U.S.D.A. FOREST SERVICE RESEARCH PAPER FPL 96 JULY

U.S. Department of Agriculture Forest Service Forest Products Laboratory Madison, Wis.

PROPERTIES OF NO. 2 DENSE KILN- DRIED SOUTHERN PINE DIMENSION LUMBER

ABSTRACT

Earlier work on strength and related properties of southern pine 2-inch dimension lumber in relation to structural grading of lumber showed that the No. 2 Dense KD grade gave average modulus of rupture values that were not appreciably higher than those of No. 2 KD. This report presents the results of tests conducted on a similar sample of No. 2 Dense 2- by 4-inch and 2- by 8-inch southern pine lumber that was obtained 2 years later from the same lumber mills.

The average of rupture obtained in the second series of 2 by 4’s agreed with that of the first series but was somewhat higher for the 2 by 8’s of the second series (7,020 p.s.i.) than for the first series (6,130 p.s.i.).

The relationships of modulus of rupture and specific gravity and of percent summerwood and specific gravity showed rather low correlation coefficients.

An evaluation of the effectiveness of visual grading for bending shows that 2 percent of the 2 by 4’s and 12 percent of the 2 by 8’s had a measured strength less than the value assigned to the grade. Similarly in the initial series, the percentages 7 percent for the 2 by 4’s and 14 percent for the 2 by 8’s.

The average percentage of summerwood measured on a section cut from the mid-third of the lumber pieces was about 38 percent. Approximately 28 percent of the pieces as evaluated by the sections cut from them had less than one-third summerwood, and 6 percent of the pieces had less than six rings per inch.

This study provides an appraisal of the strength characteristics of No. 2 Dense KD southern pine 2- by 4-inch and 2- by 8-inch dimension lumber in relation to allowable working stresses; provides data relating to an appraisal of lumber grading based on a stiffness evaluation: and includes an evaluation of the bending strength in relation to the density rule.

PROPERTIES OF NO.2 DENSE KILN-DRIED SOUTHERN PINE DIMENSION LUMBER 1

By D.V. DOYLE, Engineer

Forest Products Laboratory 2

Forest Service U.S. Department of Agriculture

I N T R O D U C T I O N

A recent research study at the Forest Products Laboratory, in cooperation with the Southern Pine Inspection Bureau, covered the strength and related properties of southern pine kiln-dried dimension lumber in relation to the structural

3grading of the lumber (5). The study included tests in flexure and compression parallel to the grain of full-size pieces in four sizes and six grades and also various tests of small clear specimens cut from the dimension lumber. In all, 1,349 static bending and 495 compression parallel­to-the-grain tests were made on the full-size pieces, together with 1,414 tests of small clear specimens.

The results of that initial study showed that the average modulus of rupture of the lumber in grades No. 1 Dense KD and No. 2 Dense KD was not appreciably higher than that obtained for corresponding grades of No. 1 KD and No. 2 KD. The 5 percent exclusion limit by count of the

1 -This research sponsored by and conducted in

modulus of rupture values (the five lowest strength values of 100 specimens) was approximately the same for the No. 1 KD and the No. 1 Dense KD grades, but was lower for the No. 2 Dense KD than for the No. 2 KD grade. The design strength levels assigned for No. 1 KD and No. 1 Dense KD were substantiated by the test values, but about 10 percent of the pieces in the No. 2 Dense KD grade had strength values that were below the recommended strength level for the grade (table 1, Initial series).

In an effort to determine if the lumber samples were fully representative of the grade, the Forest Products Laboratory in cooperation with the Southern Pine Inspection Bureau collected and tested another similar sample of No. 2 Dense KD dimension lumber. The second sample was obtained from the same mills about 2 years after the collection of the initial sample. This report presents the results of these tests and coordinates the data with those obtained in the initial study. It also presents a summary of the datapertaining to the grade-strength ratio and the application of the density rule for the dimension lumber.

was cooperation with the Southern Pine Inspection Bureau. 2Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 3Underlined numbers in parentheses refer to Literature Cited at the end of this report.'

SELECTION OF MATERIAL

The material used in this investigation was collected in the 10 major southernpine-producing states in conjunction with the collection of material for an extensive study of the tension parallel-to­the-grain properties of seasoned southern pine dimension lumber (6). For the most part, the lumber samples were obtained at the same mills and in the same manner as those previously obtained for the initial flexure and compression parallel-to-the-grain propert ies of full-size dimension lumber (5).

