AbstractDestructive evaluation of 20 beams provid-

ed data on which to base strength factors for thecoarse-grain southern pine. This lower qualitymaterial, which is defined as having fewer thanfour rings per inch, has not previously been usedin the manufacture of engineered glued-laminated timber meeting a U.S. commercialstandard. In comparison to existing glued-laminated combinations, beams containingcoarse-grain lumber are likely to have slightlyreduced bending strength and stiffness valuesand substantially reduced shear strength.

The research will provide a basis for thedevelopment of an industry specification forglued-laminated timbers permitting the use ofcoarse-grain southern pine lumber.

FLEXURALSTRENGTH OFGLUED-LAMINATEDTIMBERBEAMSCONTAININGCOARSE-GRAINSOUTHERNPINE LUMBERByR.C. MOODY, EngineerForest Products LaboratoryForest ServiceU.S. Department of Agriculture

IntroductionProblem

Current industry specifications for struc-tural glued-laminated timber require that alllumber used in their manufacture be “mediumgrain” or better (1) Although a number ofstudies on glued-laminated timber have beenconducted within the past few years, none haveconsidered the use of “coarse-grain” material(that having fewer than four rings per inch).

Historically, coarse-grain lumber has oc-curred in limited amounts and could be exclud-ed from material used in laminated timbers.Recently, however, laminators of southern pinereport difficulty in purchasing material whichcontains no coarse-grain lumber.

Coarse-grain structural lumber ismarketable but generally at design stresseslower than those for comparable medium-grainvisual grades (20). As the inner laminations ofglued-laminated timber beams are subjected torelatively low bending and axial stresses, a

significant amount of coarse-grain materialcould possibly be used for these innerlaminations. Development of combinations ofglued-laminated timber utilizing coarse-grainsouthern pine lumber would result in betterutilization of our forest resources.

Estimates of the amount of coarse-grainlumber now available vary throughout thesouthern pine growing region but this materialmay amount to about 10 percent of overallcurrent production. In some regions, estimatesare much higher, with reports of coarse-grainlumber amounting to 25 to 30 percent of produc-tion. Advancements in forestry which result infaster grown trees will mean an increasingamount of coarse-grain material (11).Background

Within the past 5 years, the glued-laminated timber industry, in cooperation withthe Forest Products Laboratory and universitylaboratories, has evaluated over 200 largebeams (40 or more feet long). Much of thisresearch was directed at describing tensionlamination grades but a significant amount alsoinvolved developing methods for using non-destructively tested lumber for laminating (see,for example, 9, 15, 16). In these studies, the twoprincipal species groups evaluated wereDouglas-fir and southern pine. In general, ex-isting and currently available grades were usedin these tests. However, one requirement of allthe research was that the southern pine lumberbe medium grain or better, i.e., four or more an-nual rings per inch. Therefore, no information isavailable that relates directly to the use ofcoarse-grain southern pine lumber forlaminating.

In an evaluation of southern pine structurallumber in 1963, Doyle and Markwardt (7) in-cluded material that was referred to as a“Special” grade. Of the information available,this grade most nearly approximates the“coarse-grain” grade and, although limited, thestudy provides an engineering evaluation of thistype material. The results give an indication ofthe bending strength and modulus of elasticitythat might be expected from coarse-grainlumber.

This research was conducted in cooperation with the Ameri-can Institute of Timber Construction (AITC).

Maintained at Madison, Wis., in cooperation with the Uni-versity of Wisconsin.

Numbers in parentheses refer to literature cited at theend of this report.

Lumber that would also be classified ascoarse grain was evaluated by Koch (10) in astudy of sawing southern pine studs from veneercores. Average strength and stiffness propertiesreported for 15 No. 3 grade or Stud grade 2 by4’s having an average growth rate of three orfewer rings per inch are summarized as follows:

Specific gravity: 0.49Modulus of rupture: 4,020 pounds per

square inch (p.s. i.)Modulus of elasticity: 1.18 million p.s.i.Based in part on the results of these

studies, the SPIB (18) used the following ratiosof properties of coarse-grain lumber to those ofmedium-grain lumber in developing their 1968grading rules:Property FactorBending strength 0.80Modulus of elasticity .75Shear strength .80Coarse-grain lumber was not consideredseparately in developing the 1970 grading rules(19).

