Timber Stud Wall Design for AS1720

Design timber wall studs to AS 1720.1. Go beyond the limits of AS 1684 and engineer tall walls, jamb studs, and high-wind framing using MGP10, MGP12, and choosing your grades!

When the Tables Aren't Enough

For standard houses, AS 1684 (Residential Timber Framed Construction) is the bible. But modern architecture—with high ceilings, large window openings, and extreme wind categories—often pushes framing beyond the scope of the standard tables.

The SPEC Toolbox Stud Wall Calculator performs a first-principles engineering analysis to AS 1720.1. It allows you to verify studs for specific heights (e.g., 3.6m), localized wind pressures, and heavy point loads that simplified span tables simply cannot handle.

What This Calculator Does

This tool analyzes a vertical timber stud as a structural Beam-Column under combined Gravity (Roof/Floor) and Lateral (Wind) loads. It verifies:

Axial Capacity (Nd): Checks compression capacity, calculating the column stability factor (k12) based on the effective buckling length.
Bending Capacity (Md): Checks resistance to out-of-plane wind pressure.
Combined Actions: The critical engineering check. It verifies the interaction between Axial Compression and Bending Moment, ensuring the stud doesn’t buckle under the combined stress.
Bearing (Np): Checks compression perpendicular to grain on the wall plates (top and bottom plates).
Wind Deflection: Calculates the lateral deflection limits (e.g., Height/180) to protect glazing and linings.
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Timber Stud Wall Design

Technical Guide: Designing to AS 1720.1

1. Buckling Stability (k12)

In AS 1720.1, the capacity of a stud is defined by its “Slenderness.”

The calculator determines the Slenderness Coefficients (S3, S4) for:

  • Major Axis: Buckling out of the wall plane (controlled by stud depth).
  • Minor Axis: Buckling in the plane of the wall (controlled by stud width).
  • Restraint Logic: The tool allows you to specify “Continuous Restraint” (e.g., Plasterboard/Cladding) on the minor axis, which usually forces buckling to govern on the major axis, significantly increasing capacity.

2. Combined Actions Equation

When a stud supports a roof truss and resists a wind gust, it is stressed in two ways.

We apply the AS 1720.1 interaction formula (Clause 3.3.3):

$$\left(\frac{N^*}{N_d}\right) + \left(\frac{M^*}{M_d}\right)^2 \le 1.0$$

Note: For slender studs ($S > 20$), the bending term is squared, meaning even small wind loads can drastically reduce the axial load the stud can carry.

3. MGP vs. F-Grades

The Australian market is dominated by Machine Graded Pine (MGP).

  • MGP10: The standard for wall framing. Low stiffness ($E=10,000$).
  • MGP12: High strength, often required for jamb studs or tall walls.
  • F17/F27: Seasoned Hardwoods used for high-load architectural framing.

The calculator includes the latest characteristic values for all these grades.

Key Formulas & Parameters

Design Capacity (Nd):

$$N_d = \phi \cdot k_1 \cdot k_4 \cdot k_6 \cdot k_{12} \cdot f’_c \cdot A_c$$

  • k1 (Duration): Checks Short Term (Wind + Dead) vs. Long Term (Dead Only). A stud might pass for wind but fail under long-term creep if the dead load is high.
  • k12 (Stability): The tool calculates effective length (Lay) based on nogging spacing (typically 1350mm) to determine the reduction factor.

Bearing Check (k7):

$$N_{p,d} = \phi \cdot k_1 \cdot k_7 \cdot f’_p \cdot A_p$$

The tool applies the k7 bearing factor. For a standard 35mm or 45mm stud on a continuous plate, this factor enhances bearing capacity, accounting for the load spreading through the plate fibers.

Tutorials

Vertical Bracing Member Design

Learn how to use the CLT Toolbox Member Calculator to design a vertical bracing diagonal. We’ll walk you through importing analysis results from external tools, identifying maximum tension and compression forces, and selecting the right inputs—grades, sections, code specifics, and forces. Plus, we’ll break down the results, covering material properties and all key design checks. CLT Toolbox is here to be your partner in designing timber projects!

Vertical Bracing Member Design

Frequently Asked Questions

Is this compliant with NCC?

Yes. While AS 1684 is a “Deemed-to-Satisfy” manual, a design to AS 1720.1 (which this tool produces) is a primary reference standard in the NCC and is fully accepted for certification.