The grading of the lumber was in accordance with the SPIB 1963 Standard Grading Rules (7) including Supplement No. 4 dated April 5, 1965. These rules conform to the requirements of the American Lumber Standards. The lumber was taken without differentiating between the various southern pine species. It was selected to conform to the kiln-dried grade requirements of not exceeding 15 percent moisture content. The lumber in grade No. 2 has an assigned working stress of one-half the basic stress for the species or, in other words, it has a 50 percent minimum grade- strength ratio.

Twenty test pieces consisting of ten 2 by 4’s, each 8 feet long, and ten 2 by 8’s, each 14 feet long, were randomly chosen from the lumber stock at each of 10 mills. One or more bundles of lumber containing a total of 100 to 200 pieces, all of one general grade in one size and length, were selected for sampling. At all mills, the No. 2 and No. 2 Dense KD lumber were bundled together. All pieces meeting the dense require­ments for lumber were removed from the bundles under the supervision of the SPIB inspector to provide a stock of No. 2 Dense KD dimension for the random sampling of the test material. The moisture content (by meter) and the grade of each piece were determined by the SPIB inspector and recorded on each piece at time of selection. In all, 100 2 by 4’s and 100 2 by 8’s were selected for test.

Each piece selected in the test sample was identified on one end by a code symbol to show the geographic location, grade, type of test, and specimen number. No reference was made to any particular grain orientation or growth charac­teristic when pieces were code marked. At some mills, the lumber had been given a surface treat­

ment of a commercial water-repellent solution. The lumber pieces were placed in small bundles,

wrapped with a moisture-resistant covering, and shipped by motor freight to the Forest Products Laboratory. Upon receipt, the bundles were opened and the lumber placed on stickers. It was stored in an atmosphere of 72° F. and 65 percent relative humidity for several weeks before test for con­ditioning to an approximate 12 percent moisture content.

TEST METHODS

The full-size dimensionlumber specimens were nondestructively tested for stiffness in flatwise bending and in torsion, and then tested to destruc­tion in edgewise bending. All tests were made in accordance with the methods previously used in the tests of the initial series of dimensionlumber specimens (5). In addition to these tests, the grade-strength ratio was visually determined (3), the amount of summerwood was measured, a ring count was made, and a static bending test of a small clear specimen was included for each of the full-size pieces.

M 97 813 F Figure 1.--Traversing microscope for

measuring percentage of summerwood.

FPL 96 2

P r o v i s i o n

The measurement in pieces containing pith but not a 5-inch radial line shall be over 3 inches farthest from pith.

The measurement i n p i eces t h e p i t h and 5- inch r a d i a l l i n e s h a l l be ove r t h e 3rd, 4 th , and 5 t h inches f rom p i t h a l ong r a d i a l l i n e .

The measurement i n p i eces c o n t a i n i n g t h e p i t h i n p i eces less t han 2 by 8 inches s h a l l be measured on second i nch from p i t h .

The measurement i n p i eces n o t c o n t a i n i n g t h e p i t h shal I be on second i nch nea res t t h e p i t h i n lumber 3 inches o r less i n t h i c kness .

Figure 2.--Basis used in determining the percentage of summerwood and the number o f rings per inch. (Provisions are based on SPlB grading rules.)

The small clear 1- by 1- by 16-inch static from within the middle third of the length of each bending specimens were cut from the undamaged full-size piece. The measurements were made in portion of each full-size specimen and tested in accordance with the density rule provisions as accordance with the provisions of ASTM D 143 (1). shown in figure 2. The number of rings per inch The amount of summerwood was measured with was also counted. a traversing microscope (fig, 1) on a section cut

3

PRESENTATION OF DATA

Tables 1 to 4 and figures 3 to 10 present the results obtained from the tests of the full-size dimension lumber and the small clear specimens of both the earlier and the present series. All data are based on the actual dimensions of the specimens. Tables 1 and 2 list the average data obtained for the 10 locations from which the test material was sampled, Also listed are adjusted modulus of rupture and modulus of elasticity values. The formulas used in computing and adjusting data and the notations are given in Appendix I. The modulus of rupture values for the different sizes of specimens were adjusted to conform to a beam 2 inches in depth. The modulus of elasticity values both in flatwise and edgewise flexure were adjusted for shear deflec­tion to the true modulus of elasticity. The modulus of rigidity values obtained in the torsion tests were used in computing the true modulus of elas­ticity of the dimension lumber specimens. A value of 16 for the ratio of modulus of elasticity to modulus of rigidity was used in computing the true modulus of elasticity of the small clear specimens.

