2013 greenwood eng treehouse

8/13/2019 2013 Greenwood Eng Treehouse

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Copyright, 2013, Charles S. Greenwood, P.E., LLC

TREE H!SE E"G#"EER#"G $R C%E &PPR'&L

&n (er(iew )y Charles S. Greenwood, P.E.

A. STRATEGIES FOR CODE APPROVAL AND THE NEED FOR SPECIFIC

CODE LANGUAGE

*+ilding Standards, an #C* p+)liation, presented in the -+ly&+g+st 2000 edition an

artile a+thored )y /ysel and %a(id *assett, P.E,. C.*., entitled Ha)ita)le

Treeho+ses "ot as Si/ple as Swiss $a/ily Ro)inson. This s+//ari4ed nearly a

deade o wor5 in this ield.

#n the thirteen years sine there has )een an e6plosion in the onstr+tion o tree7

/o+nted str+t+res aro+nd the world. 8+h has )een learned a)o+t what to do and

what not to do. 8any per/its ha(e )een granted, e(en or large p+)li7+se wheelhair

aessi)le pro9ets )y reerening #RC R202 :;.other independent syste/s<,

R301.1.2 :;..aepted engineering pratie<, R =02.1 :;rot7resistant wood

o+ndations<> li)eral +se o the peror/ane7)ased pro(isions as well as alternati(e

testing proed+res /ay also )e re?+ired. Howe(er, typial Codes are "T a good it to

treeho+se engineering and onstr+tion /ethodologies.

SIGNIFICANT DIFFERENCES COMPARED TO “GROUND-MOUNTED”

STUCTURES INCLUDE:

@@.@A $ TREE H!SES H&'E "T G"E THR!GH &" E"G#"EER#"G R

C%E RE'#EB.

""7C8PL#&"CE * TREE H!SE B"ERS C&" *E TR&CE% T :1< THE

L&CD $ & RE&S"&*LE C%E PRCESS T $LLB, &"% :2< THE#R ""7

C"$R8#ST ETH#C.

C"ST&"T CCL#C L&%#"G &"% 8ET&L $&T#G!E C&" LE&% T

C&T&STRPH#C $&#L!RE. *!#L%#"G PR$ESS#"&LS 8& "T *E TR&#"E%

#" TH#S TPE $ &"&LS#S.

$LE#"G #" *E&8S, B&LLS, -#STS, ETC., REF!#RE TH&T 8E8*ERS *E

%ES#G"E% $R 8&#8!8 %E$LECT#" B#TH#" THE#R STRE"GTH L#8#T. L30

#S L#DEL T CRE&TE !"S&$E ST#$$"ESS #" TREE H!SE STR!CT!R&L

SSTE8S.

'&R#&T#"S #" L&% CLLECT#" &"% %#STR#*!T#" 8& REF!#RE

EF!&L#&T#" *E&8S &"% THER !"!S!&L TECH"#F!ES.

Copyright 2013, Charles S. Greenwood, P.E., LLC %o "ot Copy witho+t Britten

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&LL *!T THE S#8PLEST TREE H!SES 8!ST *E E"G#"EERE% !S#"G $#"#TE

ELE8E"T &"&L#S PER$R8E% * &" EPER#E"CE% PR&CT#T#"ER T

%ETER8#"E %"&8#C $LE#*#L#T &"% STRESS C"CE"TR&T#"S. &G&#",

*!#L%#"G $$#C#&LS 8& "T *E $&8#L#&R B#TH $E&.

Code lang+age /ay reate pro)le/s rather than sol(e the/. $or e6a/ple, "$P& r+lesregarding e/ergeny power syste/s +sed in hospitals and other ritial ailities

+nort+nately ontri)+ted to +nneessary in9+ry and death d+ring S+per Stor/ Sandy.

Bhat sho+ld )e the intent o *+ilding CodesI Generally aepted as the irst )+ilding

ode :3,J00 year agoK<, the Code o Ha//+ra)i pro(ided p+nish/ent to )e /ade

proportional to the in9+ry, and +lti/ately R+le 22@ delares that # a )+ilder )+ilds a

ho+se or so/eone, and does not onstr+t it properly, and the ho+se whih he )+ilt

alls in and 5ills its owner, then that )+ilder shall )e p+t to death. The intent o /odern

odes sho+ld )e to a(oid that o+to/e.

DEVELOPMENT OF TREE HOUSE SPECIFIC CODE LANGUAGE SHOULDFOLLOW INTERNATIONALLY RECOGNIZED METHODOGIES FORPERFORMANCE-BASED DESIGN.

