Flexural properties of eastern hardwood pallet parts

Transcription

1 Flexurl properties of estern hrdwood pllet prts John A. McLeod Ill Mrshll S. White Pul A. lfju Philip A. Armn Abstrct Accurte estimtes of the flexurl properties of pllet prts re criticl to the sfe, yet efficient, design of wood pllets. To develop more ccurte dt for hrdwood pllet prts, 840 stringers nd 2,520 deckbords, representing 14 hrdwood species, were smpled from 35 mills distributed throughout the Estern United Sttes. The prts were sorted by species, visully grded, nd tested in bending to destruction. This rticle describes the methods used to conduct this reserch nd presents summry sttistics of the flexurl properties for the species nd sizes tested. The modulus of rupture nd modulus of elsticity of nominl 6-inch-wide deckbords is consistently greter thn tht of 4-inch mteril. The visul stress grding procedure used in this study cn segregte pllet prts into strength nd stiffness clsses. These dt will now be incorported into the CAD procedure for pllets known s the Pllet Design System, which ids designers in selecting wood pllets for use in mteril hndling. The wood pllet nd continer industry is the lrgest user of hrdwood lumber in the United Sttes. In 1989, n estimted 6.4 billion bord feet (BBF) of hrdwood lumber ws used to construct over 400 million hrdwood pllets (14). The wood pllet is literlly nd figurtively the bse of the unit-lod mterilhndling system by which most products re trnsported nd stored. It is the principl interfce between the rigors of the hndling environment nd the pckged product. The function of the pllet is to fcilitte trnsport nd storge of, s well s protect, the product. Becuse of the reltively lrge volume of wood required to mnufcture pllets, nd the role of the pllet in mteril hndling, the design nd selection of wood pllets impcts on 1) the vilbility of timber for lterntive uses; 2) the cost to society of solid wste disposl; 3) the cost of consumer goods; 4) productivity nd competitiveness of industry; nd 5) humn helth nd sfety, which cn be jeoprdized by pllet filures. Recent coopertive reserch efforts between Virgini Polytechnic Institute nd Stte University (Virgini Tech), the USDA Forest Service, nd the Ntionl Wooden Pllet nd Continer Assocition (NWPCA) resulted in computerized, structurl design procedure tht significntly improves the design nd selection process for wood pllets. This design procedure, known s the Pllet Design System (15), incorportes dvnced structurl nlysis techniques nd is rpidly becoming ccepted by the mteril-hndling industry s the stndrd procedure for designing nd selecting wood pllets. Accurte estimtes of the flexurl properties of the pllet prts re criticl to the sfe, yet efficient, design of pllets. Historiclly, estimtes of lumber strength nd stiffness hve been bsed on results of tests of smll cler specimens, with modifictions to these smll cler vlues to ccount for the effect of size nd grde differences. However, recent study showed tht strength rtios used to ccount for the effect of defects in ok pllet prts resulted in conservtive estimtes of bending strength, prticulrly for lower grdes with The uthors re, respectively, Reserch Associte, Associte Professor, nd Grdute Assistnt, Dept. of Wood Sci. nd Forest Prod., VPI&SU, Blcksburg, VA 24061; nd Project Leder, Primry Hrdwood Processing nd Prod. Res., USDA Forest Serv., Southestern Forest Expt. St., Blcksburg, VA This study ws coopertive reserch project funded by the USDA Forest Serv., Southestern Forest Expt. St., the Ntionl Wooden Pllet nd Continer Assoc., nd VPI&SU. This pper ws received for publiction in December Forest Products Reserch Society Forest Prod. J. 41(7/8): JULY/AUGUST 1991

