Relation of strength of wood to duration of load

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Title:
Relation of strength of wood to duration of load
Physical Description:
Book
Creator:
Wood, L. W
Forest Products Laboratory (U.S.)
University of Wisconsin
Publisher:
U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory ( Madison, Wis )
Publication Date:

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Resource Identifier:
aleph - 29512506
oclc - 757710383
System ID:
AA00020694:00001

Full Text
No. 01910 '

i r

UNITED STATES DEPARTMENT OF AGRICULTURE
REST SERVICE
FOREST PRODUCTS LABORATORY
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin

IILATION OfI SPINGTH if WC)O)D
lT IOURATION O)F ILA
D cQmber 195l




Digitized by the Internet Archive
in 2013
http://archive.org/details/relagOOfore




RDA:ICN 0(2 S=nD~ 0:_ E,21 1' O U NI FIjQAD
y~ w. 1111CC, Engineer
17orest Prolacts F~rtry orest Service
U. S. oX'r:.n f Agri ur
Sum=or
ThIs re ort ~rcetfor the use of st~ractur en -ireers, a mathematical
ex- ression :'r -n i.,- ,orts-,t stn.,ct=,Ir~rt of -v.oc' the rel.-tion of its
tendin strer -t. to tl-e dur, tiorl of 1 on, Ter aionsl-d* is ntc~tically
defined, .id ic Ations in iwor .ir:-stress rro,_em~s -.re s-.own. Structural
designers nre t.:us ena, led to t,-.--e full v1t- off t-Le Ur Usllrly hi-,h bend-
ing strength of '.Dod. under short-time lo: Iinj.
Data. sho ,.in 7 the re~in'-are from reCent>_ com-,lete"I tests of sm1ll,
clear Bou~1as-fi -eatns dne qo~tm 1 from e,-rlier studies of
rapid loading and impact. -'n emAica1 I-perbolic e 7uaticn is developed to
reTuresent the trends of t',e drt_. A few ex,, lorrtcr,. tects of other sT' cies
and in ohrstren th o ti ndicate tha-t th-e relationships mray [C-e of
general lic"Ition.
Tn- tr oduoict i on
Designing engineers custoi-,L,-rily set wohn:stresses for structural materials
at levels 1ol.-I, thae i&dmor elastic liiiitt -o insure safe and satis-
factor structures under service oinj c recognize th;Dt :,pterials
loaded [->Jthe ela stic ,Li.,nit >.-lose e _.sticit.-, rmd take on charncteris-
tics o- Irittleness cnrlsict ac cor.1i-ng, to their net-,ze. It is less
c~enr, IZ, t'-at materi,-ls differ ~<~Lin th-is resect, Pnd t ,At in
sorl-e the str.,-, th I ro )er-ties o re crat'L7 ?afected b-, the Lr -,ti ,n lo, d-
in ,. WorI nsar r vaflx-,e to t'ne str-ctarn! desi.;ener in th.a 'u th.
its el,--).- Lj.dt and its ultimr-t-_ stre -th are .i-her un,,, s:.rt-time than
under r.-tiLae blm ;di this -r.ishiner workinc- stresses .hore Tive l oads
of corfn- 'rAtivelv short duration :,.ust Ie considered in str-dotur,-J Je s i~ n.
-:aintnained risn is., in cooperation with the universityy of Wiscfrnsin.

