Timber Bolt

How it calculates

The Timber Bolt calculator follows the design procedure in AS 1720.1:2010 Section 4.4 to determine the capacity of a bolted timber connection under lateral shear, direct tension, or combined loading.

Characteristic bolt capacity

For a single bolt loaded laterally (Type 1 joint), the characteristic capacity Qsk depends on the load angle relative to the grain:

• Parallel to grain: Qsk = Qsk1, taken from AS 1720.1 Table 4.9(A) based on bolt diameter and effective timber thickness
• Perpendicular to grain: Qsk = Qskp, taken from AS 1720.1 Table 4.10(A)
• At angle θ to grain: Qsk = (Qsk1 × Qskp) / (Qsk1 × sin²θ + Qskp × cos²θ)

The system capacity parameters that modify Qsk1 and Qskp for multi-ply arrangements are drawn from Tables 4.9(B-C) and 4.10(B-C).

For bolts loaded in direct tension (Type 2 joint), the maximum tensile capacity per bolt N_d,tb is taken from Table 4.11 based on bolt diameter and material grade.

Design capacity - Type 1 joint (shear)

The design capacity of the bolt group in lateral shear is:

N_d,j = φ × k1 × k16 × k17 × n × Qsk

Where:

• φ = capacity reduction factor from Clause 2.3 (0.6 for most bolted connections)
• k1 = duration of load factor for joints from Clause 2.4.1.1 (0.57 for permanent load, up to 1.14 for wind/earthquake)
• k16 = 1.2 when bolts transfer load through close-fitting holes into steel side plates; 1.0 otherwise
• k17 = group factor from Table 4.12, which reduces capacity for bolt rows with more than 4 bolts parallel to the grain
• n = number of bolts in the group resisting the design action effect in shear

The design action effect must satisfy N* ≤ N_d,j.

Design capacity - Type 2 joint (tension)

For bolts loaded in direct tension, the group capacity is:

N_d,j = φ × k1 × k7 × n × f'_pj × Aw

Where:

• k7 = length of bearing factor from Table 2.6
• f'_pj = characteristic bearing capacity for timber at joint details, from Table C6
• Aw = effective washer bearing area from Table 4.11

This check ensures the washer and timber bearing surface can transfer the tensile load without pull-through.

Combined shear and tension

When load acts at an angle to the bolt axis, both Type 1 (shear component normal to bolt axis) and Type 2 (tension component parallel to bolt axis) capacity checks are applied simultaneously per Clause 4.4.3.4. Both utilization ratios must be at or below 1.0.

Load distribution in bolt groups

For bolt groups subject to concentric direct loads, the applied force is divided equally across all n bolts. The calculator does not automatically redistribute load for eccentric patterns - eccentricity effects per Clause 4.4.6 must be assessed separately.

Washer requirements

Every bolt in a timber-to-timber bolted joint requires a washer at each end per Clause 4.4.5. The minimum washer size is governed by bolt diameter as specified in Table 4.11. The effective bearing area Aw of the washer directly enters the Type 2 tension check.

Capacity factors

• φ = 0.6 for bolted connections (Clause 2.3)
• k1 ranges from 0.57 (permanent load) to 1.14 (wind and earthquake, short duration)
• k17 = 1.0 for up to 4 bolts in a row parallel to grain; reduces progressively for larger groups per Table 4.12