Timber Beam

How it calculates

Structural model

The calculator builds a finite element model of the beam with user-defined support positions and fixity conditions (pinned or fixed). Supports can be placed at any point along the beam length, making it possible to model simple spans, continuous beams over multiple interior supports, propped cantilevers, and beams with overhangs in a single calculation.

For inclined members - rafters on a simple slope and hip or valley beams on a corner - the geometry is resolved automatically from the entered roof pitch or hip angle and tributary width, so load tributaries and gravity components are calculated consistently with the incline.

Load combinations

The calculator assembles all specified distributed, concentrated, and wind loads into the AS/NZS 1170.0 strength (ultimate) and serviceability load combinations. Permanent action (G), imposed action (Q), and wind action (W_s, W_u, both up and down) can be defined over any portion of any span. Alternate imposed loading - a reduced imposed load applied to half the span - is included in the default presets to capture pattern loading effects in continuous spans.

Capacity checks

For every load combination the calculator evaluates the following limit states per AS 1720.1:2010:

Bending capacity. The design bending moment M* is compared against the design capacity φM_d = φ k_1 k_4 k_6 k_9 k_12 f_b' Z, where f_b' is the characteristic bending strength, Z is the section modulus, and the modification factors account for load duration (k_1), moisture content at equilibrium (k_4) and under full load (k_6), member-in-system effects (k_9), and lateral stability (k_12). The stability factor k_12 is determined from the slenderness coefficient S_1 using the lateral restraint spacing and effective length inputs for the compression edge.

Shear capacity. The design shear force V* is compared against φV_d = φ k_1 k_4 k_6 f_s' A_s, where f_s' is the characteristic shear strength, A_s is the shear area, and the same duration and moisture factors apply. Shear is checked at the critical section per clause 3.2.

Bearing capacity. At each support the design reaction is checked against the bearing capacity per clause 3.2.6. The bearing length at each support is used to compute the bearing stress on the cross-section.

Deflection. Short-term (imposed action) and long-term (permanent plus imposed action with creep factor applied to the permanent component) deflections are calculated by direct FEA integration. The calculator checks each span independently against the user-specified span-to-deflection ratios - defaults are L/250 for interior spans and L/150 for cantilevers. A hard absolute limit in millimetres is also available for sensitive finishes or brittle claddings.

Section library and moisture conditions

The section database covers thousands of standard Australian sawn, dressed, and engineered timber sections - including MGP grades, F-grades, structural LVL (Nelson Pine LVL 11, Dindas LVL 13.2 and LVL 14), and I-joists - with manufacturer section properties where available. Custom sections can be entered manually.

Moisture inputs default to equilibrium moisture content (EMC) of 15% and fully-loaded moisture content (LMC) less than 25%, consistent with most non-exposed interior applications. These can be overridden for external or wet-area members.

Outputs and load linking

For each governing load combination the results show: moment, shear, and deflection diagrams with peak values; utilisation ratios (demand/capacity ≤ 1.0) for each limit state; the governing modification factors; and traffic-light pass/fail indicators.

Support reactions are available as linked outputs. When the timber beam calculation is connected to a timber column or pad footing calculation in the same project, the reaction updates automatically whenever any input in the beam calculation changes - span, section, load, or support position. Upstream linked inputs (dead load G and live load Q) from wind load calculators or other analyses update the beam automatically in the same way.