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Failure of Wood Connections and the Evolution of Connection Design
Jaret Lynch, P.E.1
& John Cocca, P.E.2
1Wiss, Janney, Elstner Associates, Inc., 2 Trap Falls Road, Suite 502, Shelton, CT
06484; PH (203) 944-9424; FAX (203) 944-6997; email: [email protected], Janney, Elstner Associates, Inc., 2 Trap Falls Road, Suite 502, Shelton, CT
06484; PH (203) 944-9424; FAX (203) 944-6997; email: [email protected]
ABSTRACT
Wood has been used as a construction material long before concrete and steel wereavailable as structural materials. Wood is easy to work with and can be used for many
different types of framing. Wood trusses have been used for long span roof framing in
the United States since the 1800s. Over time the material, wood, has remainedlargely unchanged; however, the connections have evolved as well as the standards to
which they are designed.
Before the 1850s, truss connections typically consisted of mortise and tenon joinery.These connections make good use of the inherent strength of the wood members with
little or no reliance on fasteners or engineering analysis. During the late 1800s and
early 1900s these connections evolved into side lapped joints with steel bolts whichbear on the wood in order to transfer shear through the joint. During the mid 1900s
the invention of shear plates and split rings allowed joint forces to be transferred with
fewer bolts. During the late 1900s pressed plate connections were developed whichpermited wood trusses to be readily pre-fabricated.
In addition to the evolution of the connections, advancements in the testing of
materials and creation of design standards have evolved into what they are today.
This paper provides examples of actual woods connection failures from the different
time periods mentioned above and discusses the cause of the failures. Also provided
is a comparison of historic references versus modern design methods for the
evaluation of the failed connections.
INTRODUCTION
Below are examples of four wood connections dating from 1843 to 1955 that do not
have adequate capacity to resist the anticipated loading. These examples demonstrate
the deficiencies typically found in these types of joinery. Comparisons are madebetween historical references and modern day design standards for each of these
connections.
1843 TIMBER FRAME
In January of 2011, WJE was hired to assess the condition of a 2-1/2 story rectory in
northwestern Connecticut. This timber-framed structure dates from 1843. The wood
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ap
pur
spa
frome
wastrto t
ha
Th
sin
shoof
ten
maan
Alt
tim
bra
doap
by
the
ears to be
lins which
ced at fifte
one lengmber at a ti
l formed bts are usedhe attic flo
e failed cau
struts are
le oak peg
ulder of thehe joint rev
sile force de
erial behinis caused b
hough the
ber frames
ces transmit
etail or a lropriate for
connecting
useable spa
oak. The r
onnect to t
n feet on c
th of timbe instead
the posts eto resolve tr framing.
sing the kne
onnected to
is used to s
tenon and taled that th
veloped in t
the peg hy insufficie
roportions
f the period
ting compre
rger mortisthis tension
the rafter ta
ce in the atti
oof structur
ree timber-
nter. The
r suggestinf being rai
tending abe thrust crJEs asse
e walls to le
igure 1: Tim
the wall po
ecure each
he mortisee oak peg h
he strut (Fi
le is knowt end distan
of this con
, this type o
ssion forces
e and tenonbrace conn
ils directly
c is reduce
e consists
framed ben
connecting
g that theed as comp
ve the levelated by thesment foun
an outward
ber Frame C
st with tradi
onnection.
t the top od torn out t
ure 2). Thi
as a relisce on the te
nection are
f mortise an
through dir
with morection. This
o the frami
.
f hand he
s. The ben
girts (wall t
roof framileted bents.
of the atticrafters by tthat both
at their cent
onnection
tonal morti
A gap had
the wall pohe end of th
s type of bl
failure (Mnon.
simialar t
d tenon is i
ect bearing
shear pegscondition i
ng at the at
n common
ts are 24-fe
op plate) ar
g was assThere is a
floor. Diaging the toptruts on the
ers (Figure
e and tenon
developed
st. Closere tenon due
ck shear ru
iller & Sch
those fou
eally reser
on the woo
would haves often avoi
ic floor lev
rafters and
t wide and
e fashioned
mbled oneshort knee-
onal timberof the postcenter bent
1).
