Historic, archived document

Do not assume content reflects current
scientific knowledge, policies, or practices.

Contribution from the Forest Service
HENRY S. GRAVES, Forester

Washington, D. C. PROFESSIONAL PAPER September 27, 1915

STRENGTH TESTS OF STRUCTURAL TIMBERS TREATED BY
COMMERCIAL WOOD-PRESERVING PROCESSES.

By H. 8. Berrs and J. A. Newuin, Engineers in Forest Products, Forest Products

Laboratory.
CONTENTS.
’ Page. Page.
Objechor thewbests* “sss ah each se + bel PECOSUIES OlaheSiS sys oe on asccers. = kosie sec ste Soe 6
“SEE: ETRE UL RES Cs LES OO oy 2a De mUeCHOUS ss terete te eee eee ot Se 14
Methodstotireatment=--- ==. sc =--- == 22 3 | Publications relating to strength tests of
Mephodlomtesting-. == ets S25- 52 Soest AS | WReaVALIOUS WiOOUS Shan eos s- Hce as > 15

OBJECT OF THE TESTS.

This bulletin presents the results of tests made by the Forest
Service, in cooperation with the Illinois Central Railway and one
eastern and two western wood-preserving companies, to determine
how the strength of bridge stringers is affected by commercial creo-
sote treatments. To do this, comparison was made between the
strength of treated and untreated stringers of the same size and
quality. The test timbers were selected by representatives of the
Forest Service from stock furnished by the cooperators. The For-
est Service requested that the treatments given the timbers by each
of the cooperators be that used in its regular commercial practice.
A Forest Service representative was present during the treatments
and kept a record of the various conditions to which the material
was subjected. The woods used were loblolly pme, longleaf pine,
and Douglas fir. After treatment the loblolly and longleaf pine were
shipped to the Forest Service timber-testing laboratory at Lafayette,
Ind.,1 and the Douglas fir to the Forest Service timber-testing sta-
tion, Seattle, Wash.

1 Formerly conducted in cooperation with Purdue University.
2 Conducted in cooperation with the University of Washington.

Note.—This report is of interest to users of timber where strength is an important consideration.
1035°—Bull. 256—15 :

Y) BULLETIN 286, U. S. DEPARTMENT OF AGRICULTURE.
MATERIAL TESTED. i

The material for test was selected from regular stock, in the form
of sticks 8 inches by 16 inches in section and from 28 to 32 feet in
length. The sticks were sorted in pairs, with the object of having
those in each pair as alike as possible. .At the time of treatment
each stiek was cut into two stringers of equal length, making four
test stujmgers in each group, two butt cuts and two second or top
cuts. The groups were handled as shown in figure 1, the butt ends in
one group being treated and the top ends in the next.

LONGLEAF AND LOBLOLLY PINE.

The longleaf and loblolly pine timber were cut in southern Missis-
sippi and Louisiana. About five months elapsed between the time

a a es TOR
‘Treated as received and | ;
/ tested immediately Jested as received 2
Grou [ 7
Wiireaied as received ard a
3 seasoned berore resting | Seasoved Lerore 4esti LOGY
Burr rae

ae Treated FE
3 Tested as received tested immediately i
Group I : “ye
; Treated as received and

ab — Disk./ thick, cut tron center to derermine moisture
Fig. 1.—Method of cutting and marking test material.

the logs were sawed and the time of treatment, during four months
of which the pieces were seasoned in an open pile. The treated
stringers were en route to Lafayette, Ind., for over a month. Upon
arrival they were close piled under shelter until the tests were started
about a month later. The pieces as selected were 8 inches by 16
inches in section by 28 feet long. The material classed as ‘‘long-
leaf’? was high-grade timber, considered as first-class structural
material by the railway officials, and that classed as ‘‘loblolly”’ as
less valuable. The longleaf had only a small per cent of sap and
was of comparatively slow growth, while the loblolly averaged over
30 per cent sapwood, was of more rapid growth, and contained more
knots. The number of test stringers 14 feet long was as follows:

STRENGTH TESTS OF STRUCTURAL TIMBERS. 3

Longleaf:
5 treated partially air dry and tested.
5 tested partially air dry.
5 treated partially air dry, seasoned, and tested.
5 seasoned and tested.
Loblolly:
5 treated partially air dry and tested.
5 tested partially air dry.
5 treated partially air dry, seasoned, and tested.
5 seasoned and tested.

