Gwo-Huang ChenR.C. TangE.W. Price

AbstractFlexural properties as affected by environmental

conditions were evaluated for full-sized wood compositeI-beams webbed with oriented strand board (OSB), ran-domly oriented flakeboard (RF) and 3-ply Structural Iplywood (PLY). Solid-sawn southern pine 2 by 10's, or-dinarily used in light-frame building construction, werealso tested for comparative purposes. Environmentalconditions selected were 65 percent relative humidity(RH) (dry), 95 percent RH (humid) at 75°F, and 24-hourwater-spraying (wet) at ambient temperature. In the drycondition, the OSB group carried the largest load amongthe four beam types, but the load capacities of the fourbeam types were not significantly different in the hu-mid and wet conditions. Deflections at maximum loadof the four beam types were significantly different in thethree environmental conditions. The value for the lum-ber group was consistently higher than the I-beam typesin each test condition. Loads at I-inch deflection for thefour beam types were also significantly different in thethree environmental conditions. However, the value forthe lumber group was consistently lower than those ofthe I-beam types. In the dry condition, most failures thatoccurred in the PL Y group were in shear mode, whilethe majority of the OSB and RF members failed in bend-ing, and the failures occurred in the flanges. In the hu-mid condition, most of the I-beam and lumber specimensfailed in shear and bending, respectively. In the wet en-vironment, however, most of the I-beaIns failed in webbuckling, while most of the lumber members failed inbending.

lumber as flanges, and wood-based panels as webs. Asubstantial amount of research has been reported con-cerning the engineering performance of wood compositeI-beams in recent years. Results from the tests of com-posite wood/particleboard box- and I-beams indicatedthat the beams' load-deflection curves were linear tofailure, exhibiting virtually no warning signals prior tofailure. The beams were estimated to have 80 percentof the stiffness and 50 percent of the strength of a per-fectly clear southern pine 2 by 10 (4).

The behavior of I-beams webbed with two differenttypes of 1/4-inch-thick hardboards has also been inves-tigated (10). These beams were 12 feet and 6 feet long,11-7/8 inches deep, and fabricated with two pieces of1-7/16- by 2-1/8-inch laminated veneer lumber on eachside of the panels at top and bottom. The beams wereconditioned to equilibrium moisture content (MC) at 50percent relative humidity (RH) and 68°F, then testeddestructively in bending. Analysis of the load-deflectioncurves showed hardboard webbed I-beams were linearto a load level equivalent to 60 percent of the rail shearstrength of the web material determined from smallspecimen tests. Failure of the 12-foot hardboard webbedI-beams occurred in the tension flange with essentiallyno inelastic deformation prior to failure. Later, the be-havior of I-beams with 1/4-inch plywood as a web materi-al was included in the study. The results indicated that

The authors are, respectively, Research Assistant andProfe880r, School ofFore8tr'y and Alabama Agri. Expt. Sta., Au-burn Univ., Auburn, AL 36849-5418; an~ Manager of WoodProducts Service, Georgia-Pacific Corp., Decatur, GA 30035,formerly with the USDA Forest Serv., Southern Forest Expt.Sta. The investigation reported in this paper (AAES JournalNo. 8-881547P) is in connection with a project of the AlabamaAgri. Expt. Sta., Auburn U niv ., in ~tion with SFES, andis published with the approval of the Director. This paper wasreceived for publication in March 1988.~ Forest Products Research Society 1989.

Forest Prod. J. 39(2):17-22. .

As the cost effectiveness of engineered products im-proves, wood composite I.beams are becoming more fre-quently used in medium. and light.frame wood structur-al systems (3,7,9). In general, wood composite I-beamsare composed of solid wood or parallel-laminated veneer

FOREST PRODUCTS JOURNAL 17v~...~

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groups were not significantly different. However, 1-beams and 2 by 10 southern pine lumber were statisti-cally different with respect to beam stiffness. The 1-beams carried higher loads at I-inch deflection than didthe lumber members.

It is recognized that composite I-beams in medium-and light-frame structural systems may often encoun-ter humid and/or wet environments during their serv-ice life or during on-site construction. However, most in-vestigations related to the performance of compositebeams have been conducted in ambient or dry environ-ments, as previously mentioned. For a better under-standing of the engineering performance of compositebeaJns in service environments, information with regardto their flexural properties as affected by the MC levelis needed. Therefore, the objective of this study was toevaluate the effect of various environmental conditionson the flexural properties of wood composite I -beaJns andcompare the resulting properties with those of southernpine lumber.

..po

ipo

'-beam configurations (top) and testing set-upFigure 1

(bottom).

under a short-term destructive test, the l2-foot hard-board webbed I-beams were twice as strong and 50 per-cent stiffer than the equivalent plywood webbed speci-mens (8).

