OREGON FOREST PRODUCTS !_G" :illR/TORY
LIBRAR Y
U. S. Department of Agriculture, Forest Service
FOREST PRODUCTS LABORATORY
In cooperation with the University of Wisconsin
RESULTS OF SOME STRENGTH TESTS
ON WOODEN POLES
By T. R . C . WILSO N
Senior Engineer
C :EGON
FM1
FO !,(IS LABORATORY
RESULTS OF SOME STRENGTH TESTS ON WOODEN POLE S
By
T . R, C . Wilson
Senior Enginee r
Summary and Notes on PoleLine Design
The compilation of the data presented in this pape r
was made at the Forest Products Laboratory, maintained i n
cooperation with the University of Wisconsin at Madison, Wisconsin, and includes tests made by the Forest Service i n
cooperation with the University of Colorado at Boulder, Colorado, and in cooperation with the University of Washington a t
Seattle, Washington . These tests were made chiefly for th e
purpose of comparing the strength of other species propose d
for pole timber with that of western red and northern whit e
cedar, and to furnish a basis for pole design ; also to compar e
two seemingly different methods of test (see figs . 1 and 2) .
The tests were on 25-foot poles and were so made a s
to simulate the conditions which exist when a pole of thi s
length is set about 5 feet in the ground and given a side pul l
at the point of attachment of the cross arm . Seventeen different groups of from 15 to 20 poles each, making a total o f
291 poles were tested . Six species, western red cedar, norther n
white cedar, lodgepole pine, western hemlock, Douglas fir, an d
Engelmann spruce, were included .
Table 1 is a summary of the results . These show that
the two methods of test illustrated in Figures 1 and 2 ar e
practically identical . Study of Table 1 emphasizes the nee d
for care in choosing the basis for making comparisons betwee n
two groups of poles as comparisons on one basis may lead-t o
conclusions diametrically op p osite to those supported by comparison on another basis . This is illustrated by Groups 1 an d
6, for instance . On the basis of "top reaction at maximu m
load" as found in test Group 6 slightly excels Group 1 whil e
in top reaction at maximum load for poles of 7-inch top diameter this is reversed . Moreover, in modulus of rupture an d
top reaction at maximum load for poles of 9 inches diamete r
at the ground line Group 1 is nearly twice as high as Group 6 .
Comparison is further complicated when it is noted that Grou p
1 was tested at a considerably lower moisture content tha n
Group 6 and for that reason gave higher values . Somewhat mor e
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comparable figures are given
Table 1 have been adjusted to
the comparisons are,
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t1 v r
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ultimate breaking slROIIR%i
These figures app
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series o f
sumed maxim
The suggested stresses are based olt the is
report, together with results of test* OIL Wm.' ngam
mens of the species listed .
Codulus *If
Lbs . pet
-
,ur e
. ..
Cedar,
.*kite
,
ppd
Chestnut
Cypress, bald
Douglas fir, dense
~,_
"
" , Rocky Mountain region
Hemlock, western
Pine, to dgepole
" , southern yellow (dense)
s:
(sound) .
Spruce, Engelmann
Tamarack . . . i
Pine, Norway-
:
:-A ;
Pine, J ackl
:
"Dense' southern yellow pine as listed above refer s
to poles having not less than one-third summerwood and no t
less than six annual growth rings per inch as measured ove r
the outer two inches of radius of the butt end of the pole or ,
if the number of rings per inch ig less than six, the propo rtion of summerwood must be not j ee .than e-half . "Sound "
-See letter Oct . 26, 1925, to Northern Tie and Treating Co .
R763
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nd .computed top reactions at maximum load for green pole s
Gt'Awp
:
,e-i e
:Weight :
E,sti a.ted
t par
:cubic :d4 ui s
:foot
o-'
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?•
s
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:Pounds : Pounds per
sq . in.