Table 3 compares the average data obtained from the tests of the No. 2 KD, the matching small clear specimens cut from the No. 2 KD dimension lumber, and the No. 2 Dense KD speci­mens of the initial series; these values are also compared with those of the No. 2 Dense KD and matching small clear specimens of the second series. Also listed for comparisons are previously published properties for small clear southern pine specimens (9).

A summary of the average grade-strength ratio, the amount of summerwood, and number of rings per inch for the second series is given in table 4.

The modulus of rupture adjusted to a 2-inch depth of beam and the true modulus of elasticity values are used in the relationships given in figures 3, 4, 5, 7, and 8. In table 1 and figure 9, the modulus of rupture values obtained in test (unadjusted to 2-inch depth of beam) are compared with the recommended stress level for No. 2 Dense KD The recommended stress level was obtained by increasing the design stress (7) of 1,750 p.s.i. (pounds per square inch)

4All correlation coefficients given in this report are

by the inverse of the reduction factor 2.1 ( 91

16 for

10longtime loading times 13

for other factors, 11including factor of safety, times 10

for normal 1loading =

2.1 ). This value is the same as that

recommended by ASTM D 2018 (2) for normal loading except that the factor for beam depth was not included. It is also the same factor that was used in making comparisons of the data in the previous study (5).

DISCUSSION OF RESULTS

Comparison of Data from Initial and Second Series A comparison of the results by lumber sampling

locations between the initial and the second series, and with the small clear specimens of the second series, is given by tables 1 and 2. The relation­ship between the modulus of rupture adjusted to a 2-inch depth of beam and the flatwise modulus of elasticity, true, is presented in figure 3. The

4coefficient of correlation between modulus of rupture and flatwise modulus of elasticity for the full-size specimens was 0.529 for the initial series, 0.531 for the second series, and 0.556 for both series combined. It was 0.685 for the small clear specimens.

In both the initial and the second series, the average modulus of rupture adjusted to a 2-inch depth of beam for the 2 by 4’s was 7,640 p.s.i. (table 1). For the 2 by 8’s, the modulus of rupture adjusted to a 2-inch depthofbeamwas 7,090 p.s.i. for the initial series and 8,120 p.s.i. for the second series.

While the strength ratios were not determined for the specimens of the initial series, the results indicate that the average quality of the 2 by 8’s in the initial series was lower than that of the second series. The 2 by 8’s of the second series had a higher average specific gravity (0.544 vs. 0.530), a slightly lower average moisture content (11.8 vs. 12.3 percent), and about an 8 percent higher average modulus of elasticity than those of the initial series. The average visual grade-strength ratio for the second series of No. 2 Dense KD 2 by 8’s was 71 percent.

significant at the 95 percent level of confidence.

FPL 96 4

1 2 3 4 5 6

Tabl

e 1.

--A

vera

ge

flexu

ral

prop

ertie

s of

N

o.

2 D

ense

K

D

sout

hern

pi

ne

dim

ensi

on

lum

ber1

,2

Para

grap

h 21

6,

Stan

dard

G

radi

ng

Rul

es

for

Sout

hern

Pi

ne

Lum

ber,

1963

an

d Su

pple

men

t N

o 4,

ef

fect

ive

Apr

. 5,

19

65

Each

va

lue

is

the

aver

age

of

spec

imen

s B

ased

on

vo

lum

e of

fu

ll-si

ze

piec

e at

tim

e of

te

st

and

wei

ght

whe

n ov

endr

y.

Piec

es

with

m

odul

us

of

rupt

ure

(una

djus

ted

for

dept

h of

be

am)

less

th

an

reco

mm

ende

d st

ress

le

vel

of

3,67

5 p.

s.i.