2 TABLE 1. Common nd scientific nmes of wood species nd species groups tested. Common nme Blck locust Hickory Ash Hrd mple Americn beech Birch Blck cherry Sweetgum Blckgum Soft mple Hckberry Aspen Cottonwood Bsswood Scientific nme Robini pseudocci Cry spp. Frxinus spp. Acer spp. Fgus grndifoli Betul spp. Prunus serotin Liquidmbr styrciflu Nyss spp. Acer spp. Celtis spp. Populus spp. Populus spp. Tili spp. Figure 1. Geogrphic loction of pllet mills from which estern hrdwood pllet prts were smpled. lrger defects (12). The pplicbility of the trditionl size fctor hs been questioned due to trends in the reltive strength of pllet stringers versus deckbords (11). Much better estimtes of lumber strength nd stiffness cn be obtined from tests of full-size pieces. The structurl lumber industry hs tested full-size members of ech species nd grde in order to develop more ccurte design vlues (6). For pllet prts, however, only estern oks (10) nd yellow-poplr (9) hve been evluted through full-size testing. Flexurl property estimtes for ll other species re currently bsed on modified clerwood vlues. In order to generte more ccurte flexurl strength nd stiffness estimtes for estern hrdwood pllet prts, testing progrm ws initited. The resulting dt will llow development of flexurl property estimtes for estern hrdwood pllet prts bsed on in-grde testing. In turn, performnce estimtes for hrdwood pllets will be more ccurte nd relible, nd the hrdwood resource cn be used more efficiently. Hevy-demnd species cn be more efficiently utilized, while underutilized species cn be substituted with confidence in predicted performnce. Dt from this study cn lso be used to evlute the reltive qulity of estern hrdwood pllet prts from the grde distributions. Historicl chnges in qulity distribution cn be monitored by comprison with previous studies (7). The effectiveness of the visul grding rules in segregting estern hrdwood pllet prts by strength nd stiffness cn be evluted. Clibrtion studies used to set sfety levels in the Pllet Design System (8) cn lso be improved using this new dt. Finlly, species groupings for estern hrdwoods will more ccurtely reflect similrities nd differences in mechnicl properties of pllet prts. Objectives The objectives of this reserch project were the following: 1. To determine modulus of rupture (MOR) nd modulus of elsticity (MOE) of commonly used species of estern hrdwood pllet prts bsed on full-she tests of pieces of typicl dimension nd qulity. 2. To evlute the bility of visul grding system to segregte estern hrdwood pllet prts by MOR nd MOE. 3. To determine the proportions of currently produced estern hrdwood pllet prts tht fll within ech grde, or qulity clss. 4. To crete n estern hrdwood pllet prts dtbse for use in developing flexurl design vlues nd for use in clibrting the structurl nlysis methodology used in the Pllet Design System. Mterils nd methods Pllet prts were smpled from 35 pllet mills in 26 sttes throughout the estern hrdwood growing region. Pllet mills were selected primrily from the NWPCA membership list (13). The selection ws bsed on willingness to prticipte in the study by donting mteril nd llowing the reserch tem to visit their site. Figure 1 shows the geogrphicl loction of ech mill. Smples were rndomly selected from inventory of cut-to-size pllet prts t the mills. A totl of by 4- by 48-inch stringers, 1,3181- by 4- by 40-inch deckbords, nd 1, by 6- by 40-inch deckbords were collected nd tested. Species smpled were representtive of the mteril t ech mill t the time of smpling, except tht no ok or yellow-poplr ws selected. Ech piece ws identified s to genus nd, if possible, species. Tble 1 lists the common nd scientific nmes of the species smpled. The smples collected t ech mill were representtive of the qulity distribution before ny segregtion by grde or removl of cull pieces. Hrdwood pllet prts re produced from cnts or lumber t green moisture content (MC). Typiclly, the mteril is ssembled into pllets without drying, nd the pllets re initilly used while the mteril is still bove fiber sturtion. Currently, ll Pllet Design FOREST PRODUCTS JOURNAL Vol. 41, No. 7/8 57

