The Three Hollow Section Profiles Under AS/NZS 1163 and How to Specify Each Correctly
AS/NZS 1163 covers three distinct cross-sectional profiles of cold-formed steel hollow sections: circular hollow sections (CHS), square hollow sections (SHS), and rectangular hollow sections (RHS). Each has its own dimensional notation, tolerance requirements, and preferred applications in structural engineering. Specifying them correctly on a drawing or purchase order requires knowing which profile you need, what dimensions define it, and how the standard’s tolerances apply to what you actually receive.
Circular hollow sections (CHS)
CHS is designated by its outside diameter and wall thickness. The notation reads as: OD × t, where OD is the outside diameter in millimetres and t is the nominal wall thickness in millimetres. A 114.3 × 6.0 CHS, for example, is a circular hollow section with an outside diameter of 114.3 mm and a 6.0 mm wall.
The outside diameter of CHS follows a standardised series derived from historical pipe sizing, which is why the values don’t follow a neat arithmetic progression — common sizes include 26.9, 33.7, 42.4, 48.3, 60.3, 76.1, 88.9, 101.6, 114.3, 139.7, and 168.3 mm, continuing up through large structural sizes. When specifying CHS, the OD must match one of the standard sizes in AS/NZS 1163 — there is no such thing as a 100 mm OD CHS in this standard; the nearest is 101.6 mm.
CHS is preferred in applications where load direction varies and the section must perform equally in all orientations — column applications with multi-directional load, space frames, and exposed architectural structures where the circular profile is visually preferred. It is also the standard profile for truss chords in many industrial structures.
Under AS/NZS 1163, the outside diameter tolerance for CHS is ±0.5% of the OD or ±0.5 mm, whichever is greater. Wall thickness tolerance is −10% / +unlimited for thicknesses up to 10 mm, with tighter controls applying above. These tolerances affect section properties and should be accounted for when section capacity is close to the design limit.
Square hollow sections (SHS)
SHS is designated by the outside dimension of one side and the wall thickness: D × D × t, where D is the outer face-to-face dimension in millimetres. In practice, the notation is typically simplified to D × t since both outer dimensions are equal — a 100 × 100 × 6.0 SHS is often written as 100 × 6.0 SHS, though the full form is preferred on fabrication drawings to avoid any ambiguity.
SHS provides equal section properties in both principal axes, making it appropriate for columns carrying biaxial bending or for architecturally exposed elements where four identical faces are needed for connection and appearance. It is widely used for secondary structures, posts, frames, and connection details where the flat outer face simplifies bolted or welded connections compared to CHS.
Corner radii are specified in AS/NZS 1163 as a function of wall thickness. The external corner radius is typically around 2.0t, and this radius reduces the flat face width compared to the nominal outer dimension. For connection designs that rely on the flat face width — such as slotted plate connections or through-bolt connections — the effective flat width should be checked, not just the nominal outer dimension.
The dimensional tolerance for SHS outer dimensions under AS/NZS 1163 is ±1% of the nominal outer dimension or ±0.5 mm, whichever is greater. This is slightly looser than the OD tolerance for CHS, reflecting the manufacturing difference between round and square cold-formed profiles.
Rectangular hollow sections (RHS)
RHS is designated by the longer outer dimension, the shorter outer dimension, and the wall thickness: D × B × t. A 150 × 100 × 6.0 RHS has an outer depth of 150 mm, an outer width of 100 mm, and a wall thickness of 6.0 mm. By convention, D (depth) is the larger of the two outer dimensions when the section is oriented in its primary bending direction.
RHS provides different section properties about each principal axis, which is an advantage in applications where the dominant load direction is known. A beam loaded primarily in vertical bending benefits from an RHS oriented with the greater depth vertical — the larger second moment of area about the major axis provides more bending resistance per unit weight than an equivalent SHS. This makes RHS the standard choice for horizontal spanning members, purlins, girts, and beams where the loading direction is fixed.
The orientation of an RHS must be specified on drawings, because rotating the same section 90 degrees changes which axis carries the primary bending load. A drawing that notes “150 × 100 × 6.0 RHS” without indicating orientation leaves the fabricator to assume — and the assumption may not match the design intent.
Corner radius requirements for RHS are the same as for SHS. The aspect ratio of RHS under AS/NZS 1163 is limited to D/B ≤ 2.5, beyond which the section would be classified differently under the standard.
How to specify the grade alongside the profile
Profile designation alone is not sufficient — the grade must also be stated. Under AS/NZS 1163, the standard grades are C250L0, C350L0, and C450L0, where the number represents the minimum yield strength in MPa and L0 indicates a minimum Charpy impact energy requirement at 0°C. For most structural applications in Australia and New Zealand, C350L0 is the default grade. C250L0 is used for lighter-duty applications. C450L0 is specified where higher strength and weight reduction are the priority.
A complete specification for a structural hollow section should read, for example: “150 × 100 × 6.0 RHS C350L0 to AS/NZS 1163.” This gives the manufacturer and inspector everything needed to confirm the product: profile, dimensions, grade, and governing standard.
