The practical answer
AS 3990 is commonly investigated for steelwork forming part of mechanical equipment, while AS 4100 is the principal limit-states Standard for steel structures. Real projects can sit close to the boundary or include parts that naturally align with different frameworks.
The question is not which Standard is 'newer' or 'better' in the abstract. It is which design basis matches the equipment function, the loading regime, the fabrication and verification framework, and the obligations imposed by the project or owner specification.
If you cannot describe the scope clearly enough to choose the basis, that is the signal to step back and define the engineering problem more precisely before proceeding.
When this question usually matters
You are designing or modifying skids, frames, supports or machine structures made from structural steel sections and plate.
An owner or reviewer asks why one Standard was selected over another.
The steelwork supports machinery and also behaves like a conventional structure.
The job involves repeated loading, fatigue or serviceability limits driven by equipment performance.
Red flags that make the question more significant
The design basis was selected because 'we always use that one'.
The project specification nominates one Standard but the engineering team is checking against another without a documented reason.
The steelwork is part of a machine, lifting device or crane support but the loads are being treated like simple dead load plus static live load.
Fatigue, vibration or alignment requirements are important but absent from the design note.
How to choose the starting Standard
The decision is rarely solved by one keyword. Work through the steelwork's function and the source of the real actions.
Define what the steelwork is part of
Is it a machine, a conveyor frame, a skid, a pressure-equipment support, a building structure, a pipe-support frame or a hybrid assembly?
Identify where the loads come from
Separate general structural actions from machine-driven actions such as torque, braking, impact, vibration, maintenance loads or crane-induced actions.
Check nominated requirements
Review contracts, owner specifications, equipment Standards and previous basis documents before selecting the steel design framework.
Consider serviceability and fatigue early
If alignment, vibration, repeated cycles or dynamic response matter, those issues should influence the Standard and methodology choice, not appear as late surprises.
Document the basis before design proceeds
State why the selected basis suits the scope, and note any interfaces where another Standard or framework also needs to be considered.
Detailed practical examples
These examples show how the question changes a real engineering decision. They are not universal answers; they show what needs to be clarified before the right answer becomes obvious.
Conveyor transfer station with integrated equipment frame
A transfer station frame supports drive equipment, pulley arrangements and attached maintenance access steelwork. The frame is made from conventional steel sections and plates, and the team initially assumes AS 4100 must apply because the members look 'structural'.
The physical form is not enough. The frame function and load sources are closely tied to the mechanical equipment, making AS 3990 a natural investigation point even though AS 4100 remains relevant as a comparison and interface reference.
What made the job difficult
- Belt forces, torque reactions and maintenance access all affect the frame.
- Fatigue is credible because the equipment cycles continuously.
- The platform steelwork interfaces with AS 1657 requirements.
How the review should proceed
- Define which parts of the structure are integral to the equipment and which parts are broader supporting works.
- Build the load schedule from mechanical actions first, then assess which steelwork framework best suits the design basis.
- Check whether the project specification imposes a preferred methodology or interface requirement.
- Document any mixed-basis areas clearly so the verification path remains coherent.
A defensible answer may still involve both Standards conceptually, but the engineering package should explain why the equipment frame is being treated primarily through the AS 3990 pathway and how the interfaces are handled.
Pipe-support portal frame beside a process plant
A new steel portal frame supports process piping over a roadway. The piping team supplies reactions and thermal loads, but the frame itself is a conventional load-carrying structure with no integral machine function.
This is a stronger AS 4100 starting point because the steelwork behaves primarily as structural support rather than as part of the machine itself.
What made the job difficult
- The loads are process-driven but the structure's role is structural support.
- Serviceability still matters because pipe movement and support deflection interact.
- Future additions are likely, so the basis needs to be documented clearly.
How the review should proceed
- Confirm the frame role and the interface loads coming from the piping discipline.
- Apply a coherent structural design framework for capacity, stability and serviceability.
- Record any assumptions or future load allowances explicitly.
- Check whether local equipment-specific details need supplementary criteria beyond the primary structural basis.
The practical answer is usually AS 4100 as the principal structural design basis, supported by properly defined piping loads and any relevant interface requirements rather than a simple structural-only assumption.
Information to gather before deciding
A clear description of what the steelwork does
Load schedule showing general and equipment-specific actions
Project or owner specification and any OEM requirements
Serviceability limits such as deflection, vibration or alignment criteria
Duty cycle and fatigue relevance
Fabrication and verification basis expected for the project