Circular Hollow Section (CHS)

CHS (Circular Hollow Section), SHS (Square Hollow Section), and RHS (Rectangular Hollow Section) are all closed tubular steel sections providing high torsional stiffness and enclosed cross-section geometry, but they differ fundamentally in structural behaviour, connection detailing complexity, and aesthetic characteristics. CHS provides equal second moment of area about all axes through the centroid (Ixx = Iyy = I in any direction), making it the structurally optimal section for members subject to combined axial load, biaxial bending, and torsion simultaneously — particularly columns, struts, and space frame members where the dominant load direction is not predetermined. CHS also provides the minimum surface area per unit cross-sectional area of any closed section, reducing coating and fire protection material consumption by 20–40% compared to SHS or RHS of equivalent structural capacity. SHS provides equal section modulus and moment of inertia about the two principal axes (Ixx = Iyy) but lower values than CHS of equal perimeter due to the corner geometry, and delivers flat faces that greatly simplify beam-to-column connections, cladding attachment, and equipment bracket fabrication. RHS provides unequal section properties about the two axes, allowing the designer to orient the stronger axis in the plane of dominant bending for maximum efficiency in beam applications. Choose CHS when: equal multi-directional structural resistance is required; torsional loading is significant; architectural aesthetics favour the circular profile; surface area minimisation for coating economy is important; or CHS-to-CHS welded connections are used in exposed truss structures. Choose SHS for symmetric column applications requiring flat face connections and cladding attachment. Choose RHS for beam and chord applications where bending is predominantly in one plane and flat face geometry is needed for straightforward connection detailing.

Hot-finished CHS (EN 10210) and cold-formed CHS (EN 10219) differ in manufacturing process, residual stress state, dimensional accuracy, corner geometry (for square/rectangular — both appear as smooth circular for CHS), and mechanical property distribution. Cold-formed CHS (EN 10219) is produced by continuously ERW roll-forming steel strip into a circular tube and cold-sizing to the final diameter. Cold-working introduces residual stresses and non-uniform property distribution, with work-hardening particularly pronounced at the seam weld zone and former contact zones from the forming rolls. EN 10219 specifies mechanical properties from specimens cut from the tube body and permits larger dimensional tolerances (outside diameter ±1.0% for OD >220mm, wall thickness ±10%) and higher ovality (the difference between maximum and minimum outside diameter at any cross-section) than hot-finished sections. Cold-formed CHS is more economical and available in a wider range of small to medium diameters. Hot-finished CHS (EN 10210) is produced by forming a tube and hot-sizing above 580°C (or by the seamless piercing and sizing process for seamless CHS), relieving residual stresses and producing uniform, isotropic mechanical properties throughout the cross-section. Dimensional tolerances are tighter (OD ±1.0% for OD >219mm but with tighter ovality limits), and the absence of a visible longitudinal weld seam on the external surface (for seamless) or internal weld bead (for hot-finished welded) simplifies corrosion protection inspection and improves aesthetic appearance for exposed architectural applications. Hot-finished CHS is specified for: fatigue-critical offshore structures where residual stress accelerates crack initiation; precision structural connections using the full cross-section in tension; architecturally exposed applications requiring uniform surface appearance; and high-strength grades S420MH and S460MH only available in hot-finished form per EN 10210. Hot-finished commands a 20–40% price premium over equivalent cold-formed CHS.

Grade selection for structural CHS depends on design loads, applicable design code, operating temperature, welding requirements, and project economy objectives. S235JRH (yield ≥235 MPa, per EN 10210-1 / EN 10219-1) is the minimum structural grade suitable for lightly loaded secondary members, handrail posts, agricultural building structural members, light conveyor supports, and temporary works where design stresses are low and section sizes are governed by stiffness or minimum practical dimensions rather than strength. S275J0H (yield ≥275 MPa) provides a modest 17% strength increase over S235JRH and was historically used in the UK and some European markets for medium-load structural CHS, but has largely been superseded by S355J2H in modern structural design. S355J2H (yield ≥355 MPa) is the dominant structural CHS grade worldwide, providing approximately 50% higher yield strength than S235JRH for the same cross-section, enabling significant section size and weight reduction (typically 25–35% less steel weight for equivalent structural performance). The J2H suffix specifies Charpy V-notch impact energy ≥27J at −20°C, suitable for cold-climate outdoor structural applications down to approximately −15°C ambient. S355J2H is specified for primary structural members in building frames, roof trusses, space frames, bridges, offshore platforms, and wind turbine transition pieces where structural efficiency and weight minimisation are important design objectives. S420MH and S460MH (yield ≥420/460 MPa, hot-finished EN 10210 only) are TMCP thermomechanically processed high-strength grades providing further weight savings of 15–20% versus S355J2H at a material cost premium of 20–35%. These grades are specified for weight-critical applications including offshore platform structures where topside weight directly affects jacket and foundation design cost, high-strength truss chord members in long-span structures, and crane boom and jib sections where self-weight reduction improves crane capacity. For arctic and sub-arctic applications below −20°C, supplementary impact testing at −40°C or dedicated arctic structural grades per EN 10225 should be specified.

