• Approximate method: The approximate deflection (⌬A) of simply supported, formly loaded wood-structural-panel beams may be found by multiplying thebending deflection (⌬b ) by a shear deflection factor, S F, depending on the span-depth ratio, to account for shear deflection The bending deflection is found byconventional formulas, using the elastic modulus of the flange lumber tabulated

uni-in the NDS and the moment of uni-inertia of all effective material uni-in the section,

regardless of any butt joints The shear-deflection factors (S F) given in Table 3.5may then be applied to the bending deflection, with interpolation permitted

• Refined method: The total deflection (⌬R) may be calculated by separately puting the bending deflection (⌬b) and shear deflection (⌬s) and adding the two

com-Bending Deflection In calculating the bending deflection, the tabulated elastic modulus (E) of the flange lumber may be increased by 3% over the values tabulated

in the NDS1(E⬘ ⫽ 1.03E) to obtain a ‘‘true’’ modulus of elasticity The moment

of inertia used for computing the bending deflection is I T, the moment of inertia ofthe effective material in the section, regardless of butt joints

Shear Deflection The shear deflection for simple beams is shown in Fig 3.2

and may be calculated using the formula

KC

AG

where⌬s⫽shear deflection (in.)

K⫽a constant determined by the beam cross section, shown in Fig 3.2

C⫽a coefficient depending on the manner of loading, shown in Fig 3.2

A⫽Aflange⫹Aweb⫽ cross-sectional area of the beam (in.2)

When calculating area of wood structural panel webs for shear, use

(Nwebs)(G t C ) v v G

(Aweb)⫽冋 Glumber 册h (3.8)

where Glumber⫽shear modulus of the webs (psi)

If deflection is critical for loading conditions other than those shown in Fig 3.2,refer to Ref 2

30 β 31β 2sβ 308β +8s

5 30 +

(

1 – s 3 + s 3 1 P

β =Gflange

G web where

a

P P

Curves based on sections symmetrical about the horizontal and

vertical axes, with G flange assumed equal to G web ( =1) ␤

4

Stiffeners

Bearing Stiffeners Lumber bearing stiffeners are required over reactions and

where other heavy concentrated loads occur, to distribute such loads into the beam.They should fit accurately against the flanges, and the webs should be securelyattached to them

Bearing Stiffeners at Ends of Beams Bearing stiffeners at ends of beams should

be the same width as the lumber flange at that section Their dimension parallel tothe beam span should not be less than that given by the following two considera-tions

• Compressive strength: The thickness of stiffener must be at least equal to x in

the following equation

P

F b c⬜

where x⫽thickness (in.) of stiffener (parallel to beam span)

P⫽concentrated load or reaction (lb)

F c⬜⫽allowable stress in compression perpendicular to grain for the flangelumber (psi)

b⫽flange width (in.)

• Rolling, or planar shear: For beams with one or two webs, the thickness of stiffeners must be at least equal to x in the following equation For beams with

more than two webs, the rolling (also called planar) shear stress will be less likely

to govern

P

2hF s

where P⫽concentrated Load (lb)

h⫽depth of beam (in.)

F s⫽allowable wood structural panel planar / rolling shear stress (psi)

Bearing Stiffeners Not at Ends of Beam For bearing stiffeners not at ends of

beam, factors given in 1997 NDS1Section 2.3.10 may be applied to F c⬜

Intermediate Stiffeners Intermediate stiffeners are required to stabilize the

flanges, space them accurately during fabrication, reinforce the webs in shear andprevent their buckling, and serve as backing for gluing of web splice plates whereprespliced or scarfed webs are not used Such stiffeners are usually of 2 in (50mm) dimension lumber and are equal in width to the lumber flange between webs,allowing for splice plates, if any

Intermediate stiffeners spaced 48 in o.c (1220 mm) or less on centers willdevelop all, or nearly all, the shear strength of a beam of normal proportions

