Download - Lecture notes 8-9-10-11-12-13
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LECTURE NOTE SNo. 8-9-10
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TIMBER JOINTS DESIGN
CONTENT:
1. INTRODUCTION: TIMBER JOINTS
Classification of timber joints
Traditional joinery joints
Mechanically fastened joints
Metal connectors
Glued timber joints
2. STEPS OF TIMBER JOINTS DESIGN
3. DESIGN OF TIMBER JOINTS
General design rules
Nailed joints design
Bolted joints design
Screwed joints design
Carpentry joints design
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MEMBERS + CONNECTIONS = STRUCTURAL SYSTEM
Structural connections are points in a structure where components are
joined together.
Each structural connection contributes to the overall strength of the finished
structure and can make the difference between catastrophic failure and the
ability to successfully resist both internal and external stresses.
The connection is more than a point where two pieces of a structure are
connected. They are used to add strength and support to the finished
structure. Structural connections also provide an opportunity to transfer loads from
different areas of the structure.
Each connection presents an area of potential weakness in the structure
therefore this must be addressed by selecting an appropriate connection for the
task.
When structures are designed, designers evaluate the loads that will be
encountered in various areas to determine which structural connections should
be used for maximum stability, strength, and safety.
INTRODUCTION: TIMBER JOINTS (CONNECTIONS)
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CLASSIFICATION OF TIMBER JOINTS
The common connecting systems in structural timber may be classified as:
A. Traditional joinery joints
B. Mechanically fastened joints: connectors
C. Glued joints
carpentry joints (framed joints)
connectors metal connectors
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JOINTS IN A TIMBER STRUCTURE
They have the following functions:
1. To connect timber members in a structure;
2. To assure the structural strength & stability;
3. To transfer the actions through structural members;
4. To allow an increased timber member length when lengths of
timber products are not suitable.
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TRADITIONAL JOINERY JOINTS
CARPENTRY JOINTS (FRAMED JOINTS)
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Scarf joint
( oblic )
Cogging joint
( )
Framed joint (lap joint)
( )
Tenon joint
( )
Finger joint
TRADITIONAL JOINERY JOINTS
CARPENTRY JOINTS (FRAMED JOINTS)
The most utilized carpentry joints are:
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lv
tv
lv
tv
lv2
tv2tv1
lv1
(a) (b)
(c)
Fd
Fd
Fd
Rafter
Purlin
Wall plate
TRADITIONAL JOINERY JOINTS
CARPENTRY JOINTS (FRAMED JOINTS)
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TRADITIONAL JOINERY JOINTS
CARPENTRY JOINTS (FRAMED JOINTS)
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MECHANICALLY FASTENED JOINTS
METAL FASTENERS
1. Dowel-type fasteners (made of timber or steel):
(a) nail; (b) screw; (c) bolt; (d) dowel (peg)
MECHANICALLY
FASTENED JOINTS
1. Dowel-type fasteners
2. Metal connectors
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Elements of a Nail and Nail Types
DOWEL-TYPE FASTENERS: NAILS
New nail style
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DOWEL-TYPE FASTENERS: NAILSConnection in shear
Connection in double shear
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Self-Drilling Tapping Screw
Self-Piercing Screw
DOWEL-TYPE FASTENERS: SCREWS
Screws styles:
Screw Head Types
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DOWEL-TYPE FASTENERS: SCREWS
Different shear loading conditions double shear failure
Different shear loading conditions single shear failure
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DOWEL-TYPE FASTENERS: SCREWS
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Dowels (pegs)
DOWEL-TYPE FASTENERS: BOLTS & DOWELS
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Single-sided toothed plate
Double-sided toothed plate
Split-ring connector
Punched
metal
plate
Shear-plate connectorMild steel hangers
METAL CONNECTORS
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METAL CONNECTORS
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METAL CONNECTORS
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Single stepped scarf jointPlain scarf joint
Structural finger
joint
GLUED TIMBER JOINTS
Type of adhesive
Direction of actions
Adhesives for structural
purposes shall produce
joints of such strength
and durability that the
integrity of the bond is
maintained in the
assigned service class
throughout the expected
life of the structure.
