Timber nail connections are widely used in structural and non-structural applications due to their simplicity, cost-effectiveness, and ease of installation.

These connections are crucial in framing, flooring systems, roof structures, and even temporary applications like formwork.

This article explores key design principles, compliance with Australian Standards AS 1720.1 - 2010 Timber Structures Part 1: Design Methods, and how ClearCalcs can streamline the design process.

Applications of timber nail connections

Timber nail connections are widely used in structural and non-structural applications due to their simplicity, cost-effectiveness, and ease of installation. Key applications include:

• Framing and load-bearing walls: Nails connect studs, plates, and sheathing, providing lateral stability and shear resistance in walls.
• Flooring systems: Used to fasten floor joists, subflooring, and decking, ensuring structural integrity under loads.
• Roof trusses and rafters: Nails join timber elements in trusses and rafters, handling both vertical loads and lateral forces from wind or seismic activity.
• Non-structural elements: Often used in cabinetry, furniture, and cladding where high loads aren’t critical but simple connections are beneficial.
• Temporary construction and formwork: Nails are effective in short-term applications, such as concrete formwork and scaffolding.

Nail connections are ideal for both permanent and temporary applications, especially in residential buildings.

AS 1720.1:2010 is the standard that governs the design of timber structures in Australia, including nail connections. It provides guidance on calculating the strength and capacity of connections, considering factors like timber type, moisture content, loading conditions, and nail characteristics.

Design parameters for nail connections

Nail connections are designed to resist loads that are either parallel or perpendicular to the grain of the timber. The key parameters include:

Load directions

• Parallel to grain: Load is along the timber's length, utilizing the timber’s tensile capacity.
• Perpendicular to grain: Load is across the grain, utilizing the timber’s shear capacity.

Nail characteristics

• Diameter: The thickness of the nail shank.
• Length: The total length of the nail, including head.
• Penetration depth: The depth the nail penetrates into the timber member.

Timber properties

• Timber species: Determines the characteristic strengths.
• Moisture content: Affects the connection capacity.
• Density and grade: Impacts the holding capacity of the nails.
• ‍Member thickness: Influences nail embedment depth.

Joint type

There are two joint types:

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

Joint groups in timber nail connections

What are joint groups?

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.

Joint group classifications

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.

As per AS1720.1(2010) Clause 4.1.2, the joint group appropriate for the calculation of the design capacity of the joint shall be determined such that:

1. where a joint comprises more than one species of timber, the species with the lowest joint group classification shall be used to calculate the capacity of the joint; except
2. where the capacity of each part of the joint can be determined independently, it is permissible to calculate the design capacity of the joint as the lesser of the capacities of the individual parts based upon their individual joint group classification.

Impact of joint groups on design

Joint Groups impact the values of characteristic strengths, such as:

• Lateral Capacity (Shear Strength): The timber's resistance to forces perpendicular to the fastener.
• Withdrawal Capacity (Pull-Out Strength): The ability of timber to hold a nail or screw without it being pulled out.

These strengths are given in tables within AS 1720.1:2010 and vary according to the Joint Group classification of the timber species.

Selecting joint groups for timber species

Each timber species is assigned to a specific Joint Group based on its density and structural properties. Here’s how you can determine the Joint Group for a particular timber:

1. Refer to AS 1720.2: This standard contains detailed tables assigning specific Joint Groups to commonly used timber species.
2. Based on Moisture Content:
   
   • If timber is seasoned (moisture content ≤ 15%), use the JD classification.
   • If timber is unseasoned (moisture content > 15%), use the J classification.

For example:

• Radiata Pine: Typically falls under JD4 for seasoned and J4 for unseasoned.
• Spotted Gum: Typically classified as JD2 for seasoned and J2 for unseasoned.

The joint group classifications for timber species and moisture condition shall be as listed in Tables H2.3 and H2.4, Appendix H of AS1720.1(2010)

Eccentric joints

As per AS1720.1(2010) when it is impracticable to ensure that all the members meeting at a joint are arranged symmetrically, with their centre-lines intersecting on a common axis, which is also the axis of resistance of the fastener or group of fasteners, the combined effects of primary stresses and secondary stresses due to the resulting bending and shear stress shall be checked.

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Design process for nail connections

Step 1: Determine load capacity and requirements

• Identify the loads acting on the structural members (e.g., dead load, live load, wind load).
• Calculate the resultant forces on each connection.

Step 2: Choose joint type & timber member type

• Select the nail type, diameter, and length based on the timber species.
• Select the Joint Type as per Section 2.4 above.