(1,7

50

p.s.i

. x

2 1)

V

alue

s ad

just

ed

to

conf

orm

to

a

2 -In

ch-d

eep

beam

(f

orm

ula

in

App

. I)

va

lues

ad

just

ed

to

a tru

e m

odul

us

of

elas

ticity

(f

orm

ula

in

App

. I)

.

Table 2.--Average flexural properties of small clear specimens cut from No. 2 Dense KD southern pine dimension lumber of second series1

1 Values are the average of 10 specimens, 1 by 1 by 16 inches in size. 2 Based on ovendry weight and volume at t e s t . 3 Adjusted to a 2-inch depth of beam. 4 Adjusted for shear deflection on the basis of E/G = 16 (formula in

App. I).

A frequency distribution of the specimen popula- small clear specimens of the second series agreed tion on the basis of 100,000 p.s.i. classes of closely with that previously published for loblolly modulus of elasticity is given in figure 4 for the and shortleaf southern pine (table 3), and was dimension lumber of the initial and second series about 5 percent higher than that of the matching and for the small clear specimens of the second specimens of the No. 2 Dense KD dimension series. The average modulus of elasticity of the lumber. In the previous study (5) with No. 2 KD

FPL 96 6

Tab1e 3.--Comparison of average flexural of No. 2 KD and No. 2 Dense KD southern pine dimension and of small clear specimens

1Based on ovendry weight and volume a t time o f t e s t . 2 Small c l e a r specimens were 1 by 1 by 16 inches. 3U.S. Forest Products Laboratory. Wood Hdbk. U.S. Dept. of Agr., Agr. Hdbk. 72. 1955. (Specimens were 2 by 2 by

30 in.) 4 Values ad jus ted to conform t o a E-inch-deep beam (formula i n I) 5Values ad jus ted to a t r u e modulus of e l a s t i c i t y (formula

lumber in 2 by 4 and 2 by 8 sizes, the average modulus of elasticity of the small clear specimens was 15 and 16 percent higher than that of the dimension lumber,

The modulus of elasticity for the small clear specimens was adjusted for shear deflection using a modulus of elasticity to modulus of rigidity ratio of 16, which is an average value commonly used when specific data for a given species are not available. The average EFT/G ratio for the

dimension lumber in this study was 13.5. The use of this smaller ratio in the adjustment formula would decrease the adjusted value of the true modulus of elasticity for the small clear speci­mens by about 2 percent compared to the values

i n App. I).

Ebased on the assumed G ratio of 16.

An evaluation of how well the No. 2 Dense KD grade of dimension lumber met the bending strength requirements of the current grading rules is given in table 1. Columns 6 and 15 list the percentage of lumber pieces in the initial and second series, respectively, whose modulus of rupture (unadjusted for depth of beam) fell below the recommended strength level (1,750 x 2.1 = 3,675 p.s.i.). The results show that, in the initial series, 7 percent of the 2 by and 14 percent of the 2 by 8’s were below the recommended strength level: in the second series, 2 percent of the 2 by 4’s and 12 percent of the 2 by 8’s, were below. AS may be noted, the proportion of

7

Table 4.--Summary of average strength ratio, summerwood, and rings per inch of No. 2 Dense KD southern pine dimension lumber in the second series

1Each value is the average of 10 specimens.2Ring count and summerwood measurements were made on a section cut

from the middle third of the length of each piece.

low values was higher for the 2 by 8’s than for exclusion limit determined by count (five lowest the 2 by 4’s in both series of No. 2 Dense KD strength values of 100 specimens) forthedifferent dimension lumber tests. grades and sizes of specimens were

The modulus of rupture values at the 5 percent Series Size Grade Modulus of rupture

p.s.i.

1 2 by 4 No. 2 KD 3,600 1 2 by 4 No. 2 Dense KD 3,150 2 2 by 4 No. 2 Dense KD 4,300

1 2 by 8 No. 2 2,850 1 2 by 8 No. 2 Dense KD 2,300 2 2 by 8 No. 2 Dense KD 3,350

FPL 96 8

A comparison of the modulus of rupture adjusted to a 2-inch depth of beam for the dimension lumber specimens of the second series, and the matching small clear specimens, shows that the maximum modulus of rupture values were about twice the minimum values for the small clear specimens and about five times the minimum values for the dimension lumber. The wide range in values for the dimension lumber reflects the effect of the strength-reducing features associated with the grade.