3 System performnce estimtes reflect this prctice by using green mteril property vlues. It ws desired in this study to test the mteril while bove fiber sturtion. Therefore, once the mteril ws selected from mill, it ws wrpped in plstic to prevent chnge in MC. The mteril ws then trnsported to Willim H. Srdo Jr. Pllet nd Continer Reserch Lbortory t Virgini Tech. Once t the lbortory, the mteril ws stored submerged in wter tnks to prevent drying nd to prevent biologicl deteriortion during the time required to test the mteril. Given the lrge smple size, some dely between time of smpling nd time of testing ws unvoidble. In order to minimize this dely, the smpling/testing process ws divided into three phses. AU the mteril smpled during prticulr phse ws lso tested before beginning the next phse, keeping the storge intervl prior to testing under 60 dys. Cross-sectionl dimensions were mesured to the nerest inch. Ech piece ws visully grded ccording to specifictions developed by Srdo nd Wllin (16). This is the sme grding scheme used in the ok prts study (10). A summry of this grding scheme is shown in Tbles 2 nd 3. The prts were then tested to destruction in third-point bending using procedures outlined in ASTM D (1). Deckbords were tested fltwise t 36-inch spn nd stringers on edge t 45-inch spn, with the tension fce rndomly selected. Lod nd centerspn deflection were mesured continuously throughout the test. Devitions from the ASTM stndrd were rte of unloded crosshed trvel, which ws 1.1 nd 3.2 inches per minute for stringers nd deckbords, respectively. This ws necessry in order to test the lrge number of smples in n economicl time frme. Adjustments for pllet prt MOR nd MOE from this rte of loding to the ASTM stndrd test rte hve been developed by McLeod (11). After testing, n MC nd specific grvity (SG) smple ws tken from every third piece. MOR nd MOE were clculted s in ASTM D MOE ws corrected for sher effects using procedures in ASTM D (3). Results nd discussion This study is n unblnced fctoril design with three fctors: species, size, nd grde. It is unblnced becuse it ws imprcticl to control the number of Size of knot Loction of knots Clusters of knots Type of knots Cross grin Splits, checks, nd shke Wne Decy Pith Mismnufcture TABLE 2. Grding criteri employed for stringers. Grdes of prts Defect Description 2&BTR 3 4 Mximum portion of cross section ffected 1/4 of cross section l/3 of cross section Over notch or in end 6 in. of the stringer 1/2 in. mx. dimeter 1/4 of cross section Knots over 1/2 in. in dimeter spced 3 in. or less prt re mesured s one defect None 1/3 of cross section Knot holes, unsound or loose knots, nd holes 1/8 of cross section 1/6 of cross section Slope of generl cross grin 1 in. in 10 in. 1 in. in 8 in. Mximum dimension of locl cross grin 1/4 of cross section 1/3 of cross section Mximum length singly or in combintion Defects 3 in. or less re ignored 1/4 of length of prt 1/2 of length of prt Mximum portion of cross section 1/16 of cross section 1/8 of cross section Portion of nil fce width 3/16 effce 1/4 of fce Mximum portion of cross section None 1/8 of section Length in fce None Length boxed None 1/3 length Mximum portion of cross section, nd 1/2 of cross section 1/3 of cross section 1/2 of cross section 1/4 of cross section 1 in. in 6 in. 1/2 of cross section 3/4 of length of prt 3/16 of cross section 5/16 of fce 1/4 of section mximum of 10 in. long 1/16 of cross section 1/8 of cross section 3/16 of cross section TABLE 3. Grding criteri employed for deckbords. Grdes of prts Defect Description 2&BTR 3 4 Size of knot Mximum dimension cross width of the bord 1/4 of bord width 1/3 of bord width 1/2 of bord width Loction of knots 1/2 in. dimeter 1/4 of bord width 1/3 of bord width Clusters of knots 1/4 of bord width 1/3 of bord width 1/2 of bord width Type of knots Cross grin Splits, checks, nd shke Wne Decy Pith Mismnufcture Knots in the edges nd end 3 in. of the bords Knots over 1/2 in. in dimeter spced 3 in. or less prt re mesured s one defect Knot holes, unsound or loose knots, nd holes Slope of generl cross grin Mximum dimension of locl cross grin Mximum length singly or in combintion Defects 3 in. or less re ignored Mximum portion of cross section ffected t point of deepest penetrtion Cross section deepest penetrtion In fce of bord Boxed Mximum 10 in. in length: nd mximum cross section 1/8 of bord width 1 in. in 10 in. 1/4 of bord width 1/4 of bord length 1/16 of cross section None llowed None None 1/16 of cross section 1/6 of bord width 1 in. in 8 in. 1/3 of bord width 1/2 of bord length 1/8 of cross section 32 units 1/3 length 1/8 of cross section 1/4 of bord width 1 in. in 6 in. 1/2 of bord width 3/4 of bord length 3/16 of cross section 64 units 1/2 of cross section 58 JULY/AUGUST 1991