joinery. A
etween the
xaminationto the large
pture of the
idt, 2004)
d in other
ed for knee
. A lapped
been moreed entirely
l, although
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18
In
maco
reswo
at
Th
bea
(Pi
hortyp
not
the
ironar
thithe
WJ
coout
2 SCISSO
une of 201
sonry wallsdition date
rain the sprsened in re
ne of the ti
roof of th
ring walls.
ea sp.).
izontal tieically side
able except
sloping top
fixtures.row soffits
k unreinforlevel of the
Es assess
nections wward. The
TRUSS
0, WJE wa
at a churchback to a
reading of tcent years, i
rod anchor
church is
Samples t
he trusses
eams. Thlapped and
on is the lo
chord and i
shallow bunning the
ced brick msidewalk.
ent found
as causingalls were f
Figure 2:
hired to i
in New Jersleast 1902
he north ancluding lo
ages.
upported b
aken from
are varian
trusses arbolted wit
wer horizo
s drawn up
arrel vaultelength of th
asonry and
that wid
the roof tund to be l
Failed Conn
vestigate t
ey. The owhen thre
d south wacalized fail
nine woo
the truss m
s of a sci
e comprise5/8-inch
tal tie bea
ight with s
ceiling (wnorth and
ise to a hei
spread fail
sag andeaning a m
ection
e cause of
ner reportee metal tie
lls. The cre of the m
trusses sp
embers we
ssor truss
of machiiameter sq
which en
uare heade
ood lathe asouth walls.
ght of appr
ure of th
hrust theaximum of
two outwar
that the hirods were
ndition hasasonry and
nning 46-f
e identified
hich inco
e sawn luuare heade
ages a bird
bolts and
d plaster) sThe walls
ximately 3
bolted s
asonry beapproximat
dly leaning
tory of thisinstalled to
reportedlylaster wall
et between
as Spruce
porate two
ber that isbolts. A
s mouth in
and forged
rings fromare 16-inch
-feet above
issor truss
aring wallsly 5-inches
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eac
loc
at
an
Altofpro
Thfor
lu
meap
topthr
h with muc
ated approx
ll nine of t
implement
hough damthe scissorblem. Thes
two slopi. Each sc
ber measur
mber is faroximately
chord hasugh two fa
of the tilt
imately 28
he roof trus
ed.
ge was fouchord connconnection
g bottomissor chord
ing approxi
shioned fro3-7/8 inche
ailed in nuteners in a
ccurring a
t. below the
ses. The ro
d throughoction to ths are locate
Figure 3
hord memmember is
ately 2-1/4
m one cowide by 9-
erous locatow.
Figure 4:
ove the lev
truss beari
of was sho
t the trussee top chorat Joints 2
Truss Geo
ers of eacashioned fr
inches wid
tinuous le5/8 inches
ons. Figur
Splitting F
el of the sa
g. Failed
ed until re
s, our analywas likel& 4 as sho
etry
truss exeom one con
by 9 inche
ngth of saeep. The s
4 shows a
ilure
ctuary floo
onnections
airs could
is indicatedthe root cn in Figure
plify the stinuous len
s deep. Eac
n lumberissor conne
splitting fail
diaphragm
were found
e designed
that failureause of the3.
cissor trussth of sawn
h top chord
measuringction to the
ure passing
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Fig
the
wa
Ouov
an
be
stawit
bol
acctru
It icau
ins
fouitse
191
In
in
buisty
bet
ure 5 show
remaining
hers.
r analysis inrstressed fo
snow load
carrying th
dards. Noth insufficie
ts were let i
ommodates members.
s importantse of the ro
ead of trad
nd in the trlf.