The peror/ane7)ased *C& :*+ilding Code o &+stralia< was drated ollowingonsideration o n+/ero+s o(erseas /odels :inl+ding the "ew ealand, *ritish,Swedish and %+th e6a/ples< to s+it the &+stralian )+ilding reg+latory en(iron/ent.This has /eant that the peror/ane7)ased *C& s+)stantially inl+des the tehnialre?+ire/ents ro/ the pre(io+s *C&@0, with a peror/ane hierarhy )+ilt aro+ndthe/. The hierarhy o the *C& is shown )elow.


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See also &SCESE# J710, Chapter 1, where the onept o Peror/ane *ased %esignEngineering is introd+ed, and the ter/inology o +pany Category is replaed )yRis5 Category.

Please pro(ide yo+r ontat inor/ation i yo+ wo+ld li5e to wor5 on Tree Ho+se Codelang+age. This will )est )e ao/plished )y the P.E. and C.*.. o//+nity.

B. STRUCTURAL PROPERTIES OF LIVING TREES

Tree ste/s and li/)s are highly le6i)le (ertial and hori4ontal antile(ers> they pro(ide

the )iologial /ass transport syste/ or n+trients )etween the solar7powered engine

o+nd in the lea(es and the o/ple6 )iohe/ial atory o+nd in the root syste/.

Roots also at as a di+se soil anhoring syste/, apa)le in al/ost all ases o

resisting enor/o+s repetiti(e loads witho+t ail+re. Please see #/pro(ing Tree Health

8yths M Eli6ers )y la D. Ri)eiro, Ph.%. or an introd+tion to root physiology.

Trees grow +pward ro/ the top and o+tward ro/ their enterline.

1. The tree dia/eter a)o(e the root )+ttress, typially e6pressed as %*H

:%ia/eter at *reast Height<, /ay )e ta5en as the /ost a+rate /eas+re o the

at+al strength o that parti+lar tree> gi(en the speie properties yo+ /ay


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al+late the pro)a)le /a6i/+/ strength and thereore the load the tree is

designed )y nat+re to resist.

2. Trees respond to i/posed loads, adding )oth ?+ality and ?+antity growth at

stress points. This o+rs ?+ite rapidly, e(en in one season. Try that with yo+r

typial engineering /aterials.

3. o+ng trees grow +pward witho+t any saety ator as nat+re dee/s it /ore

i/portant to esta)lished solar ontat at its rown. Ste/ ail+re will si/ply plae

the n+trients )a5 into the ar)on yle and a new tree will try again to reah the

s+nlight.

=. The saety ator or an esta)lished tree is appro6i/ately 1.N. This is inerred

ro/ the a)ility o the tree to s+ess+lly resist loads that it has ne(er pre(io+sly

e6periened.

N. Hyper7elasti )eha(ior is deined as the a)ility to te/porarily withstand loads

that are well a)o(e the la)oratory deri(ed r+pt+re strength. This is si/ilar to the

)eha(ior o in9etion /olded plasti parts where the addition o water inreases

the short7ter/ load resistane, al)eit at lower total strength.

. The 8E o li(ing trees is a)o+t J0A or less o the sa/e tree +t and dried or

l+/)er.

7. Load *earing *iologial Str+t+res always try to grow into a state o onstant/ehanial stress. The orollary to this is a /ar5ed red+tion in strength ater

+tting the li(ing tree into on(enient shapes O two thirds o the li(ing strength/ay )e lost.

. Trees do not neessarily e6hi)it si/ilar deletions and nat+ral re?+enies inresponse to wind loads. They +se torsional osillation :/+h li5e an egg )eater<to d+/p energy )y rition into the o/parati(ely thi5 (iso+s air, and ineessary, they shed li/)s preerentially to preser(e the /ain ste/. This /a5es)eha(ior +npredita)le, e(en or the sa/e speies and si4e.

The e6at /ehanis/s ontrolling li(ing tree )eha(ior are yet to )e +lly +nderstood, )+tthis does not interere with +sing what we ha(e learned a)o+t plaing signiiant loadsinto li(ing trees o(er the last twenty years.

LetQs loo5 at the engineering properties o the tree itsel


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Th+s a /o+nting syste/ plaed 3Q a)o(e grade in two trees with harateristisdeined a)o(e /+st ahie(e eight eet o total translation apa)ility. Red+tions in treedia/eter will o o+rse inrease deletion )+t this is li/ited )y the +lti/ate r+pt+restrength o the ste/.