Studies by Koch (see, for example, Kochand Woodson (12)) demonstrate that 1/6-to 1/3-inch-thick southern pine material with a widevariation in stiffness properties and,presumably, growth rate can be utilized insmall glued-laminated beams.Objective

The objective of this study was to evaluatethe performance of large glued-laminated south-ern pine timbers with coarse-grain inner lamina-tions.

Beam Design, Materials,and Manufacturing

Description of BeamsTwenty beams, 10 each of two grade com-

binations, were approximately 23-1/2 inchesdeep, 5-1/8 inches wide, and 40 feet long. Theyconsisted of 17 laminations about 1-3/8 inchesthick manufactured from nominal 2- by 6-inchlumber.

In order to determine the amount of coarse-grain lumber to be used, a number of assump-tions of its properties based on available datawere necessary. The two grade combinationswhich are shown in figure 1 were developedfollowing general principles of USDA Bulletin1069 (8) as modified by procedures discussed inappendix I. The main feature of these com-binations in comparison to those presentlymanufactured (1) was that a significant amountof coarse-grain material could be utilized in theinner laminations. In both of these com-binations over 60 percent of the material iscoarse grain.

Based on the assumptions and proceduresgiven in appendix I, the theoretical designstresses for these combinations are:

Combination Bending Modulus ofStress elasticity(P.s. i.) (Million p.s.i.)

I 2,000 1.5II 2,400 1.7

Selection of MaterialFor this study, the three density/rate-of-

growth classifications for southern pinelumber—dense grain (D), medium grain (MG),and coarse grain (CG)—are taken from the 1970SPIB Rules (20). These classifications are defin-ed here simply:

GradeD >6 rings per inch and > 1/3 summerwood or

>4 rings per inch and > 1/2 summerwoodMG >4 rings per inchCG

Table 1. --Summary of properties of 2 by 6 southern pine lumberused to manufacture 20 glued-laminated beams

Material PreparationThe material for manufacturing the beams

was processed as follows:I. The No. land No. 2 1umber was graded

by an SPIB grading supervisor at thelaminating plant where the beams weremanufactured. Most of the No. 3 grade lumberwas graded by the sawmill grader and checkedby AITC staff at the mill. All grading wasaccording to the 1970 SPIB rules (20) and con-ducted such that maximum yield of the highestvisual grade was obtained, i.e., no No. 1 waspermitted in the No. 2 grade and no dense ormedium grain in the coarse-grain grades, etc.This required that the coarse-grain Iumber begraded coarse grain on both ends.

2. Each 2 by 6 was numbered and itsmoisture content, approximate dimensions,weight, and modulus of elasticity determined.

Moisture content was determined byaveraging three readings taken along the lengthwith a power-loss type moisture meter. Weightwas determined by doubling the reaction of oneend while the lumber was simply supported as a

beam. The modulus of elasticity was deter-mined with a vibration technique using an “E-computer. None of this information was usedfor positioning lumber in the beams but wastaken to aid in an analysis of results.

3. Appropriate grades of lumber were fingerjointed to produce full-length lamination. Thelaminations were then assigned to the properlocations, depending upon their grade, withinbeams.

4. The location of each piece of lumberwithin the beams was then recorded and thewidth of knots and other major strength-reducing characteristics within the midlength20 feet of each lamination was determined.Knot measurements were made on the face ofthe laminations farthest from the beam neutralaxis. An estimate was made of the effective sizeof spike knots and those not visible on two faces.Both knots and local grain deviation were mea-sured on both faces of the outer tension lamina-tions.

Equipment described in product information availablefrom Irvington-Moore, Inc., Portland, Oreg,

4

Manufacture of BeamsThe beams were manufactured in June 1972

by a commercial laminator experienced in glu-ing southern pine. Except for the lumber grade,the manufacturing conformed to the re-quirements given in U.S. Commercial StandardCS 253-63 for Structural Glued LaminatedTimber (21). Just prior to end jointing, qualitycontrol tests were made to assure adequatestrength of joints. The lumber was end jointedusing a finger joint with its profile visible on thenarrow face of the 2 by 6 lumber. Finger jointdimensions were 1.11-inch length, 0.03-inch tip,and 0.25-inch pitch.