8 King Pos
he winter o
upstate Ne
lding code.e roof that
een unrei
one squar
two bolts a
Figure
dicated thatr dead load
on the roo
ee to four
only were tt distance
n to the thi
bolts which
to realize tof truss fail
itional timb
ss member
t Truss
f 2009, WJ
York to
The mainis supporte
forced bric
headed bo
re tilted an
: Splitting
the truss malone and s
. More imp
times what
here not enrom the ed
kness of th
were too s
at the seveure. The bu
er joinery i
would not
was asked
erify its a
uilding waby six tim
masonry
lt which ha
d crushing
ailure with
mbers (in tgnificantly
ortantly, th
would be
ugh bolts,ge and/or e
e side mem
hort to pas
rely overstrilders relia
s notewort
have been
to analyze
bility to ca
s originallyber trusses
earing wall
split free
the wood f
Crushing
eir undamaoverstresse
bolted con
ermitted b
ut many ofnd of the
ber by chis
through t
essed boltence on thro
y because
nough to p
the roof of
rry loads s
constructehat span in
s. Wood p
rom the me
ibers benea
olts
ged state)for the ex
nections we
modern
the bolts wember. Se
ling away t
e full thick
connectiough bolted
the level o
recipitate th
an existing
ecified in
in 1918the east/we
rlins span
mber while
h the plate
ere slightlyected wind
re shown to
ood design
re installederal of the
he wood to
ness of the
s were theonnections
overstress
e failure by
rain station
the current
ith a gablest direction
etween the
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tru
spa
bot
in.di
7 iwoan
Th
apint
tha
ofnot
co
traAt
by
Strthe
AScap
bot
snolar
the
pre
De
valno
ses in the
n approxim
tom chord
x 12 in. Tensions of
. x 9 in. aod memberits membe
top chor
roximatelythe top of
pass throu
onnectionch in the
trols the d
sfer force tthe time of
hand.
ctural analadequacy o
CE-7-02. Tacity to res
tom chords
w loads ine splits in t
bottom ch
dicted by o
ign refere
es for thisexcess capa
orth/south
ately 40.5
f the truss i
e top chorin. x 14 i
d the cente(Figure 6).s were grad
Fi
to botto
1 in. thick.the bottom
h the top a
he thrust frottom chor
sign of thi
hrough theur inspecti
sis of the ef the truss t
he truss mist the dead
was found t
luded in thhe wood sp
ord (Figur
r analysis o
ces from t
type of conity for win
irection su
. and bear
fashioned
of the tru. The diago
vertical mThe wooded using vis
gure 6: Typi
chord c
The platechord. Add
d bottom c
om the topd. Shear a
type of c
onnection,n, the bolts
ntire truss ao carry bot
embers anload of the
o be signifi
present dnning from
8). This
f the connec
is time fr
ection howor snow lo
porting the
on blueston
of two mem
s is fashional web me
ember is awas identifal observat
al Truss and
nnection c
raps the eitionally, th
ords near t
chord is resd bearing
nnection.
hey are mewere found
nd its conneuniform a
connectioroof; howe
antly overs
y design cthe tip of t
distress is
tion.
me (Kidde
ever in theads.
common r
e pads at t
bers with n
ed of twombers have
teel rod enied as densions of the
Geometry
onsists of
d of the toere are four
eir bearing
isted by thef the woo
he purpose
ely there toto be loose
ctions wasd unbalanc
ns were fover, the co
ressed whe
des. Closee notched c
consistent
r 1916) pr
oofs curren
afters. The
e top of th
ominal dim
members wnominal di
cased by nograined sooods natu
an end pl
chord and, 7/8 in. dia
(Figure 7).
cantilevered parallel t
of the bol
keep the pand the nut
erformed ted loads as
und to havnection of
subjected
up inspectionnection t
ith the fa
vide allow
t configurat
oof trusses
e wall. The
nsions of 4
ith nominalensions of
n-structuraluthern pineral defects.
ate that is
is notchedmeter bolts
In this type
d plate andthe grain
s is not to
rts aligned.removable
determinespecified in
e adequatethe top and
o wind and
n revealedthe end of
ilure mode
ble design
on, there is
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Fig
Figur
re 7: Truss
e 8: Location
onnection
of Split in F
onfiguration
ield Inspecti
n
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19
WJ
Ne
res
Th
sou
unrwe
parCotop
co
co
Str
ch
sigtyp
stri
ad
A s
thiof
pro
sno
sno
5: BOW S
E was calle
York tha
lted in 39 i
roof was
th direction
einforced he fashione
abolic arch.nections bchord to bo
nectors. Th
sists of stee
ctural anal
rd of the tr
ificantly ee of failure
ng trusses
ition of live
econd type
particularnbalanced
vide provisi
w. The co
w load ac
RING TR
d to investi
t occurred
n. of snow i
Fi
onstructed
and spaced
llow concrof glue l
Web memtween thettom chord
e bottom ch
l side plates
sis verifie
ss. The fail
ceeded thender heavy
ypically pr
load, (i.e. s
of failure c
xample isload from
ons for esti
es typicall
oss the en
SS
ate the cau
during a
less than a
gure 9: Bo
of wood bo
at 8 ft. on
te masonryaminated l
ers were faeb member
connection
ord is splic
on each sid
our obser
re was loc
capacity osnow load i
ovide adeq
now and wi
mmon to t
ailure of thrifting sno
ating the u
only prov
ire roof sp
e of a bow
ajor snow
48 hour pe
string Trus
wstring tru
center. The
unit (CMUmber with
shioned fros and chordonsists of a
d 3 ft. fro
e of the me
ations that
ted at the s
the shears not unco
ate capacit
d) can resu
ese bowstri
web mem. Prior to
nbalanced l
ded guidan
an. These
tring truss
storm in th
iod (Figure
Collapse
ses spanni
ends of th
) walls. Thethe top ch
m solid saconsist ofsteel beari
the center
ber and bo
the failure
lice conne
late connemon in our
y under de
lt in membe
ng trusses t
er to chord1980, most
oads on roo
ce for the
unbalanced
roof collaps
e winter o
9).