Modulus of The modulus of rigidity was, for the mostpart,

comparable for the initial and the second series. In the initial series, the average modulus of rigidity was 137,000 p.s.i. for the 2 by 4’s and 140,000 p.s.i. for the 2 by 8’s. In the second series, it was 128,000 p.s.i. for the 2 by 4’s and 143,000 p.s.i. for the 2 by 8’s. The modulus of rigidity did not correlate with flatwise modulus of elasticity (true) for either size in either series of tests. In the second series, the modulus of rigidity correlated rather poorly with specific gravity and with the modulus of rupture (0.219).

Modulus of Rupture Versus Specific Gravity The relationship of modulus of rupture (adjusted

to a 2-inch-deep beam) to specific gravity for the full-size dimension pieces and for the small clear specimens cut from the dimension pieces is given in figure 5. The specific gravity is based on the volume of the entire piece and its weight when ovendry. These data show that the coefficient of correlation was for the dimension lumber and 0.611 for the small clear specimens. These coefficients are somewhat smaller than those obtained in the initial series (0.494 and 0.707); they illustrate that specific gravity can be used to predict the modulus of rupture more accurately in clear wood than in wood containing growth defects.

Specific Gravity--Dimension Lumber Versus Small Clear Specimens A graph of the frequency distribution of the

specific gravity for the dimension lumber and

the small clear specimens on the basis of 0.01 classes is shown in figure 6. These data illustrate the range in specific gravity of the southern pine dimension lumber and the small clear specimens of this study.

The average specific gravity of the full-size pieces was 0.538 for the 2 by 4’s and 0.544 for the 2 by The average specific gravity of the matching small clear specimens was 0.529 for the 2 by and 0.535 for the 2 by 8’s. These differences in specific gravity, as in the previous study (5), are less than 2 percent and may result from a combination of such factors as knots, resin deposits, nonuniformity in moisture content, and nonuniformity in size. Previous standard tests of small clear specimens at the Forest Products Laboratory have shown an average specific gravity of 0.51 based on ovendry weight and volume at 12 percent moisture content for loblolly and shortleaf pine (9).

Strength Ratio The bending tests of full-size dimension lumber

specimens and matching small clear specimens of the second series provide data for comparing the visual and the actual strength ratio of No. 2 Dense KD lumber.

The visual strength ratio was established by a visual examination of each piece of dimension lumber (3). The actual strength ratio was deter­mined by dividing the modulus of rupture of the dimension lumber pieces by the modulus of rupture of the matching small clear specimens after the data for both sizes of specimens were adjusted to a 2-inch depth of beam.

The average values of visual and actual strength ratios are listed in table 4 for the 10 locations from which the test material was obtained. For the most part, the visual strength ratio was higher than the actual strength ratio, with the average for all specimens being 68 percent for the visual and 58 percent for the actual strength ratio.

The relationship of the visual strength ratio with the adjusted modulus of rupture is given in figure for the individual specimens. For some pieces, there were rather large differences between the visually estimated grade-strength ratio and the actual test data. The correlation coefficient between adjusted modulus of rupture and visual strengthratio was 0.460 (fig. 7).

Figure 7 also presents a comparison of the

9

strength ratio data with the 50 percent grade-strength ratio which has been assigned as the minimum strength ratio for southern pine lumber of No. 2 grade. While 10 percent of the specimens were below the 50 percent grade-strength ratio, only 5 percent were substantially below.

Density and Growth Rate The SPIB grading rules (7) state, “Dense lumber

averages on either one end or the other not less 1than 6 annual rings per inch and not less than 3

summerwood .... Contrast in color between sum­merwood and springwood must be distinct. Pieces averaging less than 6 annual per inch and not less than 4 meet dense requirements if

1averaging 2 or more summerwood.” This rule

was first put into effect about 50 years ago (4, 8). A summary of the wood density in terms of the

percent summerwood and the number of rings per inch for the 10 locations from which the lumber was obtained in this study is listed in table 4. The average percentage of summerwood was 37.7 per­cent for the 2 by 4’s and 38.8 percent for the 2 by 8’s. In this study, the measurements of the amount of summerwood and the ring count were made on a section cut from the middle third of the length of each piece. These data therefore do not necessarily conform to the density rule criteria but relate more closely to the zone of maximum stress.