4 pieces smpled of prticulr species nd size. Insted, the number of pieces of ech were representtive of current production nd inventory t ech mill. Positive species identifiction nd visul grding did not tke plce until the smpled mteril ws t the lbortory. Lrger smple sizes for certin species reflect greter volumes of use over lrger geogrphicl re. The greter smple size for deckbords thn stringers reflects the number of pieces of ech required to construct pllet. It ws undesirble to control the number of pieces in ech grde since qulity distribution ws vrible of interest in this study. Mill or geogrphicl re ws not considered fctor in this study due to the vrible number of pieces of ny given species nd size selected from ech mill. The MC of ll mteril tested ws bove fiber sturtion. Since moisture level bove fiber sturtion is ssumed not to ffect flexurl strength nd stiffness MC is not fctor in this study. The generl liner model procedure of the Sttisticl Anlysis System (17) ws used to perform n nlysis of vrince to test for ll min effects nd interctions on MOR nd MOE. Tble 4 presents the results of this nlysis. As expected, species, size, nd grde hd significnt effect on both MOR nd MOE. All interctions between the three min fctors were lso significnt with the exception of the size x grde interction, which ws not significnt for MOE. Size effects Tble 5 contins MOR, MOE, SG, nd MC dt for ech size clss of pllet prts tested. Men nd nonprmetric lower fifth percentile MOR nd men MOE were gretest for 6-inch deckbords, followed by 4-inch deckbords, nd then stringers. A Duncn s multiple- TABLE 4. Anlysis of vrince for modulus of rupture (MOW nd modulus of elsticity (MOE) of estern hrdwood pllet prts. Fctor MOR MOE Species p < p < Size p = p = Grde p < p = Species x size p = p = Species x grde p = p = Size x grde p = p = Species x size x grde p = p = The null hypothesis sttes there is no effect of the fctor on MOR or MOE. These p-vlues represent the probbility tht if the null hypothesis ws true, the differences In MOR nd MOE would be observed. rnge test (α = 0.0 1) indicted the men MOE of ech size clss ws significntly different, while the men MOR ws not significntly different between the 4-inch nd 6-inch deckbords. Pllet deckbords re most commonly produced in two nominl size clssifictions: 1 inch thick by 4 inches wide or 1 inch thick by 6 inches wide. For hrdwood pllet deckbords, the ctul dimensions will vry widely nd depend on mnufcturer nd minimum cceptble dimensions in prticulr pllet specifiction. In the previous ok prts study ( 10), the men MOR nd MOE were significntly greter for 6-inch thn 4-inch deckbords. Bsed on sttisticl size-effect (19), 6-inch deckbords re more likely thn 4-inch deckbords to contin strength-reducing defect due to their greter volume. However, there mybe other rtionl resons for differences in the flexurl properties between 4-inch nd 6-inch deckbords when production method is considered. Most hrdwood pllet deckbords re reswn from 4 by 6 cnts. It my be resonble to expect tht the verge knot dimeters might not differ between the two deckbord sizes in proportion to their width. Indeed, in the ok prts study the verge knot dimeters were 1.44 nd 1.54 for the 4-inch nd 6-inch deckbords, respectively (1 1). Knots of equivlent dimeter will more severely reduce the flexurl strength nd stiffness of 4-inch thn 6-inch deckbord. Tble 6 contins MOR, MOE, nd SG dt by individul species nd size. For most of the species in this study, the verge nd nonprmetric lower fifth percentile MOR nd verge MOE were greter for the 6-inch thn 4-inch deckbords. The lck of sttisticl significnce