DOUGLAS FIR.

The material was selected at two western mills. In both cases
the test timbers were shipped to the creosoting companies within a
few days after they were sawed from logs at the mill, and were treated
within a few days after arrival at the creosoting plants. The pieces
as selected were 8 inches by 16 inches in section and 32 feet long,
and included three grades of material—select, merchantable, and
common, as classified by the grading rules of the West Coast Lum-
ber Manufacturers’ Association. It is customary to use only select
and merchantable timbers in permanent structures. These pieces
were cut in two just before treatment, so that the test stringers
measured 16 feet. Two processes of treatment were used, the “‘ boil-
ing’’ process and the “‘steaming”’ process. The material which was
seasoned before testing was piled in a shed with open sides. The
number of 16-foot test stringers used in studying the effect of the
two processes, and their condition when treated Pail tested, was as
follows:

Boiling process:
20 treated green and tested.
20 tested green.
19 treated green, seasoned, and tested.
19 seasoned and tested.
Steaming process: :
15 treated green and tested.
15 tested green.

treated green, air seasoned, and tested.
seasoned and tested.

METHODS OF TREATMENT.

The preservative treatments to which the three species of struc-
tural timber were subjected were briefly as follows:

LOBLOLLY PINE.!

Steamed for 4 hours under 29 pounds pressure; vacuum of 26 inches applied for 1
hour; cylinder filled with creosote and pressure of 125 pounds applied for 44 hours at
a temperature of 140° F.; vacuum of 234 inches applied for } hour. Absorption of oil,
134 pounds per cubic foot of wood.

1 Run made Mar. 4, 1908.

4 BULLETIN 286, U. S. DEPARTMENT OF AGRICULTURE.

LONGLEAF PINE.!

Steamed for 6 hours. at 30 pounds pressure; vacuum of 26 inches applied for 1 hour;
cylinder filled with creosote and pressure of 128 pounds applied for 54 hours at a
temperature of 140° F. Absorption, 12? pounds per cubic foot of wood. :

DOUGLAS FIR.!

Boiling process.—Boiled in creosote for 21? hours at temperature of 215° F.;? loss of
moisture during boiling, 1.2 pounds per cubic foot of wood; pressure raised from 0 to
145 pounds per square inch in 53 hours; temperature about 190° F. Absorption of
oil, 11.2 pounds per cubic foot of wood, as determined by measuring tank readings.
Steaming process.—Steamed at 90 pounds pressure per square inch for 4} hours;
temperature about 325° F.; vacuum of 20 inches applied for 184 hours; temperature
220° F. at end of period; cylinder filled with oil and pressure raised from 0 to maximum
pressure of 140 pounds per square inch; pressure period, 24 hours; temperature of the
oil, about 208° F. Absorption, 3.1 pounds per cubic foot of wood, as figured from
increase in original weight of stringers. The stringers were not weighed after steam-
ing, so that the probable loss can not be taken into account in computing the absorption.

METHOD OF TESTING.

The stringers were tested in bending by supporting them at the
ends and applying the load at two points located one-third of the
span from each of the end supports. This system corresponds
closely to conditions of practice. In testing the beams the load was
applied gradually and a record kept of the deflections corresponding
to regular load increments. Four factors were calculated from the
data derived from each bending test, all in terms of pounds per
square inch:

FIBER STRESS AT ELASTIC LIMIT.

This is the greatest stress that can occur in a beam loaded with
.an external load from which it will recover without permanent
deflection.

MODULUS OF RUPTURE.

This is the greatest computed stress in a beam under a breaking
load.

MODULUS OF ELASTICITY.

This is a factor computed from the relation between load and
deflection within the elastic limit, and represents the stiffness of the
wood.

LONGITUDINAL SHEAR.

This is the stress tending to split the beam lengthwise along its
neutral plane * when under maximum load.

1 Run made Mar. 5, 1908.

2 Some time after the treatments were made it was reported by the treating-plant officials that the ther-
mometer giving this reading registered 40° F. too low.

3 Plane between upper and Jower halves when beam is horizontal.

STRENGTH TESTS OF STRUCTURAL TIMBERS, 5
MOISTURE DETERMINATIONS.