Li/)s an )e analy4ed in a si/ilar /anner. Be /+st aept the root o+ndation

strength on the )asis o pre(io+s s+r(i(al o the speii tree in ?+estion and o si/ilartrees at the site.

Discussion of the Derivation of Allowable Working Stresses:


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3.1 U.S.F.S Circular #213, presented in Mechanical Engineers' Handboo, !ionel S.Mars, Editor in Chie", First Edition, Seenth $%pression, 1&1 ()lso re"er to %orerecent in"or%ation*

Species Fiber Stress (psi*

)t elastic li%it )t +upture ME (1,---'s o" psi* Co%p. erp. /rain, psiCedar, $ncense (green* 330- 2-- &- 310-Cedar, +ed (green* 310- 02-- &- 2-ouglas Fir (green* 30- -- 101- 34-Spruce, Sita (green* 2- 0-- 123- 2-

3.2. Unit Stresses in Structural Materials, ) S5%posiu%, 6ransactions o" the )%ericanSociet5 o" Ciil Engineers, 7ol. &1, 1&2 containing 8Unit Stresses in 6i%ber8, 9. ).:e;lin, U.S.F.S. Forest roducts !aborator5

:e;lin had at that ti%e conducted --,--- strength tests o" ;ood at the U.S.F.S. Forestroducts !aborator5. He states that .20 reduction "or 8ariabilit58, .20 "or %a<i%u%de"ect allo;able in grade, and .32 "or long ter% dead load. 6his creates an allo;able;oring stress "or dead loads applied to shapes cut "ro% ;hole logs at 31.2= o" testalues.

3.3 7alues "or "iber stress in bending at rupture ;ere obtained "or %an5 species ingreen and dr5 condition "ro% tests conducted oer a "ort5 5ear period b5 the U.S.F.S.Forest roducts !aborator5. 6heir research lead to the ;idel5 adopted de>ratednu%bers ;e see in toda5's codes. For e<a%ple, i" rupture occurs at ,3- psi and ;euse 31.2= o" that as the design li%it, ;e get a design alue o" 1,&4 psi.

!iing (or cut ;hole* trees hae b5 de"inition gro;n their "ibers to achiee e?ual stressdistribution. :ature proides %an5 do@ens (hundreds in so%e cases* o" tightl5 spacedla%inations naturall5, each gro;n in response to the actual stress "ound at that point inthe tree.

We can de-rate the tree itself, based upon its ultimate rupture strength x 0!"

#sing this value as the $%ree Design Strength& we can calculate the acceptablemaximum moment at ground level

'ature establishes an approximate safet( factor of ") for an established tree*using the same de-rating logic suggests that an( loads added to the living treeshould not exceed 0!" x )0+ the $extra& strength %his is the basis of the$ ")+ .ule& that has been respected as the /aximum oad in most engineereddesigns %his can also be applied to the vertical strength of the stem

1 2'34'22.2D S#556.% S7S%2/S A'D /6#'%4'3 4' %.22S


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%82 ")+ .#2 A6WS %82 /A94/#/ 2.%41A A'D 86.4;6'%A 6ADS%6 <2 D2%2./4'2D <2=6.2 76# <234' %6 D2S43' %82 %.22 86#S2

!$7E ES$/: !)S EE: U: 6HE !C)6$:, S$AE ): U+SE

66)! !) MUS6 $:C!UE 6HE 6B$C)! 1- SF F!+ ): +F E)!), )6 !E)S6 4 SF )!! !)S, ): ) 7)!UE F+ 6HE FU:)6$:SBS6EM 6H)6 $S )6 !E)S6 1-= F 6HE !D FU: )7E. 6HE+ !$7E!)S $:C!U$:/ $:, S:, $CE, )6E+ E:)6+)6$:, E6C. MUS6 )!SE )CCU:6E F+. /)AES C): H)7E ) FU!! S: !) : 6HE +F

): ):6HE+ $:>!: FU!! S: !) : 6HE EC.

)CCU:6 F+ U:USU)! )$6$:)! !)S $:C!U$:/ 6$ME+ F+)M$:/, )+6 9EC6S, HE)7B FU+:$6U+E, +$/EGA$ !$:E C::EC6$:S, E6C.

CC)S$:)!!B 6HE SE$SM$C !) $!! +U!E, U6 6+EES $6H 6HE$+ +6

SBS6EMS )+E 7E+B / )6 ECU!$:/ 6HE SH+6 6E+M /+U:M6$: 6HE+EF+E US$:/ 6HE 1>SEC: 7)!UE $S )CCE6)!E.