Lateral Stability Deep, narrow beams, particularly those used on long spans,

may require definite lateral restraint to prevent buckling The ratio of the totalmoment of inertia of all effective material about the horizontal neutral axis to thatabout the vertical axis will determine the minimum lateral support required, as given

in Table 3.6

3.2.4 Fabrication of Glued Wood Structural Panel Lumber Beams

General. This specification covers the fabrication of glued wood structural panellumber beams, in which flanges are stress-graded lumber or glulam, and webs aretrademarked wood structural panels

TABLE 3.6 Provisions for Lateral Bracing

兺I x

兺I y Provision for lateral bracing

Up to 5 None required

5 to 10 Ends held in position at bottom flanges at supports

10 to 20 Beams held in line at ends (both top and bottom flanges restrained

from horizontal movement in planes perpendicular to beam axis)

20 to 30 One edge (either top or bottom) held in line

30 to 40 Beam restrained by bridging or other bracing at intervals of not more

than 8 ft (2440 mm) More than 40 Compression flanges fully restrained and forced to deflect in a vertical

plane, as with a well-fastened joist and sheathing, or stressed-skin panel system

Wood structural panel / lumber beams should be designed by a qualified designprofessional in accordance with the latest edition of this Handbook, using themethod set forth in Sections 3.2 and 3.3 of this chapter Other design methods may

be employed, provided they are supported by adequate test data

Wood structural panel / lumber beams shall be fabricated and assembled in cordance with engineering drawings and specifications, except that minimum re-quirements herein shall be observed

At the time of gluing, the wood structural panel shall be conditioned to a ture content between 7% and 16% Pieces to be assembled into a single beam shall

mois-be selected for moisture content to conform to Assembly, mois-below

Surfaces of wood structural panels to be glued shall be clean and free from oil,dust, wax, paper tape, and other material that would be detrimental to satisfactorygluing Medium-density overlaid surfaces shall not be relied on for a structural gluebond

Lumber Grades shall be in accordance with current lumber grading rules

Knot-holes up to the same size as the sound and tight knots specified for the grade bythe grading rules may be permitted When lumber is resawn, it shall be regradedbased on the new size Lumber for stiffeners shall be 2 in (50 mm) minimumnominal thickness and of a grade equal to that of the flanges, except for extrastiffeners used only to supply pressure behind splice plates

At the time of gluing, the lumber shall be conditioned to a moisture contentbetween 7% and 16% Pieces to be assembled into a single beam shall be selectedfor moisture content to conform to Assembly, below

Surfaces of lumber to be glued shall be clean and free from oil, dust, and otherforeign matter that would be detrimental to satisfactory gluing Each piece of lum-

ber shall be machine finished, but not sanded, to a smooth surface with a maximumallowable variation of1⁄32in (1 mm) in the surface to be glued Warp, twist, cup,

or other characteristics that would prevent intimate contact of mating glued surfacesshall not be permitted

Glue Glue shall be of the type specified by the designer for anticipated

temper-Fabrication

Webs Only plywood wood structural panels shall be scarf or finger jointed.

Scarf and finger joints shall be glued under pressure and over their full contact areaand shall meet the requirements of Section 5.9 of PS 1.4In addition, no core gapshall intersect the sloped surface of the joint

Unless otherwise noted in the design, butt joints in wood-structural-panel websshall be backed with wood-structural-panel shear-splice plates centered over thejoint and glued over their full contact area The plate shall extend to within1⁄4in.(6 mm) of each flange on the inside of the beam and shall be at least equal inthickness to the web being spliced Strength axis of the splice plate shall be parallel

to that of the web Length of the plate shall be at least 12 times the web thickness.Surfaces of high-density overlaid wood structural panel to be glued shall beroughened, as by a light sanding, before gluing

Framing Scarf and finger joints may be used in flange lumber, provided the

joints are as required for the grade and stress used in the design Knots or knotholes

in the end joints shall be limited to those permitted by the lumber grade, but inany case shall not exceed1⁄4the nominal width of the piece Scarf slopes shall not

be steeper than 1 in 8 in the tension flange, or 1 in 5 in the compression flange.The edges of the framing members to which the wood structural panel webs are

to be glued shall be surfaced prior to assembly to provide a maximum variation indepth of1⁄16in (2 mm) for all members in a beam (Allow for actual thickness ofany splice plates superimposed on stiffeners.)