for the difference in men MOR between 4-inch nd 6-inch deckbords, s reported in Tble 5, is likely due to the high vribility given the number of different species. If species re considered seprtely, sttisticl t-tests show the difference in men MOR, s well s men MOE, to be significnt for mjority of the species tested. As indicted by McLin et l. (10), current pllet design procedures do not differentite mteril properties between deckbord sizes. Only two size clsses re considered, stringers nd deckbords, with 4-inch nd 6-inch deckbords combined. In the future, however, trends seen in this nd previous studies my wrrnt differentition of flexurl properties between these two deckbord sizes, prticulrly given current TABLE 5. Summry of properties of estern hrdwood pllet prts by size. MOR MOE 5th Duncn Duncn Size N percentile Men COV b grouping c Men COV grouping N SG MC (x10 3 psi) (%) (x10 6 psi) (%) (OD wt./od vol.) (%) B C , A B , A A Nonprmetric lower fifth percentile. COV - coefficient of vrition. Duncn-s multiple rnge test. Mens with the sme cpitl letter re not significntly different t the level. b c FOREST PRODUCTS JOURNAL Vol. 41, No. 7/8 59

5 TABLE 6. Properties of estern hrdwood pllet prts by species nd size. MOR Species Size N 5th percentile Men COV b Men COV N SG (x10 3 psi) Blck locust Hickory Ash Hrd mple Americn beech Birch Blck cherry Sweetgum Blckgum Soft mple Hckberry Aspen Cottonwood Bsswood (%) (x10 6 psi) (%) (OD wt./od vol.) , , Nonprmetric lower fifth percentile. COV - coefficient of vrition. b MOE mnufcturing prctices for hrdwood pllet deckbords. Six-inch deckbords hve some dvntges over 4-inch deckbords from pllet performnce stndpoint; they hve greter lod-crrying cpcity due to their greter section modulus, nd they cn provide greter joint rigidity becuse they cn receive greter number of fsteners per joint without splitting. If they lso hve superior flexurl properties (independent of section modulus), this could ffect future design trends nd pllet mnufcturing prctice by fvoring production nd use of 6-inch over 4-inch deckbords. As seen in Tble 5, verge nd nonprmetric lower fifth percentile MOR nd verge MOE were lower for stringers thn deckbords. A lower MOR for stringers would be expected bsed on the trditionl depth effect (5). MOR nd MOE were lso found to be lower for stringers thn deckbords in previous studies of yellow-poplr (9) nd red lder nd biglef mple (4). In the ok prts study, stringer MOE ws lower thn deckbord MOE (10). However, stringer MOR ws ctully greter thn deckbord MOR, opposite the trend expected bsed on difference in depth. It ws postulted tht ny size effect my be species dependent since ok is quite different ntomiclly thn yellowpoplr, lder, or mple. However, in this study, only soft mple nd sweetgum stringer MOR ws greter thn the verge MOR for deckbords. Ring-porous species such s sh nd hickory, s well s ll other species tested, hd higher deckbord MOR compred to stringer MOR. MOE ws lower thn deckbord MOE for ll the species tested. No definitive explntion cn be offered for the higher stringer MOR in ok, soft mple, or sweetgum. However, it does not pper to be relted to ntomicl differences nor to differences in SG or MC. Sweetgum stringers did hve greter percentge of higher grde mteril compred to deckbords. In the ok prts study, the loding rtes used resulted in somewht higher strin rte in stringers compred to deckbords, which would tend to increse the stringer MOR. Overll, the mjority of species tested in this nd previous studies tend to hve lower MOR for stringers thn deckbords, s expected. Grde distributions In this study, the grde distribution refers to the percentge of popultion of pllet prts tht flls within ech of the grdes defined by Srdo nd Wllin (16). The grde distribution is useful for evluting the 60 JULY/AUGUST 1991

