AS 4100 vs AS/NZS 5131 — Which One Does a Melbourne Builder Actually Need to Understand?
Open almost any structural steel shop drawing on a Melbourne project and somewhere on the title block you will see two standards cited side by side: AS 4100 and AS/NZS 5131. They sit there as if everyone already knows what each one does. Most builders do not — and that is not a criticism. Until 2016, the second of those barely existed as a separate document, and the way responsibilities now divide between them has not been explained particularly well to the people who actually run building sites.
This article is not a structural engineering tutorial. It does not pretend to replace the work that an RPEng does when calculating a beam, or the judgment a building surveyor brings to a sign-off. It is written for the people standing on site reading a shop drawing — the custom home builder, the townhouse developer, the site manager — trying to work out what these two standards mean for their week. The short version is that one is a design standard and the other is a fabrication and erection standard. Once you can hold that distinction in your head, a surprising amount of the rest of the documentation falls into place.
The Construction Category framework inside AS/NZS 5131 — CC1 through CC4 — quietly determines how thick the documentation pack is going to be, which welder qualifications the shop needs, and how much inspection work has to happen before steel can be signed off. Builders who learn to read those categories early generally save themselves a fortnight of friction later.
1. The 30-Second Version Most Builders Never Get Told
AS 4100 is the steel structures design standard. Its job is to tell the structural engineer how big the beam needs to be, what the connection capacities should be, and how the structure should behave under load. The output of AS 4100 lives in the engineer’s calculation pad and on the structural drawings.
AS/NZS 5131 is the structural steelwork fabrication and erection standard. Its job is to tell the fabricator how to make that beam — what welding qualifications apply, what tolerance bands are acceptable, what inspection regime governs the welds — and to tell the erector how to put it together on site. Its output lives in shop drawings, Inspection and Test Plans, welder qualification records, mill certificates, and the erection methodology.
Both standards live on the same project because a project has to be designed correctly and then made and assembled correctly. A builder does not need to compute a member capacity to AS 4100 — that is the engineer’s job. But a builder absolutely does need to read the AS/NZS 5131 references on a shop drawing, because those references tell you what your fabricator has to deliver and what paperwork has to be on file before the building surveyor signs anything off.
2. A Bit of History — Why They Used to Live in One Book
Before 2016, AS 4100 carried fabrication and erection content inside itself. Designers, fabricators and inspectors all worked from the same document, and the distinction between a design rule and a fabrication rule was blurred. AS/NZS 5131 was then published as a standalone fabrication and erection standard, with AS 4100 amended to remove most of its overlapping content and refer to 5131. Subsequent amendments have put more shape around shop drawing requirements, Inspection and Test Plans, and welder qualification records.
For a builder, today’s structural steel documentation pack is noticeably thicker than it was a decade ago, and the responsibilities for delivering each part of that pack are more clearly assigned. When the title block cites both AS 4100 and AS/NZS 5131, it is telling you that the design has been done to one standard and the make-and-assemble has been done to the other. A.C. Steel Construction’s in-house CAD team carries a fair amount of this load on behalf of builders — the shop drawing set, welder qualifications, Inspection and Test Plan, mill certificates and welding procedure references arrive in a package organised against AS/NZS 5131 expectations rather than left for the builder to assemble from fragments.
3. CC1 to CC4 — The Number That Decides Your Document Stack
The single most important number on a structural steel shop drawing — from a builder’s point of view — is the Construction Category. AS/NZS 5131 defines four. CC1 covers the lowest-risk work: simple, small, low-consequence-of-failure structures. CC2 is the workhorse category for most residential and smaller commercial projects. CC3 covers higher-risk work — multi-storey residential, commercial buildings, more public-facing structures. CC4 is reserved for the highest-risk work: large public buildings, bridges, specialised industrial structures.
The category is assigned by the structural engineer based on the project’s risk profile. Once that number is set, it determines the welder qualifications required, the level of NDT, the documentation depth, and the tolerance bands. A CC3 job is not just a CC2 job with more paperwork — it is a different operating regime for the fabricator, with implications for material selection, weld quality and inspection effort.
For builders, the question to ask early — at contract stage if possible — is simple. What Construction Category has the engineer assigned to this project? If the answer is not clearly written on the structural drawings or in the specification, that is a conversation to have with the engineer before anything is fabricated. As a rough indication of where projects tend to land, a two-storey custom home with significant cantilevers or long spans often sits at CC2, and townhouse rows or low-rise residential blocks can be assigned anywhere from CC2 to CC3 depending on the engineer’s read of risk. The final call is the engineer’s.
4. What a Compliant Shop Drawing Set Should Contain
A shop drawing set delivered against AS/NZS 5131 expectations is not just a few pages of GA. It is a structured pack, and a builder who knows what to look for can tell within a few minutes whether the fabricator has been thorough.