Assembly. The range of moisture content of the various pieces assembled into asingle beam shall not exceed 5%

All side-grain wood joints at flanges and stiffeners shall be glued over their fullcontact area

Scarf and finger joints in stress-grade lumber flanges shall be well scatteredthroughout Unless otherwise specified, they shall not be spaced closer than 16times the lamination thickness in adjoining laminations, measured from center tocenter (Ignore wood structural panels between laminations.) In flanges of three orfewer laminations, only one joint shall be allowed at any one cross section; inflanges of four or more laminations, two joints may be allowed at the same crosssection

Unless otherwise specified, butt joints in lumber flanges shall be spaced at least

30 times the lamination thickness in adjoining laminations and at least 10 timesthe lamination thickness in nonadjoining laminations (Ignore wood structural pan-els between laminations.) No butt joints shall be allowed in portions of beams

intended for mechanical splices or other stressed connections, unless specificallycovered in the design.

Stiffeners shall be placed as shown in the design, but in any case, they shall bespaced not to exceed 4 ft (1220 mm) on center, and at reactions and other concen-trated load points Stiffeners shall be held in tight contact with the flanges bypositive lateral pressure during assembly

Unless otherwise specified by the designer, web butt joints shall be staggered atleast 24 in (610 mm) When glued during assembly, web splice plates shall bebacked with one or more lumber stiffeners accurately machined in width to obtainadequate pressure Where the design calls for the stiffeners to act as the web splices,web butt joints shall be located over the center of the stiffener, within1⁄16 in (2mm), and webs shall be glued to the stiffener

Where two adjacent webs are used, their contacting surfaces shall be gluedtogether over the full flange and stiffener area Wood structural panel webs shall

be glued to framing members over their full contact area, using means that willprovide close contact and substantially uniform pressure Where clamping or otherpositive mechanical means are used, as required where webs are enclosed bothsides with lumber laminations or where flanges are being glued simultaneously withthe beam assembly, the pressure on the net framing area shall be sufficient toprovide adequate contact and ensure good glue bond with 100–150 psi (690–1,035kPa) on the net glued area recommended and shall be uniformly distributed by caulplates, beams, or other effective means Where webs enclose a lumber flange oranother web, nail gluing may be used in place of mechanical pressure methods.Nail sizes and spacings shown in the following schedule are suggested as a guide.Nail sizes shall be:

• At least 4d (4-penny common) for wood structural panels up to3⁄8 in (10 mm)thick

• At least 6d (6-penny common) for1⁄2–7⁄8in (12.7–22 mm) wood structural panels

• At least 8d (8-penny common) for 1 inch to 11⁄8in (25–30 mm) wood structuralpanels

Nail spacing shall not exceed:

• 3 in (75 mm) along the flanges for wood structural panels through 3⁄8 in (10mm)

• 4 in (100 mm) for wood structural panels1⁄2in (10 mm) and thicker

Lines of nails shall be set in3⁄4in (19 mm) from the lumber edge

• Two lines shall be used for 4 in (100 mm) nominal flange lumber

• Three lines for lumber 6, 8, and 10 in (150, 200, and 255 mm) nominal

• Four lines for 12 in (305 mm) nominal

Application of pressure or nailing may start at any point but shall progress to

an end or ends In any case, it shall be the responsibility of the fabricator to produce

a continuous glue bond that meets or exceeds applicable specifications

Unless otherwise specified, width of beams shall equal, within1⁄16in (2 mm),the sum of the lumber and wood structural panel dimensions, allowing for resur-facing The net flange dimension in the plane of the laminations shall be no morethan 1⁄ in (6 mm) less than the standard surfaced lumber width To allow for

resurfacing for finish appearance and uniformity, actual beam depth may be up to

3⁄8 in (10 mm) less than nominal for beams up to 24 in (635 mm) deep, 1⁄2 in.(12.7 mm) less for beams 24 in (635 mm) and deeper, with tolerances of⫺1⁄8in.(3 mm) and⫹1⁄4in (6 mm) Length of beam shall be as specified1⁄4in (6 mm)