All laminations were planed to l-3/8-inchthickness prior to spreading adhesive with anextruder-type spreader. No attempt was madeto control end joint location in the tensionlamination — it was the intent that a number ofbeams have finger joints in the highly stressedregion during test. Beams were not cambered.

Melamine adhesive was used in the fingerjoints and phenol resorcinol was used in face-gluing the laminations. Following gluing, thebeams were planed to a width of 5-1/8 inches toclean the sides. An end view of five beams readyfor tests is shown in figure 2. Combination Ibeams were designated Nos. 61-70 and Com-bination II beams were designated 71-80.Comment on Manufacture

The No. 3 CG lumber presented problemsthroughout the manufacture of the beams.Twist and bow present in the grade causedalinement problems during finger jointing andbeams stacked in dry layup were quite unstable.Also, handling of the 41-foot-long laminaewithout breakage was difficult because the largeknots, which are characteristic of the grade,made them quite fragile. Based on the ex-perience gained in manufacturing these re-search beams, it is recommended that amanufacturer considering the use of largeamounts of this grade of lumber also considerthe added fabricating problems associated withit.

Research MethodsEquipment

The beams were two-point loaded on a 38-foot span with 8 feet between loading heads andtested in conformance with principles given inASTM D 198 (3) and methods previously used(14). A preload of 2,000 pounds was applied tothe beams to assure full contact and properalinement of equipment and gages, then thebeams were continuously loaded to failure at adeflection rate of approximately 1 inch perminute.

Data ObtainedDimensions and weight of each beam were

determined prior to test. Dimensionmeasurements were averages of those taken atmidlength and at each load point. Data takenduring tests included maximum load, centerlinedeflection between supports, and centerlinedeflections of the beam over a 7-foot span be-tween load points.

Centerline deflections between supportswere measured relative to the supports using awire deflectometer (3). Readings were taken tofailure at 2,000-pound load increments. Deflec-tions between load points were measured with ayoke deflectometer (2). Deflection data were ob-tained at 2,000-pound load increments up to12,000 pounds total load when the yoke wasremoved. Immediately after failure, moisturecontent readings were taken for each laminationnear the failure region using a resistance-typemeter.

Presentation and Analysis ofResults

Flexural Strength DataResults of the 20 beam tests are given in

tables 2 and 3. All beams averaged near 12 per-cent moisture content, thus no moisture contentadjustments were applied to the data for sub-sequent analyses.

As one of the purposes of the research wasto provide a basis for developing a specificationincorporating coarse-grain lumber, it wasnecessary to analyze the data with regard toproposed design levels for beams in the twogroups. The method used to adjust modulus ofrupture values to the level of proposed designvalues is the same procedure as previouslyfollowed using the currently accepted size effectfactor of 0.921 for the test beams (1) and areduction factor of 0.475 (14).

The data given in table 4 show that all 10Combination I beams would have justified adesign stress of 1,690 p.s.i. based on the loweststrength beam. Similarly, the 10 Combination IIbeams would have justified a design stress of2,000 p.s.i. Both of these design levels aresomewhat below the 2,000 and 2,400 p.s.i.stresses proposed for the combination (in De-scription of Beams). A review of the preliminaryassumptions, given in appendix I, suggests pos-sible reasons.

One preliminary assumption was that knotfrequency data available for dense andmedium-grain lumber would also apply tocoarse-grain lumber. Data analysis indicatedthat this assumption may be justified for theNo. 3 lumber but not for the No. 2 lumber. A

5

considerable difference was found between theknot frequency properties of No. 2 CG lumberand that published for MG and D lumber.Therefore, knot data given in appendix I shouldbe used to calculate strength ratio factors forcombinations containing No. 2 CG lumber.

Another preliminary assumption was thatthe clear wood bending strength of coarse-grainlumber was 0.8 that of medium grain. Areanalysis of the combinations by assumingfailure initiated at the interface of the coarse-grain and medium-grain lumber in the lowest

strength beam in each combination indicatedthat this factor was too high.

Calculations given in appendix I indicatethat the clear wood design stress in bending forcoarse-grain material should not exceed 2,000p.s.i. or about two-thirds of that for medium-grain material. Coarse grain factors of between0.70 and 0.75 were obtained for modulus ofelasticity (appendix I) and horizontal shear(appendix II). Such factors may not always be

conservative and will not necessarily apply tosingle pieces of lumber.

