Timber Column

How it calculates

The Timber Column (AS 1720.1:2010) calculator designs timber columns and studs to the current Australian Standard for Timber Structures using Limit State Design. It resolves axial forces, bending moments, and combined interaction demands then applies AS 1720.1 capacity equations for all relevant limit states.

Structural analysis

The calculator performs a first-order elastic analysis on the column as a beam-column element, resolving axial forces, bending moments about both principal axes, and deflections under the applied design actions. End conditions and effective length factors are specified separately for each axis to reflect the actual restraint conditions of the column. Concentrated axial loads and lateral loads can be applied along the member height. The member is assumed to be of uniform cross-section throughout its length.

Axial compression capacity

Section capacity in compression is checked per AS 1720.1:2010 Section 3.3:

utilization = N / (phi × Nd) ≤ 1.0*

Where phi = 0.85 for compression and Nd = k1 × k4 × k6 × k12 × fc' × A. The modification factors account for load duration (k1), partial seasoning (k4), temperature (k6), and column stability (k12). The characteristic compressive strength fc' is obtained from the section's assigned stress grade.

Stability factor k12

The column stability factor k12 is the central slenderness check for timber columns per AS 1720.1:2010 Clause 3.3.3. It accounts for elastic buckling and is a function of the slenderness ratio S4 for each axis:

S4 = Leff / d

Where Leff is the effective length for the axis under consideration and d is the corresponding cross-section dimension. k12 is evaluated for both the major and minor axes; the governing (lower) value is used in the axial capacity check. For short, stocky columns k12 approaches 1.0; for slender columns k12 reduces significantly, reflecting increased buckling sensitivity.

Bending capacity

Major- and minor-axis bending capacities are checked per AS 1720.1:2010 Section 3.2:

utilization = M / (phi × Md) ≤ 1.0*

Where phi = 0.85 for bending and Md = k1 × k4 × k6 × k8 × k9 × fb' × Z. The stability factor k8 is applied to the bending capacity to account for lateral buckling of the compression edge of the member. For sections with continuous lateral restraint to the compression edge, k8 = 1.0. For unrestrained members, k8 is a function of the slenderness coefficient S1 per Clause 3.2.3.

Combined bending and axial compression

The combined interaction check follows AS 1720.1:2010 Clause 3.3.4. Both major- and minor-axis bending demands are included:

(N / (phi × Nd))² + (Mx / (phi × Mdx)) + (My* / (phi × Mdy)) ≤ 1.0**

The squared axial term reflects the AS 1720.1 interaction formulation. All demands are assessed simultaneously under each load case; the governing utilization across all load cases is reported.

Modification factors

The following modification factors are evaluated and applied where relevant:

• k1 - load duration factor, reflecting the reduced strength under sustained loading relative to short-duration test conditions
• k4 - partial seasoning factor for unseasoned or partially seasoned timber, applied when moisture content may exceed the design threshold
• k6 - temperature factor, reducing capacity for members in high-temperature environments such as enclosed roof spaces
• k8 - stability factor for bending members subject to lateral buckling of the compression edge
• k12 - stability factor for compression members, derived from the column slenderness and used to reduce axial capacity for buckling

Section database

Sections are selected from the Australian timber section database, which includes standard sawn and structural sections in seasoned and unseasoned form across the full range of stress grades (F4 through F34 and MGP grades). Multi-ply configurations are supported; the calculator adjusts section properties and stability factors for the combined ply assembly. Custom dimensions with user-specified stress grades can also be entered directly.

Outputs

Results are presented as colour-coded utilization ratios for each limit state - axial compression, major-axis bending, minor-axis bending, and combined interaction - with AS 1720.1:2010 clause references. Stability factors k8 and k12, effective lengths, slenderness ratios, section properties, and governing demands are tabulated for straightforward report documentation.