Step 3: Calculate characteristic capacities

• Use Tables 4.1 & 4.2 of AS1720.1(2010) to find the characteristic capacity of the nails

Step 4: Calculate design capacities

• Use Section 4.2.3 of AS1720.1(2010) to find the design capacity of the nail group.

Step 5: Compliance with AS 1720.1

• Ensure that the calculated capacities are greater than the applied loads.
• Ensure spacing, edge, and end distances comply.

Design of nailed joints

Characteristic capacities for Type 1 Joints Q_k

The characteristic capacities for nails, listed in Tables 4.1 and 4.2, apply to plain shank low carbon steel nails, as specified in AS 2334.

Design capacity for Type 1 Joints

As per AS1720.1(2010) Clause 4.2.3.2, the design capacity (N_dj) for a Type 1 joint containing n nails designed to resist direct loads, as illustrated in Figure 4.5(a), for strength limit states shall satisfy the following:

$$N_{dj}\ge N^*$$

$$N_{dj}= \phi k_{13}k_{14}k_{16}k_{17}nQ_k$$

Where

$\phi$=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

$k_{13}$=1.0 for nails in side grain, 0.6 for nails in end grain

$k_{14}$=1.0 for nails in single shear, 2.0 for nails in double shear

$k_{16}$=1.2 for nails driven through close fitting holes into metal side plates

$k_{16}$=1.1 for nails driven through plywood gussets, 1.0 otherwise

$k_{17}$=factor for multiple nailed joints given in Table 4.3(A) AS1720.1(2010)

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

$Q_k$=characteristic capacity as per Section 3.1 above

As per AS1720.1(2010) Clause 4.2.3.3, the design capacity (M_dj) per shear plane interface for a Type 1 joint containing n nails designed to resist in-plane moment, as illustrated in Figure 4.5(b), for strength limit states shall satisfy the following:

$$M_{dj}\ge M^*$$

$$M_{dj}=k_1k_{13}k_{14}k_{16}k_{17}r_{max}Q_k\sum_{i=1}^{n}(\frac{r_i}{r_{max}})^\frac{3}{2}$$

Where

$r_{max}$= max value of r_i which is the distance of the i-th nail to the centroid of the nail group

Characteristic capacities for Type 2 Joints Q_k

The characteristic capacities for Type 2 joints, where fasteners are subjected to axial loads where the fastener is installed into the side or end grain of connected members are detailed in the table below.

Design capacities for Type 2 Joints

As per AS1720.1(2010) Clause 4.2.3.3, the design capacity (Nd,j) for a Type 2 joint containing n nails designed to resist axial loads tending to cause withdrawal, as illustrated in Figure 4.4, for strength limit states shall satisfy the following:

$$N_{dj}\ge N^*$$

Where

$$N_{dj}=\phi k_{13}l_pnQ_k$$

$k_{13}$=1.1 for withdrawal from side grain,  0.25 for withdrawal from end grain for straight driven nails

$k_{13}$=0.6 for withdrawal from end grain

$l_p$=depth of nail penetration into supporting member in millimetres

$n$=total number of nails in joint

$Q_k$=characteristic capacity as per Section 3.3 above

Spacing, edge and end distances for nails

AS1720.21(2010) Table 4.4 provides recommended minimum spacings, edge and end distances for nails in terms of nail diameter (D).

Nail length and timber thickness

As per Clause 4.2.5 of AS1720.1 (2010), the characteristic capacities given in Section 3.1 and 3.3 shall only be applicable, where timber thicknesses and nail length as shown in Figure 4.6 are as follows:

1. Two-member joints (nails in single shear) Thickness of first member, tl >10D; depth of penetration of nail into second member, tp >10D.
2. Three-member joints (nails in double shear) Thickness of central member, tm >10D; thickness to outer member, to >7.5D; depth of penetration of nail into outer member, tp >7.5D.

Avoidance of splitting

As per Clause 4.2.6 of AS1720.1 (2010), the characteristic capacities for nails have been derived on the assumption that splitting of the timber does not occur to any significant extent. In unseasoned timber that shows a marked tendency to split, the use of prebored holes of 80% of the nail diameter is recommended.

For a deeper understanding of timber connections, including different joint types and fastening methods, check out ClearCalcs' detailed article: An Introduction to Timber Connections in Structural Design.

Hand calculation timber nail connection design example

Design a Type 1 shear connection using nails for a timber joint subjected to a dead load shear force of 2 kN as well as a live load shear force of 1kN. Builder has requested to use 145 x 45 MGP10 Pine with a moisture content of 12% for both supporting members. Nails available on site are 3.06 mm in diameter and effective penetration depth is 30mm.

