“Can I still use AS 3990 for this conveyor frame, machine skid or equipment support — or should I be using AS 4100?”
This is usually the real question. The difficult part is often not calculating a beam or checking a weld. It is choosing a defensible design basis before the calculations begin.
The practical answer
The Standard is still listed as current. Its scope covers the design, fabrication, erection, repair and alteration of steelwork associated with mechanical equipment, and it expressly recognises two possible design pathways: limit-states design to AS 4100 or working-stress design to AS 3990.
That does not mean every machine frame should automatically be designed to AS 3990. The better question is: what is the steelwork, what equipment-specific requirements apply, what does the project specification require, and which design method is compatible with the real loads and service conditions?
For new general structural steelwork, AS 4100 is often the more natural starting point. AS 3990 remains particularly relevant where the steelwork is genuinely associated with mechanical equipment and an application Standard, legacy design basis, equipment classification or international machinery code is built around working-stress concepts.
What AS 3990 is commonly used for
AS 3990 is aimed at steelwork associated with mechanical equipment. The scope gives examples such as boilers and pressure vessels, lifts, cranes, mining equipment, petroleum piping systems and bulk handling equipment. In practice, the first question is whether the steelwork forms part of, supports, restrains or is functionally integrated with the mechanical equipment.
- Conveyor support frames and transfer structures
- Machine frames and equipment skids
- Steelwork associated with cranes and lifting machinery
- Frames supporting boilers and pressure equipment
- Mining equipment steelwork
- Bulk handling equipment and associated supports
- Steelwork associated with gas and liquid petroleum piping systems
- Every steel frame located inside an industrial plant
- Every building structure that happens to support mechanical equipment
- Every lifting device or crane component without checking the relevant application standard
- A substitute for machinery safety, access, pressure equipment or welding requirements
- A reason to ignore a client specification that nominates another design basis
Scope limits matter
AS 3990 also contains explicit limits. Among them, it excludes road and railway bridges, most steel elements thinner than 3 mm, designs using a yield value above 450 MPa, and cold-formed members other than those complying with the nominated hollow-section Standard. These limits are a reminder that “mechanical equipment” is not enough on its own; the material, member form and application still have to fit the Standard.
Is AS 3990 still valid?
Yes — it is old, but it is still listed as current.
The age of the document is one of the main reasons engineers hesitate to use it. AS 3990 dates from 1993 and its preface explains that AS 4100 is the preferred limit-states method for steelwork, while the working-stress method was retained for certain mechanical equipment applications.
“It is from 1993, therefore it cannot be used.”
Check current status, scope, application requirements, contract requirements and whether the working-stress basis is appropriate.
“It is still current, therefore I should use it for every machine frame.”
Compare the equipment function and governing requirements before selecting AS 3990 or AS 4100.
A Standard being current does not, by itself, make it mandatory for every project. Equally, a newer Standard does not automatically make every older item of plant non-compliant. Legislation, contracts, client specifications, application Standards, original design basis, modification scope and risk all matter.
AS 3990 or AS 4100: where should you start?
There is no single one-line test. A defensible selection is usually made by working through the function of the steelwork and the requirements surrounding it.
| Question | AS 3990 | AS 4100 |
|---|---|---|
| Primary design philosophy | Working stress / permissible stress | Limit states |
| Typical context | Mechanical equipment steelwork where the application supports that basis | Steel structures and structural steelwork more broadly |
| Document age | 1993; still current | 2020; pending revision, with Amendment 1 current |
| Mechanical and dynamic loads | Relies strongly on the applicable equipment / loading requirements | Uses limit-state actions and combinations, with equipment loads still needing proper definition |
| Fatigue | Included through stress-range provisions and detail categories | Dedicated fatigue design section |
| Fabrication framework | Contains fabrication and erection requirements in the older code framework | Modern framework linked to AS/NZS 5131 and construction categories |
The bigger trap is using an inconsistent design basis: one document for loads, another for capacities, an old drawing for duty, and no clear treatment of dynamics or fatigue.
Key engineering concepts to understand
Working-stress design is not just “AS 4100 with lower stresses”
AS 3990 is built around permissible stresses and working-stress design. The load basis, combinations, stress limits and checking method need to be treated as one coherent system.
The application Standard can control the real load case
AS 3990 requires the structure to resist the most adverse combination of static and dynamic forces reasonably expected from the specified loads. For equipment steelwork, the critical loads may come from operation: acceleration, braking, impact, unbalance, test loads, blocked chutes, maintenance positions or transient events.
Strength is only one part of adequacy
The Standard explicitly brings stiffness, stability and serviceability into the design basis. A frame can be “strong enough” yet still be unsuitable if deflection causes misalignment, vibration affects operation, or restraint assumptions are unrealistic.
Deflection limits should come from function, not habit
AS 3990 places responsibility for selecting deflection limits on the design engineer and points users toward application requirements. A generic span ratio may not protect shaft alignment, belt tracking, seals, pipe nozzles or machine tolerances.
Fatigue may govern even when static stresses look comfortable
For frequently fluctuating live loads, AS 3990 requires fatigue to be considered using stress range, number of cycles and constructional detail. Weld terminations, attachments, abrupt geometry changes and local repairs can be more important than the nominal member stress.
Fabrication and supervision are part of the design outcome
The Standard addresses fabrication, erection and supervision, not calculations alone. A design assumption about weld quality, fit-up, restraint or material cannot be separated from how the work is actually executed and checked.
Detailed engineering case studies
AS 3990 becomes most useful when it is applied to a real equipment problem. These examples show how the engineering decision changes once load path, fatigue, damage and modification history are considered together.
