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New ZealandNZS 3603:1993 (Amdt 4)

Timber Beam

Run the calc
The easy to use online Calcs.com Timber Beam Calculator enables you to do quick and powerful design and analysis for simple and continuous timber beams, with unlimited supports and loads. Choose from our library of thousands of common timber sections, or create your own and get instant results with our easy to understand traffic light checks for moment, deflection, and shear.

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Method & scope

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Calculation method

Structural model
The calculator models the timber beam as a one-dimensional element and computes bending moment, shear, and deflection diagrams under each load combination. Loads are entered by type, dead (G), live (Q), wind (W), snow (S), and governing strength (ULS) and serviceability (SLS) combinations are generated automatically per NZS 1170.0/1170.1. Three independent serviceability combinations cover short-term, long-term, and imposed-load deflection checks.
Moment capacity (NZS 3603:1993 Cl 3.2.4)
The factored moment capacity phi × M_n is computed from the characteristic bending strength k_1 × f_b × S, adjusted by:
  • k_1, load duration factor
  • k_4, partial seasoning factor (where applicable)
  • k_8, member restraint / stability factor (Cl 2.10)
The stability factor k_8 accounts for lateral buckling of the compression edge. The effective length for minor-axis buckling is computed from the unbraced segment length and end conditions (Cl 3.2.5). When k_8 is less than 1.0, the moment capacity is reduced proportionally. For curved glulam members, radial stress induced by bending is checked against the allowable radial tension or compression stress per NZS 3603:1993 Eq 8.4-5, a limit state specific to curved and pitched cambered beams. moment utilization = M* / phi × M_n ≤ 1.0
Shear capacity (NZS 3603:1993 Cl 3.2.3.1)
Factored shear capacity phi × V_n is derived from the characteristic shear strength f_s applied to the effective shear area. Duration-of-load and moisture factors are applied consistently with the bending checks. shear utilization = V* / phi × V_n ≤ 1.0
Bearing capacity
At each support, bearing demand is checked against the characteristic bearing strength applied over the contact area. Moisture-condition adjustments are applied. Supports with zero contact area (e.g., hangers) are excluded from the bearing check.
Deflection checks
Three independent deflection limits are verified for each span:
  1. Short-term deflection, governed by live and imposed loads; checked against a user-defined span/n ratio
  2. Long-term deflection, includes creep under sustained dead and live loads; checked against a separate span/n ratio
  3. Imposed-load deflection, live and variable loads only; useful for floor vibration and partition-damage limits
An optional absolute limit in mm can be applied alongside any span-ratio limit.
Service condition and moisture content
Service condition is selected as dry or wet. For outdoor or ground-contact applications where expected moisture content exceeds threshold values over a 12-month period, the calculator applies moisture-adjustment factors per NZS 3603:1993 Cl 6.3.3, reducing strength and stiffness properties accordingly.
Load linking
Support reactions are exported as linked outputs. Connected column and footing calculations placed downstream in the same project receive updated reactions automatically whenever the beam inputs change.

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How to use it

1

Open the calculator

Open it from Run calc in the About this calculator panel above. To start from a typical setup, choose one of the presets listed there.
2

Enter your inputs

Work through the input sections from top to bottom. Click any input label to see its reference explanation, clause, conditions and assumptions. See Checks, References, Conditions and Assumptions.
3

Review the results and export

Check the utilization of each governing check in the summary, then export a PDF report. See Views and Export.

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Available presets

Each preset opens the calculator with a typical setup already entered.

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Common questions

The calculator designs timber beams to NZS 3603:1993 (Amendment 4). Load combinations follow NZS 1170.0 and AS/NZS 1170.1. Wind pressure coefficients reference AS/NZS 1170.2:2011 for tributary wind load inputs.
Key inputs include the timber section (species, grade, and product type from the built-in database or a custom section), beam length, number of members in a group or laminate, service condition (dry or wet), support layout, loads by type (dead, live, wind, snow), lateral restraint for stability checks, and three independent deflection limit ratios.
The calculator checks moment capacity (NZS 3603:1993 Cl 3.2.4), shear capacity (Cl 3.2.3.1), bearing capacity at each support, and three deflection limits: governing short-term deflection, long-term deflection, and imposed-load deflection. For curved glulam members, radial stress under bending is also checked. A stability factor check (Cl 2.10) governs when the compression edge is insufficiently braced.
Yes. The calculator supports single solid timber members, laminated groups, and glulam. For glulam curved beams, it checks radial stress capacity per NZS 3603:1993 Eq 8.4-5 and verifies lamination thickness and radius of curvature limits per Cl 8.7.5. Multiple members in a group or laminate are entered via the member count input, and the calculator applies the group modification factor from Cl 2.9 automatically.
Service condition (dry or wet) is selected as a top-level input. For sites with expected moisture content exceeding threshold values, the calculator applies the appropriate duration-of-load and moisture-adjustment factors per NZS 3603:1993 Cl 6.3.3. Dry-service properties apply to covered structures; wet-service applies to exposed or ground-contact conditions.
Yes, beam reactions link directly to connected column and footing calculations in the same project. When you update a span, load, or section, all downstream calculations update automatically, no manual re-entry of reactions required.

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Next steps

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