Determine the Characteristic Capacity Q_k

The characteristic capacities for nails, listed in Tables 4.1 and 4.2, apply to plain shank low carbon steel nails, as specified in AS 2334. As we are using seasoned timber (MGP10) we will use Table 4.1B. MGP10 is classified in AS 1720 to have a common joint group of JD4. And based on our nail diameter (taken as 2.8mm to be conservative), gives us a characteristic capacity per nail of 665N.

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Determine the Design Capacity

As per AS1720.1(2010) Clause 4.2.3.2, the design capacity (N_dj) for a Type 1 joint containing n nails designed to resist direct loads, as illustrated in Figure 4.5(a), for strength limit states shall satisfy the following:

$$N_{dj}\ge N^*$$

Where

$$N_{dj}= \phi k_{13}k_{14}k_{16}k_{17}nQ_k$$

Where

$N^*$=1.2G+1.5Q for strength design = 1.2(2kN)+1.5(1kN)=3.9kN=3900N

$\phi$=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

$k_{13}$=1.0 for nails in side grain

$k_{14}$=1.0 for nails in single shear

$k_{16}$=1.0

$k_{17}$=1.0 as per AS1720.1(2010) Table 4.3(A) as we will use 4 or less rows of fasteners

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

$Q_k\text{(characateristic capacity)}$=665N

$$n\ge \frac{N^*}{\phi k_{13}k_{14}k_{16}k_{17}nQ_k}\ge \frac{3900}{0.6*0.57*1.0*1.0*1.0*665}\ge 17.15$$

Therefore, we must use 18 nails spaced at min 5 times the nail diameter, so we will use a nail spacing of 300mm. These could be arranged into 3 rows of 6 nails or 4 rows of 5 nails.

ClearCalcs timber nail connection design example

For this section, we will demonstrate how much faster and more accurate the design process is with the ClearCalcs Timber Nail Calculator to AS 1720.1(2010). Additionally, the calculator allows us to be more creative with our design, ensuring we achieve the most structurally efficient and cost effective outcome. We will use the same example as previous.

Design a Type 1 shear connection using nails for a timber joint subjected to a dead load shear force of 2 kN as well as a live load shear force of 1kN. Builder has requested to use 145 x 45 MGP10 Pine with a moisture content of 12% for both supporting members. Nails available on site are 3.06 mm in diameter and effective penetration depth is 30mm.

Input the loads

The loads should first be input as these parameters will not change (unless there is a major redesign of the structure). ClearCalcs has load linking available, so the loads from other members can be added in here or manually entered.

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Input the member, joint and nail type

Detail the timber members, check with the builder what their preference is or specify the supporting members determined to be suitable in the ClearCalcs Timber Beam Calculator & Timber Column Calculator which ensure the member itself is adequate for the load applied.

ClearCalcs also has design guides for determining the supporting members including Designing a Timber Beam to AS 1720.1(2010) and Designing a Timber Column to AS 1720.1(2010).

The builder should be consulted about the supporting member types, as they may have a preference of what materials they have on hand or can easily procure. The same is true for the nail type. But for this example they have been specified and are entered below.

Specify the number of nails and spacing to achieve a compliant design

The designer can adjust the number of nails and spacing of nails to ensure the utilisation for both shear and moment is below 100%.

The designer can be creative here, and provide the builder different compliant options, which may make construction easier on site. The designer can get instantaneous feedback on the impact different nail numbers and spacings have on the joints design capacity, which makes designing much faster, rather than having to manually perform all the calculations again in Section 5.

By iterating we have found that 4 nails in each direction is compliant (total number of nails is 16) which is slightly more efficient than the hand calculation design, which shows why being able to try different options with ClearCalcs can deliver value to clients.

Export your calculation to append to your design

Many jurisdictions require the engineer to provide detailed calculations to meet regulatory compliance and design standards.

Some clients even specify this as a requirement of the design. ClearCalcs automatically exports the calculations completed in the background in accordance with AS 1720.1(2010) to save you time and ensure there are no calculation errors.

Additional considerations

Corrosion protection

Nails should be hot-dipped galvanized or stainless steel if used in environments with high moisture content or exposed to weather.

Construction quality

The engineer and building surveyor should inspect some of the joints when constructed to ensure on site quality is meeting the structural design requirements.

Conclusion

Designing nail connections in timber structures requires understanding the loading conditions, timber properties, nail characteristics, and the correct use of the design equations outlined in AS 1720.1:2010. Proper calculation, adherence to standards, and compliance with minimum requirements ensure a safe and effective connection. ClearCalcs Timber Nail Calculator to AS1720.1(2010) greatly increases the speed, efficiency and cost effectiveness of timber nail connection designs.

Grab a free trial to experience the joys of designing with ClearCalcs today!

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