Moisture determinations on the untreated wood were made by
taking either borings or disks from the tested pieces, weighing them,
and then drying them to constant weight. The difference between
the original weight and the dry weight divided by the dry weight
times 100 is taken as the per cent of moisture at the time of test.
Disks taken from the untreated stringers were cut into a number of
pieces and the moisture separately determined for each in order to
find the distribution of moisture throughout the cross section. The
method of dividing the disks is shown in figure 2. The moisture ©
determinations made on treated specimens were handled by dis-
tilling the treated shavings cut from the test pieces with water-
saturated xylol. For such determinations
a definite quantity of treated borings was
taken. Inall cases a corresponding volume
of untreated shavings was obtained, and
the dry weight of this sample determined
as a basis for computing the moisture con-
tent of the treated sample. All test pieces
were weighed and measured, the number
of rings counted on a radial line, and the
per cent of summerwood and sap deter-
mined.t Sketches were made and photo-
oeraphs taken, showing the size and loca-
tion of knots, checks, and shakes.

TESTS ON SMALL STICKS.

After failure occurred in the stringers,
small pieces 2 inches by 2 inches in section
ads feet lone were cut from the unbroken "'¢- 2 Molsture distribution, disk

3 5 for 8-inch by 16-inch stringer.

portions. These small pieces were selected

so as to be free from defects and with straight grain. Their location
im a cross section of the stringer was noted, so that data could be
secured on the relative strength of the inner and outer portions. The
tests of small pieces included bending tests on specimens 2 by 2 by 30
inches, compression tests in which specimens 2 by 2 by 8 inches were
crushed endwise parallel with the grain, compression tests at right
angles to the grain, and shearing tests in which a projecting portion
of a small block was sheared off parallel to the grain while the main
portion of the block was held firm.

1 Determinations of summerwood and sap were omitted for some of the Douglas fir.

6 BULLETIN 286, U. S. DEPARTMENT OF AGRICULTURE.

RESULTS OF TESTS.

The results of the bending tests on the natural and treated stringers
are shown in figures 3 to 7.

» The diagrams were made by first plotting the values for modulus of
rupture of the natural beams (solid lines) arranged from the highest
to lowest, beginning with the highest value on the left at the top of
the figure. The modulus of rupture of the treated half (dotted
lines) of the test pieces was then plotted in the same vertical line as
the untreated pieces. The two values are marked to distinguish
butts (6) from corresponding tops (7). The other values (fiber
stress at elastic limit and modulus of elasticity) for the same beams
are plotted in the same vertical lines.

Conclusions should not be drawn regarding the comparative effect
of creosoting on the strength of the different woods, since they were
not treated under similar conditions. It should se be kept in mind
that the test material was not selected for the purpose of comparing
the various species.

LOBLOLLY PINE.

Figure 3 gives a comparison of the strength and stiffness of natural
and treated loblolly pine stringers for partially air-dry and seasoned
material. In drawing conclusions from the diagrams it should be
kept in mind that butt strmgers are naturally stronger than second-
cut or top stringers. This point was considered when the method of
selecting the test material was determined upon and butts and tops
were arranged to alternate in serving as treated and untreated
material. It will be noted from figure 3 that when the butts were
treated the breaking strength of the butts and tops fell rather close
together, while when the tops were treated the breaking strength
values were much farther apart. This shows an evident weakening
due to the treatment, even when the lower breaking strength of the
top stringers is taken into account. The tests are too few to make
a definite statement as to the amount of weakening for the specific
treatment under consideration. It is probably not more than 17
‘per cent. The fiber strength at elastic limit and the stiffness both
show a greater weakening due to treatment than does the breaking
strength. The weakening is more marked in both strength and
stiffness in the air dry than in the partially air-dry stringers. Both
the treated and untreated stringers showed a strength about 30 per
cent greater in the seasoned material than in the partially air-dry

material.
LONGLEAF PINE.

In figure 4 the strength of treated and untreated longleaf pine
stringers is compared for both partially air-dry and seasoned material.
It does not appear that the breaking strength was affected by the
treatment used with these stringers. There is aslight reduction in the

STRENGTH TESTS OF STRUCTURAL TIMBERS. “i

average strength at elastic limit and stiffness. In the air-seasoned
beams the untreated butt cuts were higher in strength and stiffness
than the treated top cuts, but, on the other hand, the untreated top
cuts fell below the treated butts in strength and stiffness in nearly
every case. In the partially seasoned stringers the treated and
untreated material falls together somewhat more closely.