Identification. Each member shall be identified by the appropriate trademark of

an independent inspection and testing agency, legibly applied to be clearly visible.Locate trademark approximately 2 ft (610 mm) from either end, except appearance

of installed beam shall be considered If the strength of one flange is different fromthat of the other, the top flange shall be clearly marked on the outside surface ofthe finished beam

3.2.5 Glued Box Beam Design Example

This example is intended for use as a general guide Review of those sectionspertinent to your specific design is recommended before proceeding

Preliminary considerations as to the grade of wood structural panel and lumber

to be used for a given design should include a check on availability Where fullexterior durability is not required for the wood structural panel, the wood structuralpanel may be specified exposure 1 (interior panel with exterior glue), generallypermitting the use of higher allowable wood structural panel shear stresses

Problem

Design a 28 ft (8.5 m) simple-span roof beam to support a total uniform load of

290 plf (265 N / m) Maximum design depth for the beam is 24 in (610 mm).Allowable deflection under total load⫽L / 240 Panel strength axis parallel to beam

length Duration of load⫽ 1.15 as for snow loads

290⫻28

M⫽ ⫽28,420 lb-ft⫽341,040 lb-in (38,500 N-m)

8Try flanges consisting of two Douglas fir-larch No 1 and Better 2 ⫻ 4s withtwo unspliced webs of23⁄32-in (18.3 mm) 48 / 24 OSB rated sheathing (Fig 3.3)

Trial 1

Check Bending Strength

Mavailable⫽Mflange⫹Mwebs⫽F S t flanges⫹F S t webs (3.12)

1

F t⫽F C t F⫽allowable tensile stress in flanges (NDS tabulated value)

⫽800⫻1.5⫽1200 psi (8.27 MPa)

b = 2.75

h = 23.5

d = 3.25 c

d⫽net depth of flanges⫽3.5 in.⫺冉 冊2 ⫽3.25 in (85 mm)

d1⫽net beam depth⫺2(d)⫽23.5⫺2(3.25)⫽17 in (430 mm)

b⫽net width of flanges⫽2⫻(1.50 in.⫺0.125 in.)⫽2.75 in (70 mm)

wheret⬘web⫽effective thickness of a web

EAwebC G⫽tabulated axial stiffness for 23⁄32 in 48 / 24 OSB rated sheathing ⫽

5,850,000⫻ 1.0⫽5,850,000 lb / ft of width (85.40 kN / m)

Eflange⫽1,800,000 psi (12.4 GPa)

Calculating the effective thickness of a web by transforming the web into lumber,

EAweb/ Eflange, and dividing by 12 in / ft of width,

Section moment capacity:

(F⬘t S)total⫽F (C )S t D Total⫽1200⫻1.15⫻(157.3⫹24.9)⫽251,436 lb-in.Converting to lb-ft:

Total 2141.3 2434.4 182.2 20,953

Because this is considerably less moment capacity than required, the beamstrength must be increased Because bending controls the design and the beam depth

is limited, full-depth web splices may be considered It is often more convenient,however, to increase the number of flange laminations and / or specify a higher-grade flange lumber.