NOT BASED ON WATER
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TIMBER JOINTS DESIGN
The behaviour of wood structures is very complex because of non- linearity,
sensitivity to creep, biological degradation, and variability of the material
and connections.
Joints are often the most critical components of any engineered structure and
can govern the overall strength, serviceability, durability, and fire resistance.
Joints often are the weakest link in timber structures.
The behaviour of wood members in a load-carrying system depends on the
material properties of wood and on the connection type between the
members.
A joint is an assembly of two or more structural elements which transfer shear,
axial loads (compression or tension) and moments from one member to
another.
The selection of fasteners is not only controlled by the loading and the load-
carrying capacity condition but also includes some construction consideration
such as aesthetics, the cost efficiency of the structure and the fabrication
process.
In Romania timber design is currently going through a major period of change as
a result of the introduction of EC5 / SR EN 1995: Part 1&2, both limit state design
codes rather than the permissible stress approach used in the past.
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STEPS OF TIMBER JOINTS DESIGN
Step 1: To choose the type of connection (there is almost
always a suitable connection for a particular purpose) based
on the technical consideration or environmental conditions.
For a given structure, the selection is controlled by:
the loading and the load-carrying capacity conditions;
some construction considerations;
aesthetics;
the cost-efficiency of the structure;
the fabrication process.
There is no standard procedure from which the best connection
can be designed for any structure.
Step 2: To pre-design the joint following the rules given by
standards.
Step 3: To verify the load-carrying capacity of the joint.
The main idea in design may be:
he simpler the joint and the fewer the
fasteners, the better is the structural result
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Joints should be designed so that the load induced in each
fastener or timber connector unit by the design loads appropriate
to the structure should not exceed the permissible values.
When more than one nail, screw, bolt, etc. are used in a joint, the
permissible load is the sum of the permissible loads for the
individual units.
If the load on a joint is carried by more than one type of fastener,
due account should be taken of the relative stiffness.
The effective cross-section of a joined member should be used
when calculating its strength.
GENERAL DESIGN RULES
F
AAA nef
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TYPE OF LOADS CARRY BY
FASTENER
Dowel-type fasteners, such as
nails, screws, bolts and dowels,
are used to hold two, three or
more members together to form
a joint.
In general, they are designed to
carry lateral shear loads, but
there are occasions where they
might be subjected to axial
loads (withdrawal loads).
MECHANICALLY FASTENED TIMBER JOINTS DESIGN
JOINTS WITH DOWEL-TYPE FASTENERS:
NAILS, SCREWS, BOLTS AND DOWELS
Axial load
Shear loads
GENERAL DESIGN RULES
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SINGLE SHEAR IN DOWEL-TYPE FASTENERThe crushing area of
timber member
Shear plan
MODES OF FASTENER FAILURE
GENERAL DESIGN RULES: SHEAR LOADS
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DOUBLE SHEAR IN DOWEL - TYPE FASTENER
Double
shear plans
The crushing area of
timber member
MODES OF FASTENER FAILURE
GENERAL DESIGN RULES: SHEAR LOADS
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The axial load or the withdrawal load is the load to pull out the dowel-type
fasteners.
Metal
connector
The value of fastener withdrawal
strength is influenced by the
direction of timber member grain
related to direction of fastener and
is defined as embedding strength
of fastened timber member.
GENERAL DESIGN RULES: AXIAL LOAD WITHDRAWAL
LOAD
ta,point
ta,head
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A slip modulus of a joint with dowel-type fasteners for the serviceability limit
states Kser (per shear plane per fastener or connector) should be used.
m m,1 m,2
If the densities of the
two jointed members
are different
finst = 1 mm + F/Kser
The final deformation of a
joint, f3 , made from
members with different
creep properties (kdef,1,
kdef,2), should be
calculated as:
c321finalmax,efmax,ffffff
Fastener type Kser
Dowels
Bolts with or without clearance
Screws
Nails (with pre-drilling)
m1.5Pd/23
Nails (without pre-drilling) m1.5d0.8/30
Staples m1.5d0.8/80
Split-ring connectors type A according to EN 912
Shear-plate connectors type B according to EN 912m dc/2
Toothed-plate connectors:
Connectors types C1 to C9 according to EN 912 1.5 m dc/4
Connectors type C10 and C11 according to EN 912 m dc/2
The clearance should be added separately to the deformation.
