TIMBER CONNECTIONS.

INTRODUCTION.

It is commonly stated that a structure is a constructed assembly of joints separated by members

and in timber engineering is generally the critical factor in the design of a 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 requirement of the member material.

Why connections:

1. Joints are crucial points in many timber structures because they can determine the overall
strength and performance.
2. The length of the structural timber is generally shorter than the required spans and as s
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 literally loaded dowels-type fasteners (nails, bolts, screws, dowels,
or nail plates).
Examples of connection systems.

column/post beam system.

Increasing the spans through connections

Examples of different truss systems where connections have been used to combine timber
elements of different lengths to achieve longer spans.
Connection types.

These are permitted in solid or laminated timber if their presence is allowed in a design.
Examples include;
a) Scarf joints.
• The joints are clamped by nailing.
• This must be sized to avoid damage on the surface.
b) Finger joints. (b & c)
• Used to maximize the utilization of materials
• Fingers may be of long or short profile.
 c) Framing anchors
• Cut and punched from thin pieces of galvanized metal.
• Used to join pieces of timbers at right angles.
• Ordered by leg length and handled in the required direction.

d) Flat straps
• Galvanised metal with pre-punched holes are regular intervals.
• Comes in various thicknesses.
 • Used for holding down joining length and lateral restraints.

e) Pre-punched plates
• They are generally galvanized plates with pre-punched holes
• Used for end-to-end jointing of splices.

f) Truss clips.
• As the name suggests, it is used for holding down the end of trusses over wall
plates.
• These are thin galvanized metal strips bent to shape.
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 a wire coil having a minimum tensile strength of 600N/mm 2. 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 galvanizing.

Nails may be driven by hand or by pneumatically operated machines. When nails are to be driven
into dense timbers there is a danger that excessive splitting may occur. Methods of avoiding
splitting are bluing the pointed end of the nail so that it can cut through the timber fibres rather
than separating them or to pre-drill a hole in the timber less than 80% of the nail’s diameter. Pre-
drilling is not normally carried out on timber 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 distance between the nails and the end and the
edge of the timber may be reduced thus producing more compact joints.
• Less slip occurs in the joints.

Disadvantages of pre-drilling.

• Labour intensive and as a result expensive.

• reduces the cross-sectional area of the member.

SCREW

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 screw has over a nail is its additional withdrawal capacity.
DOWELS.

Dowels are circular rods of timber, steel, or carbon-reinforced plastics that have a minimum
diameter of 6mm.

Dowels are driven into identically or marginally undersized holes. These holes must be drilled
through all members in operation or made using CNC machines.

Joints with dowels are used in timber constructions to transmit high forces. Dowels are an
economic type of joint that is easy to produce.

BOLTS

Bolts are dowel-type fasteners with heads and nuts. Bolts are normally ordinary bolts (M12-M14
with a coarse head) with washers that have a side length of about 3d and a 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 the 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.

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 a fire than mechanically fastened
connections.

Key disadvantages are;

• Stringent quality control is required.

• Unsuitable in conditions of fluctuating moisture content.
• Unsuitable if there is a significant component of load perpendicular to the plane of
adhesion.
TIMBER CONNECTORS.

Bolted joints can be strengthened by connectors in the joint surface.

The following are defined as “Timber connectors”

1. Split ring connector joints.

• Timber to timber only
• Installed in pre-cut grooves
• Pressed into the timber.
Timber connectors are load-transferring devices that 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-treated wood or in wet service conditions.
Specification and installation of the bolt are important as it clamps the joints together so that the
connector acts effectively.
 2. Shear plate connector joints

Available in 50mm to 100mm diameter.

Used to steel-to-wood connections or for mountable wood-to-wood connections.

Installation is similar to a split ring but sits flash to the timber surface once installed.

May be used singly when connected to a steel plate, and also used in connecting both bolts to
the footing.

• Timber to timber only

• Installed in pre-cut grooves
 3. Toothed-plate connector joints.

Most suitable for softwood as they are easily embedded.

Consist of a thin gauge plate with the edges cut at regular intervals and the separate pieces of
the edge, then twisted to produce pointed teeth ready for penetration into a timber surface.

2 types are available; two-sided and single-sided, either squared or circular.

Double-sided is suitable for wood-to-wood connection, generally employed for permanent joint
on or off sites.

Singles-sided are for wood-to-metal connections or wood-to-wood joints in case of mountable

joints or where fabrication is involved.

• Timber to timber only

 4. Spiked grid

Similar in performance and installation to the double-sided toothed plates.

Generally recommended where powered tools are not available.
Wood-to-wood connections are installed by pressure hence no groves are required.
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 characteristics of the connection.

SLIP.

When a small load is applied to a nailed joint, the nail begins to bend in the hole accompanied by
movement of the joined timber members relative to each other. The relative movement of the
joined members is referred to as a joint slip. As the load increases the wood fibres in contact with
the nail begin to crush and the bend in the nail increases. The point side end of the nail tends to
pull out of the wood. The timber is in this regard subject to compressive stress while the nail is
subjected to shear and tensile stresses. The joint strength is therefore an interaction of:

• The withdrawal resistance of the nail.

• The shear strength of the nail.
• The compressive strength of the wood.

The difference between a nailed and a bolted joint is that in the latter, the tendency of the
connector to pull out is restrained by the washers which bear on the timber surface.