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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 Comparisons are madebetween historical references and modern day design standards for each of these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

1970Structures Congress 2012 ASCE 2012

Structures Congress 2012


2/10

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




3/10

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




4/10

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




5/10

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




6/10

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




7/10

Fig

Figur

re 7: Truss

e 8: Location

onnection

of Split in F

onfiguration

ield Inspecti

n




8/10

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




9/10

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




10/10

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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