About 27 percent of the 2 by 4’s and 29 percent 1of the 2 by 8’s did not meet the 3 summerwood

requirement for dense lumber. A number of the pieces would have met the density rule require­ments had the measurements been made at one end or the other of the pieces. The 2 by had an average of 12.4 rings per inch with 3 percent of the pieces showing less than 6 rings per inch. The 2 by 8’s had anaverageof 10.2 rings per inch with 10 percent of the pieces showing less than 6 rings per inch.

An evaluation of the effect of density, expressed as the proportion of summerwood, in the speci­mens on the flexural properties of dimension lumber is given in figures 8 and 9. Figure 8 shows the relationship of flatwise modulus of elasticity, true, to modulus of rupture adjusted to a 2-inch depth of beam for the specimens of the second

series; it also compares the relationship for the

specimens containing 3 1 summerwood or more

1with those containing less than 3 summerwood.

Figure 9 shows the relationship of modulus of rupture unadjusted for depth of beam and the percent summerwood.

The relationship between proportion of sum­merwood and specific gravity (based on the volume of the full-size pieces and weight when ovendry) is given in figure 10 for the individual pieces of dimension lumber of the second series. These data show that the amount of summerwood in an individual piece of lumber is not closely related to its specific gravity. The correlation coefficient for these data was

The summerwood measurements were made on a section of wood cut from the mid-third of the length of each piece. Examination of other sections cut from different locations along the length of the pieces shows largedifferences in the amount of summerwood within individual pieces of dimension lumber. One problemin the determina­tion of summerwood is the difficulty encountered in establishing the boundaries of the summerwood bands in some specimens, even after dye has been applied to the ends of the pieces. In the deter­mination of density, it is recognized that the presence of knots, pitch pockets, and other features is reflected in the specific gravity determinations for the full-size pieces.

Multiple Cor relations Multiple correlations were made to determine

the effect of combining two independent variables such as specific gravity (S.G.), modulus of elasticity (E FT), strengthratio (S.R.), andpercent

summerwood (S MWD) in evaluating the modulus

of rupture (R 2) for the No. 2 Dense KD southern

pine specimens of this study. The correlation coefficients for R versus the different variables2 are

CorreIations CorreIation c o e f f i c i e n t

R2 versus S.G. 0.454 EFT .531 S.R. .460 SMWD .451

FPL 96 10

Correlations CorreIation coefficient

R2 versus S.G. + S.R. .583 S.G. + SMWD .572 S.G. + E FT .573 EFT +SMWD .577 EFT + S.R. .626

S.R. + SMWD .573

As expected, the combination of two independent variables provided some improvement, with the combination of E FT and S.R. giving the highest

correlation coefficient.

SUMMARY A N D CONCLUSIONS

The study provides data on the strength and related properties of No. 2 Dense KD dimension lumber and matched small clear specimens of southern pine from 10 locations. Data are provided for the appraisal of structural grading and the establishment of design bending stresses on the basis of grade-strength ratio procedures, density measurements, and bending strength data obtained on the full-size lumber pieces. The results are based on data obtained in an initial series of tests of southern pine lumber consisting of 100 2 by 4’s and 100 2 by 8’s, and on a similar series of tests of a subsequent sample of lumber obtained about 2 years later from the same lumber mills.

The modulus of rupture for the 2 by 4’s was the same for both series of tests. For the 2 by 8’s, the modulus of rupture was higher for the second series (7,020 p.s.i.) thanforthefirst (6,130 p.s.i.), perhaps due in part to the quality of the second sample being a little higher in the grade. The relationship of modulus of rupture to flatwise modulus of elasticity for the full-size pieces, expressed as a linear regression, gave correlation coefficients of 0.529 for the initial series, 0.531 for the second series, and 0.556 for both series combined.

Because specific gravity is a general index of the properties of clear wood, an evaluation of its relationship to the properties of the dimension lumber was also made. The data show a correla­

tion coefficient of 0.454 between the modulus of rupture and specific gravity for the full-size specimens and 0.611 for the small clear speci­mens. These data reflect the decreasing effect of density on bending strength in the lower grades as compared with clear material. The data also show that the percentages of summerwood and specific gravity of the full-size pieces were not closely related. The correlation coefficient was 0.474.

