Timber connections

Andrew Livingstone
Centre for Offsite Construction + Innovative Structures
A.Livingstone@napier.ac.uk
1) Introduction
It is commonly stated that “a structure is a constructed assembly of
joints separated by members” (McLain ,1998) and in timber engineering
the joint is generally the critical factor in the design of the structure. The
strength of the connectors in the joint will normally dictate the strength of the
structure; their stiffness will greatly influence its overall behaviour and member sizes
will generally be determined by the numbers and physical characteristics of the
connector rather than by the strength requirements of the member material.

Forces will act on a system which is

constructed of members.

The inner forces caused by the external

actions will be transferred from one
member to another at a node point.

The transfer of forces at the node point will

be via a joint.

1. Pinned Joint 2. Semi- 3. Rigid Joint

Rigid Joint
1. Joints are crucial points in many
timber structures because they can
determine the overall strength and
performance.

2. The length of structural timber is

generally shorter than the required
spans and as a result splicing or
composite structures (e.g. trusses) must
be used.

3. Forces between members are most often

transferred through lap joints,
either by adhesives (glues) or by laterally
loaded dowel-type fasteners (nails, bolts,
screws, dowels or nail plates).
1.1 Examples of connections in systems

Top left Post & beam system

Top right Sibelius Hall, Lahti, Finland

Bottom Scottish Parliament,

Edinburgh, Scotland
1.2 Increasing spans through connections
Examples of different truss systems where connections have been used to combine
timber elements of different lengths to achieve longer spans.
1.2 Increasing spans through connections

Bolted Flitch beam fabrication

1.2 Increasing spans through connections

Bolted Flitch beam fabrication

1.3 Connection types

Traditional timber joint

(a) Scarf joint (b) Horizontal finger joint (c) Vertical finger joint

Glued joints
TRADA Wood information sheet 31
1.3 Connection types

(b) Nail

(c) A modern self-drilling wood screw

(a) Dowels

(b) Hexagonal head bolt

Dowel type connectors
1.3 Connection types
2) Nails, Screws, Bolts & Dowels
2.1 Nails
Nails are the most commonly used fasteners in timber construction and are available
in a variety of lengths, cross-sectional areas and surface treatments .

The most common type of nail is the smooth steel wire nail which has a circular cross-
section and is cut from wire coil having a minimum tensile strength of
600N/mm2. It is available in a standard range of diameters up to a maximum of 8mm
and can be plain or treated against corrosion, for example, by galvanising.
2.1 Nails
Nails may be driven by hand or by pneumatically operated portable machines. When nails
are to be driven into dense timbers there is a danger that excessive splitting will occur.
Methods of avoiding splitting are blunting the pointed end of the nail so that it cuts through
the timber fibres rather than separating them or to pre-drill a hole in the timber less than
80% of the nail diameter. Pre-drilling is not normally carried out on timbers with a lower
characteristic density of 500kg/m3.

Advantages of pre-drilling:

•The lateral load carrying capacity of the nail is increased.

•The spacing between the nails and the distances between the nails and the end and
edge of the timber may be reduced thus producing more compact joints.
•Less slip occurs in the joints.

Disadvantages:
•Labour intensive and as a result expensive.
•Reduces the cross sectional area of the member.
2.2 Screws

Wood screws are especially suitable for steel-to-timber

and panel to timber joints, but they can also be used
for timber-to-timber joints. Such screwed joints are
normally designed as single shear joints.

Screws are inserted by turning and this can be done

either by hand or by power actuated tool depending on
the situation.

The main advantage a screw has over a nail is its

additional withdrawal capacity.
2.3 Dowels
Dowels are circular rods of timber, steel, or
carbon-reinforced plastics which have a
minimum diameter of 6mm.

Dowels are driven into identically or marginally

undersized holes. These holes must either be drilled
through all members in one operation or made
using CNC machines.

Joints with dowels are used in timber construction

to transmit high forces. Dowels are an economic
type of joint which is easy to produce.
2.4 Bolts
Bolts are dowel-type fasteners with heads and nuts. Bolts are normally ordinary machine
bolts (M12 – M14 with a coarse head) with washers that have a side length of about 3d
and thickness of 0.3d, where d is the bolt diameter.

Bolts will be placed through pre-drilled holes which are 1-2mm oversized and the
bolt and washer tightened on application such that the members of the connection fit
closely together. If necessary bolts will be required to be re-tightened when the timber has
reached equilibrium moisture content.

Another type of bolt is a lag screw which has a sharp end and coarse threads designed to
penetrate and grip wood fibre.

a) Carriage bolt b) Hexagonal head bolt

c) Square head bolt d) Lag screw

a, b & c TRADA Wood information Sheet 52 d Canadian wood council (www.cwc.ca)

3) Glued joints
Key advantages: of glued joints

•Structural glued joints are generally stiffer, require less

timber and have a better appearance than
mechanically fastened connections.
•They are resistant to corrosive atmospheres
•Joints made with thermosetting resins are safer in fire
than mechanically fastened connections.

