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# Design a Timber Screw Connection to AS 1720.1:2010
> Design timber screwed connections to AS 1720.1:2010. Checks screw patterns under direct shear, in-plane moment, and tension capacity.
Design rectangular screw patterns in timber connections to AS 1720.1:2010. Checks direct shear capacity (Cl 4.3.3.2), in-plane moment capacity (Cl 4.3.3.3), and withdrawal/tension capacity (Cl 4.3.3.4) for Type 1 and Type 2 joints.
The Calcs.com Timber Screw Calculator enables the easy design and analysis of timber screwed connections to AS 1720.1:2010. The calculator assumes rectangular screw patterns subject to direct shear, in-plane moment or tension, and that detailing requirements are checked separately.
## Method & scope
| Property | Detail |
| ---------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Design standards | AS 1720.1:2010 |
| Regions | Australia & New Zealand |
| What it checks | Joint type and screw position, Characteristic fastener capacity, Load combinations and load duration factor k\_1, Rectangular pattern capacity, Type 1 (shear) joints, Type 2 (tension/withdrawal) joints, Joint category factor |
#### Calculation method
##### 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 color-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.
## How to use it
Open it from **Run calc** in the About this calculator panel above.
Work through the input sections from top to bottom. Click any input label to see its reference explanation, clause, conditions and assumptions. See [Checks, References, Conditions and Assumptions](/docs/running-calculations/checking-results/checks-references-conditions-assumptions).
Check the utilization of each governing check in the summary, then export a PDF report. See [Views and Export](/docs/running-calculations/exporting-reports/views-and-export).
## Common questions
AS 1720.1:2010 (Timber Structures). The calculator applies Section 4.3 of that standard to determine the design capacity of screwed connections, using characteristic values for lateral (shear) and withdrawal (tension) fastener capacity, combined with joint category factors and load duration factor k\_1.
Joint type and screw position (side grain or end grain; single or double shear for Type 1, or withdrawal for Type 2), screw size, metal type, timber species and joint group for both members, rectangular pattern geometry (n\_x screws in X, n\_y in Y, spacings s\_x and s\_y), penetration depth t\_p, joint category, and applied shear/moment or tension loads with load types for AS/NZS 1170 load combinations.
For Type 1 (shear) joints, the calculator outputs direct shear demand V\* and capacity V\_d,j (AS 1720.1 Cl 4.3.3.2), and in-plane moment demand M\* and capacity M\_d,j (Cl 4.3.3.3), each with a utilization ratio. For Type 2 (tension) joints it outputs withdrawal demand N\* and capacity N\_d,j (Cl 4.3.3.4). The governing utilization ratio is flagged in the calculation header.
The calculator covers Type 1 and Type 2 joints as defined in AS 1720.1:2010 Figures 4.2 and 4.4, including screws in side grain and end grain for single and double shear. Characteristic capacity values are drawn from AS 1720.1 Section 4.3 tables based on selected screw size and joint group, covering structural screws used in Australian timber framing.
The calculator checks pattern capacity under the specified load case. Minimum spacing, end distance, and edge distance requirements per AS 1720.1 must be verified separately, the calculation notes this assumption explicitly. The pattern geometry diagram updates visually as you adjust n\_x, n\_y, s\_x, and s\_y to help you lay out a pattern before detailing checks.
## Next steps
Complete list of Calcs.com design calculators available for Australian and New Zealand projects (AS 3600, AS 4100, AS 1720, AS/NZS 1170, and related standards).
Design timber bolted connections to AS 1720.1:2010. Checks bolt patterns under direct shear, in-plane moment, and tension for Australian timber construction.
Design timber beams with multiple spans and load cases to AS 1720.1:2010 (Amdt 3). Checks bending, shear, bearing and deflection.
Design timber truss chords, frame members and rafter ties to AS 1720.1:2010 (Amdt 3), with combined bending, axial and shear checks.
Check the capacity of nailed timber connections to AS 1720.1:2010.
