Carpentry Joints_Thierry Descamps.pdf

7/24/2019 Carpentry Joints_Thierry Descamps.pdf

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Universit de Mons 1

University of Mons - BelgiumFaculty of engineering - Department of Structural Mechanics

Carpentry connectionsTraining school on assessment and reinforcement of timber elements

Thierry DESCAMPS


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What aout strength !

Design and reinforcement

What aout stiffness!

Structural assessment of old timber structures

What is it !


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Widespread

Natural material (timber): ariability

Decay and M!

Structural comple"ity: !omple" geometry and #oints (more comple" thanmasonries $)

% simplified analysis considering only plane parts ofthe system& is often hard to reali'e or completelyimpossible

eometry and #oints are characteristic of

%n area and a period of time

!arpenters and engineers *no+ledge: daringengineering ,

Mind blo+ing timbre structures are not al+aysne+ ,

Timer frame"or#s$


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Carpentry joints connect timber elements, often without any dowel type fasteners.

Forces are transferred within the joints via contact pressure and friction. The smartcutting of the joint by the carpenter create notches and contact surfaces between the

connected members.

Within the connections, there is an interaction in terms of stiffness and strength betweenthe different pathways in which the forces are transferred.

Eccentricities are inherent in this kind of connections to be considered.

Carpentry connections can be classified in families but there is a huge amount ofcarpentry joints probably as much as major timber buildings and carpenters.

Carpentry %oints$

et.s ha/e a loo* at some carpentry #oints (non e"hausti/e)0



Pinned Tenon and

mortise (wooden pegs)

Pin number and placement varied

with the size of the memberand the preferences of carpenter

Drawings: Sobon J.



Through tenon with outside wedges(flatwise bending of the tenon)

!f one piece (here the post) is wider than the the

other" the tenon can be housed into it.

The tenon can be centered or be flush with the

la#out face of the post.

Drawings: Sobon J.



$edged Dovetail Through %ortise and Tenon

&rom tenon an' mortise %oints to 'ovetails %oints

Partiall# housed wedged dovetail

through mortise and tenon

Drawings: Sobon J.



Through %ortise and Tenon with Dovetailed Shoulder

Drawings: Sobon J.



!nverted lapped dovetail

&apped half'dovetail girder oint

Drawings: Sobon J.



Drawings: Sobon J.

otched beams (the simplest oint to craft and insert" and

conse*uentl# the most common)



+alved and unders*uinted scarf

(to improve bending strength and resistance

to seasoning twist)

Drawings: Sobon J.

+alved scarf with four pins (simplest to fashion).



+alved and bladed scarf

with pinned tenons

+alved" bladed and cogged scarf(helps align the scarf and increases its bending

strength against horizontal loads)

Drawings: Sobon J.


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he sloped& lapped portion isstopped before it feathers out to

nothing

!ompared +ith the half-lap&shear strength is /astly impro/ed

by the sloped surface

Spla#ed scarf oint (the lapped surfaces are sloping).

Drawings: Sobon J.


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Spla#ed" unders*uinted and cogged scarf oint

Drawings: Sobon J.


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$edges act as a reinforcement.

The tensile capacit#" torsion" andbending strength in both directions are

greatl# increased (cog).

The pins (and their position) increase theoint,s overall performance.

The butts need not be unders*uinted.

Spla#ed" -nders*uinted and $edges

Drawings: Sobon J.

P. &eml#n


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The conscientious builder locates the scarf where bending forces are low...

Drawings: Sobon J.

Spla#ed with $edges and %ultiple lapped surfaces


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What is it !


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Step %oints (rafter an' tie-eam

%oints or purlin plate)


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Step %oints

the slope of the notch must minimi'e the angle bet+een the stresses and the grain direction

for both connected elements (bisector)

the depth of the notch (t/) should not e"ceed h23 for s*e+ angles 456 and h27 for s*e+

angles 8 756 (9t' et al (1;)& D (=557))


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Precompression of horizontal beam due to


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Support reaction

N tie beam


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The rear face under compression (/0) is generall# neglected

?o+e/er& @arisi and @ia''a (=555) suggest to consider a reduced length d

(possible concentration of high stresses in a limited length)

,

1

5

1

3


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3 points to check

Single posterior step

!rac* $

ap


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*einforcement of step %oints

(a) binding strip1 (b) internal bolt1 (c) stirrup1 (d) tension ties ' 2ranco (0344).