Test FailuresAs expected, test failures were generally

sudden, and it was not always possible to deter-mine exactly the principal cause of failure. Thefollowing comments are based on close observa-tion during tests and examination of the failedsections after tests.

The 20 beam failures could be classifiedinto three categories: (1) Failure through knotsand associated grain deviation in the outer ten-sion lamination, (2) failure through finger jointsin the outer one or two tension laminations, or(3) failure of inner laminations. Actually, threebeams (Nos. 62,64, and 79) failed for part of thewidth of the tension lamination through a fingerjoint and through a knot about 2 feet away forthe remainder (fig. 3). Two of these (Nos. 62and 64) were believed to have initiated at thefinger joint. Failure of beam 79 was believed to

6

7

9

Figure 3.—Location of failure in the midlengthportion of the outer tension laminations ofsix beams. Beams 62, 64, and 79 failed forapart of the width of the tension laminationthrough a finger joint and through graindeviation associated with a knot for theremainder. Failure of beams 63, 65, and 73was believed to initiate in the tension lami-nation at the near-maximum-size strength-reducing characteristics permitted in thegrade.

(M 140 74I)

have initiated at a knot. Another beam (No. 72)failed through a knot in the tension laminationand through a finger joint in the second lamina-tion. It was also believed to have a failed first atthe knot.

The beams represented near lowline quali-ty for the grade, as all 20 midlength tensionlaminations had near-maximum-size strength-reducing characteristics permitted. However,only five beams (Nos. 63, 65, 72, 73, and 75) fail-ed through these locations. A total of 13 beamsfailed primarily through finger joints in the ten-sion lamination (fig. 4).

Two beams (Nos. 61 and 67) were believedto have failed initially in the third, fourth, orfifth lamination from the tension face (fig. 5).Examination of all the failed beams revealedthat the coarse-grain lumber used in theselaminations tended to fail differently than hasbeen previously found with medium-grainlumber. The wide-ringed laminations generallyfailed in a brash manner as shown in figure 6.Quite possibly the failure of more than twobeams initiated in the inner plies andpropagated through knots or finger joints in thetension lamination. It seems certain that these

10

Figure 4.—Beam failure through a finger jointin the outer tension lamination near a loadpoint. About two-thirds of the cross sectionfailed across the base of the fingers whilethe remainder failed along the joint profile.

(M 140 445-2)

inner laminae influenced the strength of allbeams.

Low strength beams of each combination(Nos. 65 and 75) both failed through the selectednear-maximum-size characteristics. Fingerjoints were involved in failure of the next threelowest strength beams in each group.

The incidence of failure at finger joints iscause for concern because finger joint quality isa controllable factor in beam manufacture. Ex-amination of finger joint failures did not revealany consistent pattern in joint failures. Somefailed across the base of fingers, others along thefinger joint profile, while others suggested in-adequate bonding — especially on the latewood.Quality control test of joints just prior to beammanufacture indicated that the equipment wasproducing joints which met the bending

strength requirements of CS 253-63 (21). Also,full-size tension tests of finger-jointed lumber(app. III) indicated that the finger jointstrength was as good as two groups of materialpreviously evaluated.

Therefore, whereas the incidence of failureat finger joints might indicate strength deficien-cies, tests on finger-jointed lumber produced atabout the same time on the same equipment in-dicate satisfactory strength. One possible ex-planation for this apparent contradiction is thatrelatively low stiffness inner plies will notassume their proportionate share of the bendingstress thereby requiring the stiffer outer plies toassume an even greater share. Thus, beamshaving low stiffness middle laminations requirestronger outer laminae and consequently higherquality finger joints for equal beam strength.

11

Figure 5.—Unusual failure of beam 67 resultedin no damage to the outer three tensionlaminations within the middle one-third ofthe beam. The failure was believed to initi-ate in the inner laminations containingcoarse-grain lumber.

(M 140 450- 10)

Modulus of Elasticity (MOE)Full-span MOE averaged 1.35 and 1.49

million p.s.i. for the Combination I and Com-bination II beams, respectively (tables 2 and 3).These values are 10 and 13 percent belowpredicted values given in appendix I which arebased partly on published (19), and partly onassumed (7, 10), lumber MOE values.

