The Difference Between a Steel Tube and a Structural Hollow Section — and Why AS 1163 Defines That Line
Two pieces of steel tube can look identical and be fundamentally different products. Same shape, same dimensions, similar surface finish — but one is a structural hollow section manufactured to a specific standard, and one is a general-purpose tube with no structural qualification. In a warehouse or on a job site, you can’t tell them apart by looking. On paper, the difference is a mill certificate and a standards mark. In a structure, the difference can be the one between a compliant installation and one that shouldn’t have been built the way it was.
AS/NZS 1163 is the Australian and New Zealand standard that defines where the line between these two categories sits for cold-formed steel hollow sections. Understanding what the standard actually requires — and what it doesn’t require of general-purpose tube — explains why specification matters in ways that aren’t obvious from the outside.
What AS/NZS 1163 covers
The standard applies to cold-formed steel hollow sections: circular hollow sections (CHS), rectangular hollow sections (RHS), and square hollow sections (SHS). “Cold-formed” refers to the manufacturing process — the tube is formed from flat steel strip at or near room temperature rather than at elevated temperature. The cold-forming process introduces residual stresses and work hardening that affect the mechanical properties of the finished section.
The standard specifies three strength grades: C250, C350, and C450. The number in each grade designation indicates the minimum yield strength in megapascals — 250 MPa, 350 MPa, and 450 MPa respectively. Each grade also has a minimum tensile strength requirement: 320 MPa for C250, 430 MPa for C350, and 500 MPa for C450.
In addition to mechanical properties, the standard specifies dimensional tolerances for wall thickness, outside dimensions, length, squareness, and straightness. It also specifies requirements for chemical composition, impact testing at specific temperatures for certain grades, and testing frequencies and methods.
What general-purpose tube doesn’t guarantee
A steel tube manufactured without reference to AS/NZS 1163 may meet the dimensional requirements of the standard by accident — the size might be right, the wall thickness might be within tolerance. What it doesn’t have is verified mechanical properties, documented testing, or traceable certification.
This matters for structural use specifically because structural calculations in Australia and New Zealand are written to assume compliance with referenced standards. When a structural engineer designs a connection using 100×100×4 SHS C350, the capacity calculations assume the section meets the yield strength, tensile strength, elongation, and dimensional tolerances specified in AS/NZS 1163 for C350 grade. If the actual section doesn’t meet those properties — because it was sourced as general-purpose tube rather than compliant structural hollow section — the calculated capacity doesn’t apply.
The problem isn’t that the general-purpose tube is necessarily weaker. It might have adequate properties. The problem is that without tested and certified compliance, there’s no basis for assuming those properties exist. Structural design relies on documented material performance, not inferred performance.
The mill certificate distinction
For AS 1163 steel pipe compliant hollow sections, the mill certificate documents the heat of steel used, the chemical composition, the mechanical test results from the production run, and the dimensional inspection results. This certificate provides traceability from the finished product back to the tested material properties, which is what allows a structure to be certified as built to specification.
A general-purpose tube sourced from a non-certified manufacturer comes with no such documentation. In some cases, it comes with a document that resembles a mill certificate but contains only nominal specifications rather than actual test results — a description of what was ordered rather than a record of what was tested and found to comply.
The distinction is important when structures are audited, inspected, or involved in insurance claims or litigation. A certifying engineer or building surveyor looking at the documentation can verify compliant hollow section through its mill certificate. They cannot verify non-compliant tube through nominal specifications, because nominal specs don’t confirm that the material was actually tested.
Where the categories overlap and where they don’t
For non-structural applications — general fabrication, furniture manufacture, gates and fencing, handrails in non-critical applications — general-purpose tube is entirely appropriate and using AS/NZS 1163 compliant material is unnecessary cost. The standard was written for structural applications, and applying its requirements to applications that don’t need them adds expense without benefit.
The categories overlap most ambiguously in light structural applications where formal engineering certification may not be required but where structural performance still matters. A steel carport, a mezzanine floor in a workshop, a pergola with a roof load — these applications sit in a space where the formal structural certification process may not apply but where the consequences of material underperformance are real.
In these cases, the decision to use compliant hollow section comes down to risk appetite and documentation. Using AS/NZS 1163 compliant material in these applications provides the same benefit it provides in formally certified structures: documented evidence that the material meets defined properties, available if the structure is ever questioned.
Why the specification gets overlooked
In practice, AS/NZS 1163 compliant hollow section and general-purpose tube often sell alongside each other at steel distributors and building suppliers, sometimes at similar prices. The price difference for common sizes isn’t large, and buyers who don’t know to ask for compliant material may not realize there’s a distinction to make.
The point of failure is typically the purchase decision, not deliberate substitution. A fabricator sourcing tube for a structural application buys from the stock that’s available without specifying the standard, the supplier supplies what’s in stock, and compliant or non-compliant tube may arrive depending on what the supplier happened to have. Without a specification on the purchase order and documentation requested on delivery, there’s no guarantee that what arrives is what the structure requires.
The fix is straightforward: specify AS/NZS 1163 and the required grade on the purchase order, and require a mill certificate on delivery. This doesn’t add cost to the procurement process; it only adds documentation requirements that a legitimate supplier of compliant product will have no difficulty meeting.