6 reltive qulity of the mteril. Currently, most pllet mnufcturers do not seprte individul grdes. Insted, only lower qulity mteril is removed, resulting in combined grde, such s 4&BTR or 3&BTR. The grde distribution cn lso be used in development of design vlues for these combined grdes (11). Tble 7 contins the grde distribution by size for the estern hrdwood pllet prts smpled in this study. There ws greter percentge of 4-inch deckbords of the highest grde compred to 6-inch deckbords. This might be expected bsed on Weibull s sttisticl size-effect theory (19) in which 6-inch deckbords re more likely thn 4-inch deckbords to contin strength-reducing defect due to their greter volume. However, there ws lso slightly greter percentge of 4-inch deckbords of the lowest grdes. A greter percentge of 6-inch deckbords were of mid-qulity grde 3. This tends to support the previous discussion tht bsed on current hrdwood deckbord mnufcturing prctice, verge knot sizes might not differ between 4-inch nd 6-inch bords proportionl to their size. Equivlent knot sizes would downgrde 4-inch more thn 6-inch deckbord. Deckbords hd greter percentge of pieces in the highest grde nd smller percentge of pieces in the lower two grdes compred to stringers. Since stringers nd deckbords re typiclly reswn from cnts in similr fshion, but with greter thickness for stringers, the difference in grde distribution between these two sizes my be due to the greter volume in stringers nd therefore the greter likelihood of defects. Tble 7 contins two dditionl grde distributions mesured in erlier studies. The first is from qulity distribution study of estern hrdwood deckbords published in 1974 (7). In 1985, McLeod (11) compiled ll dt sets of grded pllet prts to dte nd computed the weighted verge grde distribution shown in Tble 7. This verge represents both stringers nd deckbords, hrdwoods nd softwoods, nd estern nd western U.S. species. Coincidentlly, the percentges in these two grde distributions re identicl (rounded to the nerest 0.5%). The grde distributions indicte tht currently produced estern hrdwood pllet prts hve smller proportion of very high qulity pieces compred to pllet prts produced over the pst 15 to 20 yers. There ws more of decrese in the percentge of high grde prts for stringers compred to deckbords. Pllet prts re typiclly produced from low qulity timber resources. Nevertheless, current s well s previous studies hve found tht the mjority of pieces produced meet the higher qulity pllet prt grdes, with smller proportion flling into the lower grdes. This indictes the wood pllet industry s bility to utilize nd even upgrde low qulity resource through remnufcture. A notble exception to this trend ws the estern ok prts study (10), which hd nerly TABLE 7. Comprison of grde or qulity distribution for estern hrdwood pllet prts smpled in this study with similr distributions mesured in previous studies. Grde distribution N 2&BTR 3 4 CULL (%) s s 1, s 1, Previous study of estern hrdwood deckbords 17, Averge for ll vilble dt sets of grded pllet prts b 43, Species include mple. beech, birch, ok, nd gum (7). Includes deckbords nd stringers, hrdwoods nd softwoods, estern nd western U.S. species (11). b TABLE 8. Flexurl properties of estern hrdwood pllet prts by size nd grde. Duncn Duncn Size Grde N L5EL Men COV b grouping c Men COV grouping (x10 3 psi) (%) (x10 6 psi) (%) 2&BTR 324 4, A A B B C C CULL D D 2&BTR A A B B C C CULL C C 2&BTR A A B B B BC CULL C 1, C Nonprmetric lower fifth percentile. COV - coefficient of vrition. Duncn's multiple rnge test. Mens with the sme cpitl letter re not significntly different t the level. b c MOR MOE FOREST PRODUCTS JOURNAL Vol. 41, No. 7/8 61

7 Figure 3. Comprison of MOE of estern hrdwood pllet prts by species. Refer to Figure 2 for n explntion of the smll cler specimen vlues. Figure 2. Comprison of MOR of est- ern hrdwood pllet prts by species. Smll cler specimen vlues re from ASTM D (2) except for cherry, which is from the Wood Hndbook (18). Smll cler vlue for hickory is the verge for pecn, wter, mockernut, pignut, shgbrk, shellbrk, bitternut, nd nutmeg; for sh, the verge is for green nd white; for hrd mple, it is for sugr nd blck; for birch, it is for pper, sweet, nd yellow; for soft mple, it is for red nd silver; nd for spen, it is for bigtooth