Beam MOE values calculated from theknown lumber properties in each beam given intable 5 show MOE values for the two groups tobe 1.47 and 1.60 million p.s.i., respectively. Ac-tual full-span MOE values were below thesefigures by 7 to 8 percent. This discrepancy isconsistent with previous research in that it hasbeen found that a transformed section analysisof lumber MOE values can result in predictedbeam MOE values up to 7 percent higher thanactually obtained. This difference can be at-tributed to shear deflections if the lower

modulus of rigidity of the inner laminations isconsidered. Therefore, by reducing modulus ofelasticity values calculated from a transformedsection approach, accurate beam MOE valuescan be predicted.IK/ IGRatios

The ratios of moment of inertia of knotsto the total moment of inertia (IK/IG) of the 20beams are given in table 6. These werecalculated using the method proposed byWilson and Nottingham (8, 22) at each 0.1 footincrement for the midlength 20 feet of eachbeam. Near-maximum values of IK / IGassociated with the proposed design levels of 2,-000 p.s.i. and 2,400 p.s.i. would be 0.242 and0.168, respectively (17). These values wereequaled or exceeded at one or more sections fortwo of the Combination I beams and four of theCombination II beams.

The bending test results previously discuss-ed suggested somewhat lower design levels for

12

the two combinations. If a design value of 1,600 suggest, the associated near-maximum IK/IGp.s.i. was assigned to the Combination I beams values would be 0.314 and 0.242. All actual IK/IGand 2,000 p.s.i. to the Combination II beams as ratios in the respective beam groups are belowthe near-minimum-strength beams would these values.

13

14

Figure 6.—Failure of beam 61 was believed tohave initiated near a load point (24-ft. loca-tion) in the interior coarse-grain lamina-tions. The type of brash failure shown in thethird, fourth,typical of thebeams.

(M 140 448-8)

ConclusionsBending tests on southern pine glulam

beams manufactured with large amounts ofcoarse-grain lumber and near minimum qualitytension laminations revealed the following:

1. Clear wood stresses for bending, shear,and modulus of elasticity for coarse-grainlumber should not exceed about 70 percent ofthat for medium-grain lumber when used toderive glued-laminated combinations. In fact, a

and fifth laminations wascoarse-grain lumber in all

more conservative value of two-thirds or less issuggested for bending.

2. Existing knot data for No. 2 southernpine lumber do not apply to No. 2 CG lumber.Knot data given for No. 2 CG lumber should beused in deriving glued-laminated combinations.

3. Tension lamination requirements andquality control procedures for glued end jointsmust be modified when significant amounts ofcoarse-grain lumber are used in glued-laminated combinations.

15

APPENDIX IDEVELOPMENT OF THEBEAM COMBINATIONS

Theoretical DerivationsPreliminary Assumptions

Several assumptions were made in order toarrive at Combinations I and II (fig. 1). They in-cluded:

(a) The clear wood design stress in bendingand shear of coarse-grain material (10 years’loading) is 2,420 p.s.i., about 0.8 of the value of 3,025 p.s.i. published in Bulletin 1069 (8). Itshould be recognized that the stresses for bend-ing and shear listed therein are not consistentwith current ASTM practice for structurallumber. Therefore, these clear wood stressesshould be considered as subject to review.

(b) The modulus of elasticity of coarse-grain material is between 0.70 and 0.75 of thatfor medium-grain material (7,18).

(c) Slope-of-grain strength ratios for com-pression parallel-to-grain published in Bulletin1069 (8) may be conservative for the compres-sion side of laminated bending members. Thisassumption is based on observation of manytest failures in which strength-reducingcharacteristics in the outer compressionlaminations did not appear to affect theminimum strength of beams.

(d) The most recent knot surveys for D andMG southern pine lumber (17) approximatelyapply to the revised 1970 structural grades andmay be approximately applied to CG lumber.

The study did provide an opportunity toevaluate the validity of these assumptions asdiscussed later in this appendix.Positioning of Grades

Using the above assumptions, the strengthratios of the combinations (fig. 1) were derivedfollowing procedures similar to those given inreference (17) for a 17-lamination beam.Calculations are summarized in table I-1.