1
2
3
4
b
:Western red cedar
:
u
u
It
tt
n
n
:
it
u
it
:
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---
27
--
6 :Northern white cedar :
7
8
9
10
11
12a& b
13
:Lodgepole pine
n
:
u
tt
n
:
n
tt
:
n
tt
:
it
:
"
:Western hemlock
14 :Douglas
15 :Engelmann spruce
5200)
:
:
5400)Average :
: 4700)
:
:
4500) 5000 :
:
: 5200)
1580)
1690)Average
1130)
1740) 1620
1980)
:
:
:
:
:
1630 )
1690)Average
1470 )
1410) 157 0
1630 )
: 3000
: 1560
:
:
:
:
40
4100
5500
: 5000
:
4000
4400
:
5500
: 6200
1280
1450
2120
2420
2070
: 1340
: 2010
:
:
:
:
:
:
:
38
: 6700
: 1900
: 210 0
39
: 3200
: 1220
: 1000
28
39
---
--
:
Top2 reaction at maximum
;load-. Load foawles of
:
"7''-7
d I incit top r
: diame
::round line DJ
.,_ .
<~
•- -- .
Pounds!
Mind
s
:
:
940
1280
172 0
1570
125 0
138 0
172 0
1940
1For description st t
- Top reaction at maximum load is equivalent to the pull which if applie d
1 foot from the tip end would break a pole 25 feet long set 5 feet i n
the ground .
-The figures here are for Pacific Coast Douglas fir as the poles of Group 1 4
were cut in this region . Douglas fir from the Rocky Mountain regio n
averages about 4 pounds per cubic foot lighter when green, 2 pounds pe r
cubic foot lighter when thoroughly dry, and the clear wood is abou t
,?4 ant weaker than that from the coast region .
R763
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en
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Method of Tes t
Forest Service Metho d
With , th,o
1±'x of Groups 5a and 12a, tilt
.vel: in Table 1 . pie made• ty the metheril ox.,Irg-j, .
in Ise lay the Forest ServieA Binae 19.11 . (More it.-pitaiLIOdr Anformlatida on Grottp€ 1, 7,
and 1_5 a,V :given in 'United. . 1tate s
Departmeat of AleW ,j4ultWe
Oa, ''Te0q ,40t Rocky Mmtain Joedis for 'thgeplaoce
.whioas no 004 1von thA
other grot. .a.iae have hexett.0ore. been publis'.w.d .')
;g
,sunfaleir
r.
In . t.1teftsts the 2Z-foot 'pole is s€imported 1 f'aO.e
froth title 1k-eft .satvO 1 foot Es-ma the top elrtql". making the span
23 feet .
(See IL
-k-t
L.) 6A.ul ifs !ppled 4 -fete* fr4m t.,lip.e
support (al. 5 Net front
butt e) 'wJ10iollt
q*10oximately
the gzatmd
fir a 36Q*P*-b OAtwit..
• _TA
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to,*
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e4ries of tests-40ta f
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Sex-14m 4110k, Wer e
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,epregmiNa wiriet4ft
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inwreityrrrreuri
aV•
4114qp* co
ept
ent .
ti
1*Pe-a-e ir4varie 604ti eni~ 100mg ellf a variety of diameters an d
lengths. . A brief siftlpaqp
Bulletin 67 previously m&ntiosed .
_
R7 63
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Fig . 2 . - Pacific Telegraph and Telephone Company's metho d
of testing poles .
Fig . 1 . - Forest Service method of testing poles .
ZM137F
Comparison of the Two Method s
Apparently the conditions in an actual pole lin e
where the top of the pole is pulled sideways by the tensio n
of dead-end wires, or by wind pressure on the wires is mor e
nearly reproduced by the Pacific Telephone and Telegrap h
Company's method than by the Forest Service method . However ,
the difference between these two methods is more apparen t
than real and they are essentially identical .
In a pole line or in the Pacific Telephone an d
Telegraph Company's test method the butt portion of the pol e
is held more or less rigidly and the top pulled sideways .
In the Forest Service method as previously described the to p
and butt are supported and bending produced by a force applie d
at the ground line . In tests by either method it is the relative motion of butt, top, and ground line points that result s
in the breaking of the pole . The force or pull acting on th e
top of the pole is definitely known in either case .