The visual and actual strength ratio was deter­mined for the material. The visual strength ratio consisted of a visual examination of each piece in accordance with ASTM D 245. The actual strength ratio consisted of the ratio of the modulus of rupture of the dimension lumber to that of the small clear specimens after both sizes were adjusted to a 2-inch depth of beam. In comparison with the minimum grade-strength ratio of 50 per­cent which has been established for No. 2 southern pine lumber, the average visual strength ratio was 68 percent and the actual strength ratio was 58 percent. The relationship of the actual to the visual strength ratio as expressed as a linear regression gave a coefficient of correlation of 0.332.

With the modulus of rupture unadjusted for depth of beam, 2 percent of the 2 by 4’s and 12 percent of the 2 by 8’s had a modulus of rupture lower than the recommended strength level for the No. 2 Dense KD grade of dimension lumber. In the initial series, 7 percent of the 2 by 4’s and 14 percent of the 2 by 8’s had a modulus of rupture lower than the recommended strength level.

The average percentage of summerwood measured within the mid-third of the length of the dimension lumber pieces was about 38 percent. Approximately 28 percent of the pieces did not meet the density rule requirement of one-third summerwood, and about 6 percent of the pieces had less than the required 6 rings per inch. These data suggest that the density rule used in lumber grading has some limitations in evaluating the quality of dimension lumber because of the variability in the wood structure along the length of the pieces, the difficulty encountered in visually estimating the extent of the summerwood, and the much greater significance of strength-reducing characteristics.

11

Figure 3.--Relationship of modulus of rupture (adjusted to a 2-inch depth of beam) to flat-wise modulus of elasticity (true) of the initial and second series of No. 2 Dense KD southern pine 2 by 4's and 2 by 8's, and the small clear specimens of the second series.

M 134 349

FPL 96 12

Figure 4.--Frequency distribution of the dimension lumber and the small clear specimens of No. 2 Dense KD southern pine lumber by 100,000 p.s.i. classes of modulus of elasticity (true).

M 134 343

13

Figure 5.--Relationship, to specific gravity, of the modulus of rupture (adjusted to a 2-inch depth of beam) of second series of No. 2 Dense KD southern pine specimens and of the matching small clear specimens.

M 134 348

FPL 96 14

Figure 6.--Frequency distribution of the second series of No. 2 Dense KD and the matching small clear southern pine specimens by 0.01 classes of specific gravity.

M 134 344

15

Figure 7.--Relationship of the modulus of rupture (adjusted to a 2-inch depth of beam) to the visual strength ratio of the second series of No. 2 Dense KD southern pine dimen-sion lumber.

M 134 346

FPL 96 16

Figure 8.--Effects of various amounts of summerwood on the relationships of modulus of rupture (adjusted to a 2-inch depth of to flatwise modulus of elasticity (true) for specimens of the second series of No. 2 Dense KD southern pine lumber.

M 134 350

17

Figure 9.--Relationship of modulus of rupture (unadjusted for depth of beam) to percent summer-wood of the second series of No. 2 Dense KD southern pine lumber.

M 134

FPL 96 18

Figure 10.--Relationship of the percent summerwood to specific gravity of the second series of No. 2 Dense KD southern pine lumber.

M 134 345

19

L I T E R A T U R E C I T E D

1. American Society for Testing and Materials 1952. Standard methods of testing small

clear specimens of timber. ASTM D 143-52.

2. 1962. Tentative recommended practice for

determining design stresses for load-sharing lumber members. ASTM D 2018-62T.

3. 1964. Tentative methods of establishing

structural grades of lumber. ASTM D 245-64T.

4. Betts, H. S. 1915. Discussion of the proposed Forest

Service rules for grading the strength of southern pine struc-tural lumber. Amer. Soc. Test. Mater. Proc., 18th Annual Meeting., Vol. XV, Part 1 Comm. Rep.

5. Doyle, D. V., and Markwardt, L. J. 1966. Properties of southern pine in rela-

tion to strength grading of dimen-sion lumber. U. S. Forest Serv. Res. Pap. FPL 64. Forest Prod. Lab., Madison, Wis.

