Timber Member (Design Only)

How it calculates

Member inputs and section properties

The calculator accepts either a standard section from the AS 1720.1 timber section library (species, grade, and nominal dimensions) or a custom rectangular section defined by breadth b and depth d. For unseasoned timber, AS 1720.1 requires production tolerance deductions - the calculator automatically reduces nominal dimensions by 3 mm per face when the loaded moisture content (LMC) is ≥ 25%.

Structural category (Category 1, 2, or 3) determines the capacity factor φ used throughout: φ = 0.85 for Category 1, φ = 0.70 for Category 2, and φ = 0.45 for Category 3 structural members.

Modification factors

AS 1720.1 capacity is expressed as a product of the reference strength, section modulus or area, and a set of modification factors:

• k1 (load duration factor) - applied to reference strength based on the shortest-duration load type present, consistent with AS 1720.1 Table 2.1.
• k4 and k6 (moisture content factors) - k4 adjusts for in-service moisture conditions relative to the standard test moisture content; k6 applies a further reduction when the member may be wetted in service (LMC ≥ 25%).
• k9 (member system factor) - accounts for load-sharing in parallel member systems. When two or more members of similar stiffness share load at spacings of 450 mm or less, k9 > 1.0 increases the design capacity.
• k12 (stability factor for compression and bending) - calculated from the slenderness coefficient S1 for bending or the slenderness ratios Lay/b and Lax/d for compression. k12 reduces capacity below that of a fully restrained section and is ≤ 1.0.

Bending capacity check

Design bending capacity about the major axis is:

φMd = φ · k1 · k4 · k6 · k9 · k12 · f'b · Z

where f'b is the characteristic bending strength and Z is the elastic section modulus. The utilisation ratio M* / φMd must be ≤ 1.0.

The slenderness coefficient S1 used to derive k12 depends on the effective length between lateral restraints Lay, the member depth, and the end-moment condition at the restraint. Discrete restraints at the compression edge or continuous restraints significantly improve k12 relative to an unrestrained member.

Axial tension and compression checks

Tension capacity uses the net cross-sectional area At (reduced at connections if applicable):

φNtd = φ · k1 · k4 · k6 · f't · At

Compression capacity is governed by buckling about the minor and major axes. The calculator evaluates stability factors for both axes from the effective lengths Lax and Lay, and uses the lesser of the two resulting compression capacities:

φNcd = φ · k1 · k4 · k6 · k12 · f'c · Ag

Combined action checks

For beam-column members carrying both bending and axial force, AS 1720.1 requires the following interaction checks.

Combined bending plus compression (Clause 3.3.2):

(N*c / φNcd) + (M* / φMd) ≤ 1.0

Combined bending plus tension (Clause 3.2.2):

(N*t / φNtd) + (M* / (φ · k1 · k4 · k6 · k9 · f'b · Z)) ≤ 1.0

Both checks must pass simultaneously. For members with biaxial bending, the secondary bending term is added to the interaction equation.

Shear capacity check

Design shear capacity:

φVd = φ · k1 · k4 · k6 · f's · (2/3) · b · d

Shear demand V* must satisfy V* / φVd ≤ 1.0.

Load linking

Member forces can be entered manually or linked from a structural analysis calculation within the same Calcs.com project. When linked, N*, M*, and V* update automatically whenever the upstream analysis is modified. Member reactions can link downstream to timber connection calculators (Timber Nail, Timber Screw, Timber Bolt) to keep the full load path live within the project.