Timber Screw

How it calculates

Joint type and screw position

The first input is joint type and screw position, which sets two key modification factors drawn from AS 1720.1:2010 Figures 4.2 and 4.4:

• Type 1 (shear) joints - screws are loaded laterally. Side grain positions use k_13 = 1.0; end grain positions use k_13 = 0.6 to account for reduced holding capacity. Single shear yields k_14 = 1.0; double shear yields k_14 = 2.0.
• Type 2 (tension) joints - screws are loaded axially in withdrawal. Side grain uses k_13 = 1.0; end grain uses k_13 = 0.6.

Characteristic fastener capacity

For the selected screw size and joint group of the receiving timber member, the calculator looks up the characteristic lateral capacity (Q_k for Type 1) or characteristic withdrawal capacity (Q_wk for Type 2) from AS 1720.1:2010 tables. The joint group is determined by the timber species - for example, a seasoned hardwood such as F17 carries a higher joint group rating than a softwood like MGP10.

Load combinations and load duration factor k_1

Applied loads (dead, live, wind, earthquake, etc.) are entered per load type. The calculator assembles AS/NZS 1170.0 strength load combinations and identifies the governing combination. The load duration factor k_1 is applied to the demand side - it reflects the characteristic strength reduction for sustained loads.

Rectangular pattern capacity - Type 1 (shear) joints

For a rectangular pattern of n_x columns and n_y rows with spacings s_x and s_y, the calculator derives:

Direct shear capacity (Cl 4.3.3.2):

V_d,j = joint_category_factor × k_13 × k_14 × n × Q_k

where n = n_x × n_y is the total screw count.

In-plane moment capacity (Cl 4.3.3.3):

The polar second moment of area of the screw pattern about its centroid is computed from the spacings. Moment capacity M_d,j accounts for the most critically loaded screw at maximum distance from the centroid, scaled by the same characteristic fastener capacity.

The combined utilization for simultaneous shear and moment loading is checked as:

(V* / V_d,j)² + (M* / M_d,j)² ≤ 1.0

ensuring neither limit state is exceeded individually or in combination.

Type 2 (tension/withdrawal) joints

The withdrawal capacity per screw is derived from the characteristic withdrawal capacity Q_wk and the penetration depth t_p into the receiving member. For the full pattern:

N_d,j = joint_category_factor × k_13 × n × Q_wk × t_p

The demand N* is compared against N_d,j (Cl 4.3.3.4) and the utilization ratio is reported.

Joint category factor

The joint category (Category 1 to 4 per AS 1720.1 Table 2.5) applies a capacity reduction factor that reflects structural risk and inspection level. Category 1 (house construction) uses base characteristic values; higher categories apply increasing reduction factors to the calculated capacity.

Outputs and pattern diagram

The calculator displays a visual cross-section diagram of the screw pattern, updating in real time as geometry inputs change. Output utilization ratios are colour-coded: green for pass, red for fail. Each check - Shear, Moment, and Tension - shows the demand, capacity, and utilization ratio with the corresponding AS 1720.1 clause number, making outputs directly traceable to the standard for documentation and peer review.