Design Guide for Timber Bolt Connections to Australian Standards

Timber bolted connections, are commonly used in timber structures, from residential to larger commercial applications, because they allow for efficient load transfer between members and flexibility during assembly.

The main benefits of timber bolted connectors are:

High load-carrying capacity
Ease of assembly and disassembly
Durability and reliability
Versatility in applications (can be applied at angles)
Compatibility with engineered wood products
Reduced splitting risk
Performance under cyclic and dynamic loads
Cost-effectiveness

Figure 1: Timber bolted connections in a queen post truss (Reference)

This guide is a resource for structural engineers. It covers the fundamentals of Australian standards AS 1720.1, its scope, and the key design parameters for timber bolted connections.

Note: This design guide is intended to be informative, but compliance with AS 1720.1(2010) design standards must be ensured in every design. The designer must have the appropriate qualifications depending on the jurisdiction in which they are designing.

AS 1720.1 Timber Structures

AS 1720.1 provides structural design methods for timber and engineered wood products used in building construction. It covers:

Structural design principles for timber and wood-based materials.
Requirements for designing connections, including bolted, nailed, and screwed timber joints.
Load-carrying capacity criteria for connections subjected to various loads.
Provisions for different timber species, moisture conditions, and service environments.
The standard applies to all types of solid timber, including softwood and hardwood, as well as engineered wood products such as plywood, laminated veneer lumber (LVL), and glued laminated timber (glulam). Specifically, AS1720.1 provides guidelines for joint types, spacing, and bolt sizes to achieve optimal performance for timber connectors, such as bolted connections.

In this article, we will cover the design procedure for timber bolted connections in accordance with AS 1720.1(2010) Timber Structures.

Design parameters

Timber properties

Joint group: Joint groups categorize timber species based on their density, hardness, and connection performance. The performance is primarily focused on the timber's capacity to hold fasteners like nails, bolts, or screws under loading conditions. The Joint groups used in AS 1720.1:2010 include the following:
- JD1 to JD6: Used for seasoned (dry) timber.
  - JD1 is the strongest, usually representing high-density, seasoned hardwoods.
  - JD6 is the weakest, usually for lower-density, seasoned softwoods.
- J1 to J6: Used for unseasoned (green) timber.
  - J1 is the strongest, usually high-density, unseasoned hardwoods.
  - J6 is the weakest, usually low-density, unseasoned softwoods.
Grain direction: The strength of timber varies depending on loading parallel or perpendicular to the grain, influencing the design of bolted connections.

Bolted connection types

Single shear and double shear connections: Bolted timber joints can either be single shear (two connected members) or double shear (three connected members). AS 1720.1 provides formulas and factors for each connection type.
Spacing and edge distances: The standard specifies the minimum spacing between bolts and from the edge of timber members to avoid splitting and ensure load distribution.
Bolt size: Bolts must meet minimum size requirements based on load and timber type. Common bolt diameters in timber design range from 10mm to 24mm.

Load conditions

Axial load: Loads acting along the member's length. Bolted connections in axial loading are typically less demanding than those under perpendicular load.
Lateral load: Loads acting perpendicular to the timber member. Lateral loads significantly impact bolted connections due to potential shear forces on bolts.
Combined loads: When axial and lateral loads are combined, AS 1720.1 provides interaction equations to ensure the connection can safely support both loads simultaneously.

Connection detailing

End and edge distances: AS 1720.1 specifies minimum distances from the bolt center to the end or edge of timber members to prevent splitting.
Bolt spacing: Minimum and maximum spacing between bolts is critical to ensure even load distribution across the connection. Excessive spacing can cause uneven loading, while tight spacing may reduce load-carrying capacity.
Number of bolts: The standard outlines the maximum allowable number of bolts in a connection, balancing load distribution and timber’s splitting resistance.

Service factors and duration of load

AS 1720.1 adjusts the design capacity based on load duration, with different factors for permanent, short-term, and instantaneous loads. These adjustments ensure the connection’s longevity.

Exposure conditions, including weathering and potential decay in untreated timber, are accounted for with adjustment factors. For example, an outdoor bolted connection may require corrosion-resistant bolts and larger safety factors.