For convenience, refer to Table 3.2 This table gives the moment, shear and

stiffness capacities of the most commonly used glued box beams The table C Dis1.0, so the moment and shear capacities may be multiplied by 1.15 for this exampleproblem The sheathing capacities are based on 15⁄32 in (12 mm) 32 / 16 ratedsheathing, as opposed to 23⁄32in (18 mm) 48 / 24 rated sheathing in the previousexample The capacities will be altered when different thicknesses and types ofpanels and flanges are used

is the same as already shown

Assume trial section as shown in Fig 3.4

Before calculating the moment of inertia (I) and the statical moment (Q) for a

given trial section, the probable location of butt joints (if any) in both the web andflange members must be determined and adjustments applied For this example,consider scarf joints of 1:12 slope for both the tension and compression flangesand butt joints in the wood-structural-panel webs staggered 24 in (635 mm).Where the beam design is controlled by horizontal shear, possible revisionsinclude a specification of thicker wood structural panels, use of Structural I sheath-ing (such as in this example), or the addition of web member(s) to the end quartersections of the beam

Where flange-web (planar / rolling) shear controls the design, Structural I websshould be considered In addition, greater flange-web area may be required

Check Bending Strength

Mavailable⫽Mflange⫹Mwebs⫽F S t flanges⫹F S t webs

Tabulated E for lumber flanges⫽1,900,000 psi (13.1 GPa)

F C t F⫽allowable tensile stress in flanges⫽1000⫻1.3⫽1300 psi (8.96 MPa)

where F t⫽ tabulated tensile stress (psi)

d = 5.25

b = 2.75

h = 23.5

Y Y

where b⫽net width of flanges ⫽2⫻(1.50 in.⫺0.125 in.)⫽2.75 in (70 mm)

h⫽Net depth of beam⫽24 in ⫺0.5 in.⫽23.5 in (597 mm)

d⫽Net depth of flanges⫽5.5 in.⫺冉 冊0.52 ⫽5.25 in (133 mm)

Iweb⫽冉 12 冊⫽196.8 in (81.91⫻10 mm )

Section Modulus of Beam Components

Section Moment Capacity

M T⫽F⬘t S⫽(F⬘t S)total

F⬘t (C )S D ⫽1300⫻1.15⫻(210.3⫹16.8)

⫽339,514 lb-in / ft (125.85 kN-m / ft)339,514

F (C )S t D ⫽

12

⫽28,292 lb-ft (38,359 N-m)艑28,420 lb-ft (38,532 N-m) OKNote that the use of five-ply, structural I did not improve the bending capacity.The primary benefit from using structural I will be when the beam capacity is

controlled by planar / rolling shear, F s , or shear through the thickness, F v

Check Horizontal or Shear-Through-the-Thickness and Planar Shear Capacity

wL

2290(28)

in (21.7 N / mm)

⫽

F t v v⬘C G F v t v C G adjusted for C Dand adjustment for adhesive on only one

side of wood structural panel web and glued areaⱖ30% of beamdepth

N⫽number of webs

F t v v⬘C G⫽F t C (C )(F ) v v G D A ⫽124(1.15)(1.19)

⫽169.7 lb-in (29.72 N-mm)

where F A⫽allowable increase for continuous glued edge framing parallel to

face grain per PDS 3.8.1.3

d⫽5.25 in (133 mm)(2)(89.6)(1.15)(0.5)(5.25)(2864.2)

V s⫽

131.7

⫽11,764.8 lb (52.33 kN)⬎4,060 lb (18.06 kN) OK

Check Beam Deflection

Both the approximate and the Refined methods for determining deflection are lustrated below As a rule, if the deflection calculates to near the allowable limit

il-using the approximate method, recalculate by the refined method before alteringthe trial conditions.

Check Deflection—⌬A —Approximate Method

384(1,900,000)(2864.2)

Check Deflection—Refined Method

Even though the approximate deflection is only2⁄3of the allowable deflection, thefollowing calculation is provided as an example of calculating a more precise de-flection:

⫽45,900 lb / in (8,038 N / mm) for five-ply Structural I plywood

Glumber⫽Eflanges(0.06)⫽1,900,000(0.06)⫽114,00 psi (786 MPa)

A T⫽2(2.75)(5.25)⫹冋 册(23.5)⫽47.80 in (30.84⫻10 mm )

114,000

4 35(290)(28) (12) 2.21(341,040)

I provides the maximum effective area for a given panel thickness, making it the

stiffest grade for the webs Also, consider using lumber with a higher E.