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/orce'displacement curves for unstrengthened and strengthened connections

with a 536 s7ew angle under monotonic loading. 2ranco et al (0344)


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/orce'displacement curves for unstrengthened and strengthened connectionswith a 836 s7ew angle under monotonic loading. 2ranco et al (0344)


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Dovetail %oints


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*oun'e' Dovetail %oint

De/elopment than*s to !N! +ood-processing machinery

>sthetic

>asy to use on site (plug and play #oint)


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(allAe et al& =515)

From e"perimental researches& Werner proposed a design guideline (Werner& =55=)

Since rounded dovetail oints can fail either b# brea7ing of the oist or main beam" thetwo members are designed separatel#:

,!"#%

3& '(& ),

,!* + -,-.& /!* /(0(%

Where %1 is the effecti/e do/etail area

hmain& h1 and b1 are in mm and F is in *N& and:

'( (0 2

4

% & /(

(

% & /(

(

% 0 &

(6

7


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annert proposed later on a design guideline that ta*es into account the si'e effect

for timber strength in brittle failure (annert& =55B)

%s rounded do/etails #oints are /ery similar to end notched beam supports& he

proposed a modified design formula based on >urocode 4 formula for notched

beams +ith the definition of a specific reduction factor */ (>N 14:=554)

1,5 & 8

'( ), &

3,9::6

'(

,


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he failure is typically brittle& and occurs in the elastic range reinforcements needed:

tuning the geometry (to reduce stress concentrations)

using additional reinforcements

9einforcement with self'tapping screws at an angle of ;< (left)1

reinforcement with an adhesive la#er between oist and beam (middle)1

combination of self'tapping screws and adhesive la#er (right). (Tannert 0340).

*einforcement of roun'e' 'ovetails


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Ceinforcement +ith self tapping scre+s is /ery efficient (increase stiffness& capacity ofconnections perpendicular to the grain and produce a more ductile failure mode)


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Tenon an'

mortise %oints


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Tenon an' mortise %oint

he bearing capacity of tapered tenon #oints is a function of the angle

of the connection& and length of the toe and mortise depth (%man et

al& =55B& udd et al& =51=& itos et al& =51=)


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m is the ratio of G to F

dditionall#" =+ is the coefficient of friction at the front side" =>the coefficient of

friction at the bottom face" ? the connection angle" hs the height of the strut" ls thelength of the strut and t@the distance between the bottom surface and the loading

point of +.


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What is it !


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Widespread

Natural material (timber): ariability (in the same frame)

Decay

Structural comple"ity: !omple"ity of the global geometry and the connections(more comple" than masonries $)

ac* of technical dataHs : Sections (I /ariations)

Strength classes (/isual on site strength grading)

oints (contact areas& nodes& crac*s0)

Structural assessment and morpho-chronology:

% good understanding of the history of the structure isnecessary

I dendochronology

+l' timer frame"or#s$

Drawings P. &eml#n

C t th '


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Component metho'

@rediction of the stiffness of the #oint For M& N and :

Definition of a model statically eEui/alent

>ach component is an area +here contact appears

% stiffness is associated to each component

The component 44, is the pair of surfaces 4 and 4,"respectivel# belonging to timber elements and 2.

This component is composed of two stiffnesses as 4correspond to a compression parallel to grain for the

element and 4, is perpendicular to the grainA

E,uivalent spring mo'el

Three areas of contact appear when theconnection is bend (%B)

ICR = peg


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Jnder the assumption of small rotations :

( )2

k k k k k k k k

k k k

tot k k

k

F z k z k z zM

k k z

= = = = =

The gloal rotational stiffness can easily e estimate'from the geometry an' the stiffness of all pairs of

surfaces in contact$.

imitations of the method (strictly analytical) :

he deformation of contact surface can not be estimatedas it is an infinite half space of +ood ,

/irst enhancement of the method: ssumption of an infinite half space A


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First method: F>M

Komatsu (=55) proposed a pure analytical

correction in his study on traditional apanese

mud shear +all

LNu*i #oint LNu*i effect

modified analytical model

cut factor m"Cis defined with the help of elastic finiteelement models (/C%) using an orthotropic material"contact and different slenderness of contact areas

Second method: additional length effect


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C*uilibrium of forces and a additional length effect E Fomatsu 033G

Moreo/er& Komatsu proposed a post yielding beha/iour +hich allo+s the definition of a poly-linear%O relationship


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omparison of initial stiffness obtained from anal#tical and e@perimental results

(hang 033H)

h i b t thi fi t h d


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&EM an' e/perimentation on real si0econnections give goo' results.