Safety and capacity factors

AS 1720.1 uses safety factors to account for uncertainties in material properties and construction variability. Typical factors range between 0.6 and 0.9 for timber bolted connections.

Design procedure

Step 1: Determine characteristic capacity of bolts

As per Clause 4.4.2.4 of AS 1720.1, the characteristic capacity for a laterally loaded single bolt in a bolted joint system (Qsk) shall be derived as follows:

a) For systems loaded parallel to grain:

$$Q_{sk}=Q_{sk1}$$

Where

$$Q_{sk1}\text{ is given in Table 4.9(A) of AS 1720.1}$$

b) For systems loaded perpendicular to the grain:

$$Q_{sk}=Q_{skp}$$

Where

$$Q_{skp}\text{ iis given in Table 4.10(A) of AS1720.1}$$

c) For systems loaded at an angle to the grain, (θ):

$$Qsk=\frac{Q_{sk1}Q_{skp}}{Q_{sk1}sin^2(\theta)+Q_{skp}cos^2(\theta)}$$

The system capacity parameters are determined from AS 1720.1(2010) Table 4.9(B-C) and Table 4.10 (B-C).

The maximum tensile load capacity for a bolt subject to direct axial loading shall not exceed the value appropriate to the diameter and metal from which the bolt is manufactured, as given in AS 1720.1(2010) Table 4.11.

Step 2: Determine joint type

There are two joint types:

Type 1 joint where fasteners subject to shear loads where the fastener is into the side or end grain of connected members
Type 2 joint where fasteners subject to axial loads where the fastener is installed into the side or end grain of connected members

Step 3: Determine design capacity of Bolted Connection

Type 1 Joint

The design capacity (N_d,j) for a Type 1 joint containing n bolts in shear to resist lateral loads, as illustrated in AS 1720.1 (2010) Table 4.9 and Table 4.10, shall satisfy the following.

$$N_{d,j}\ge N^*$$

$$N_{d,j}=\theta k_1k_{16}k_{17}nQ_{sk}$$

Where

$\theta$= capacity factor (Clause 2.3 AS1720.1(2010)), 0.6 is most conservative

$k_1$= the factor for duration of load for joints (Clause 2.4.1.1 AS1720.1(2010)), 0.57 is most conservative

$k_{16}$= 1.2 for bolts that transfer load through close fitting holes into metal side plates, 1.0 otherwise

$k_{17}$= factor for multiple bolted joint given in Table 4.12 AS1720.1(2010) shown below

$n$= total number of bolts in connection resisting design action effect in shear

$Q_{sk}$= characteristic capacity as per Step 1 above

3.3.1 Type 2 Joint

The design capacity (N_d,j) for a Type 2 joint in which bolts are loaded in direct tension shall satisfy

$$N_{d,j}\ge N^*$$

Where

$$N_{dj}=nN_{d,tb}$$

$$N_{d,j}=\theta k_1k_7nf'_{pj}A_w$$

$N^*$= design action effect in direct tension

$n$= total number of bolts in connection resisting design action effect in shear

$N_{d,tb}$= design capacity of bolt in tension as per Table 4.11 in AS 1720.1(2010)

$\theta$= capacity factor (Clause 2.3 AS1720.1(2010)), 0.6 is most conservative

$k_1$= the factor for duration of load for joints (Clause 2.4.1.1 AS 1720.1(2010)), 0.57 is most conservative

$k_7$= length of bearing factor as per Table 2.6 AS 1720.1(2010)

$f'_{pj}$= characteristic bearing capacity for timber in joints as per AS 1720.1(2010) Table C6

$A_w$= effective area of washer for bearing as per AS 1720.1(2010) Table 4.11

Bolted joints with loads at an angle to the bolt axis

As per AS 1720.1(2010) Clause 4.4.3.4, the design capacity (Ndj) for a joint in which bolts are loaded at an angle to the axis of the bolts (see Figure 4.8) shall satisfy:

AS 1720.1(2010) Clause 4.4.3.2, which is the Type 1 Joint criteria captured above for that component of the load normal to the bolt axis; and
AS 1720.1(2010) Clause 4.4.3.3 which is the Type 2 Joint criteria captured above for that component parallel to the bolt axis.