$: !)S C+E)6E M7EME:6 $:C!U$:/ 6+):S!)6$: ): 6+S$:)!SC$!!)6$:, ): 6HESE M6$:S )+E SU9EC6 6 +):M +E7E+S)!S.

$ %A<& > %.2286#S2 A%%A18/2'% <6%S

For a s%all tree house ;e can assu%e an e""ectie !D (including ;alls and supportingbea%s, etc.* o" 1-- SF in %ost cases. )boe 10- S.F. a %ore detailed tall5 ;ill bere?uired. 6his %eans the 6)s ;ill need to support 10 ips, and ;ith true ratings in the

3,--- pound range or less, at least or 0 o" the %ost co%%on t5pe ;ill be re?uired.

6he design is e""ectiel5 the co%bination o" a 3I .. b5 1I long 6EC>lie shear ring%ounted on a threaded stud, usuall5 1.20I in dia%eter lengths and collar details %a5ar5 ;idel5.

) one inch dia%eter lag bolt ;ith a I long thic ;alled tube (holobar* ;G 2I .. < 1I$.., ;ith at least 3I i%bedded into co%petent sap;ood and heart;ood, ;ill support 0 or ips and allo; "or so%e "uture tree dia%eter gro;th. $t is also easier to install andless e<pensie. 6his 6) design taes adantage o" the %uch greater pull>outresistance o" the lag thread, and there"ore %aes a great upper %ounting point "or aturnbucle attached to a long sha"t lo;er 6).

Since it is pre"erable to eep the load centered upon the ertical a<is o" the tree ste%, itis good practice to %ount support bea%s on both sides o" the host tree. n s%all treesthe onl5 option is to use a through "astener st5le 6), ;hich can si%pl5 be a solid sha"tplaced through a suitable hole bored through the tree ;ith a ship auger. 3M 1&-epo<5 is "re?uentl5 used to "i< the through "astener to the tree, as ;ell as to seal the;ound. 6his %ethod is gaining ground ;hen used ;ith larger trees, and is essentiall5


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%andator5 ;hen lateral tension loads are attached to the tree such as @ip lines, cableand chain bridges, etc.

!arge co%petent hard;ood trees can utili@e a pair o" Jside %ountI 6)s rather than athrough "astener, but the %a<i%u% aailable co%pression strength perpendicular to the

grain %ust be ;atched closel5.

6he total !D load is the best deter%inant o" the 6) sha"t dia%eter. Strengthincreases b5 the ratio o" the s?uare o" the dia%eter, so a 1.0I dia%eter sha"t greatl5e<ceeds the load capabilit5 o" a 1.20I dia%eter sha"t, and so on.

Since the sha"t itsel" is supported b5 its o;n bending strength plus the support proidedb5 the collar bearing upon the crushing strength o" the grain (both "or ertical andhori@ontal loads*, it is obious that large and long collars proide substantial increase in6) load rating.

For a critical non>redundant pri%ar5 support 6), it %ust be assu%ed that loadreersals during the li"e o" the structure ;ill e<ceed %an5 tens o" thousands, i" not %ore.6his %eans that spring steel is %andator5, and that is acco%plished b5 speci"5ing theproper allo5 "ollo;ed b5 proper heat treat%ent. Such a part should :6 be installed;ith a pipe ;rench or si%ilar tool since scarring ;ill allo; stress cracs to "or%,potentiall5 leading to catastrophic "ailure. $t is not di""icult to proide a %ounting sche%eon the end o" a 6) that eli%inates this dangerous practice.

/4'4/4;4'3 %82 %6%A '#/<2. 6= %A<S 4S 4/56.%A'% %6 %.22 82A%8

For each 3I collar in. s?. o" the ca%biu% la5er is sacri"iced "or the 6). For an

eighteen inch dia%eter tree ;ith t;o si%ilar 6) penetrations, I o" the 0.0I totalcircu%"erence, or about 1-=, has been sacri"iced. $" the 1.0 inch dia%eter JH!I st5le6) is used in a co%petent tree, as %uch as 3- ips can be supported rather thanabout ips ;ith the short>collared 1.20 inch dia%eter 6).

6he practice o" ;rapping the tree ;ith cable cinched around ;ood blocs, as is done inso%e areas, is a particularl5 egregious "or% o" %ounting since little ertical support isdeeloped ;hile at the sa%e ti%e causing a %assie a%ount o" inKur5 to the ca%biu%la5er b5 s?uee@ing it enough to stop biological transport "lo;. :eer do this.