Component metho' largely overestimate the

stiffnessInce again" observations made on real size tests

(bro7en peg) suggest that the peg is not the center ofrotation of the connection

he comparison bet+een this first enhanced

model and e"perimentations is disappointing :

M1 M-

&EM Component

metho'

E/perime

ntation

&EM Component

metho'

E/perimen

tation

Stiffness

23.m4mr

a'P

56.78 99.6: 5;.


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Second enhancement of the method:

% geometrical research field is defined and an iterati/e process is implemented et.s call u& / and the

displacements and rotation of any point into this field

For each supposed position of

From their stiffness it is possible to computed the contact forces :

Cesulting forces at the tenon is :

We finally sol/e this system and the

( )2

1

, ,

y

i

i i

iy

kF dl f u v

l = =

11 12 13

21 22 23

31 32 33

V k k k u

N k k k v

M k k k

=

u v+


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Why this focus on stiffness ?

5. >nfluence on the 'esign of the %oint


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he #oint is statically indeterminate ,

>Eui/alent model

;. >nfluence on the 'esign of the "hole structure


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Most of the time& connections are supposed to bemoment free (hinges) his assumption :

Simplifies the computations


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fortified castle of >caussinnes-alaing

!athedral Q- of ournai

%bbatiale de la @ai"-Dieu

1;&;; m

4&7 m

B& m

7&;4 m

R&7 m

R&7 m

%ll are statically indeterminate frames: R"

!hteau d.>caussinnes-alaing


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@inge Carpent. Do"els Stiff.Ben'ing

stiffness

gBorne infrieure

Borne suprieure

Variations de contraintes en fonction de la rigidit de lassemblage


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g g

K. CANT et F. MIN!N" # $er%ice de gnie ci%il et mcani&ue des structures

%bb ti l d l @ i Di


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%bbatiale de la @ai"-Dieu


stiffness

!athedrale Q- of ournai


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stiffness


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han* you for yourattention

%man C& West ?& !ormier D (=55B)& %n e/aluation of loose tenon #oint strength /or Prod J& 4B(;):71U73 laV ?& e#t*a


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imber Structures& Stuttgart

ranco M (=55B)&


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%aterials and Structures DQ< 15171R2s114=R-51;-5547

9t' K-?& ?oor D& M9hler K& Natterer (1;)& !onstruire en ois - !hoisir& conce/oir& realiser PressesPol#techni*ues et -niversitaires 9omandes& ausanne& S+it'erland

udd & Fonseca F& Wal*er !& horley @ (=51=)& ensile strength of /aried-angle mortise and tenon connections intimber framesJ Struct Cng 1;R(4):7;7U733

Komatsu K& Kitamori %& ung K and Mori (=55)& >stimation of he Mechanical @roperties of Mud Shear WallsSub#ecting to ateral Shear Force& c*elman !& >rdil & Q'cifci % (=51=)& >ffect of tenon geometry& grain orientation& andshoulder on bending moment capacity and moment rotation characteristics of mortise and tenon #oints $ood

/iber Sci33(3):1UB

Meisel %& Moosbrugger & Schic*hofer (=515)& Sur/ey and Cealistic Modelling of %ncient %ustrian CoofStructures


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conditions !anadian ournal of !i/il >ngineering& ;3(1=):1755-1754

annert (=55B)& Structural performance of Counded Do/etail !onnections @hd hesis& Jni/ersity of ritish!olumbia& ancou/er& !anada& %pril =55B

annert & am F (=55)& Self-tapping scre+s as reinforcement for rounded do/etail connections Structural!ontrol and ?ealth Monitoring& 17(;): ;R3-;B3

annert& & am& F& and allAe& (=515) Strength @rediction for Counded Do/etail !onnections !onsidering Si'e>ffects >ng Mech& 1;7(;)& ;4BU;77

annert & Keller N& Frei C& allAe (=51=)& =51=)

J'ielli (=553)&

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Carpentry Joints_Thierry Descamps.pdf