2,def1,definst3k1k1ff
The deflection of a beam:
MECHANICALLY FASTENED TIMBER JOINTS DESIGN
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The load-carrying capacities and deformation of fasteners shall be determined
on the basis of tests carried out in conformity with EN 26891, EN 28970, and the
relevant European test.
It shall be taken into account that the load-carrying capacity of a multiple-fastener
joint will frequently be less than the sum of the individual fastener capacities.
The arrangement and sizes of the fasteners in a joint, and the fastener spicing, edge
and end distances shall be chosen so that the expected strengths can be obtained.
The effective characteristic load-carrying capacity of the joint is given by a
combination of:
Fax,Rk = characteristic withdrawal capacity for fasteners;
Fv,Rk = characteristic load-carrying capacity per shear plane for fasteners.
The standard verification for a connection, according to EC 5, is: n - is the effective
number of fasteners
1,
,
,
,
Rdv
Edv
Rdax
Edax
F
F
F
F
Rk,vmod
Ed,vM
Fk
Fn1
F
F
Rd,v
Ed,v
dEd,v NFdEd,ax VFM
modRk,vRd,v
knFF
M
modRk,axRd,ax
knFF
MECHANICALLY FASTENED TIMBER JOINTS DESIGN
1F
F
F
F2
Rd,v
Ed,v
2
Rd,ax
Ed,ax
For nails other than smooth nails, as defined in EN 14592, for screwed connections subjected to a
combination of axial load and lateral load, expression should be satisfied.
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FASTENER SPACINGS AND DISTANCES
Grain
direction t2
t1
MECHANICALLY FASTENED TIMBER JOINTS DESIGN
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The minimum spacing and distances recommended by EC5, are presented
in tables (the following table presents an example with general conditions):
Distance No pre-drilling Pre-drilled
k
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B,Ed,vA,Ed,v
n
1ii,Ed,vEd,v
FFFF
B,u
B,Ed,v
A,u
A,Ed,v
K
F
K
F
Ku,A and Ku,B which are the ultimate slip moduli for limit state design, for
each group of fasteners.
where:
nA = number of A type fastener;
nB = number of B type fastener.
A
Ed,vAA,Ed,vFnF
COMBINATION OF MULTIPLE TYPES OF FASTENERS
B
Ed,vBB,Ed,vFnF
The design force is distributed proportionally to:
MECHANICALLY FASTENED TIMBER JOINTS DESIGN
nA
nB
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ishiseriu nKK )()(,where:
(nsh)i = the number of shear planes per fastener type i;
(Kser)i = the slip modulus per fastener per shear plane for fastener type i.
The design load distributed to each fastener, Fv,Ed,A and Fv,Ed,B , is:
d
B,uBA,uA
A,u
A,Ed,vN
KnKn
KF
d
B,uBA,uA
B,u
B,Ed,vN
KnKn
KF
where Nd is the design value of the load acting on connection.
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CHARACTERISTIC LOAD-CARRYING CAPACITY PER SHEAR PLANE FOR
FASTENER IN SINGLE SHEAR (Fv,Rk):
DESIGN OF TIMBER-TO-TIMBER AND PANEL-TO-TIMBER
JOINTSt1
t2
t1 = timber/panel/steel plate thickness;
t2 = timber penetration;
fh,1,k , fh,2,k = characteristic embedding strengths;
d = fastener diameter;
= fh,2,k / fh,1,kMy,Rk = characteristic yield moment of fastener;
Fax,Rk = characteristic withdrawal capacity for fastenersTimber/panel
Timber
Single shear