Load Combinations — AS 4100 Australian Standard

AS 4100 / AS/NZS 1170 strength, serviceability, and stability load combinations for Australian and New Zealand structural steel design. Educational use only.

This page documents the scope, inputs, outputs, and approach of the AS 4100 Load Combinations tool on steelcalculator.app. The interactive tool runs in your browser; this documentation ensures the page is useful even without JavaScript.

What this tool is for

Generating strength and serviceability load combinations per AS/NZS 1170.0 for use with AS 4100 steel design.
Understanding load factors for permanent action (G), imposed action (Q), wind action (W), earthquake action (E), and other actions.
Identifying the governing combination for a given set of characteristic loads.

What this tool is not for

It does not determine characteristic load values (those come from AS/NZS 1170.1, 1170.2, 1170.3, and 1170.4).
It does not produce combination-specific load paths or member forces.
It does not cover all special combinations for cranes, fatigue, or construction loads.

Key concepts this page covers

AS/NZS 1170.0 load combination framework
permanent action factor (1.2G or 0.9G)
imposed action combination factor psi_c
strength (ULS) and serviceability (SLS) limit states

Inputs and outputs

Typical inputs: characteristic permanent action G, characteristic imposed action Q, wind action W, earthquake action E, snow action S, and the importance level of the structure.

Typical outputs: all applicable ULS and SLS combinations with computed factored values, the governing combination and its factored demand, and the load case identifier.

Computation approach

The tool applies the load combination equations from AS/NZS 1170.0 Table 4.1 (strength) and Section 4.3 (serviceability). Each combination multiplies the characteristic actions by the appropriate partial factors and combination factors. The tool evaluates all applicable combinations and identifies the one producing the maximum demand for the selected effect (axial, shear, moment, etc.).

Frequently Asked Questions

How do AS/NZS 1170 load combinations differ from ASCE 7? The general framework is similar (factored combinations at ULS, unfactored at SLS), but the partial factors and combination rules differ. AS/NZS 1170.0 uses psi factors (combination factor psi_c, long-term factor psi_l, short-term factor psi_s) to reduce imposed actions when combined with other variable actions, whereas ASCE 7 uses a single set of load factors for each combination. The companion action concept is equivalent but the numerical values are calibrated to different reliability targets.

What is the companion action combination factor psi_c? When multiple variable actions are present in a load combination, the dominant action is taken at its full characteristic value while companion actions are reduced by psi_c. For most floor live loads, psi_c = 0.4; for storage, psi_c = 0.6; for wind, psi_c = 0.0 (wind is either the dominant action or absent in the combination). This reflects the low probability of two variable actions reaching their peak values simultaneously.

When do I use 0.9G instead of 1.2G? The 0.9G factor applies in combinations where permanent action is stabilising and the variable action (such as wind uplift or overturning) is destabilising. For example, the combination 0.9G + W_u checks whether wind uplift exceeds the stabilising effect of self-weight. Using 0.9G with destabilising actions is essential for checking uplift, sliding, and overturning stability.

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

The site operator provides the content "as is" and "as available" without warranties of any kind. To the maximum extent permitted by law, the operator disclaims liability for any loss or damage arising from the use of, or reliance on, this page or any linked tools.