Trial 2 Summary

Section Properties (transformed)

Inet(in 4 ) Igross(in 4 ) Snet(in 3 ) Qgross(in 3 )

Moment capacity (lb-ft) Flanges 2470.6 2470.6 210.3 131.7 —

Stiffener thickness required for planar / rolling shear at bearing ends:

Check Lateral Stability

As final steps in the overall design, review the structural adequacy of beamsupports and develop beam connection details

There are several differences between glued beam design methods and nailed pled) box beam design One difference is that the nailed connections between theflanges and the webs are not rigid, as with the glued connections This permits anindeterminate amount of slip between the flanges and the webs To maintain aconservative design, therefore, the webs are not typically considered in calculatingmaximum moment capacity In nailed design, the flanges are assumed to carry alltensile and compressive stresses and the webs carry only shear

(sta-Another difference is that planar / rolling shear does not enter into the tions Consequently, glued area need not be maximized and the flanges may beoriented either flatwise or edgewise in the beam In addition, the flanges need not

calcula-be planed down to smaller dimensions to accommodate optimal glue thickness and

B

C

beam design example.

curing pressure This saves some labor and gives a larger flange cross-sectional area

to work with Some of this labor savings is lost, however, because nailed beamsrequire a considerable amount of nailing

A more efficient beam can be designed with flatwise orientation of the flangelumber because this orientation maximizes the distance of the flange lumber fromthe neutral axis of the beam (Fig 3.5) In addition, a beam with a single flatwise-oriented flange on the top and bottom gives more depth of penetration to accom-modate maximum design capacity from each fastener Oriented vertically, a singleflange may not be thick enough to develop full fastener lateral (shear) capacity.Nails may be spaced along the flange according to the amount of shear thatmust be transferred at each point In a uniformly loaded simple span beam, nailingwill be heaviest at the ends and least through the middle The design capacities of

the nails may be adjusted for C D Care must be taken to avoid splitting the lumber.Staples have less tendency to split lumber and may be spaced much more closely,without splitting, than nails To simplify construction, consider spacing nails todevelop the required maximum shear transfer in the outer 1⁄4 of beam length and

at twice that spacing through the center 1⁄2 of beam length Maximum fastenerspacing shall be 6 in o.c (150 mm)

Glue-bond quality, so easily compromised under field-assembly conditions, isnot an issue with nailed box beam designs Nailed box beams may, therefore, befield assembled and consistently achieve full design capacity They should be de-signed by a design professional, but third-party inspection is not generally required,provided that the beam is assembled according to engineered drawings (Fig 3.6).

3.3.1 Nailed Box Beam Design Example

Using the design example from Trial 2 of the glued wood structural panel lumberbeam above, design a 28 ft long (8.53 m) nailed box beam with 32 / 16 rated,15⁄32

in (12 mm) five-ply Structural I plywood webs The strength axis of the woodstructural panel (plywood in this case) is oriented parallel to the beam span and thecontinuous double 2⫻6 Douglas fir select structural flanges are oriented flatwisefor maximum efficiency (Fig 3.7) Determine the maximum moment capacity,

F t S, of the beam, the maximum shear capacity, and the nailing requirements The

lateral design capacity for 8d common nails is 74 lb / nail (329 N) (unadjusted for

C D) Assume one row of nails in each flange lamination

Check Bending Strength

d⫽depth of flange (in.)