6. , and Markwardt, L. J. Tension parallel-to-grain properties

of southern pine dimension lum-ber. U. S. Forest Serv. Res. Pap. FPL 84. Forest Prod. Lab., Madison, Wis.

7. Southern Pine Inspection Bureau 1963. Standard grading rules for southern

pine lumber. New Orleans, La. 8. Swan, O. T.

1915. Southern yellow-pine timber and den-sity grading rules. Amer. Soc. Test. Mater. Proc., 18th Annual Meeting, Vol. XV.

9. U. S. Forest Products Laboratory 1955. Wood handbook. U. S. Dep. Agr.,

Agr. Handb. 72.

FPL 96 20

APPENDIX 1

Notation

a--Span, inches a --Gage length, inches n E--Modulus of elasticity in flexure of small clear specimens, pounds per square

inch EE--Modulus of elasticity of dimension lumber in flexure edgewise, pounds per

square inch EET--Modulus of elasticity of dimension lumber in flexure edgewise, adjusted

for shear deflection, pounds per square inch EF--Modulus of elasticity of dimension lumber in flexure flatwise, pounds per

square inch EFT--Modulus of elasticity of dimension lumber in flexure flatwise adjusted

for shear deflection, pounds per square inch ET--Modulus of elasticity in flexure of small clear specimens adjusted for

shear deflection, pounds per square inch G--Modulus of rigidity associated with shear strain parallel to the grain and

radial or tangential to growth rings, pounds per square inch K n = 0.239 for 2 by 4 A constant based on standard dressed size of dimen-

= 0.288 for 2 by 8 } sion lumber

ℓ--Length, inches M.C.--Moisture content, percent n--Number of specimens P--Maximum load, pounds PPL --Load at proportional limit, pounds

r--Coefficient of correlation R--Modulus of rupture, pounds per square inch R2 --Modulus of rupture of dimension lumber adjusted to a 2-inch depth of beam,

pounds per square inch S.G.--Specific gravity at test SMWD --Summerwood

SPL --Flexural stress at proportional limit, pounds per square inch

SPL2 --Flexural stress at proportional limit of dimension lumber adjusted to a 2-inch depth of beam, pounds per square inch

21

S.R.--Strength ratio for stress in extreme fiber in bending S --Standard error of estimate, a measure of the variation of data about the y.x regression line

t--Thickness, inches T--Torsional moment, inch-pounds w--Width, inches WD --Weight of sample ovendry, grams

WG--Weight of specimen at test, grams

WP --Weight of specimen at test, pounds

WT --Weight of sample at test, grams

ΔF --Center deflection relative to supports at load of 112 pounds, inches Δ

PL --Center deflection relative to supports at proportional limit load, inches

θ --Angle of twist, radians

FPL 96 22

F o r m u l a s Used in Computing and Adjusting Data

Flexure -,-Quarter Point Loa ding - -Dimension Lumber

Flexure- -Center Loading--Small Clear Specimens

23

PUBLICATION LISTS ISSUED BY THE

FOREST PRODUCTS LABORATORY

The following lists of publications Forest Products Laboratory or relate to able upon request:

Architects, Builders, Engineers, and Retail Lumbermen

Box and Crate Construction and Packaging Data

Chemistry of Wood

Drying of Wood

Fire Performance

Fungus and Insect Defects in Forest Products

Furniture Manufacturers, Woodworkers, and Teachers of Woodshop Practice

Glue, Glued Products, and Veneer

Growth, Structure, and Identification of Wood

deal with investigative projects of the special interest groups and are avail-

Milling and Utilization of Timber Products

Mechanical Properties and Struc-tural Uses of Wood and Products

Modified Woods, Paper-Base Laminates, and Reinforced Plastic Laminates

Sandwich Construction

Thermal Properties of Wood

Wood Fiber Products

Wood Finishing Subjects

Wood Preservation

Wood

Note: Since Forest Products Laboratory publications are so varied in subject matter, no single catalog of titles is issued. Instead, a listing is made for each area of Laboratory research. Twice a year, January 1 and July 1, a list is compiled showing new reports for the previous 6 months. This is the only item sent regularly to the Laboratory’s mailing roster, and it serves to keep current the various subject matter listings. Names may be added to the mailing roster upon request.

FPL 96 24 1.5-29