) better approach is possible ;hen used onl5 "or hori@ontal load support this approachplaces a "or%ed steel ring around the tree so%e distance a;a5 "ro% the bar, ;ith atleast three threaded pins tightened into the sap;ood to position the deice. $t o""erslittle ertical support "or tree house and plat"or% loads, but achiees better tree healththan the cinched bloc %ethod.

32'2.A D4S1#SS46' 6= %.22 86#S2 2'34'22.4'3 4SS#2S


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Charles S. Greenwood, P.E., LLC, /a5es no lai/ or warranty that the inor/ation

presented herein is diretly applia)le to any other sit+ation or ir+/stane sa(e or

the ed+ational e6erise o the attending engineers and )+ilders. # yo+ are onsidering)+ilding a tree ho+se that /ay need *+ilding Code appro(al, it is neessary that yo+

e/ploy a Proessional Engineer who has de/onstrated their s5ills in this area o

pratie. #t is ar /ore e6pensi(e to engineer an as7)+ilt str+t+re than to properly

engineer it prior to onstr+tion. # yo+ need these ser(ies, /y ontats are N=1 N@2

=100 and gnwdenga(enet.o/.

L&% CLLECT#" How )ig is itI Bhat do the o/ponents weighI Bhat does itall weighI How is the weight distri)+tedI BhatQs the seis/i loadI BhatQs the sailarea :wind load<I How a)o+t the li(e loadI Snow loadI


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L&% TR&"S$ER How do yo+ ta5e that entire load yo+Q(e olleted and p+t intoa ew rather s/all asteners and ha(e it wor5 properlyI Be are seeing a (ariety opro)le/s with inade?+ate /o+nting /ethods. Bhat an )e done a)o+t itI

L&% S!PPRT "ow yo+r loads are )earing on the trees the/sel(es. How do

trees reat to e6ternal loadsI How /+h is too /+hI Bhen is a tree too s/all to atas part o the s+pport syste/I Bhat happens when we /i6 r+theshelpers andtrees togetherI

PER8#TS %o yo+ need oneI How do yo+ get a per/it i yo+ need itI Bhy is the#nternational Residential Code not (ery +se+l, )+t #nternational *+ilding Code isI

S#"GLE TREE S!PPRT SSTE8S

Tree Ho+ses /o+nted in a single tree will +se the lassi 5nee )rae or hanging

a)le sol+tion> so/e o the (ertial loads and typially all o the hori4ontal loads /ay

)e transerred diretly into the ste/ o the host tree. The /a9or disad(antage is that atso/e point the tree /+st penetrate the plan (iew, and pro)a)ly :e6ept in the ase o

(ery large s+pport trees< the roo as well.

%eletion o the host tree o(er the (ertial distane o the triangle reated )y a 5nee

)rae syste/ will atte/pt to )end the )+ilding ra/e andor the 5nee )rae itsel as the

tree is li5ely to )e a great deal stronger than the 5nee )raera/e syste/. So/ething

has to gi(e. Ca)les o o+rse are laterally non7s+pporting )+t all o the hori4ontal load

/+st )e resisted )y the interae )etween the ho+se ra/e and the tree itsel> so/e

5ind o astening syste/ will at least pre(ent destr+ti(e )ar5 )anging.

'ery s/all )a5yard tree ho+ses are good andidates or an ade?+ate single tree.

TB TREE S!PPRT SSTE8S

& seond s+pport tree oers /a9or ad(antages, along with a ew added dii+lties.

Si/ple twin7)ea/ s+pport syste/s will oten s+ie, losely /o+nted at eah side o

larger dia/eter trees, or with the additional o/ple6ity o a /o/ent ra/e :pro)a)ly

steel< with /+ltiple astening points. Ro)+st 9oists or ross )ea/s an antile(er

o+tward in a )alaned onig+ration on eah side o the parallel )ea/ lines, pro(iding

loor area ?+i5ly.

ne again, so/ething has to gi(e, so it is typial pratie to /o+nt the )ea/s solidly to

one tree and with !H8B :!ltra High 8ole+lar Beight Polyethylene< wear plates at the

other. $or o//erial and p+)li +se pro9ets it is reo//ended to o/pletely enlose

)oth the i6ed and sliding onnetions with steel lasp )ra5ets that inl+des !H8B

on all s+raes> this will allow /o(e/ent &"% pre(ent /oist+re ind+ed )ea/ rot as will

)e the ase in a steel7to7wood interae.


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Bhile this arrange/ent a(ors a retang+lar plan (iew, one /ay easily get reati(e.