Conveyor drive upgrade on an existing transfer frame
A mine replaces a conveyor drive with a larger motor and gearbox. The original frame is decades old, the duty has changed and a new maintenance platform is also proposed.
The job sits squarely in the grey area that makes AS 3990 valuable: existing mechanical equipment steelwork, uncertain original design basis, dynamic actions and a modification that changes both loads and access.
What made the job difficult
- The existing drawings show member sizes but not the original load assumptions.
- The new drive changes mass, torque reactions, start/stop behaviour and belt tensions.
- The attached maintenance platform has different access and structural requirements from the equipment frame itself.
- Existing weld details may have accumulated fatigue damage over years of cyclic service.
How the engineering review should proceed
- Recover the original equipment function, steelwork design basis and available duty information before checking capacity.
- Create a changed-load schedule covering dead load, operating torque, acceleration, braking, maintenance and abnormal cases.
- Check serviceability and vibration effects where alignment or machine performance may govern before member strength.
- Separate the design basis for the mechanical equipment frame, access platform and any building-supporting steelwork.
A defensible modification assessment may use AS 3990 for the equipment steelwork while coordinating AS 1657 for access and AS 4100 for other structural components. The key is to document why each basis applies and keep the load methodology consistent.
Cracking around a heavy mobile plant axle mounting plate
Repeated hairline cracking develops in thick steel plates around a front axle mounting region on heavy mobile plant. Similar cracking has appeared on more than one machine over time, and previous repairs have largely consisted of local weld restoration.
The visible crack is in mechanical equipment steelwork, but the real engineering problem is repeated dynamic loading, local stiffness, stress concentration and fatigue—not simply whether the plate can carry a static axle reaction.
What made the job difficult
- Cracking recurs near a highly restrained welded load-transfer region.
- The machine operates over uneven surfaces and experiences variable dynamic loading.
- Previous weld repairs may restore section but also change local residual stress and geometry.
- The original detailed fatigue basis may not be available.
How the engineering review should proceed
- Map crack location and direction, then compare it with the actual force path from axle into frame.
- Use NDT to define the crack extent before removing evidence by grinding or welding.
- Assess local stiffness transitions, weld terminations and repeated stress range rather than relying on nominal plate stress alone.
- Separate short-term repair needed to restore integrity from long-term design changes intended to reduce recurrence.
A strong engineering response is usually a staged plan: quarantine or operating controls where necessary, NDT and controlled repair, then a permanent modification based on the fatigue mechanism. Simply filling the crack may return the machine to service without removing the cause.
Hydraulic cylinder mounting plate torn from a machine boom
A heavy hydraulic cylinder connection suffers a major tear at the mounting plate on mobile plant. The immediate temptation is to weld the plate back into position and reinforce it with gussets.
This is a mechanical equipment steelwork failure where the repair needs to restore a credible load path under hydraulic, eccentric and dynamic actions. The failure itself is evidence that the previous local force transfer was inadequate for the actual service.
What made the job difficult
- Hydraulic cylinder force can be high and may not act concentrically through the plate and surrounding structure.
- The tear can extend beyond the visibly separated plate into heat-affected or parent material.
- Oversized or abnormal operating loads may have contributed to the event.
- A heavily reinforced repair can move the next failure into adjacent weaker structure.
How the engineering review should proceed
- Quarantine the equipment and characterise the full damage using suitable inspection methods.
- Reconstruct the cylinder force envelope and the actual three-dimensional load path into the boom or frame.
- Check whether the original plate, welds and surrounding parent structure had adequate strength, stiffness and fatigue resistance.
- Design the permanent repair or replacement so local reinforcement is integrated into the wider structure rather than simply making one plate thicker.
The preferred long-term solution may be OEM replacement or a fully engineered replacement detail supported by drawings, weld procedures, NDT and verification. Temporary restoration and permanent redesign should be treated as different engineering decisions.
Common mistakes
“Skid”, “platform”, “frame” and “structure” are labels. The function and governing requirements matter more.
AS 4100 is newer, but AS 3990 is still current. The correct starting point depends on the application and project requirements.
Load combinations, analysis and resistance checks need a consistent basis.
Mechanical equipment can introduce impact, acceleration, braking, unbalance, torque, test and maintenance actions.
Repeated stress range and detail geometry can govern long before a static yielding check becomes critical.
Machine alignment and equipment function can require tighter or simply different serviceability criteria.
A weld can remove the visible symptom while leaving the load path, stiffness discontinuity or fatigue driver unchanged.
Material identification, welding, fit-up, inspection and supervision are part of achieving the design assumptions.
What information should you gather?
Before deciding which Standard to use—or whether the existing plant is adequate—collect enough information to understand the equipment and the real load path.
What does the steelwork actually do?
Is it part of the machine, or part of the building / supporting structure?
Which equipment-specific or application Standard applies?
What design Standard was used originally?
Does the client, owner or project specification nominate AS 3990, AS 4100 or another basis?
What are the static, dynamic, impact, braking, acceleration and operating loads?
What load cycles and duty history are expected?
Could fatigue govern the design or remaining life?
Are vibration, deflection or alignment limits critical to machine operation?
Are there existing cracks, distortion, corrosion or previous repairs?
Are material grades and weld details known?
Is the work a new design, repair, alteration or modification of existing plant?
What should happen next?
Source basis and limitations
This guide was prepared from AS 3990—1993, Mechanical equipment — Steelwork, and cross-checked against AS 4100:2020 including Amendment No. 1 for the comparison between working-stress and limit-states pathways.
AS Applied does not reproduce the Standard. Dimensions, equations, permissible stresses, detail classifications, acceptance criteria and project-specific design requirements should be checked in the official documents and any other applicable Standards.
Status basis: official catalogue status checked July 2026. Always confirm the current edition, amendments and project requirements before use.
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