Timber Screwed Joint Designer to AS 1720.1

This calculator designs timber screw joint connections by checking the timber species and screw group geometry. It checks the capacity of screw joints connecting two or three timber members, subject to applied axial or shear forces and moments.
All calculations are performed in accordance with AS 1720.1-2010.
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CalculationJoint Configuration
Define screw properties:
﻿﻿Joint type
:Type 1 - Single shear​
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﻿﻿Load direction
:Perpendicular to the grain​
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﻿﻿Extra strengthening measures
:Driven through plywood gussets​
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﻿﻿Screw position
:End grain​
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﻿﻿Screw material
:Steel and 18-8 stainless steel​
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﻿﻿Screw size number
:8.00​
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﻿﻿Screw diameter
:4.17mm​
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﻿﻿lp
:180mm​
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﻿﻿Penetration check
:Ok ✅​
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Define geometry:
Parallel to the Grain (x-axis)
﻿﻿Nx (columns of screw)
:3​
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﻿﻿Sx
:50.0mm​
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﻿﻿Minimum Sx
:41.70​
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﻿﻿Sx check for spacing
:✅ ​
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﻿﻿Minimum edge distance
:20.85​
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Perpendicular to the Grain (y-axis)
﻿﻿Ny (rows of screw)
:4​
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﻿﻿Sy
:70.0mm​
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﻿﻿Minimum Sy
:12.51​
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﻿﻿Sy check for spacing
:✅ ​
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﻿﻿Minimum end distance
:41.70​
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The plot assumes the x-axis is the axis parallel to the grain.
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Member Properties
Design Capacity
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ExplanationWhen used to join timber elements, screws provide an extremely strong connection. The threads in the screws increases contact surface area, increasing grip and anchorage within the timber fibers.
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Common timber screwed joint arrangements
Code Parameters and EquationsAS 1720.1 categorises a connection type into either "type 1" or "type 2" for the purpose of design. The code also prescribes minimum dimensions for the screw set-out and for the timber thickness.
Type 1 Joint
Type 2 Joint
A type 2 joint is a system where screws are resisting withdrawal (axial) forces, ﻿N∗
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Type 2 joints
The strength of a type 2 joint is primarily governed by the screw embedment depth, unlike type 1 joints which depend on the screw size.
Capacity of a type 2 joint to resist direct axial or tension loads causing withdrawal is calculated as:
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ϕNj=min(ϕ k13 lp n Qk , n Ndts)(Cl. 4.3.3.4)\large \phi N_j = \text{min(}\phi \space k_{13} \space l_p \space n \space Q_k \space , \space n \space N_{dts} \text{)}\hspace{0.5cm} \text{(Cl. 4.3.3.4)}ϕNj​=min(ϕ k13​ lp​ n Qk​ , n Ndts​)(Cl. 4.3.3.4)
Where:
﻿﻿k13​
, again, accounts for the effect of the screw position.
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→k13=1.0for withdrawal from side grain→k13=0.6for withdrawal from end grain\hspace{0.5cm} \rightarrow \hspace{0.5cm}k_{13}=1.0 \hspace{0.5cm}\text{for withdrawal from side grain}\\\hspace{0.5cm} \rightarrow\hspace{0.5cm}k_{13}=0.6 \hspace{0.5cm}\text{for withdrawal from end grain}→k13​=1.0for withdrawal from side grain→k13​=0.6for withdrawal from end grain
﻿﻿lp​
 is the depth of screw penetration, into primary member (i.e. the member in which the screw ends)
﻿﻿Qk​
 is determined from Table 4.2 (A) and (B):
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AS1720.1 - Table 4.7 (A) and (B) - ﻿Qk​
 for screws in Type 2 joint
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AS1720.1 - Table 4.7
Minimum Spacing, Edge and End Distance
Screw Length and Minimum Timber Thickness
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Related Resources🔗 Timber Nailed Joint Designer to AS 1720.1
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