Avr Dried and ested

M37 CIRC, 26 TFA JG TF/ 23
m7 38 so 22 25 39 £ SEAM NUMBERS —FZ0 32 24. oe see

O— 8 V97V#AL O----O 7PLATED
2 = Butts T= Tops
Fic. 3.—Effect of preservative treatment on the strength and stiffness of loblolly-pine stringers treated
partially air dry.

DOUGLAS FIR.

Figures 5 and 6 show the strength and stiffness of treated and
untreated stringers of green and seasoned Douglas fir, respectively,
treated by the so-called ‘‘ boiling” process as used in this case. There
appears to be a marked weakening of the breaking strength with the
particular treatment. used. The average breaking strength of the
stringers tested green and after seasoning is 33 per cent and 39 per
cent, respectively, less than the average strength of the natural
stringers. The fiber stress at elastic limit also appears to be reduced,
although to a somewhat less extent. In the green material no
weakening is apparent in the stiffness. The seasoned stringers,
however, show a falling off in stiffness in the treated material.

Figure 7 shows the strength and stiffness of green * Douglas fir
treated by the so-called ‘‘steaming”’ process. The breaking strength

1 The air-seasoned material is not yet tested, July, 1915.

+

S : BULLETIN 286, U. S. DEPARTMENT OF AGRICULTURE.

gested Gicectly atrer Teatuent ae Aur bried and Tested
(427

9000 9000

8000 8000
<7 eigas 10D UL is
8 S $F
W600 < 6000 AUPTURE
& 2 i
N 500 t 5000
. x UBER STRESS
9200 94000 LOTn eee
N g LLA8TIC LIMIT

S)

g 5 L300

ZA
Q
SS
S
hy
S
S

SF.

ST00WLUS
OF
LLASTIOTY.

1000 OUNDS FEF
Xx
SS
N
Ss
Ss

4000 4000
Nee) Oi ON aS 7 72 72
PAVE LT 7 9T NOL te SEAM NUMBER seh oir yap gi oe

@—@ WATURAL O---0O TZREATED
B= L775 7 = Togs

Fic. 4.—Effect of preservative treatment on the strength and stiffness of longleaf-pine stringers treated
partially dry.

S

Ss6000

&

NY 5000

g /QouLus
S 4000 "8 OF

N we FUPTURE

3000

FIBER STRESS
AT

LFLASTIC LIMIT

ae a a
oO
ir Sande 0) ,
fy
s

/ O
lp ©
| /
4
7

U
7) ,

was
Fol fe Sf], ial Sete ses lal
OA EL aS a
fife Joop [it [alan el es

10 UP 2! Sg Se OE Ze et ae)
BEAM NUMBER

@—9 WATUPAL O---O 7REATED (20x16 PROCESS )
B= Butts 7 = Tops
Fig. 5.—Eflect, of “‘boiling process” of preservative treatment on the strength and stiffness of Douglas-fir
stringers treated green and tested without seasoning.

=)

1400

1200

1000 POUND PER 59. My.
8
SS
8

ie
be

STRENGTH TESTS OF STRUCTURAL TIMBERS. @g

aS Js modern ees oe ee
CO So so shi thabas Ssh
&
*) 5000 ss eS
= ~=2a3
N x ()
© emacs suse Se PNA | Vpecasees
X 4000 = -- 2 | as
CREEPER TER pet Beate
b) D
S OOO 0=--0, 5 O
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g

ms o 2UdeRe Ea a

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2000
S 1800
> YobuLUus
N OF
: 1500 LASTICITY
g 7400
N
S
x 1200
S
N

4000

Fr) 10 15 46 l? 18 79
© Beat Wusnee
@—O “4ATUPAL O---D FFEATEO poor ——
_@ = Butts 7 = Tops

Fig. 6.—Effeci of ‘‘boiling process” of preservative treatment on the strength and stiffness of Douglas-fir
stringers treated green, air seasoned and tested.