The bending strength of fabricated beams withflanges of minimally machined whole or half stems oflodgepole pine and webs made of lodgepole pine flake-boards has also been evaluated (5). The results indicat-ed that the beams conditioned and equilibrated at 7 to10 percent MC carried more load at failure than thoseof Douglas-fir and larch 2 by 10's, 2 by 12's, and9.5-inch-deep beams fabricated with parallel-laminatedDouglas-fir veneer flanges. Moreover, the pine beamshad significantly less deflection than 2 by 10's or beamsfabricated with laminated-veneer flanges, and wereequal in stiffness to 2 by 12's.

More recently, the flexural properties of four typesof I-beams webbed with plywood, waferboard, and OSBmade of aspen, were evaluated at 55 percent RH and75°F, and compared with southern pine lumber data (6).The I-beams were 16 feet long, 10 inches deep, and werefabricated with solid southern pine lumber flanges withcross-sectional dimensions of 2-5/8 by 1- m inches. It wasfound that OSB I-beams carried more load and higherstress at failure than the other I-beam types. Failuresof members webbed with OSB, waferboard, and plywoodwere not frequently associated with the failure of theflange-web joint in tension. Comparisons between 1-beams and 2 by 10 southern pine lumber indicated thatthe load capacities and maximum stresses for the two

Materials and methodsThe I-beams were built up from solid wood flanges

and wood composite panels as webs. The flange stock wassouthern pine lumber of grades No.1 Dense and SelectStructural with a minimum machine stress rating (MSR)MOE of2.2 x 10- psi. The flange elements, 2.625 incheswide and 1.5 inches deep, were finger-jointed at randomintervals along the beam's length to accomplish theproduction of long beams, but the intervals between thefinger joints were never less than 72 inches. The webmaterials were 3/8-inch wood composite panels, i.e.,Structural I C-C grade southern pine plywood (PLY), 3-layer oriented strandboard (OSB), and randomly orient-ed flakeboard (RF) fabricated with a 50/50 mixture ofsweetgum and red oak flakes and bonded with liquidphenol-formaldehyde. Plywood was commercial board,but the OSB and RF were noncommercial boards. Panelswere sawn and used in beam fabrication such that themajor panel axis (8-ft. direction) was perpendicular to thelength of the beam. All webs were butt-jointed at 4-footintervals. Flange-web joints and butt joints in webs werebonded with phenol-resorcinol adhesive. The solid lum-ber specimens tested in this study, No.2 southern pine2 by 10's, ordinarily used as standard supporting mem-bers in light-frame building Structures, were purchasedat a local retail lumberyard in 16-foot lengths. The I-beams were fabricated with a depth of 10 inches and atotal length of 40 feet by Trus Joist Corp. at Valdosta,Ga., and each beam was later trimmed to two 16-foottesting members at Auburn University's Forest ProductsLaboratory. The I-beam configuration, as well as the lo-cation of butt joints, is pictured in Figure 1.

Three environmental conditions were chosen for thisstudy: 1) dry condition (65% RH at 75°F); 2) humid con-dition (95% RH at 75°F); and 3) wet condition (24-hr.water-spraying). The dry condition was used to simulatea normal use condition, the humid condition was usedto simulate protected exterior use during rain or veryhumid weather, and the wet condition was intended tosimulate on-site construction immediately after heavyrain.

FEBRUARY 198918

Agure 2. - Aexural properties for four beams types evaluated under three envorinmental conditions. A. Load capac-

ities: B. Deflections at maximum load: and C. loads at 1..jnch deflection.

TABLE 1. - Simplc .tan.aa ~ lamber.all , in ,.,. ~lWiIv,,-"-Ial~-- ~-

Be8m 9t~~-type (ft~

At I-m.Maximum d8OeetioD ~. MCCCBIditioD

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0.1

12.8

0.1

11.6

0.2

10.0LO

18.1

0.6

17.9

0.3

17.7

0.7

13.8

0.6

8.8

0.9

~

6.1

4.

1.2

8.0

~1

(in.)

2.37

0..2.370.132.130.-US0.83

~0-11I.-0.-U60.57a.-0.88

4-100.87~0.-2.330..-4-820..

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4,646-6,3632.1667,3801'-6,508

797

4,-682

..-982

6,3151,4186-1,497

..-867

S.686-

3,634-

3,8901.407

Dry PLY MeaDsri"

RF MUDSD

~ MMDSD

2 by 10 MMDSD

Humid PLY MMDSD

RF MeaDSD

~ ManSD

2 by 10 MeaDSD

W. PLY w.aSD

RF MeaDSD

~ MeanSD

2 by 10 MeaDSD

. DeOectiOD at ~um 1O8d.~ SD - ItaDd8N deYiation.

6O-ton Tinius-Olsen testing machine <Fig. 1). All beamswere tested on a 15-foot span with the best flange or edge(visually less defects) in tension. Loads and correspond-ing deflection measurements were obtained by using acomputer-controlled data acquisition system. A 25-k.ipuniversal load cell was used to sense the loads, while alinear potentiometer was used to measure the midspandeflection. All tests were conducted with a constant loadrate of 0.20 inch per minute, and the data acquisitionsystem made load and deflection readings at 3-second in-tervals. After a beam failed, the test data were printed.and a load-deflection curve was plotted. A 5-inch seg-ment was cut from the end of each beam to determinethe beam's MC. Failure patterns of beams were record-ed and photographed for use in failure mode analysis.

