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📝 This CalcTree template includes the calculation of timber screw connection capacities in accordance with AS 1720.1 - Design of Timber Structures.
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.




Calculation

Joint Configuration

Define screw properties:


Joint type
:Type 1 - Single shear​



Load direction
:Perpendicular to the grain​



Extra strengthening measures
:Driven through plywood gussets​



Screw position
:End grain​



Screw material
:Steel and 18-8 stainless steel​



Screw size number
:8.00​



Screw diameter
:4.17mm​



lp
:180mm​



Penetration check
:Ok ✅​



Define geometry:
Parallel to the Grain (x-axis)


Nx (columns of screw)
:3​



Sx
:50.0mm​



Minimum Sx
:41.70​



Sx check for spacing
:✅ ​



Minimum edge distance
:20.85​


Perpendicular to the Grain (y-axis)


Ny (rows of screw)
:4​



Sy
:70.0mm​



Minimum Sy
:12.51​



Sy check for spacing
:✅ ​



Minimum end distance
:41.70​


Can’t display the image because of an internal error. Our team is looking at the issue.

The plot assumes the x-axis is the axis parallel to the grain.




Member Properties

Define member properties and geometry:

Member #1


Grade 1
:F14​



Species 1
:Ash, alpine​



Condition 1
:Seasoned​



Thickness 1
:90mm​



Timber type 1
:Hardwood​



Strength group 1
:SD4​



Joint group 1
:JD3​



Additional mechanical properties for Member #1



Characteristic strength in bearing parallel to grain, f'l1
:51.00MPa​



Characteristic strength in bearing perpendicular to grain, f'p1
:17MPa​



Characteristic strength in tension perpendicular to grain, f'tp1
:0.6MPa​



Characteristic strength in bending, f'b1
:22MPa​



Characteristic strength in compression, f'c1
:27MPa​



Characteristic strength in shear at joints, f'sj1
:6.1MPa​



Characteristic strength in shear in a beam, f's1
:3.3MPa​



Modulus of elasticity parallel to grain, E1
:12000MPa​



Modulus of rigidity, G1
:800.00MPa​


Member #2


Grade 2
:F27​



Species 2
:Ash, mountain​



Condition 2
:Seasoned​



Thickness 2
:75mm​



Timber type 2
:Hardwood​



Strength group 2
:SD3​



Joint group 2
:JD3​



Additional mechanical properties for Member #2



Characteristic strength in bearing parallel to grain, f'l2
:59MPa​



Characteristic strength in bearing perpendicular to grain, f'p2
:19MPa​



Characteristic strength in tension perpendicular to grain, f'tp2
:0.6MPa​



Characteristic strength in bending, f'b2
:42MPa​



Characteristic strength in shear in a beam, f's2
:5.1MPa​



Characteristic strength in shear at joints, f'sj2
:7.6MPa​



Modulus of elasticity parallel to grain, E2
:18500MPa​



Modulus of rigidity, G2
:1230MPa​


Member #3 (for Double Shear only)


Grade 3
:F27​



Species 3
:Hemlock western​



Condition 3
:Seasoned​



Thickness 3
:60mm​



Timber type 3
:Softwood​



Strength group 3
:SD7​



Joint group 3
:JD4​


Additional mechanical properties for Member #3



Characteristic strength in bearing parallel to grain, f'l3
:23.00MPa​



Characteristic strength in bearing perpendicular to grain, f'p3
:8.6MPa​



Characteristic strength in tension perpendicular to grain, f'tp3
:0.4MPa​



Characteristic strength in compression, f'c3
:51MPa​



Characteristic strength in bending, f'b3
:34MPa​



Characteristic strength in shear in a beam, f's3
:5.1MPa​



Characteristic strength in shear at joints, f'sj3
:3.8MPa​



Modulus of elasticity parallel to grain, E3
:18500MPa​



Modulus of rigidity, G3
:1230MPa​



Design Capacity



is dependent on the condition of the member (for Type 1) and the nail penetration length (for Type 2). The below values are as per Table 4.1 (A), (B) and Table 4.2(A), (B).
  1. 
    
    = capacity for a bolt parallel to the grain in member
    
    
  1. 
    
    = capacity for a bolt perpendicular to the grain in member
    
    

Member #1


Qk1_1
:810N​



Qk2_1
:73N/mm​

Member #2


Qk1_2
:575N​



Qk2_2
:56N/mm​

Member #3


Qk1_3
:575N​



Qk2_3
:56N/mm​

❗Capacity for Member #3 is only applicable for joints in double shear

Type 1 joint:

ϕNj≥ϕ k1 k13 k14 k16 k17 nQkϕM=ϕ k1 k13 k14 k16 k17 rmax Qk [Σ(rirmax)]32\phi N_j \geq \phi \space k_1 \space k_{13} \space k_{14} \space k_{16} \space k_{17} \space nQ_k \\ \phi M = \phi \space k_1 \space k_{13} \space k_{14} \space k_{16} \space k_{17} \space r_{max} \space Q_k \space [\Sigma (\frac{r_i}{r_{max}})]^\frac{3}{2}
Type 2 joint:

ϕNj=min( ϕ k13 lp n Qk , n Nd,ts )\phi N_j = \text{min(} \space \phi \space k_{13} \space l_p \space n \space Q_k \space , \space n \space N_{d,ts} \space \text{)}


N*
:6.00kN​



N_Utilisation
:1.71​



N_Check
:❌ ​





M*
:0.13kN m​



M_Utilisation
:3.18​



M_Check
:❌ ​



Parameter Values



phi
:0.85​



k1
:1.00​



k13
:2.00​



k14
:1.00​



k16
:1.00​



k17_M
:1.20​



k17_N
:1.00​



Maximum tensile capacity
:3180N​



Explanation

When 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.
Common timber screwed joint arrangements


Code Parameters and Equations

AS 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

Minimum Spacing, Edge and End Distance

Screw Length and Minimum Timber Thickness



Related Resources

  1. 🔗 Timber Nailed Joint Designer to AS 1720.1
  2. 🔗 Timber Beam Calculator to AS1720.1
  3. 🔗 Timber Column Calculator to AS1720.1
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