Step 4: Specify spacings, edge and end distances for bolts

As per AS 1720.1(2010) Clause 4.4.4, the edge, end and between-fastener spacings are not less than those shown in Figure 4.9.

End Distance (lpar)

As per AS 1720.1(2010) Clause 4.4.4.2, the required end distance (lpar) shall be at least 8D in tension joints in unseasoned timber, 7D in tension joints in seasoned timber, and 5D in compression joints and in joints subject to bending moment for both moisture conditions.

It is appropriate to use lesser end distances in tension joints provided the characteristic capacity is reduced in proportion to the reduction in end distance.

In no case shall the end distance for tension joints be less than 6D for unseasoned timber and 5D for seasoned timber

Spacing between bolts perpendicular to load (a)

As per AS 1720.1(2010) Clause 4.4.4.3, the distance, a, shall be at least 2.5D for a b/D ratio of 2, and it shall be increased proportionately so that it is at least 5D for a b/D ratio of 6 or more, where b is the effective thickness of the member loaded perpendicular to the grain.

Loads acting at an angle to the grain

As per AS 1720.1(2010) Clause 4.4.4.4, for loads acting at an angle 0° to 30° to the grain, the spacings, edge and end distances shall be taken as for loads parallel to the grain. For loads acting at an angle of 30° to 90° to the grain, the spacings, edge and end distances shall be taken as for loads acting perpendicular to the grain.

Step 5: Specify washers

As per AS 1720.1(2010) Clause 4.4.5, in all timber-to-timber bolted structural joints, every bolt shall be fitted with a washer at each end, of a size not less than that given in Table 4.11 which is shown in Step 1.

Step 6: Eccentric joints

As per AS 1720.1(2010) Clause 4.4.6, when it is impracticable to ensure that all the members meeting at a joint (see Figure 4.9 in Step 4) are arranged symmetrically, that is, the members’ centroidal axes intersecting on a common axis which is also the axis of resistance of the bolt or group of bolts, then the combination of primary stresses (induced by axial loads) and secondary stresses (induced by bending moment resulting from bolt eccentricities) shall be checked to ensure that no member or fastener is excessively stressed.

In addition, the design capacity in transverse shear at an eccentric joint (Vd,sj) shall satisfy the following:

$$V_{d,sj}\ge V^*_{sj}$$

$$V_{d,sj}\ge \theta k_1k_4k_6f_{sj}'A_{sj}$$

Where

$V^*_{sj}$= design action effect in shear on the joint

$\theta$= capacity factor (see Clause 2.3)

$k_1$= the factor for duration of load for joints (Clause 2.4.1.1 AS1720.1(2010)), 0.57 is most conservative

$k_4$= length of bearing factor as per Table 2.6 AS1720.1(2010)

$k_6$= 1.0 (0.9 if coastal Queensland or northern Australia) as per Clause 2.4.3 of AS1720.1(2010)

$f_{sj}'$= characteristic value in shear at joint details appropriate to species strength group

$A_{sj}$=$\frac{2bd_s}{3}$ (see Figure 4.10 from AS1170.1(2010)

Timber bolted connection design by hand calculation

Design a timber bolted connection that will connect two 90x45 JD2 species timbers together with the load applied parallel to the grain. The bolted joint is for a timber residential building and is subjected to a shear load of 2kN (dead load). The live loads consist of 1 kN shear in the same direction as the dead load.

Step 1: Determine characteristic capacity of bolts

As per Clause 4.4.2.4 of AS 1720.1, the characteristic capacity for a laterally loaded single bolt in a bolted joint system (Qsk) shall be derived as follows:

For systems loaded parallel to grain for M6 bolts (which we will try first as they are smallest/cheapest available):

$Q_{sk}=Q_{sk1}=Q_{k1}$= 3500N for a two member joint configuration as per Table 4.9(A) & 4.9(C) of AS1720.1

This provides us with a round washer size of 30mm with a thickness of 1.6mm and an effective bearing area (Aw) of 200 mm².