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Key Sections
Introduction
Using hardwood & cypress
Designing for Appearance
Designing for Structural Strength
Structural Properties
Sizes and Availability
Span Tables
Timber Joints
Insulating Against Fire
Check Structural Selection
Designing for Durability
Formalising specification
Bibliography
Glossary
 
Related Documents
Cladding
Domestic decks
Expressed hardwood structures
Timber flooring
Non-domestic decks
Joinery, furniture and fit-out
Internal lining boards
Piles and poles
Stairs, handrails and balustrades
 
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Home > Technical & Detailing Guide > Designing for Structural Strength > Timber Joints & Connectors

Designing for Structural Strength

Timber Joints & Connectors
Timber must be able to make strong joints as well as having strong spanning ability. This is determined by strength developed parallel or perpendicular to grain. If one direction is weaker than the other then joint strength is reduced. Splitting may occur if connectors are placed too close to the edge or too close to each other. The Species Guide referenced in Section 5 of this document advises on the 'joint group' to which each species belongs.
This is a measure of the timbers ability to hold fasteners and bear joint loads. For domestic construction AS 1684 advises on connector arrangements for various joint groups in standard framing situations. In other instances joint group information must be used in conjunction with AS 1720.1 and structural engineering principles, to confirm jointing and connector requirements.

Choosing an Efficient Connector Between Timber Members
An efficient connector maximises structural performance, minimises cost, and minimizes installation effort. Many small connectors spaced close together – such as nails – are generally more efficient than fewer large connectors spaced further apart – such as bolts. In addition, nails have the added benefit of being fast to install, especially when used with a nail gun and nailing template.

Connector efficiency can also be aided by the design of the timber joint. For instance, beams that bear directly onto columns are more efficient than cleat joints. If a cleat joint can not be entirely avoided then the derivation shown using a corbel support is preferable (refer Figure 12).

Figure 12: Timber connector efficiency through load bearing joints
Spec-guide - Figure 12: timber connector efficiency through load bearing joints

Types of Connectors
An advantage of timber is its ability to make use of a wide range of connectors, including nails, spikes, screws, bolts and nail plates.

Nails
Nails are the most common form of mechanical fasteners used in construction.
There are many types, sizes and forms. When using them the number, spacing, depth of penetration, pull out strength and resistance to lateral movement, influences overall connection strength. Nail options include bullet head, flat head, hardboard, wallboard, fibre cement, clout and plasterboard nails – as shown in Table 10. Different types of corrosion resistant materials can be matched with certain nail types including stainless steel, silicon bronze, monel and galvanised coating. Nails have a tendency to split timber when being driven and the use of blunt or chisel headed nails may alleviate the problem, as will predrilling of kiln dried timber. The metal in some nails can react with the extract from timbers, forming stains. For instance, uncoated steel nails can cause black stains while copper leaves green stains. To avoid this problem galvanised nails are recommended.

Screws
Screws are commonly classified by head type and by the method of drive. They are mainly used for light to medium scale structural situations. They offer superior pull-out strength compared to nails but take longer to install. In recent years self drilling screws – especially Type 17 screws – have made installation faster. Types include countersunk, raised counter sunk, round head hexagon, washer head – as shown in Table 11. Screws are manufactured from low-carbon steel, brass and stainless steel. The latter two are often used for highly corrosive environments. An alternative is to make use of hot-dipped galvanising. Yet another alternative – for low corrosion situations – is electroplating in either zinc, zinc–chrome, cadmium nickel or chromium.

Bolts
Bolts function by bearing on the surface of the timber and the shearing action within the bolt itself. Common types are shown in Tables 12, and in most instances should be accompanied with washers. Common washer sizes for timber are shown in Table 13.

Bolts are commonly used to fix large timber members together, or timber to steel. Coach screws are used for slightly lower strength situations and are essentially heavy duty screws but are sized similar to bolts. They are useful where nuts cannot be placed onto bolts and are only suitable for timber to timber joints, or steel to timber joints.

Bolts can also be used to attach timber to concrete or masonry. This is typically through the use of special masonry or chemical anchors. Here no nuts are used on the end in the concrete or masonry. Instead the bolts rely on friction or adhesion to the substrate. Care is required to make sure the anchor strength is sufficient to resist pull out loads. In addition, washers at the bolt heads must be large enough to prevent timber fibres from crushing when exposed to pull-out loads.

In general bolts and coach screws are made from low-carbon steel and in some instances, brass and stainless steel. The choice is often driven by the need for corrosion resistance. Where steel is used this can be improved using hot-dipped galvanising and electro-plating.

Spec guide - Table 10: Nail selection guide

Spec guide - Table 11: Screw selection guide

Spec guide - Table 12: Bolt selection guide

Spec guide - Table 13: Washer selection guide