nd quking spen. equl percentges in ech grde. As noted by the uthors of tht study, this my hve been due to hevy demnd for ok by the furniture industry t the time, reducing the overll qulity of mteril vilble to the pllet industry for remnufcture into pllet prts. While the grde distribution for ech species in the current study ws slightly different, most were resonbly close to the verges shown in Tble 7. Effectiveness of visul grding The primry purpose of the pllet prt grdes used in this study, which hve specific limits on size nd loction of knots, slope of grin, nd other defects, is to segregte mteril by flexurl strength nd stiffness. Tble 8 provides MOR nd MOE dt by size nd grde. The lower fifth percentile nd verge MOR nd verge MOE decresed for both stringers nd deckbords with decresing grde. Four-inch deckbords of grde Cull were the only exception, hving slightly higher verge MOR nd MOE thn grde 4. However, the lower fifth percentile MOR ws lower for grde Cull. The coefficient of vrition of the flexurl properties tended to increse with decresing grde. These trends re s expected nd indicte tht the grding specifictions llow seprtion of mteril with differing flexurl properties on visul bsis. To determine which grdes hd sttistics.lly significnt differences, Duncn s multiplernge test ws performed (α = 0.05). Results of this test, s given in Tble 8, indicte the men MOR nd MOE re significntly different between ech grde of string- ers. For 4-inch deckbords, the men MOR nd MOE were significntly different between grdes 2&BTR, 3, nd 4. For 6-inch deckbords, the men MOR nd MOE were significntly different between grdes 2&BTR, 3, nd Cull. Effect of species on flexurl strength nd stiffness Figures 2 nd 3 llow visul comprison of the MOR nd MOE, respectively, for the species tested in this study. While dt for ll species tested re contined in Tble 6, only those with the lrgest smple sizes re shown in Figures 2 nd 3. Within ech figure, the verge experimentlly determined MOR nd MOE re shown seprtely for stringers, 4-inch, nd 6-inch deckbords of ech species group. The verge MOR nd MOE vlues determined from tests of smll cler specimens t green MC nd published in ASTM D (2) re lso shown. These smll cler vlues represent expected trends between species, which re shown in decresing order of smll cler vlue. As indicted in Figure 2, the reltive rnking of species groups by experimentlly determined MOR generlly followed the rnking by smll cler vlue. The verge MOR for stringers nd deckbords ws typiclly less thn the smll cler vlue, s would be expected given the presence of defects nd differences in size. Exceptions include soft mple stringers nd deckbords nd 6-inch spen deckbords, which hd higher MOR vlues thn expected. It should be noted 62 JULY/AUGUST 1991

8 tht the incresed loding rte used to test the stringers nd deckbords in this study probbly incresed the verge MOR between 5 nd 10 percent compred to the stndrd ASTM loding rte (11). The effect of size on MOR, s previously discussed, is evident with higher 6-inch thn 4-inch deckbord MOR nd higher deckbord thn stringer MOR for most species. As indicted in Figure 3, the reltive rnking of species groups by experimentlly determined MOE did not follow the rnking by smll cler vlue prticulrly well. MOE ws typiclly less thn clerwood MOE, s would be expected due to the presence of defects. Deckbord MOE ws typiclly higher thn stringer MOE, nd 6-inch deckbord MOE ws typiclly higher thn 4-inch deckbord MOE. Deckbord MOE ws lso much higher thn cler wood MOE for most species. Soft mple, blckgum, nd spen hd higher MOE thn expected. The incresed loding rte used to test the mteril in this study probbly resulted in 3 to 8 percent increse in MOE compred to the stndrd ASTM loding rte (11). The substntilly different spn-to-depth rtio for the deckbord tests versus the stringer nd smll cler specimen tests my lso hve ffected reltive MOE, lthough correction for sher effects ws mde to the stringer nd smll cler dt. The trend observed in previous studies of higher deckbord thn stringer MOE ws