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Steelwork

AS 3990 vs AS 4100

A practical comparison for machine frames, skids, conveyor steelwork and equipment supports.

Pathway at a glance

AS 3990 or AS 4100?

The first split is functional: mechanical equipment steelwork, general steel structure, or a mixed/project-nominated case.

AS 3990 or AS 4100 — which design pathway Ask what the steelwork is part of: mechanical equipment points to AS 3990 working stress; a building or structure points to AS 4100 limit states; anything at the interface needs the boundary defined and documented. Cyclic loading brings fatigue provisions to the front either way. What is the steelwork part of? equipment both structure AS 3990 mechanical equipment working stress method Interface / hybrid define the boundary document who governs AS 4100 buildings, structures limit states method Cyclic or dynamic loading? Fatigue provisions lead the design conversation either way
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How to use this summary

  • If the steelwork is functionally part of mechanical equipment, AS 3990 is a natural starting point.
  • If it is mainly a steel structure, AS 4100 is usually the more natural starting point.
  • If the project specification or application standard nominates the basis, follow that requirement and document the interfaces.
  • For existing plant, recover the original basis before mixing methods or assumptions.

The tree helps you pick a starting point. It does not justify mixing design philosophies without a written basis.

01

Short practical answer

AS 3990 is not simply the “old version” of AS 4100.

AS 3990 is a working-stress Standard written for steelwork associated with mechanical equipment. AS 4100 is a limit-states Standard for steel structures. AS 3990 itself recognises that steelwork within its scope may be designed using either AS 3990 or AS 4100.

02

The key difference

AS 3990AS 4100
Design philosophyWorking stressLimit states
Natural starting contextMechanical equipment steelworkSteel structures
Status in July 2026CurrentPending revision; Amendment 1 current
Age19932020
Modern fabrication frameworkOlder embedded fabrication frameworkLinked to AS/NZS 5131 and construction categories
03

Screening questions

  1. 1
    Is the steelwork actually part of mechanical equipment?

    If it is mainly a building or general structure, AS 4100 is usually the more natural starting point.

  2. 2
    Does an application Standard nominate or assume a design method?

    Crane, bulk handling, pressure equipment and other machinery requirements can shape the correct basis.

  3. 3
    What does the project specification require?

    A client, owner or principal may nominate AS 4100 even for equipment-supporting steelwork.

  4. 4
    Are you assessing existing plant?

    The original design basis, modification history and compatibility with the proposed assessment method matter.

  5. 5
    Can you keep the full methodology consistent?

    Loads, combinations, analysis, capacities, serviceability and fatigue should not be assembled from incompatible design philosophies.

04

What to avoid

“AS 3990 is old, so it is invalid.”

It is still listed as current.

“AS 3990 is current, so it is always acceptable.”

Scope and project requirements still have to be checked.

“The frame supports a machine, so it must be AS 3990.”

Equipment function is relevant, but not the only question.

“I can use AS 1170 loads and AS 3990 stresses without further thought.”

The design basis needs a consistent treatment of actions and permissible stresses.

05

What should you do next?

Start with the detailed AS 3990 page, then record the reason for your selected design basis in the design criteria or assessment report.

Read the full AS 3990 guide