BSL |.
Pele abet ilelobeed |
SCOR oe

OF
a Eases ha ale

g

N 3000

N Ly FaER STRESS

an

2000 S71CLINUIT
1000
2000

SS 1800

&

6S

< 4600

q.

g /4£00 /f00ULYS

S OF

é Se oy eee pease

Sg 4200
40090

40 “é 4/2
are Wie

@—8 WATURAL O----O 7EATEO ae PROCESS i
G@ = Burts 7 = ops

Fie. 7.—Effect of ‘steaming process” of preservative treatment on the strength and stiffness of Douglas-fir
stringers treated green and tested without seasoning.

BULLETIN 28
286, U. S. DEPARTMENT OF AGRICULT
URE,

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STRENGTH TESTS OF STRUCTURAL TIMBERS. “LT

_ and fiber stress at the elastic limit was considerably less in the treated
material (35 and 36 per cent, respectively), and the stiffness was
slightly less.

Table 1 gives the average values of the strength functions shown in
the diagrams, together with the highest and lowest values and some
additional data.

SMALL PIECES CUT FROM STRINGERS.

Table 2 gives the average strength and stiffness of the small pieces
cut from the main beams for both treated and natural material of the
three species under test. The average values of the small pieces cut
from the outside portions of the main beams and the average values
of the small pieces cut from the interior portions are also given. No
moisture determinations were made on the small pieces cut from the
treated longleaf and loblolly pine timbers. The determinations for
moisture in various parts of the cross sections of the treated timbers
of these two species indicate that mn general they contained slightly
more moisture than the natural pieces. The treated sticks are in
general weaker than the natural sticks, but the difference is slight
except for partially air-dry loblolly pme. Part of the apparent loss
in strength of the treated material may be ascribed to its higher
moisture content. |

In the Douglas fir treated by the boiling process and tested green,
the average for the outside sticks: shows a decrease in strength over
the natural, with but little difference in stiffness. As compared with
the natural sticks the treated sticks cut from the interior of the main
beams showed a more marked drop in strength and stiffness. The
air-dry material in all cases showed a decided decrease in the strength
of the treated sticks: The decrease in stiffness was less marked.
Part of this decrease may be accounted for by the higher moisture
content of the treated pieces.

12

BULLETIN 286, U. S. DEPARTMENT OF AGRICULTURE.

TaBLE 2.—Strength and stiffness of small pieces—natural and treated—cut trom the inside
and outside portions of longleaf pine, loblolly pine, and Douglas fir stringers.

Species, condition, and
locality.

Longleaf pine:
Partially air dry—
All

see c ec eee eee ee ee - -

Insider: 22s ase
Loblolly pine:
Partially air dry—

Douglas fir:
Boiling process—
Green—

reen—

Modulus
of rupture
(pounds per
square inch).

Fiberstress| Modulus
ofelastic | ofelasticity
limit

(pounds per! pounds per
squareinch). Squareinch).

ees

Number; Moisture | Rings per
oftests.| (per cent). inch.
IN| SS) NE Abe N. qe
28q (e290 |b2lang|sesece 1952) | 1823
DARED As eZ O ny ae 19.6 | 19.3
ABN 8 || PERO es SoSe 14.6 | 14.4
P|) 98s | WES \iecesen 19.9 | 19.6
SEAS. Oi oeaae 20.21 18.1
Ri OP I. Grleacoae 18.5 | 14.2
1 303|5287|208Sn|2e-eee 6.4 | 6.4
PMA | OPA AIS Nec code 6.9 | 6.9
G2 {GH G2 252) | ae 4.6} 5.5
SOR S26, |pL22 On Pees Too Wal 4
JAS | 21 NOM eee CoO! WoC
Gi abaliall 3: |Seeeae 5.7} 5.8
48 | 38 | 30.1 | 27.5 | 10.7 | 12.6
Ph | AI OG leacoce M7 |) 1839
DA AON ES ORGa|Berecice 9.6 | 11.4
| 66 | 54] 15.6 |.....- 1133, |)aled al
[oor oe Aan Salers APS SG
FeB3 1) 27 PGE 48 le ooo ce P252 | 1256

Omtsidesss---2- | +7
ANSIdCs 3 aes Se (eens

5,623
5, 587
5,953 |5,088 |1,581 | 1,464

8,070
8,060
8,153

es eee ee ee ee

STRENGTH TESTS OF STRUCTURAL TIMBERS. 13

SPECIAL TESTS ON SMALL PIECES.