Using the transformed section concept, thecalculated average modulus of elasticity valuesfor the two combinations are given in table I-2.Recommended Modifications

Visual grading requirements for the outertension laminations of the two combinations(fig. 1) followed AITC recommendations. Infollowing current AITC procedures for 2400flevel beams (1), the second tension laminationfor Combination II was required to be No. 1 Dinstead of No. 1 MG. This recommendation was

based on results of large beam tests where No. 2D second laminations were involved in severalfailures of beams below desired strength levels.By changing the grade requirement of the sec-ond lamination to No. 1 D (among other things),a similar problem was not found in subsequenttests (14, 15).

Based on the above experience with secondlaminations in beams designed to 2400f or 2600flevels, it was believed necessary to consider up-grading the second tension lamination of Com-bination I. With a type of tension laminationsomewhat similar to AITC 301-22 (1), limitedresults have indicated that a No. 2 D secondlamination is required to maintain a 2000f level(14). As the tension lamination for CombinationI was a somewhat lower grade than 301-22, itwas decided that the second tension laminationof Combination I be No. 2 D instead of No. 2MG.

Review of PreliminaryAssumptions

Clear Wood Design StressOne possible reason that the measured

strength for several beams fell below that pro-posed in designing them is that the clear wooddesign stress in bending for the coarse-grainmaterial, assumed to be 80 percent of that formedium-grain material, may have beenoverestimated. By considering the coarse-graininner beams independently and assuming thatthey determined beam strength, clear wooddesign stresses for coarse-grain material can becalculated for each beam. The procedure usedfollowed USDA Bulletin No. 1069 (8) and utiliz-ed the coarse-grain knot data discussed later.

The inner beam strength ratio for the 13laminations of Combination I was determinedto be 0.585 and for the 11 laminations of Com-bination II was determined to be 0.574. Usingthese strength ratios, the adjusted moduli ofrupture given in table 4 were converted to thelevel of coarse-grain clear wood values. Thesevalues of coarse-grain clear wood design stressesare listed in table I-3 for each beam.

The minimum value of clear wood designstress of the 20 beams by this analysis was 2,210p.s.i. (table I-3). This is somewhat lower thanthe value of 2,420 p.s.i. used in the initial designof the beams. Even though all 20 beams were

16

selected to have near minimum quality tensionlaminations and therefore are likely to representlower strength beams of the grade, a more con-servative value than 2,210 p.s.i. is probably ap-propriate for designing glued-laminated beamcombinations.

To statistically analyze the data from tableI-3, data for the two combinations were com-bined and assumed to be normally distributed.

17

Estimates of near minimum values are thenpossible and yielded values of 2,230 and 2,020p.s.i. for the 5 and 1 percent lower exclusionlimits, respectively.

Uncertainty associated with sampling andexperimental errors must be accepted but canbe partially quantified in analyzing the nearminimum value by another method. Results ofsuch an analysis given in figure I-1 show lower

end points of intervals said to contain at least 95percent of the population. From this figure, onecan estimate that the 5 percent exclusion limitis at least 2,000 p.s.i. with about 90 percent con-fidence.

Results of all analysis of near minimumclear wood design stresses for the coarse-grainmaterial indicate that a value of 2,000 p.s.i., orabout two-thirds of that used for medium-grainmaterial, maybe appropriate for use in design-ing glued-laminated beam combinations. Itshould be noted that such a value may not ap-

ply to single members and is valid for the interi-or lamination of glued-laminated beams only ifthe type of analysis given in Bulletin 1069 (8) isfollowed. An alternate analysis method beingconsidered which involves a transformed sec-tion concept would result in a lower clear wooddesign stress value.

See normal tolerance intervals in “Tolerance intervals I:Simulation studies of nonparametric tolerance inter-vals, “ by Habermann, Hermann. Proc. of IUFRO, sec.V, Forest Prod., Capetown, South Africa. 1973.

19

PERCENT CONFIDENCE

Figure I-1.— Tolerance limits for near-minimumclear wood design stress values in bending(5 pct exlusion limit) for use in derivingglulam combinations containing coarse-grain southern pine.