previously discussed. McLeod ( 11) lso found deckbord MOE to be higher thn smll cler specimen MOE in tests of ok nd yellow-poplr. Summry nd conclusions Eight-hundred forty pllet stringers nd 2,520 pllet deckbords of estern hrdwood species were smpled from 35 mills in 26 sttes. The prts were identified s to genus nd (if prcticl) species, visully grded, nd tested to destruction in bending. The resulting flexurl property dt were used to crete dtbse for estern hrdwood pllet prts. This dtbse cn be used to develop ccurte nd relible flexurl property estimtes for use in structurl design of hrdwood pllets. Six-inch-wide deckbords were found to hve higher verge flexurl strength nd stiffness thn 4-inch deckbords for most species, lthough the differences were not lwys sttisticlly significnt. Deckbords did exhibit significntly higher flexurl strength nd stiffness properties thn stringers, s might be expected bsed on the difference in depth. In developing design vlues for pllet prts, considertion should be given to differentiting flexurl properties between 4-inch nd 6-inch deckbords, s well s between deckbords nd stringers. Compred with the verge for dt collected over the pst 15 to 20 yers, the estern hrdwood pllet prts smpled in this study hd slightly lower percentge of pieces in the highest qulity grde, lthough the percentge in grde Cull ws unchnged. A greter percentge of the stringers in this study were of lower grde thn deckbords. A greter percentge of 4-inch deckbords were of the lowest grdes compred to 6-inch deckbords. Visul grding ws found to be effective in segregting estern hrdwood pllet prts by flexurl strength nd stiffness. Higher qulity grdes hd significntly greter verge MOR nd MOE s well s lower vribility. Nonprmetric lower fifth percentile MOR decresed with decresing grde. The reltive rnking of the species groups by stringer nd deckbord MOR followed expected trends. However, similr rnking by MOE did not follow the expected pttern s well Literture cited Americn Society for Testing nd Mterils Stndrd methods of sttic tests of timbers in structurl sizes. ASTM D ASTM, Phildelphi, P Stndrd methods of estblishing clerwood strength vlues. ASTM D ASTM, Phildelphi, P Stndrd method for evluting llowble properties for grdes of structurl lumber. ASTM D ASTM, Phildelphi, P. Bstendorff, K.M. nd A. Polensek Strength nd stiffness of red lder nd biglef mple pllet mterils. Forest Prod. J. 34(7/8): Bohnnn, B Effect of size on bending strength of wood members. Res. Pp. FPL 56. USDA Forest Serv., Forest Prod. Lb., Mdison, Wis. Forest Products Reserch Society Proc In-grde testing of structurl lumber. FPRS, Mdison, Wis. Lrge, H.R. nd R.E. Frost Qulity distribution of pllet prts from low-grde lumber. Res. Pp. NE-284. USDA Forest Serv., Northest. Forest Expt. St., Upper Drby, P. Loferski, J.R. nd T.E. McLin Development of relibility-bsed design procedure for wood pllets. Forest Prod. J. 37(7/8):7-14. McLin, T.E. nd J.S. Hollnd Preliminry evlution of the strength nd stiffness of yellow-poplr pllet shook. Forest Prod J. 32(11/12):51-56., H.W. Spurlock, J.A. McLeod, nd W.B. Wllin Flexurl properties of estern ok pllet lumber. Forest Prod. J. 36(9):7-15. McLeod, J.A Development of flexurl design vlues for pllet shook. M.S. thesis. Dept. of Forest Prod., VPI&SU, Blcksburg, V. nd T.E. McLin Evlution of bending strength rtios for estern ok pllet lumber. Wood nd Fiber Sci. 19(3): Ntionl Wooden Pllet nd Continer Assocition Membership nd product directory NWPCA Wshington, D.C Production report. NWPCA, Wshington, D.C. Pllet Design System. Copyright 1990 by the Ntionl Wooden Pllet nd Continer Assoc. Wshington, D.C. Srdo, W.H. nd W.B. Wllin The performnce of wooden pllets in pllet exchnge progrms. NWPCA, Wshington, D.C. Sttisticl Anlysis System SAS Institute Inc., Cry, N.C. USDA Forest Semite Wood Hndbook. Agri. Hndb. No. 72 (rev). USDA Forest Serv., Forest Prod. Lb., Mdison, Wis. Weibull, W A sttisticl theory of the strength of mterils. Swedish Royl Inst. for Engineering Res., Stockholm, Sweden. FOREST PRODUCTS JOURNAL Vol. 41, No. 7/8 63