&dding 5nee )raes to this type o s+pport syste/ an pro(ide or greater spans, )+t

re/e/)er that so/ething has to gi(e and it will pro)a)ly )e the ra/e itsel O so/e

a/o+nt o twisting and deletion is ine(ita)le as the trees will rarely at in onert

d+ring wind e(ents. Torsional loads also o/e in play with /+ltiple s+pport tree

designs and this ine(ita)ility /+st )e pro(ided or. $ort+nately, i the s+pport )ea/s are

ro)+st and not too rigid in plan (iew, and well7/o+nted with the )ea/ /o+nts desri)ed

a)o(e, there will li5ely )e s+iient le6i)ility to allow or nor/al tree /o(e/ents.

THREE TREE S!PPRT SSTE8S

"ow weQre getting reati(e, as the )+ilding and str+t+ral design an )eo/e /+h

/ore interesting. !s+ally one tree will at as the /aster and the others will at as

sla(es as ar as dierential /otions are onerned. *ea/ onnetions /+st )e ree to

/o(e with respet to eah other.

This is where the analogy to early &/erian railway loo/oti(e design )eo/es

signiiant. The E+ropean pratie was to )+ild (ery preise rail align/ent on (ery

sta)le road)ed, while the &/erian pratie was 9+st the opposite O there was a lot o

distane to o(er, and the in(estors wanted to /a5e the highest rate o ret+rn as soon

as possi)le, so they /ade the rappiest tra5 possi)le. !nort+nately, the engines in

parti+lar let the rails oten. Rather than i/pro(e the tra5, designers reali4ed that

le(er loating s+pport lin5ages a/ongst the wheels and engine ra/e O s+spension

with load distri)+tion and e?+ali4ation O wo+ld 5eep the loo/oti(es on the tra5 /ost

o the ti/e. $or tree ho+ses, /+ltiple sliding onnetions :with =7)ar 5nee )rae lin5s

when re?+ired< pro(ide load e?+ali4ation d+ring translation.

$!R :R 8RE< TREE S!PPRT SSTE8S

&ll o the a)o(e applies when yo+ are )lessed :or +rsed< with n+/ero+s s+pport trees.

Re/e/)er, the translational o/ple6ity goes +p geo/etrially with the n+/)er o

s+pport trees in(ol(ed.

&RT#$#C#&L TREES O GR!"% 8!"TE% S!PPRT PSTS

This an )e a handy way o+t when yo+ are +nwilling to /a5e yo+r design it what nat+re

has pro(ided, or yo+r onstr+tion is too hea(y to )e s+pported )y the a(aila)le trees.

& tether )all pole is a good starting point or this dis+ssion, as /ost o +s ha(e p+t

onrete aro+nd a ar wheel and /o+nted a pipe to it and then ro5ed its world. The

idea here is to 5eep the syste/ ro/ ating li5e a )owling pin> these pins are also a

gra(ity sta)ili4ed syste/, )+t one that is li5ely to all o(er i the hori4ontal ore e6eeds

the (ertial sta)ility /o/ent. Bind /ahine and +tility towers, and or that /atter, s5y


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srapers, are large e6a/ples o a gro+nd s+pported and gra(ity sta)ili4ed (ertial

antile(ers. Tree ho+se posts an )e rigid with slider tops, or le6i)le i yo+r design is

elegant.

TREE H!SE &TT&CH8E"T *LTS &"% THER $&STE"ERS

Trees s+pport (ertial and hori4ontal loads )y resisting the r+shing ores ating on the

grain o the or/erly li(ing wood O (ery little load is resisted )y the li(ing and reently

li(ing tiss+e. Bood is /+h stronger parallel to the diretion o the grain, and this

presents a signiiant pro)le/ when atte/pting to s+pport hori4ontal loads s+h as 4ip

lines, )ridges, wind and seis/i ores ro/ str+t+res, as the hori4ontal r+shing

strength drops typially to 10A o the a)o(e. Thereore, this engineer reo//ends

eli/inating as /+h as possi)le the s+pport o e6ternally i/posed hori4ontal loads>

o)(io+sly, a signiiant a/o+nt o hori4ontal load will transer in any ase ro/ the tree

ho+se itsel into the tree.