(M 141 540)

Modulus of Elasticity (MOE) 65 and 82 percent as stiff for the No. 2 and No.The lumber data indicated no significant

difference between the average MOE of the No.2 CG and No. 3 CG grades. If these two gradeswere combined, their average MOE would be1.17 million p.s.i. and average specific gravitywould be 0.46 as determined by the E-computer. If a reduction factor is applied to ad-just MOE values obtained by E-computer tovalues applicable for beam calculations by atransformed section analysis, an MOE value of1.1 million p.s.i. may be applicable for bothgrades. This agrees with the initially assumedMOE of the No. 3 CG grade and represents aslight reduction from the 1.2 million p.s.i. valueassumed for the No. 2 CG material.

A comparison of the actual MOE of the twogrades of coarse-grain lumber with thatpublished for medium-grain lumber (1,20) in-dicates that the coarse-grain lumber averaged

3, respectively. Direct comparison of the datafor the No. 2 MG and CG grades in this studyindicate the coarse-grain lumber to be 74 per-cent as stiff. As no No. 3 MG lumber was in-cluded, a similar comparison for this grade wasnot possible.

Slope-of-Grain Strength RatioOuter compression laminations for Com-

bination II beams were selected to be No. 1 MGlumber having near 1:10 slope of grain. As ex-pected, this did not appear to influence thefailure pattern of any beams as all failures ap-parently initiated in the tension half of thebeams. These data are insufficient to supportdefinite conclusions but do lend support to theproposal that slope-of-grain strength ratios areconservative in compression.

20

Knot Frequency AnalysisResults of a survey of the knot frequency

(17) in the No. 2 and No. 3 CG grades are sum-marized in table I-4. These data were obtainedfrom knot measurements made on themidlength 20 feet of each lamination.

The difference between knot properties ofthe two coarse-grain grades is quite small. Incomparison to results of the most recent knotsurvey on dense and medium-grain lumber boththe average and 1/2 percent exclusion limit knot

size in No. 2 CG was found to be significantlylarger. Results of the No. 3 CG lumber werequite close to the currently used values for No. 3MG lumber.

The small discrepancy between the twogroups of No. 3 lumber would have essentiallyno effect on the calculations given in table I-1.However, the changes in the No. 2 CG lumberknot properties have a significant effect due tolower predicted strength ratios for combinationscontaining No. 2 CG lumber.

APPENDIX II—LUMBERCHARACTERISTICS

Modulus of Elasticity andSpecific Gravity

The distribution of modulus of elasticityand specific gravity in the six grades of 2 by 6lumber used to manufacture the beams for thisstudy are given in the figures II-1 through II-6.Modulus of elasticity values were determinedflatwise by a vibration technique using an “E-computer.”

During the vibration evaluation, the lum-ber was supported at its ends as a simplebeam and the reaction at one end was recorded.This was assumed to be one-half of the totalweight. Specific gravity values were based onthe volume at time of test and the weight ad-justed to ovendry using a moisture contentrepresenting an average of readings taken atthree locations on the lumber.

21

Figure II-1.—Properties of No. 1 D southern pinelumber at an average moisture content of 13percent (N = 124)

(M 141 541)

22

Figure II-2.—Properties of No. 1 MG southernpine lumber at an average moisture contentof 12 percent. (N = 101)

(M 141 542)

23

Figure II-3.—Properties of No. 2D southern pinelumber at an average moisture content of11 percent. (N = 108)

(M 141 543)

24

Figure II-4.—Properties of No. 2 MG southernpine lumber at an average moisture contentof 10 percent, (N = 320)

(M 141 544)

25

Figure II-5.—Properties of No. 2 CG southernpine lumber at an average moisture contentof 10 percent. (N = 488)

(M 141 545)

26

Figure II-6.—Properties of No. 3 CG southernpine lumber at an average moisture contentof 10 percent. (N = 435)

(M 141 546)

27

Shear Strength of Coarse-GrainLumber

Development of lumber combinations forlaminating including coarse-grain lumber willrequire assignment of a stress in shear fordesign. Although using coarse-grain lumber forthe inner laminations may have a minor effecton design values in bending strength andstiffness, it is likely to have a major effect onshear strength.