Table 3 gives a condensed summary of the results of a special series
of tests on small clear speciments (2 by 2 inches in section) of Douglas
fir, longleaf pine, and shortleaf pine. The tests were made at
the Forest Products Laboratory to study the effect of the various
steps used in the treatment of the full-sized stringers. Eight sticks
were subjected to each of the processes shown in Table 3. One-half
of the sticks were tested shortly after treatment and one-half after
they had been piled in the laboratory long enough (5 months) to reach
a practically constant weight.

All the processes caused a reduction in the strength values of the
unseasoned material of the three species with, in most cases, a recov-
ery after seasoning, except in the tension tests. In these the weaken-
ing in the unseasoned material remained after seasoning in all processes
but the creosote bath.

TABLE 3.—LE fect of various treatments on small clear sticks (results expressed! in per cent
of strength of untreated material).

Steamed at Creosote bath
. ack | 2 pounds ab atmos-
: teamed 2 ours; pheric pres-
F Creosote at °
Steamed at 2 pounds 26-inch atmospheric Suet F ”
20 pounds Shours; | vacuum pressure 7 hours;
Sime 26-inch 1 hour; 200° F.. creosote at
‘| vacuum | creosote, 27 hours 145 pounds
> ihour. | 120 pounds 5 pressure,
| pressure, | 180° F.,
| | 4% hours. i? hours
‘Unsea-| Air |Unsea-| Air Tae! Air | Unsea- Air Unsea-| Air
soned.| dry. soned.| dry. |soned.} dry. | soned.| dry. |soned.| dry.
Bending:
Modulus of rupture—
Douslas tir: . 3222 - - cnn | 74 96 WBE § 1193 83 86 92 89 86 98
Longleaf pine.........---- 83 TOO |Reoees [Rees 80° Saber a 1. eee ae 81 93
Shortleaf pine..........--. 73 107 72 | 98 84 104 96 108 89 106
Modulus of elasticity— |
Douglas Mess Ps Fok sl sees i 100 84 100 | 95 98 97 105 93 99
qoncleal pine=..- =<-=--s-- 94 TOG Ral ecoser. eS fee 91 OOM eens” eseanoe 92 108
Shortleaf pine.....-..-.-..-- 84 104 92 |103 96 105 112 102 96 100
Compression:
Maximum crushing
strength—
DOUSIAS NE fo. cs sects , 68 102 76 88 80 97 83 102 90 112
Longleaf pine............. Ese 102-9 | Seco ee 82 W6Te. (exes eae 205 ees
Shortleaf pine............- Tue tOt \07W 1108 g2_—-'|104 89 —- [103 89 109
Shear with grain— | |
Wonelas AE fos Se. - 72 =| 93 74 GS ce 100 gl 118 86 125
Longleaf pine.......------ Uel TA ied ene Sel oes ley Boer ot esetse aeaarme segeanc 74 - 115
Shortleattpime: == =... moe 107 74 1057 2°80 108 88 114 90 115
Tension perpendicular to |
grain—
1D hPa 1 ae 57 _ | 69 54 64 | 48 57 47‘ |116 632s | tz
Longleaf pine._...--.----- 42 2 Peres (SS Rnaan lesaocee VG eases iocscsee|ecstoce 61 | 67
Shortleaf pine...........-- 70 81 j “1 73 | 71 64 95 130 79 88
Shrinkage! in cross section
during treatment— |
Worslashire =... Sica eee enae FBS les Sete» Vises ee gl eee ae S73 ae Gi g3 [Sea
onsleaf pine: =.-..:.---.- SAQseest owe eoonass|aoeeees ers OSs eee Ss eee |e crereyes 66) [Eos
Shortleai pine s..22=- =. --- Bt eeieorte 550) | Sasceee S744 ee eae F520) || Bere spe Bias |e ee
During treatment and sea- | |
soning— |
Worehasdiresaecss 2). 525 Sau |heree sae PR OsOS Meee eee 8: 350 Psae222 Meola seaee (Ee ll eee 11-31
Mousicatpiie:a.... 2. |. sees OF 64s) ee Ciel earings ees TIS] eRe eet teenies (Deena 7.90
Shonilear pines.) 20)..2.-|22. 25. | 10. 47 | echt 9 30n Las Caf 0G ea re (Vel hee 5.73

1 Shrinkage given in per cents of areas when first measured. Corresponding shrinkage of untreated mate-
rial: Douglas fir, 6.40; longleaf pine, 8.48; shortleaf pine, 7.29.
2 Increase in volume.