The data on Groups 5a, 5b, 12a and 12b furnish a
basis for comparing the results secured by the two tes t
methods . The evidence from such a comparison is negative ,
i .e ., it cannot be said that either test method gives hishe r
resaUUs that} the other .
Computation of Result s
The columns of Table 1 which include figures on th e
strength of the tested poles are "Modulus of Rupture" an d
"Top reaction at maximum load ." Modulus of rupture is th e
computed maximum stress in the pole at the maximum load . The
point at which this maximum stress occurs depends on th e
taper of the pole . If the taper is such that a diameter o f
one and one-half times that at the top reaction, corresponding to the point of attachment of a single cross-arm, or t o
the point where the pull of the wires is centered when mor e
than one arm is used, is found above the ground line the n
the maximum stress is at this point of one and one-half diameters . Mien the taper is less than this the maximum stres s
is at the ground line . The relation between maximum stres s
and top reaction is given by the following formula :
Maximum stress
L x top r3 actio n
. 09 8D
where L is the distance in inches frem the top reaction to th e
point of maximum. stsosom and D is the diametet in inahmS at
this point .
T.W, ptleV having the same top diameter and top re action willAot, have the Siate madUllas of' rupture unlesa 00V
. have the Me taper ; or, it their tapor$-diffez two poltt
having the same madmiua. of rupture alzdj same '00 ,el,iiameter wil l
not have the same top reaction at mzm4mvm Iaa(d since the difference in tape w4 will r'e.slult in diffe.veg t distances from tO P
reaction to poiht of maximum stress .
"Top reaetilsn at maximum load 4 is, in case
.. t elate
tests, equivale .lt to Ow pull which, it applied 1 foot from
the top end would breat a pole 25 feet long set 5 , feet in the
ground .
It is a me.agwre of the strength of a pole .aa a uni t
and is the best figure en which to balm a compaxieam of the .
strength of the various groups of pele g .
The use of Nmaxim4K fibeff'stress at poiat of failure" instead of "modulus of rupture" has been eu4ogeated .
A
comparison of these quantities for the pole-a of ,bMttpo ga and
12a is given in the following table :
Table 3 .--Comparison of "Modulus of 'Rupture "nd
_
"Maximum Fiber Strass at_Poiat_ of Fal-1 ure" - Poles tested byPacific Ta:610toDl
and
,m%
aDIRX.JP,tit
lagi
Group
5a
12a
Modulus of Rupture
5770
7850
Maximum ?ibex
stress @it poitut
of faikwr e
5S*Q
From the above table it will be seen :hat . the affeaecc e
tween the modulus of rllptu .re and maximum fiber stweag at .
point of failure is not great . This tathex small d4famewoe ,
however, might not obtain in the case of low es pales .
It seems logical to use moduius of rbw*ex* jasteali
of "maximum fiber stress at point of fmA-lure" area for. tat e
following reasons :
1 . Modulus of ruptuTe is the magetmism stress eLeveloped and it is manifestly unfair to have the strength of the
pole represented by the lesser Stress developed at the point
of failure .
2 . The point of failure cannot be definitely locate d
and each of the possible locations of this point may give a
different "maximum fiber stress at p oint of failure . "
The test data are, of course, intended to fora m
B.
a basis for design and in design the ground line or the poin t
of one and one-half diameters is taken as the critical section . (In most cases the critical section is at the groun d
line oecause moisture and decay at this point are certain t o
reduce the strength below gnat of the upper part of the pol e
since the upper part is less affected by these factors . )
4 . In design or in comparing test results the sid e
which the pole is able to withstand is the factor o f
most interest . If the modulus of rupture is known the sid e
pull for a pole of any given dimensions can be readily computed . This cannot oe done with "maximum fiber stress a t
point of failure" unless data is given or some assumptio n
made as to the location of the "point of failure ." The use
of maximum fiber stress at point of failure combined wit h
a correctly assumed location for the point of failure wil l
lead to correct results but simplicity of computation is i n
favor of modulus of rupture .
pull
Adjustment of Result s
Adjustment for size of Pole .--As will be noted fro m
Table 1, the poles tested varied considerably in diameter .