It was estimated that coarse-grain lumberwould be weaker in shear than medium-grainand, to get an indication of the amount of theexpected reduction, a number of shear testswere conducted. The material used consisted of40 pieces of 2 by 6 lumber, No. 2 and No. 3coarse grain, that remained following manufac-ture. Small clear shear blocks which, with oneexception, met ASTM requirements (4), werecut from this lumber. The exception was that afull 2-inch-width specimen could not be ob-tained; rather, the width was about 1-3/8 in-ches, the thickness of the lumber. No attemptwas made to aline annual rings to obtain purelytangential or radial shearing planes and noattempt was made to avoid the pith in thespecimens.

Shear tests were conducted on the materialfollowing ASTM (4) procedures. The followingresults were adjusted to a 12 percent moisturecontent (2) for comparison with published datafor the southern pines:

Number of tests: 40Shear strength average: 1,100 p.s.i.Standard deviation: 190 p.s.i.The expected average shear strength of

southern pine at 12 percent moisture content is1,430 p.s.i. (6). By forming a ratio of the twostrength values for the two types of lumber, a“coarse-grain factor” of 0.77 is obtained. Know-ing that the average shear strength values aresubject to sampling and experimental errors,the use of the 0.77 factor would not be conser-vative.

An analysis of the shear strength data sum-marized in figure II-7 show that a value of 0.7for this ratio could be used with a high degree ofconfidence. Therefore, these data suggest thatcoarse-grain lumber might be designed with 0.7of the shear stress that is used for other southernpine.

For this analysis, it was assumed that the error in the pub-lished shear strength was small in comparison with thatof the coarse-grain sample and could be neglected.

APPENDIX III— TENSIONLAMINATIONS

CHARACTERISTICSProperties

Data collected on lumber used in the highlystressed region of the outer tension laminationare given in table III-1. Moisture content andMOE data were not used in the selection ofthese tension laminations. Specific gravity datawere used to assure that the lumber was nearaverage, i.e., not exceptionally heavy orlightweight.

Results of measurements on the majorstrength-reducing characteristics for the outertension laminations are given in tables III-2 andIII-3. The near-minimum quality 301-20 tensionlaminations were selected to have a knot andassociated grain deviation occupying nearly 50percent of a cross section with the remainderclear and straight grained. As given in table III-2, the average clear wood at the critical sectionfor the 10 beams was 53 percent—quite close tothe desired one-half. Similarly, the 301-24 ten-sion laminations were selected to be near-minimum quality with one-third of a cross sec-tion occupied by knots and grain deviation,Clear wood at the critical section given in tableIII-3 averaged 66 percent—also quite close tothe desired value of two-thirds.

The quality of lumber used for tensionlamination for the beams is shown in figures III-1 and III-3 while the location of failure in theouter tension laminations for each beam isshown in figures III-2 and III-4.

Tension Tests of Finger JointsTension tests were conducted on 12-foot-

long 2 by 6’s which had been finger jointed dur-ing the beam manufacture. This lumber, whichmet or exceeded the AITC 301-26 visual gradingrequirements (1), was evaluated according toprocedures previously used (15). Results of the28 evaluations are given in table III-4 and shownin figure III-5.

In comparison to two previous groups offinger-jointed lumber summarized in table III-5, this group had the highest average strength.Based on favorable performance of beamsmanufactured with the quality of jointsreported in FPL 151 (15), it would be concludedthat the finger joints in this study would also beadequate for high strength beams.

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Figure III-2.—Lamination data and failure loca-tion for midlength region of three outer ten-sion laminations are shown for the Combi-nation I beams. For each piece of lumberused in this region, modulus of elasticity(MOE) is indicated first (in million p.s.i.),then specific gravity (Sp. Gr.), and finallymoisture content (M.C.) in percent. Thelocation of primary failure in each lamina-tion is indicated by (M 141 550)

Figure III-3.—Near-minimum quality AITC301-24 grade tension laminations used forCombination II beams. Midlength was at20 feet, and constant moment section wasfrom 16 to 24 feet during test.

(M I40447-10 and M 140450-6)

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Figure III-4.—Lamination data and failure loca-tion for midlength region of three outertension laminations are shown for the Com-bination II beams. For each piece of lumberused in this region, modulus of elasticity(MOE) is indicated first (in million p.s.i.),then specific gravity (Sp. Gr.), and finallymoisture content (M.C.) in percent. Thelocation of primary failure in each lamina-tion is indicated by

(M 141 549)

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