Check Web Shear

(4060)(173.3)Space two rows of 8d common nails 2 in o.c (50 mm) in the outer 1⁄4 (7 ft)(2.1 m) to develop the required maximum shear capacity Space nails at 4 in o.c.(102 mm) along the inner1⁄2 (14 ft) (4.3 m) of beam Stagger nails when used at

2 in o.c (51 mm) to minimize splitting

Bearing Capacity and Lateral Stability Check for bearing stiffeners and lateral

stability according to Wood Structural Panel Webs, above, under Section 3.2.3, andTable 3.6

When end stiffeners are inserted between flanges, nails may be spaced

3 inches on center.

nailing pattern.

STRUCTURAL PANEL / STRESSED-SKIN

PANELS

Flat panels with stressed wood structural panel skins and spaced lumber stringersact like a series of built-up I-beams, with the wood structural panel skins takingmost of the bending stresses as well as performing a sheathing function and thelumber stringers taking shear stresses

Since stressed-skin panels are usually relatively shallow, any shear deformationbetween skins and webs will contribute materially to deflection For maximumstiffness, therefore, a rigid connection is required between the wood structural paneland the lumber Thus, all panels considered in this design method are assumed to

be assembled with glue

Although it is possible to use laminated or scarf-jointed members for the ers of stressed-skin panels, such panels are usually restricted to single-laminationstringers The maximum length of lumber available, therefore, generally determinespanel maximum length

string-Headers (at the ends of the panel) and blocking (within the panel) serve to alignthe stringers, back up splice plates, support skin edges, and help to distribute con-centrated loads They may be omitted in some cases, but should always be usedwhen stressed-skin panels are applied with their stringers horizontal on a slopingroof Without headers and blocking, panels so applied may tend to assume a par-allelogram cross section

Panels with top and bottom skins, as shown in Fig 3.9, are most common sided panels, as shown in Fig 3.10, are also used, especially when special ceilingtreatment is desired, or when no ceiling is required A variation of the single-skinpanel, with lumber strips on the bottom of the stringers, as shown in Fig 3.11, iscalled a T-flange panel

or as shown on opposite end Scarf joint in lower skin is preferred method for plywood (alternate: spliced butt joint)

Lumber stringers Lumber blocking (not required

if pre-spliced skins are used)

Top skin Butt joint between wood structural panels (or scarfed)

Wood structural panel splice plate Lumber stringers

Skin Scarfed joint or splice-plated butt joint between panels Lumber stringers Stringer from next panel Lumber flange

Two-sided panels do not require bridging Stringers in one-sided panels may bebridged, as are joists of the same depth in conventional construction

Owing to the high strength of wood structural panels, calculations will oftenindicate that a thin bottom skin is structurally sufficient There is some possibility,however, of a slight bow when1⁄4in (6 mm) bottom skins are used with strengthaxis parallel to stringers on 16 in centers (405 mm) Such a bow, although of noimportance structurally, may be undesirable from an appearance standpoint Forstringers so spaced, therefore,3⁄8in (8 mm) wood structural panels are the mini-mum recommended

A number of decorative plywood surfaces are adaptable for panels where thebottom skin will serve as a ceiling for a habitable room The design of panels usingsuch decorative plywood must, of course, allow for the special properties of theproduct See Materials, above, under Section 3.2.4, for a general discussion of thewood components used in the fabrication of SIPs.

3.4.1 Design Considerations

Transformed Section. In calculating section properties for stressed-skin panels,the designer must take into account the composite nature of the unit Unless allmaterials in the panel have similar moduli of elasticity, some method must beemployed to make allowance for the differences in MOE

Different moduli of elasticity may be reconciled by the use of a transformed, oreffective, section The transformed section approach is common to structural design

of composite sections It consists of transforming the actual section into one ofequivalent strength and stiffness, as if composed of a single material Where aportion of a section is to be transformed to another material, its actual area must

be multiplied by the ratio of its MOE to that of the other material

E1

冉 冊E2

thus arriving at an effective area of the new material

For instance, assume a stressed-skin panel with the modulus of elasticity of thestringers half that of the skins Properties of the section could be stated in terms ofthose of a transformed section having the modulus of elasticity of the skins andcalculated as if the stringers were only half as wide as they actually are