Results and discussion

Beam performanceThe testing results and their statistics are summa-

rized in Table 1, and the flexural properties for the fourbeam types, evaluated under the three environmentalconditions, are plotted in Figure 2.

Analyses of variance (ANOV A) for the flexural pr0p-erties of lumber and I-beams in different environmentswere performed at the 90 percent probability level, andthe results of Duncan's multiple-range tests are summa-rized in Table 2. Load capacities for lumber and I-beamtypes were significantly different only in the dry condi-tion.1n this environment, OSB specimens carried signif-icantly greater loads than the other three beam types.while lumber members, RF, and PL Y types did not showsignificant differences in load capacities.

ANOV A for deflections at m~_.:imum load showedsignificant differences between beam types in all threeenvironmental conditions. Results of Duncan's multi-ple-range test indicated that, in the dry condition, lum-ber specimens deflected significantly more than PL Y andRF types, but the OSB deflections were not significant-ly different from the other three beam types. In the hu-mid condition, the RF type deflected significantly lessthan the other three beam types and the deflections forlumber and the other two I-beam types were not signifi-

Four specimens were randomly selected from thepopulation of each beam type and exposed to each en.vironmental condition. For the dry and humid condi-tions, beams were stored in a controlled environment ata constant temperature of 75°F for at least 6 weeks. Forthe wet condition, beams were supported at a height 5feet above the unsbaded ground and continuouslywater-sprayed for 24 hours. The average MC for eachbeam type was approximately equal or exceeded the fibersaturation point (FSP) (Table 1).

After these exposures. each beam was tested tofailure in flexure with third-point loading according tothe ASTM standard D 198-76 (2) by using a modified

19FOREST PRODUCTS JOURNAL va..",

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fected by the environmental conditions are in progressand they will be reported in separate articles.

Literature cited1. American Institute of Timber ConstnJCtion. 1985. Timber Constrw:-

tion Manual. 3rd ed. John Wiley &. Sons, N.Y.2. American Society for Testing and Materials. 1983. Static tests of

timbers in structuralsius. ASTM D 198-76. ASTM, Philadelphia,Pa.

3. Booth, L.G. 1974. The effect of flange-web joint displacement onthe design of plywood web I-beams- J. of the Institute of WoodScie~. 6(6):13-24.

4. Johnson, JA et aL 1976. The perfonnaDCe of composite wood/par-ticlelMl8rd beam under- two-point 1o&ding. Wood aDd Fiber 8(2)-.85-97.

5. Koch, P. and E.J. Burke. 1985. Strength of fabricated joists withflanges of minimally machined whole or half stems of lodgepolepine. Forest Prod. J. 35(1):39-47.

6. Leichti, R.J. 1986. A-..ing the reliability of wood composite 1-beama. Ph_D. diss. Auburn Univ., Auburn, Ala.

7. McNatt, J.D. 1980. Hardboard-webbed beams: research aDd appli.cation. Forest Prod. J. 3CXI0)-.57-64.

8. aDd MoJ. Superresky. 1983. I..ong-tenn load performanceoCl1ardIMI8rd I-beams. Res. Pap. FPL-441. USDA Forest Serv., FOI'e&tProd. Lab., Madison, Wis.

9. Sliker, A. aDd O. Suchsland. 1982. Rxamination of a sin~idaljointf« 88embling wood I-beam aJmPOnents. Forest Prod. J. ~16-20.

10. Superfesky, M.J. aDd T.J. Ramaker. 1976. Hardboard-webbed 1-beama subjected to shOl't-term loading. Res. Pap. FPL-264. USDAForest Serv., Forest Prod. Lab., Madison, Wi&.

environmental condition, with the value for the lumbermembers consistently higher than those of the I-beamtypes.

3. Significant differences in loads at I-inch deflec-tion were found for the four beam types in each environ-mental condition. Southern pine lumber consistently hadthe smallest value among the four beam types.

4. As the MC of the beams increased, the OSB grouplost a greater percentage of load capacity than the othergroups. For the PLY and lumber groups, beams deflect-ed the most in the wet environment.

5. Overall performance of all beam types in the threedifferent environmental conditions indicated that woodcomposite I-beams carried similar or slightly higherloads at I-inch deflection than No.2 grade southern pine2 by 10 lumber. Also, lumber members were not as stiffas I-beams.

6. Flexural behavior of the beams was reflected bytheir failure modes. Fibers in the wood were softened andthe strengths of the web materials were decreased as thebeam moisture increased. As a result, more I-beamsfailed in web buckling, whereas most of the 2 by 10 lum-ber failed in excessive deflection.

7. Additional studies on the long-term performanceof composite I-beams and solid lumber members as af-

21 FEBRUARY 1989