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Timber Screw Design

Australian Standard connection design tool for structural timber.

Screw Design

The AS 1720.1 Screw Calculator is a professional engineering tool designed to determine the Design Capacity ($\phi N_j$) of fastener connections in accordance with AS 1720.1:2010. This tool streamlines the design of Coach Screws and Type 17 Screws in Sawn Timber, Glulam, LVL, and CLT.

Key Features:

Automated Failure Mode Analysis: Instantly calculates Characteristic Capacity (Qk) for all AS 1720.1 failure modes (yielding, embedment, and withdrawals).
Joint Group : Input wLogicood species (e.g., Radiata Pine, Spotted Gum) or density to automatically assign Joint Groups (J1 – J6).
Capacity Factor (𝜙) Control: Apply the correct Category (1, 2, or 3) factors for structural joints instantly.
Mass Timber Ready: Calculate connections in Cross-Laminated Timber (CLT) and LVL with correct density and embedment properties.
Supplier Agnostic: Design with standard Coach Screws (AS 1393) or proprietary Self-Tapping Screws from leading suppliers.

About Screw Design

Stop Calculating k-Factors by Hand

Manual design of timber connections involves juggling multiple modification factors ($k$). The SPEC Toolbox calculator automates the provisions of AS 1720.1 Section 4, ensuring you never miss a duration or moisture adjustment.

We solve for the Design Capacity ($\phi N_j$) by evaluating:

  • Embedment Failure: Crushing of the timber fibers under the screw shank.
  • Fastener Yielding: Bending of the screw (formation of plastic hinges).
  • Withdrawal Failure: Pull-out resistance from the timber grain.

Automatic Joint Grouping (J1 - J6)

Australian timber design relies on Joint Groups to classify timber density. Instead of manually looking up species tables, our calculator allows you to:

  • Select common species (e.g., Radiata Pine, Vic Ash, Jarrah).
  • Automatically map them to their Seasoned (SD) or Unseasoned (S) Joint Group (J1 for Ironbark down to J6 for softwoods).
  • The tool applies the correct Characteristic Capacity tables based on the determined J-Group.

Coach Screws & Type 17 Screws

We distinguish between the two primary screw types used in Australian construction:

  • Coach Screws (AS 1393): Heavy-duty hex-head screws. The calculator handles the specific shank geometry and thread lengths standard in Australia.
  • >Type 17 Screws: Self-drilling screws with a specialized point. We calculate the lateral and withdrawal capacities specific to the gauge (e.g., 14g, 18g) and penetration depth.

Advanced Support for Mass Timber & CLT

CLT adoption grows in Australia (per the NCC 2019/2022 updates), engineers need tools that understand engineered wood.

  • Face vs. Edge Screw: Calculate withdrawal and shear capacities for screws driven into the narrow edge of CLT panels vs. the face.
  • Gap Filling: Account for the possibility of screws intersecting gaps in CLT laminations.
  • Effective Density: User-defined density inputs allow for accurate categorization of European Spruce CLT often imported into Australia.

Automatic Modification Factors (k-factors)

We automate the full suite of modification factors to convert Characteristic Capacity ($Q_k$) to Design Capacity ($\phi N_j$):

  • Duration of Load ($k_1$): Toggle between Permanent (0.57), 5-month (0.80), 5-day (0.94), and Instantaneous (1.0) loads.
  • Grain Orientation ($k_{13}$): Adjusts for screws loaded parallel vs. perpendicular to the grain (critical for unseasoned timber).
  • End Grain ($k_{16}$): Automatically reduces capacity for screws driven into end grain.
  • Multiple Fasteners ($k_{17}$): Calculates the group reduction factor when multiple screws are aligned in a row.

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