14 BULLETIN 286, U. S. DEPARTMENT OF AGRICULTURE.

The shrinkage measurements on the steamed material with ‘and
without vacuum showed less than 1 per cent decrease in volume dur-
ing treatment for all the species. After seasoning a shrinkage of from |
8.4 per cent for Douglas fir to 10.6 per cent for longleaf pine was re-
corded. Steaming and vacuum followed by creosote showed a some-—
what higher shrinkage for Douglas fir than for the pines, both in the
unseasoned and air-dry pieces. The creasote bath had little influ-
ence on the shrinkage, the reduction after seasoning corresponding
closely to the shrinkage of untreated pieces. The pressure treatment
following the creosote bath showed a somewhat higher shrinkage for
Douglas fir than for longleaf or shortleaf. |

While the weakening in the Douglas fir stringers is not explained
by the series of special tests, they indicate that the trouble has to do
with stresses in the full-sized stringers, probably caused by rapid and
unequal shrinkage during the process. A further series of tests is
now under way on 8-foot stringers 8 by 16 inches in section treated at
the Forest Products Laboratory, from which results that bear more
directly on the problem are expected.

DEDUCTIONS.

(1) Timber may be very materially weakened by preservative
processes.

(2) Creosote in itself aoes not appear to weaken timber.

(3) A preservative process which will seriously injure one timber
may have little or no effect on the strength of another.

(4) A comparison of the effect of a preservative process on the
strength of different species should not be made, unless it is the com-
mon or best adapted process for ail the species compared.

(5) The same treatment given to a timber of a particular species
may have a different effect upon different pieces of that species, |
depending upon the form of the timber used, its size, and its condition
when treated.

FE Nn Pane ne Oa cl me SSS ee SS
at = = RN gl ee =

STRENGTH TESTS OF STRUCTURAL TIMBERS. 15 3

PUBLICATIONS RELATING TO STRENGTH TESTS OF VARIOUS WOODS.
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Fire-ltlled Douglas Fir: A Sttidy of Its Rate of Deterioration, Usability, and Strength.
By Jcseph Burke Knapp. Pp. 18, figs. 5. 1912. (Forest Service Bulletin 112.)
Mechanical Properties of Western-Larch. By O. P.M. Goss. Pp. 45, Pls. IV, figs 14.
1913. (Forest Service Bulletin 122.)

Experiments on the Strength of Treated Timber. By W. Kendrick Hatt, Ph.D. Pp.
31, figs. 2, tables 12. 1906. (Forest Service Circular 39.)

Tests of Rocky Mountain Wood for Telephone Poles. By Norman de W. Betts and
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Rocky Mountain Mine Timbers... By Norman de W. Betts. Pp. 34, figs. 7, tables 16.
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Tests of Wooden Barrels. By J. A. Newlin. Pp. 12, figs. 1, Pls. V, tables 6. 1914.
(Department Bulletin 86.)

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_ Timber: An Elementary Discussion of the Characteristics and Properties of Wood.
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Bulletin 10.) Price, 10 cents.

Effect of Moisture upon the Strength and Stiffness of Wood. By Harry Donald
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Properties and Uses of Douglas Fir: Part I, Mechanical Properties. Part II, Com-
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Tests of Structural Timbers. By McGarvey Cline and A. L: Heim. Pp. 123, Pls.
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Holding Force of Railroad Spikes in Wooden Ties. By W. Kendrick Hatt, Ph. D.
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Tests of Vehicle and Implement Woods. By H. B. Holroyd and H. S8. Betts. Pp.
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Properties and Uses of the Southern Pines. By H.S. Betts. Pp. 30, figs. 6, 1909.
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Utilization of California Eucalypts.. By H. 8S. Betts and C. Stowell Smith. Pp. 30,
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Strength Values for Structural Timbers. By McGarvey Cline. Pp. 8, tables 4.
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