CHS-to-CHS welded connections (also called tubular connections or node connections) are designed per AISC Specification Chapter K (Connections to Hollow Structural Sections) for American practice or EN 1993-1-8 Clause 7 (Joints in hollow section lattice girders) for European practice. The principal connection types are: T-connections (single branch perpendicular to chord), Y-connections (single branch at angle to chord), K-connections with gap (two branches on one chord side with gap between branch toes — most common in truss design), K-connections with overlap (branches overlapping at toes), and X-connections (branches on opposite chord faces). Design strength equations for each connection type are provided as functions of: chord outside diameter (D) and wall thickness (T); branch outside diameter (d) and wall thickness (t); diameter ratio β = d/D; chord slenderness γ = D/(2T); angle between branch and chord θ; and gap or overlap ratio. Failure modes include: chord face plastification (local yielding of chord wall under branch load — governing for small β ratios); punching shear through chord wall; chord side wall buckling; and branch member failure adjacent to weld. Key design limitations and validity ranges per EN 1993-1-8 Table 7.1: diameter ratio 0.2 ≤ β ≤ 1.0; chord slenderness γ = D/(2T) ≤ 50; wall thickness T ≥ 2.5mm and t ≥ 2.5mm; branch-to-chord angle θ ≥ 30°; chord utilisation ratio (ratio of axial force in chord to chord yield capacity) must be within the range specified in the relevant formula. For CHS trusses, gap connections (with gap g ≥ t₁ + t₂ between branch toes) are generally preferred over overlap connections for fabrication simplicity and quality control. All CHS-to-CHS structural welds should be full circumferential fillet or partial/full penetration groove welds per AWS D1.1 or EN ISO 15614, with particular attention to weld profile at the branch saddle (point of smallest included angle) where incomplete fusion is most likely to occur in manual welding.

ASTM A500, A53, and A501 are three distinct American standards for round structural steel tubes, each with different manufacturing processes, mechanical property requirements, and appropriate structural applications. ASTM A500 Grade B round tube is cold-formed welded (ERW) carbon steel structural tubing with minimum yield strength 42,000 psi (290 MPa) and tensile 58,000 psi (400 MPa) for round sections — note that the A500 round tube yield of 42 ksi is lower than A500 Grade B square/rectangular yield of 46 ksi, a distinction that surprises many structural engineers. A500 is the standard specification for round structural HSS sections in building construction per AISC, available in outside diameters from 1.315 inch (33.4mm) to 20 inch (508mm) with standard HSS wall thicknesses. ASTM A53 Grade B is a specification for welded and seamless black and hot-dip galvanized steel pipe originally developed for pressure piping systems but widely used as structural CHS due to its availability, low cost, and AISC-listed status. A53 Grade B requires minimum yield 35,000 psi (241 MPa) and tensile 60,000 psi (414 MPa). A53 pipe is available in nominal pipe sizes (NPS) from ½ inch to 26 inch (DN 15 to DN 650), with Schedule 40, 80, and XXH wall thicknesses — thicker walls than equivalent HSS for the same OD, making A53 heavier but more economical per length for small-diameter applications. A53 is AISC-listed for structural use as round HSS and is commonly used for handrails, bracing, secondary structural members, and light columns where the pipe schedule wall is acceptable. ASTM A501 specifies hot-formed (hot-finished) welded and seamless carbon steel structural tubing with minimum yield 36,000 psi (248 MPa) and tensile 58,000 psi (400 MPa), manufactured at elevated temperature to relieve cold-forming residual stresses. A501 provides more uniform mechanical properties and larger corner radii than A500, and is preferred for applications requiring superior weldability, large bend radii, or specific section isotropy requirements, at a significant cost premium over A500. For most structural applications per AISC 360, A500 Grade B is the practical and economically preferred specification for round structural CHS.