It is manifestly incorrect to compare poles of different diameters without making allowance for the difference in diameter .
In order to put all the groups on the same basis as to siz e
the data derived directly from the tests have been used t o
compute the strength, volume, and weight of poles of .7 inche s
top diameter and poles of 9 inches ground line diameter .
These adjusted figures are given in the last eight columns o f
Table 1 . The adjustments to 7 inches top diameter are explained as follows, using Group 2 as an example : Diameter a t
load point (ground line) =
7" + 2 taper from top to load
= 7 + 2 x 1 .00 = 9 .00 inche s
(the factor 2 is used because taper i s
given in Table 1 in inches per 10 feet )
It was found from a study of the poles tested tha t
volumes were quite accurately expressed by the followin g
equation :
Vol . - (in cu . ft .) = .14-D 1 2
Mote
Where D. is the diameter in incises at the middle
compute on the assumption that the taper from t om' .
is the same as from top t? g4t*Wl line .
For the 7-inch top diameter pales of Gr !t$p,
12 . 5
D 1 = 7 .00 + 10
.0 x 1 .00 = & r ?
Vol . = 0 .14(8 .2:5) 2 = 9 .5 cu.. ft .
Then this adjusted volume m zltiplied by weight per efoibli'o Pero t
gives the adjusted weight of the pole -- 9 .5 x 27 .0 = 0&e
pounds .
fo r
Since/the same m;oc ulus of rupture tae, top r- .we iieg
at maximum load va ries as the caul ~e of the gD?our d Lim* ci4ameti
the adjusted top reaction at maximum load . equals r .00)3 2490
.45)'
21 65 .
made in a
The adjustments to 9 inches
similar manner .
r©:z.d li.ae
4
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:
Adjustment for Moisture Co•n,4itio m
The above mentioned adjustments eliminate frc t.'e
comparison of the various groups the effect of differences .
of diameter . They remain not entirely comp :raaijrle, however ;;
s
because of considerable difference in the :moisture Om
when tested . Table 2 gives the estimated modul- of 00-W%trv '
and computed top reactions at maxima m load for green egi sM1
More accurate comparisons can be made from this table ' th+ n f -Mm
any of the figures given in Table 1 .
4
-This
-Thi
s is the formula for the volume of a cylinc e ' 25 feet lon g
and of diameter D l . The maximum variation from t. 4e 'e
tion value was in the case of northern wMi tq • .e : .aLr RIVWMS
6) . The actual average volume as obtaimed . fe&m a4:;00 ^
measurements was 4-1/2 percent great :er tt.ar the .oAoM-Me
value . I t is not known that equal acelaxaLc .T would be o.lb
tained by applying a similar formula to poles of grei~
length .
.R763
-77 -
Use of Results in role Desig n
It is probable that a pole in the line will, a t
points above the ground line, be capable of developing it s
air-dry strength (approximately 1-1/2 times its strengt h
When green) while at the ground line it will, because o f
absorption of moisture from the soil, develop only abou t
the same strength as if green . If decay takes place thi s
will also be at the ground line, making it certain that, i n
a large number of cases by far the weakest section of th e
sole will be that at the ground line . Consequently, th e
strength of green 'rather than that of dry or partially dr y
material should h2 the basis for computing allowable load s
on poles .
In using the results of these tests it should b e
remembered that each group includes a comparatively smal l
number of poles and that the variability of timber is suc h
that occasionally poles of low weight -- when completel y
dry -- may be found which, because of the poor quality o f
the wood (aside from defects) have only about one-half th e
strength of other and heavier -- when completely dry - poles of the same species . (The density or specific gravity
of the dry wood is, aside from actual strength tests, th e
best criterion of the strength of the non-defective portion s
of any timber .) The effect of such defects as knots, decay ,
cross grain, etc ., is, of course, to reduce the strength belo w
that of a non-defective timber and consequently add to th e
Variations in strength .
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