Key disadvantages are:

•stringent quality control is required
•unsuitable in conditions of fluctuating moisture
content if dissimilar materials are involved or if there is
a change in the angle of grain at their interfaces.
•unsuitable if there is a significant component of load
perpendicular to the plane of adhesion.
 Setting process
Adhesive Application Advantages / Disadvantages
and cure time
Thermo-Plastic
Polyvinyl Acetate, interior non-reactive, 40 minutes
Catalyzed Polyvinyl but some special formulations at room easy to work with
Acetate (PVA) are waterproof temperature
Interior, high speed production non-reactive, sets by
Hot Melts grips on contact when hot
lines cooling
Thermo and Room Temperature Set
Resorcinol formaldehyde waterproof, high cost, marine-
(RF) fully exterior, laminating, finger plywood
jointing, wood jointing reactive, sets in 2 minutes
Phenol-resorcinol
with heat and 6 hours at
formaldehyde (PRF)
room temperature waterproof
Phenol formaldehyde fully exterior, plywood, some
(PF) particalboard
Thermo-Set
semi-exterior and Interior,
Melamine formaldehyde plywood, particleboard,
(MF) formwork panels. (not often reactive, sets with heat in
used alone in the UK) 2 minutes and 30 minutes moisture resistant, low cost
to 12 hours at room
Melamine urea semi-exterior and Interior, temperature
formaldehyde (MUF) laminating, plywood,
particleboard, finger jointing
10 to 12 hours to cure.
interior, plywood, easy to work, withsomewat gap
There are liquid catalysts
Urea formaldehyde (UF) particleboard, wood jointing, filling, moisture resistant,
that will allow the resin to
bent laminations foundry sand molds
cure in 20 minutes

Isocyanates and reactive, one component

isocyanates fully exterior, ability to set in high moisture
Polyurethanes sets with heat in 2
polyurethane semi-exterior conditions, suitable for multiple
(Most Polyurethane are minutes, from to 2 to 60
and moist interior where martials, 100% solid, good gap
thermo-set but minutes at room
temperature does not exceed filling properties, low glue
thermoplastic are temperature for two-part
50°, laminating spread rate, expensive
available) resins
Catalyst
structural repairs, suitable for
reactive, hardens between
multiple martials, timber end-
Epoxy resins semi-exterior and Interior 2 - 60 min gains full
jointing, waterproof, good gap
strength in 24 hours
filling properties
(1) An elevated temperature is required to cure PF, MF and MUF adhesives.
(2) PVA (polyvinyl acetate) adhesives should not be used for structural purposes, but in certain
limited circumstances PVAc (cross linked PVA adhesives) may be acceptable.
4) Timber connectors
Bolted joints can be strengthened by
connectors in the joint surface.

The following are defined as “Timber

Connectors”: Dapping tool forming seat Application of timber side
for split rings plates

1. Split ring connector joints

• timber to timber only
• Installed in pre-cut grooves
2. Shear plate connector joints
• timber to timber or steel
• Installed in pre-cut grooves Dapping tool forming seat Application of steel side
3. Toothed-plate connector joints for shear plate plates

• timber to timber or steel Fabrication of split ring and shear connector joints
• Pressed into the timber Canadian wood council (www.cwc.ca)
Timber connectors are load transferring
devices which rely on bolts or lag screws
to restrain the joint assembly. They are
more efficient structurally than bolts or
lag screws used alone because they
enlarge the wood area over which a load
is distributed. Mainly used to transfer
loads in heavy timber or glulam members
as in roof trusses they are not usually
protectively coated and need to be
galvanized only if used with preservation Toothed plate connector toothed plate
treated wood or in wet service connector (www.cullen-bp.co.uk)

conditions. Specification and installation

of the bolt is important as it clamps the
joint together so that the connector acts
effectively.
 a) Split ring in single shear b) Split ring in double shear

c) One shear plate – bolts in single shear d) Two shear plates – bolts in double shear

Split ring and shear plate connectors joints

Canadian wood council (www.cwc.ca)
5) Connection plates
Punched metal plate fasteners
A punched metal plate fastener is defined in
prEN1075 “Timber Structures – Joints made of
punched metal fasteners” as a fastener made of
metal plate having integral projections punched
out in one direction and bent perpendicular to
the base of the plate, being used to join two or
more pieces of timber of the same thickness in
the same plane”.

The metal used is generally galvanised or

stainless steel plate of thicknesses varying from
0.9mm to 2.5mm .
The limiting strength of a punched metal plate is
determined by one of two criteria:

1.Its anchorage (gripping) capacity in any of the

jointed members.
2.Its net sectional steel capacity at any of the
interfaces.
Dimension nailing plates
Dimensional nailing plates are made of light-gauge mild steel cut and folded to
shape and pre-punched with holes for specified nails. The most common kinds
are:

•Angle brackets
•Joist hangers
•Truss clips
6) Specification of connections
The specification of the fixing will depend on a range of factors:

-Nature of the forces being applied and their magnitude.

-Practicality and/or manufacturability
-Aesthetics
-Environmental conditions
-Cost

When specifying a connection it is important to consider how the whole system is to

function and this will depend not only on the load-carrying capacity of the connection
but also on the load-deformation characteristic of the connection.
Slip

If the system being designed is statically indeterminate then the load deformation is
influenced by the deformation of the members and slip in the joints. Slip in the
joints is often the largest contributor and can therefore be an important
criteria in specification.

Using nails, screws and bolts in combination

Also important in design is the concept of connections acting together. Nails, screws
and bolts can be used together in a joint as they have similar ductile behaviour.
However, because of the tolerance required in bolt holes to allow application they
should not be considered to be acting together with other mechanical fasteners due
to initial slip.