- A general arrangement drawing showing column grid, beam layout and level references
- Section marks identifying each member (UB, UC, PFC, SHS, RHS and so on) with size and grade
- Connection details for every typical and non-typical connection, with bolt and weld specifications
- Welding symbols referenced to AS 1101.3 and welding procedures referenced to AS/NZS 1554.1
- An Inspection and Test Plan setting out hold points, witness points and acceptance criteria
- Material specifications referencing AS/NZS 3679 or AS/NZS 1163 as relevant
- Welder qualification records covering the welders who will perform the work, referenced against AS/NZS 1554.1 or AS/NZS 2980
- Mill certificates traceable to heat numbers and material batches
When that pack arrives intact, the builder has effectively received a fabricator’s compliance dossier alongside the steel. The ITP in particular is worth reading — it tells you when the engineer or surveyor will be invited to inspect, what the inspector is checking for, and what happens if a hold point fails. A builder who treats the ITP as an active programme document, rather than paperwork to file, will find their inspection chain runs much more smoothly.
5. Who Signs What — The Responsibility Map
One of the quieter sources of programme drift is confusion about who signs off on what. The structural engineer — typically an RPEng — owns the design sign-off to AS 4100, the Construction Category determination, and the final structural inspection after erection. The fabricator owns shop drawing development, ITP execution, welder qualifications, mill certificates and dimensional accuracy. The builder owns site preparation, bolt-up coordination, erection programme, site safety, and inspection coordination. The building surveyor owns the broader building compliance sign-off that eventually leads to occupancy.
It is common for builders to assume that once the fabricator has signed off on the shop work, the structural side is closed. It is not. The RPEng’s structural sign-off after erection is a separate step, and on CC2 and CC3 jobs that step is non-negotiable. A.C. Steel Construction will deliver a compliant fabrication and a comprehensive documentation pack and coordinate the inspection chain, but the structural assessment and final sign-off belong to the engineer. The same boundary applies to Construction Category itself — the fabricator can explain what CC2 versus CC3 implies, but the assignment is the engineer’s call.
6. Where Builders Get Burned by Not Asking About Standards
A few recurring patterns catch builders out when the standards conversation happens too late. Contracts that do not specify a Construction Category: the job is priced as if CC2 applies, work begins, and somewhere in the middle of the programme it becomes clear that the engineer expected CC3. The fabricator then needs to upgrade welder qualifications, expand the NDT sampling, and rework documentation — cost and time both get hit. Shop drawings without an ITP: the fabricator completes the work, the engineer arrives for final inspection, and there is no record of intermediate NDT having been done. Welder qualification gaps: a welder whose ticket does not cover the procedure performed becomes a documentation problem at sign-off, even if the weld itself is sound. Missing mill certificates: without traceable material certification, occupancy and resale conversations get awkward.
All of these are avoidable by spending an hour at contract stage writing the Construction Category, the ITP submission schedule, and the mill certificate format into the scope. A fabricator set up against AS/NZS 5131 will not push back on this. The fabricators who push back are the ones to be cautious of.
7. Why the Standards Conversation Is Becoming a Builder Conversation
There is a shift in who actually reads the AS/NZS 5131 documentation on a residential project. A decade ago, this was almost entirely the fabricator’s domain. Today, with multi-unit residential programmes carrying longer responsibility chains, the builder is increasingly the on-site coordinator of compliance. That role works better when the builder can hold a credible site meeting conversation about CC2 versus CC3, ITP hold points, welder ticket coverage, and how a missing mill certificate gets resolved. A.C. Steel Construction typically provides a project-specific summary at kickoff that translates the standards references on the shop drawings into the language a builder uses on site.
Victoria’s residential and infrastructure pipeline is not slowing down, and the standards framework around structural steel is only becoming more detailed over time. Standards are not for engineers alone — they are for everyone in the build chain who has to live with the result. When you next see AS 4100 and AS/NZS 5131 sitting together on a shop drawing title block, you can read them with a clear head: AS 4100 is the design standard, signed off by the engineer, and AS/NZS 5131 is the fabrication and erection standard, organised around a Construction Category the engineer has assigned. A.C. Steel Construction’s CAD-to-installation delivery is built to land all of that in your hands as a single, traceable package across residential, commercial and industrial work in Victoria.
This article references standards (AS 4100, AS/NZS 5131, AS/NZS 1554.1, AS/NZS 1163, AS/NZS 3679) that are periodically amended. For current versions and authoritative interpretation, consult Standards Australia, Engineers Australia, the ABCB, and the structural engineer assigned to your project. Construction Category determination and final structural sign-off remain the engineer’s responsibility.