Co/pression against the grain :loads are parallel to the grain< s+pports gra(ity loads in

tree ho+ses. The /ost typial ase is an inserted pin o so/e dia/eter, ad!"at #$%

t& '$ad( )*+$'+d> this an )e a nail, a lag srew, an ar)orist sha5le /o+nt, a speial

lag srew or st+d with an integral shear ollar, or a pipe or t+)e going part way or all the

way thro+gh the tree. &ll ha(e )een +sed, and all wor5 when applied orretly. &ll

penetrations an in(ol(e ris5 to the health o the tree, so proper pratie is to /ini/i4e

the total area o ins+lt to the li(ing a/)i+/ layer, and to treat andor seal the in9+ry.

Cinh rings da/age a /+h larger area o the a/)i+/ and thereore sho+ld not )e

+sed.

#t is reo//ended pratie to +se do+)led asteners O one o(er the other O either in a

si/ple pin with t+rn)+5le, or a do+)le T&* with integral /o/ent )earing )ea/ s+pport

ra/e. Re/e/)er that when a a)le s+pport is +sed, the +pper astener now has a

p+ll7o+t load added )y the triang+lation inherent in that design. &lso, the a)le /o+nt

an end +p s+pporting a large portion o the load and thereore needs to )e as strong as

the lower T&*. $inally, all loads in trees reate a )iologial response that ind+es the

tree to add signiiant )olstering at the point o stress> this an o+r in as little as one

year and /ay )eo/e the do/inant ons+/ption /ehanis/.

Spea5ing o ons+/ption :where the tree literally eats the astener<, one /+st deidehow /any years o ser(ie yo+r tree ho+se is going to pro(ide )eore /a9or re7/o+nting

is re?+ired. E6periene is showing that this )eo/es an iss+e /+h sooner than /ost

)+ilders e6pet. E6tenda)le T&*s are now o/ing into +se to pro(ide or easier

planned re7/o+nting> ha(ing oneQs )ea/ lines and 9oists laid o+t in an easily /o(a)le

pattern helps to ailitate this. o+rse, /any older tree ho+ses ha(e si/ply )een

partially ons+/ed )y the host tree:s< )+t the 9+ry is still o+t on the eet o this type o


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da/age. ne shool o tho+ght is i yo+ /a5e the )ea/s o+t o the sa/e /aterial as

the tree they /ay not re9et the tiss+e as they are ons+/ed. Good l+5 with that.

Cr+shing s+pport ro/ the end grain into the T&* is analogo+s to a syste/ o oil

springs :literally eah ring< s+pporting a hori4ontal lea spring that is al/ost rigidly

attahed at its inner end. Th+s at the inner end so/e o the o/pression ore /+st )eresisted at the top o the astener to o+nter )alane the +pward s+pport along the

shan5 :or ollar< as it nears the o+tside s+rae. There is al/ost no )earing strength

o(er the o+tside inh o the astener pilot hole, so it is +p to the r+shing strength o the

wood se(eral inhes in to the tree to s+pport yo+r tree ho+se. This why it is o//on

pratie to inrease the )earing area )y +sing large dia/eter ollars o s+iient length

O indeed, the sa/e 3 dia/eter penetration thro+gh the a/)i+/ layer /ay pro(ide

three or o+r ti/es as /+h s+pport )y +sing a long ollar.

Be sho+ld dis+ss the proper /etall+rgial properties or T&*s. Speiiations

ad(oated )y this engineer are to anneal ater /ahining ollowed )y ?+enh andte/pering to prod+e a Ro5well C hardness o appro6i/ately R 3N. Bith =1=0

alloy this will ahie(e yield strengths a)o(e 100,000 psi. Sine stress re(ersals oten

o+r /any ti/es per day, it is predita)le that witho+t proper alloying and heat

treat/ent, the steel will ail O 9+st li5e p+tting a piee o /etal in a (ie and )ending it

)a5 and orth +ntil it rat+res. Sine the rat+re will )egin at a disontin+ity on the

s+rae, it is :or sho+ld )e< a)sol+tely or)idden to install T&*s with pipe wrenhes or

other de(ies that an in9+re the highly7stressed astener s+rae.

Relati(e o/pression strength parallel to the grain is shown in the ollowing hart. &

9+(enile Bhite a5 was +sed or load7to7ail+re tests.


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& speial tool was de(eloped to ta5e the a)o(e data :Pro(isional Patent &ppliation

iled<. Bith this tool an installer an he5 and (eriy the (ertial grain o/pressi(e

strength o the speii hole )eing +sed to s+pport a T&*. Rotating the tool @0 degrees

oers a si/ilar set o readings or the hori4ontal r+shing properties.

Preise load rating is then /ade possi)le )y /athe/atially analy4ing the a)o(e data>

this prod+es res+lts that are speii to the parti+lar astener design and to the at+al

loation into the tree in ?+estion. #, or e6a/ple, the design load is greater than the

load al+lated ro/ the a)o(e data, then a seond astener an )e added so/e

distane a)o(e or )elow.