g 71 feet i
trusses be
top and boord curved
n dimensioolted shearg shoe with
of the truss.
lted shear pl
occurred in
tion where
ctors (Figurexperience.
ad load alo
r or connect
at was not
connectionbuilding co
fs as a resul
inclusion o
loads can
e in upstate
2008 that
the north-
r on 12 in.
tom chordsto form a
nal lumber.plates. Theshear plate
The splice
ates.
the bottom
the demand
e 10). ThisThese bow
ne, but the
ion failure.
observed in
as a resultdes did not
t of drifting
a uniform
cause web
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me
res
C
De
moco
we
ana
Ti
Atjoi
str
du
Re
firs
pufor
waofred
Fusta
Wco
mbers to ca
lting in fail
NCLUSIO
ign of woo
rtise and tstructed by
l as knowl
lyzing and
ber Frame
the beginniery becam
sses publis
to the larg
earch and a
t National
lished in tthis is that
determineood memb
uced by a
thermore, cdards.
en evaluatinections an
rry loads m
ure of the m
Fig
NS
d connectio
enon joineskilled cra
edge passe
designing t
ngineering
g of the 19tavailable,
ed in these
factors of s
dvancement
esign Spec
is NDS areuntil the m
using a coers were deproximatel
onnection
g existingd their abilit
uch greater
embers and
re 10: Fail
s has evolv
y were gtsmen usin
down thr
imber fram
Council (T
h Century,first in Eur
references
afety which
of wood te
ification (N
significantlid-1960s, t
relation toeloped and
40% co
apacities w
ood structy to resist t
than those
or connecti
ed Bottom
ed over tim
nerally noknowledg
ugh the g
ed connecti
EC 1-07).
esign referope, then l
are surprisi
were histor
hnology br
DS) for wo
y higher thhe allowabl
he bendingperformed,pared to
ere also re
res, particue anticipate
required by
ons.
hord Splic
e. The earli
engineeregained th
enerations.
ons have b
nces for wter in Ame
gly close t
ically appli
ought about
od in 1944.
n currentlye tensile str
stress. Oncthe allowa
current sta
uced com
lar attentiond loads.
the unifor
st connecti
d. These jough trial a
Current gui
een develo
od construrica. The s
the curren
d.
the develop
The allowa
permitted.ess for woo
full scale tle tensile stdards (Kri
ared to cur
should be
load case
ns, such as
oints werend error as
delines for
ed by The
tion and itsfe working
guidelines
ment of the
ble stresses
One reasond members
ension testsresses werestie 1996).
rent design
iven to the
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REFERENCES
Kidder, F. E. (1916).Architects and builders pocket-book, Wiley, New York.Kristie, R. J., and Johnson, A. P. (1996). Investigating and repairing wood bowstring
trusses.Pract. Period. Struct. Des. Constr., ASCE, 1(1), 25-30.
Miller J. F. and Schmidt, R. J. (2004). Capacity of pegged mortise and tenonjoinery. Research Report, Dept. of Civil & Arch. Engineering, University of
Wyoming, Laramie, Wyoming.American Forest & Paper Association (AFPA). (2005). National design specification
for wood construction, Washington, D.C.American Society of Civil Engineers (ASCE). (2002). Minimum design loads for
buildings and other structures, New York.Timber Frame Engineering Council. (2007). Standard for design of timber frame
structures and commentary, Becket, Mass.
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