i\-a-icultune--.a:sci




--he r._7_7ti-on of the stren,- -th of vced to tl-e duration of _oa d h,-s beer- in-
x-otip tC +" th P-e .rut "rtr or mpn ye,- rs. Recent an; lyIsis
of irti!- >a rom ,4 series of !on,-time loa-dinF tess of small, cle.-,r
Lcu'1s-irLeas led to a restudy of the relation of duration of loal to
Sstre-::,t1_ of wo,.d nnd a revision. of cornclains, --reviLousl:7 held. 71-i s
~rs L~riestl-e (117tn considered in the restudy -)nd re-,,orts the con-
:D, t '7 onsid1ere(I herc. --re from bending tests of Do~isfrat tWO levels of
r~.~J' .rL <~j t 0ti- in the air-dr,,- ranLe. Te concl --sions ca -Tot --et
ex~n2'1 t o ot', Lr s cies a-nd strenr-th po rtes or to wood i n re
o-~ n.I~l -lhuha 1few exi-llorritor,,, tests in-1icate that _0endin,: andi other
str.-n -th -o 'orties in some oth-,er sp ecies re s elr> ff -tei A~
at~'- ,~iion of 7 th -e Conclusions is subject to revision -!s .-.ore
co1-.. lete inform,, tion is obtaij4-
Sources of Data
soris Of 1-C -)nL-time lor-,1irg tests of 1- -t- I-nch, cerDua-l
buims -t I)andf 12 -,crcent .-,,isture content was 'Le ,= in 1943 and is now
a rec'.i c':'le in. est s ,i' -ens were si;: etecl zo constant loads
rJan' a fgro:.-, to 151 erce--t of teload~s t-az ca_:usedl I ailire of mLatched
co- :ls'ecies -n rstanird static-endin- test of about 5 minutes'
d uration. D'arations uint-11 failure uriaer these loads ran-'ed fromi a few
minutes to more t =n & years. 11, ire 1 show,,s durations Jotte i air st
stress levels corrosp ornen to the al- lied loads in each- of thu tests.
jt i on va J.lues are shnown on a losarithmic scale.
S tu 1i e sc th e c ff ec ts of r a,)i rat es of lo,-d i ni- on s,--., cer Dou~laz- fir
o 1-j' c-Dortod by Dikl3 ata from thait r&Dortl a-nd a !line sh-owing.
their trend n-,-e --,ott _-d in fiLmire 2. Theo time sca le is o, -,ri t-1Mic. The
faof riue nd f "iure ? arc not exactly c L. rale ince fi ue I
dhos ur- i ns of stress increa)sinc- to 7.redetormined le-Vels aind then he ld
const _,:, while flig u-e 2 slho,.rs times o-f l~onain, continuousl- increasin,:- at.
CM costnt rat e until failure. L evartheless, it is believed that both sell
of (1a-ta nre governed by the same propertiess, of the m-,ateriz-l and should be
r evre sent cc I one continuous curve.
Inattest, in hihthie actua-l forces or lo:-ds were observc-%i., wre re-
-~orted 4 _;A-A. h dt n~c ed tha,.t the modualus of araTturc of
o'Jla7 -Li -s s -out 75 -ercent reater in im weat than in static bendinE
as' show-.n t he st ,,.! ,rd F -miaate test. Elmendorfts da-ta-- on Dou-lus-fir
Wi S k I, J. i. C,-t Of La L K i' in~ 01. t_.i ~crres and I xr~
S t r cn~ cd t : c ore st F 1 ixUc ,-ibrjr-Itor r ,. i o. J7 17f .Q 19.1
-Elmendcorf, _zAr. Stresses in Li aCt. vg.,rn,,l of the 17r,-uklin Lnstitute.

