Lecture notes 8-9-10-11-12-13

LECTURE NOTE SNo. 8-9-10

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

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)

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

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.

TRADITIONAL JOINERY JOINTS

CARPENTRY JOINTS (FRAMED JOINTS)

Scarf joint

( oblic )

Cogging joint

( )

Framed joint (lap joint)

( )

Tenon joint

( )

Finger joint



The most utilized carpentry joints are:

lv

tv

lv

tv

lv2

tv2tv1

lv1

(a) (b)

(c)

Fd

Fd

Fd

Rafter

Purlin

Wall plate



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

Elements of a Nail and Nail Types

DOWEL-TYPE FASTENERS: NAILS

New nail style

DOWEL-TYPE FASTENERS: NAILSConnection in shear

Connection in double shear

Self-Drilling Tapping Screw

Self-Piercing Screw

DOWEL-TYPE FASTENERS: SCREWS

Screws styles:

Screw Head Types


Different shear loading conditions double shear failure

Different shear loading conditions single shear failure

Dowels (pegs)

DOWEL-TYPE FASTENERS: BOLTS & DOWELS

Single-sided toothed plate

Double-sided toothed plate

Split-ring connector

Punched

metal

plate

Shear-plate connectorMild steel hangers

METAL CONNECTORS

METAL CONNECTORS

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

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.

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

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

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

SINGLE SHEAR IN DOWEL-TYPE FASTENERThe crushing area of

timber member

Shear plan

MODES OF FASTENER FAILURE

GENERAL DESIGN RULES: SHEAR LOADS

DOUBLE SHEAR IN DOWEL - TYPE FASTENER

Double

shear plans

The crushing area of

timber member

MODES OF FASTENER FAILURE

GENERAL DESIGN RULES: SHEAR LOADS

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

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:


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


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.

FASTENER SPACINGS AND DISTANCES

Grain

direction t2

t1


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

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:


nA

nB

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.

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

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Lecture notes 8-9-10-11-12-13