$#"#TE ELE8E"T &"&LS#S $ TREES &"% TREE H!SES

$or any )+t the /ost )asi tree ho+se designs, si/ple 27di/ensional statis7)ased

analysis, typially done )y hand, is inade?+ate. %+e to dyna/i loads, trees and tree

ho+ses are )est analy4ed as 37di/ensional le6i)le o)9ets a5in to wooden )oats or

e(en spae rat. This engineer has de(eloped speii $E& tehni?+es appropriate to

tree ho+se engineering that allows all /anner o load o/)inations to )e e6plored and

de/onstrated.


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THE $!T!RE $ TREE H!SE L#'#"G How do yo+ plan or the long ter/ +se oyo+r tree ho+seI #s this a trend that will ontin+eI Bhat /+st yo+ do when itQs ti/e tore7/o+nt yo+r tree ho+seI The pit+re shows the initial stage o re7/o+nting a largetree ho+se.

The asteners +sed in the installation o the a)o(e tree ho+se ha(e )eg+n to )e

aggressi(ely ons+/ed )y the s+pport trees. Bor5/en shown a)o(e are e(al+ating


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/ethods that /ight )e +sed to re7/o+nt this str+t+re in a /anner that will not re?+ire

additional /a9or wor5 in the +t+re. "ote the te/porary a)le )raing added d+ring

onstr+tion to s+pport the (ery hea(y loads eno+ntered in this three7story str+t+re.

This )and7aid an also )e per/anently re/o(ed )y e/ploying s+ita)le )ea/s in the

re7/o+nt.

The L SSTE8 was e/ployed to re7/o+nt that tree ho+se and will )e a(aila)le or

si/ilar re7/o+nting pro9ets and new onstr+tion. #t represents a )rea5 ro/ pre(io+s

/ethodologies and pro(ides n+/ero+s engineering i/pro(e/ents inl+ding load

apa)ility, speed o installation and ost per ton o load s+pported. See

treeho+seengineering.o/ or details or ontat Greenwood Engineering.

GE"ER&L R!LES $R TREE H!SE E"G#"EER#"G

The 1NA R+le 7 Trees are onig+red )y nat+re to s+pport all o their pro)a)leloads while /aintaining so/e ator o saety or i/pro)a)le loads, or or ir+/stanes

where the tree itsel is o/pro/ised thro+gh in9+ry or disease. There is generalagree/ent that a)o+t N0A saety ator is ahie(ed with trees o hardy speies O )+tonly ater the tree has esta)lished itsel a/ongst the o/peting ad9aent trees. Thisengineer has ad(oated ons+/ing no /ore than 13rd o that saety ator when addinga tree ho+se load :' andor H<.

Tree ste/ dia/eters allow load al+lation thro+gh re(erse engineering O gi(enthe speies and dia/eter it is possi)le to al+late the wind drag and weight load o thetree +nder st+dy. This is at+ally /+h /ore a+rate than esti/ating the wind dragand weight. The )ending /o/ent a)o+t this dia/eter is the starting point or the a)o(e1NA R+le.

PER8#TS There is no presripti(e path or tree ho+se onstr+tion, nor is thereli5ely to )e any ti/e soon, i e(er. There are si/ply too /any (aria)les and too /anyreati(e sol+tions to ondense this s+)9et into a proed+re hart. n the other hand,the #nternational *+ilding Code /ay allow proessionals to propose dierent/ethodologies i they are a)le to s+pport the/ with /athe/atial analysis and testing.This is why the #*C is +sed to s+pport yo+r Code tree ho+se, while the #RC /ay )e+sed to deine eat+res and o/ponents. #t wo+ld )e (ery help+l to ha(e a +ntionaland separate do+/ent speiially written to /eet the speii and +ni?+e hallengeso tree ho+se onstr+tion.

Load red+tions o//only e/ployed in Code analysis are not appropriate in treeho+se design. 8ethods that red+e or e6a/ple the i/posed wind load to so/eartiiially low (al+e are si/ply as5ing or tro+)le. The lia)ility iss+es o tree ho+seonstr+tion and +se are not to )e tri(iali4ed. n the other hand, the "ational %esignStandards and ti/)er engineering pratie in general are a good design reerene, as isBood Properties :-. Binandy, 1@@=<, !.S.$.S. $orest Prod+ts La)oratory, Bisonsin,!S&.


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2013 greenwood eng treehouse

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