Design Loads. The design live loads must not be less than required by the erning building regulations Allowance must be made for any temporary erectionloads or moving concentrated loads Roof panels must be designed to resist upliftdue to wind load, combined with internal pressure developed by wind throughopenings in the sidewalls, minus the dead load of the panels and roofing Lateralloads, which develop diaphragm action, may require special consideration, partic-ularly to fastenings between panels and to framing

gov-Allowable Working Stresses and Capacities. Wood structural panel working pacities are determined as described in Chapter 2

ca-Wood Structural Panels in Stressed-Skin Panels

Effective Sections For all panels, whether containing butt joints or not,

deflec-tion and shear stresses are based on the gross secdeflec-tion of all material having itsstrength axis parallel with the direction of principal stress

All wood structural panels and all lumber having their strength axis parallel tothe direction of stress may be considered effective in resisting bending stress, except

when butt-jointed and except as reduced for b distance below, under Section 3.4.2.

The best method for splicing skins, from both the structural and appearancestandpoint, is to scarf joint plywood to the desired length

End Joints for Tension or Bending. End joints across the face grain shall beconsidered capable of transmitting the following stresses parallel with the face plies

Scarf Joints and Finger Joints Scarf joints 1 in 8 or flatter shall be considered

as transmitting full allowable stress in tension or flexure Scarf joints 1 in 5 shall

be considered as transmitting 75% of the allowable stress Scarf joints steeper than

1 in 5 shall not be used Finger joints are acceptable, at design levels supported byadequate test data OSB panels shall not be scarf or finger jointed

Butt Joints When backed with a glued wood structural panel splice plate on

one side, centered over butt joint, and having its strength axis perpendicular to thejoint, and of a grade equal to the wood structural panel spliced, and being no thinnerthan the panel itself, joints may be considered capable of transmitting tensile orflexural stresses as in Table 3.4 (normal duration of loading) Strength may be takenproportionately for shorter splice-plate lengths

End Joints for Compression. End joints across the face grain may be considered

as transmitting 100% of the compressive strength of the panels joined when forming to the requirements of this section (normal duration of load)

con-Scarf Joints and Finger Joints Slope no steeper than 1 in 5.

Butt Joints Spliced as in Table 3.4 and with the splice lengths tabulated therein.

Strength maybe taken proportionately for shorter splice-plate lengths

End Joints for Shear

Scarf Joints and Finger Joints OSB panels shall not be scarf or finger jointed.

Scarf joints in plywood panels, along or across the face grain, with slope of 1 in

8 or flatter, may be designed for 100% of the shear strength of the panels joined.Finger joints in plywood panels are acceptable, at design levels supported by ade-quate test data

Butt Joints Butt joints, along or across the face grain, may be designed for

100% of the strength of the panels joined when backed with a glued wood structuralpanel splice plate on one side, no thinner than the panel itself, of a grade andspecies group equal to the wood structural panel spliced, and of a length equal to

at least 12 times the panel thickness

Strength may be taken proportionately for shorter splice-plate lengths

Combination of Stresses Joints subject to more than one type of stress (for

example, tension and shear), or to a stress reversal (for example, tension and pression), shall be designed for the most severe case

com-Permissible Alternative Joints Other types of glued joints, such as

tongue-and-groove joints or those backed with lumber framing, may be used at stress levelsdemonstrated by acceptable tests

Splice plates are used only between the stringers, and consequently there is acertain percentage of the panel, which is not spliced When calculating the strength

of the panel at the splices, only the portion of the skin, which is actually spliced,should be considered effective Width and thickness of plates should not be reduced

Allowable Deflection. Deflection must not exceed that allowed by the applicablebuilding code Maximum recommended deflections are given in Table 3.1 Moresevere limitations may be required by special conditions such as the installation ofmarble or ceramic tile

Camber. Camber may be provided opposite to the direction of anticipated tion for purposes of appearance or utility It will have no effect on strength or actualstiffness