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Figure 3 shows an empirical hperbolic curve passing through a point
representing a duration of stress of 0.015 second and a stress equal to
1-50 percent of ultimate strength in a standard test. This curve apTraxi-
mates the trends of data from the long-time-loading and the rapid-loading
tests. It has the equation
- 108.4 + 18.3
0.04-635
x
in wnici x is the duration of stress in seconds and y is the stress x-
r ressci as a percentage of the standard-test strength. This equation is
corn uted so that the curve ,asses t.-hrolih three selected points. The
first loint is the point just describe and is somewhat below the impact
point. The second point is at the lCO-percent stren..th level, for which
a d'ura9tion of stress of 7-1/2 minutes was assumed. The third point is
- rbitrarily selected from the long-time loading data with a strength level
o 72 percent and a duration of 3,71-0 hours (shown on fi. ). The hori-
zontzl ts;mlote of tis hyerbola is 18.3 percent, a strength level for
which the duration is Dres-umed to ue infinite.
The h1yperbolic curve of fioifre 3 is also shown on fi i re 1. It is well
within the range of the long-time loading data, though toward the up-er
side of thait range for strength Levels of 60 percent and lower. On the
other hand, a similar curve for bendine strength of Sitka. spruce shows
still higher stren-th levels for this ran-ge of durations (fig. 1). The
departure of the hyperbola from the general trend of data at the 95-
percent strength level is necessary to fit it to the maid-loading data
of fiar e 2. In t'ne absence of similar information from other species,
this h4 /erlola ma-)y be tadOn to express a general relationship between
strength aud duration of loadl for those species most used in construction.
Aoplical tion to rin4 Stre sses
The relation of duration of 1o-d to strength is iLportnt in the determina-
tion of wor-ing stresses for stracturl desi lth wood, Advantage may
be ta en of the increased strength of wood under short-time loading by
increasing the i-orking stresses where m.,oi:xr, load is of limited duration.
Fi~npie 4 illustrates a convenient ens for doing thlis. It shows the
1yncrbolie cuve of -iue 3 -lotted in a fori directly a{- licabl-e to
workin ; stress use. zasic workin, stresses recoiX..ened ly the Forest
Products _.-arratory are for the condition of lon ti_ e fu I load for
i ,,h_ ch tlhe strength is ass-Lmed to be nine-sixtent,1s of the strength in the
standdird ya-nte test. lhis 1on -tirie load level is t,-iken as 1"Q percent
in ie 4: ith other )ercentges caiculr-ted -ron it s a ase. ution
values are converted to units of time that are easily visutilized for long
as aell as short durations.
-Forest 1-rcdts 4-bortoryi Strenth and Belated Prooerties of Wood Gro -n
in the .nited States. 3Dept. of 4r. Tech. 13ull. lo. 479. 99 pp.
Illus,

-4--

Rert. 1-I1o. 19316




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0 A-
STA C1H, L 4E~ 0 W/ NG 6EMPIRICAL HYi P46RBC-IC CURVE
80iA- r
CURVE FOR 5/7T SPRUCE
415'V J 48 Al qM LT~ C ONTENT C),VTRO4.5
0 '5 "I ,~T4 T 6% IfO TU,4E CNTE'VT CO~rROLS PO11T7 SE*LEC7EO FOR
04 OE W TO T RATE OF 5Tr,4,S IA!CRLE.4 LOCATION OF HYPERPOL4
5PECMEN A7T 4&-T 7/2% /Y1 5TO L 'T GNTROL5
l~o /N TI A ES7/AlG MACHINE
5 PECIMN 4T Z-AIUT 6'% MOIST/Ri CONTENT, CONTROLS
,YON '/V A 4 TV 1 N
1 ,(LN ATA JSOT Io -U~TRE COWTENT. F4 AT
IRAT 8f fUO~f IT NTR 7 iS' RUN IN4
TE'T( ACHING,
0 7 5 TREL5 F,4LORE ~0 U
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/- ------
/ 2 0 - .- .. -
90 ,
oo
o" o "'%..
ofo
II o I
o o
/0
0// /O /0(
Z M 89381 F TIME OF LOADING TO FAILURE (SECONDS)
Figure 2 --Relation of str,.-_gth to time of loading in rapid loading
tests of small, clear Douglas-fir bending specimens.,

,0ooo00




POINrS USED IN COMPUrING rHE
EOUL)rION OF HE cURvE

QZ
130
g TrRENC OF DAr4I IN
Z RAPID LOADING~
/00
/00-
FMPIRIC1L HYPERBOLIC CURVE
108.4
ffO~n~ON X 0.046555
9O_
rRENO OF DA A IN
LONG-rlIfE LOADING
r 80-
7 0 -- -------
60
OlOo/ 0.0/ 0./ 1 /0 too ,000 /0,00 100,000 /l,0t000 10p~qq" /00000,000 .
URATION OF SRESS TO FA/LURE (SECONDS) (X)
7 M 88799 F
Figure 5.--Relation of duration of' Stress to level of Stress in long-
time loading and rapid lading of Douglas-fir bending
spec imens-; The broken ine represents an extension oll'
the curve f(--r long-time loading.

ELMENDDRF




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160
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120 __0_
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