Rectangular Steel Tube (RHS)

RHS (Rectangular Hollow Section) and SHS (Square Hollow Section) are both closed rectangular tubular sections, with SHS being the special case where width equals depth. The fundamental structural difference is that RHS has two distinct principal axes with different second moments of area (Ixx ≠ Iyy), allowing the designer to orient the stronger axis in the direction of dominant bending, while SHS provides equal bending resistance about both axes (Ixx = Iyy). Choose RHS when: (1) The member is subject to bending predominantly in one plane — orienting the RHS with the larger dimension in the bending plane provides greater section modulus and thus more efficient use of material than an SHS of equivalent weight. (2) The connection geometry benefits from different face widths — a wider face facilitates easier attachment of multiple incoming members from one direction while the narrower face accommodates connections from the perpendicular direction. (3) Architectural or dimensional constraints require a non-square profile — vehicle chassis rails, trailer longitudinals, and window frame sections often have proportional constraints that favour RHS. Choose SHS when: (1) The member is subject to equal bending in both directions — column sections in building frames, struts in space trusses, and symmetrically loaded posts benefit from SHS's equal biaxial resistance. (2) Rotational symmetry of connections is required — SHS simplifies detailing of four-way connections where equal numbers of members frame in from each direction. (3) Aesthetic uniformity is important — SHS provides a visually symmetric profile suited to architecturally exposed applications where the section is viewed from multiple directions. In practice, RHS is more economical than SHS for beam and chord applications where one bending axis dominates, while SHS is more efficient for column and strut applications under axial load or symmetric biaxial bending.

Cold-formed RHS (EN 10219) and hot-finished RHS (EN 10210) differ in manufacturing process, dimensional accuracy, corner geometry, residual stress state, and mechanical property distribution — differences that affect structural design, connection performance, and material cost. Cold-formed RHS (EN 10219) is produced by continuously cold-rolling steel strip into a circular tube, electric resistance welding the longitudinal seam, and then cold-sizing through square/rectangular forming rolls at room temperature. The cold-working process introduces residual stresses and work-hardens the steel, particularly at the corners where bending strains are concentrated — corner yield strength can be 20–30% higher than the flat face in small sections, but residual stresses and strain-hardening reduce the effective plastic rotation capacity of connections. EN 10219 specifies mechanical properties from flat face specimens only, and the standard permits larger dimensional tolerances (wall thickness tolerance ±10% for t≤5mm) and larger corner radii (up to 3t outer corner radius) than hot-finished sections. Cold-formed RHS is generally less expensive and is the standard choice for most structural applications in building frames, trusses, and secondary framing. Hot-finished RHS (EN 10210) is produced by forming a tube and hot-sizing above the steel recrystallisation temperature (>580°C), which relieves residual stresses and produces uniform, isotropic mechanical properties throughout the cross-section including the corners. Dimensional tolerances are tighter (wall thickness ±10% or 0.5mm, whichever is greater), corner radii are smaller (maximum 2.0t outer), and the section profile is more precise. Hot-finished RHS is specified for: connections utilising the corner zones in tension (T, Y, X branch connections in tubular trusses), sections subject to fatigue loading where residual stresses from cold-forming could accelerate crack initiation, high-strength grades S420MH and S460MH (only available as hot-finished per EN 10210), and precision architectural applications requiring tighter dimensional tolerances. Hot-finished RHS typically commands a 15–30% price premium over cold-formed equivalent sections.

Grade selection for structural RHS depends on design load levels, applicable design code, welding requirements, operating temperature, and project cost objectives. S235JRH (yield ≥235 MPa, per EN 10219-1 / EN 10210-1) is the minimum grade for structural applications and is suitable for lightly loaded secondary members, purlins, girts, handrails, balustrades, non-structural architectural frames, agricultural building framing, and temporary works where design stresses are low and section sizes are governed by stiffness or minimum practical dimensions rather than strength. The JRH sub-grade suffix denotes impact testing at +20°C — adequate for indoor and temperate outdoor applications. S275J0H (yield ≥275 MPa) provides a modest strength increase over S235JRH (approximately 17%) and was historically specified for medium-load structural sections in the UK and some European markets, but has largely been superseded by S355J2H in modern practice. The J0H suffix indicates impact testing at 0°C. S355J2H (yield ≥355 MPa, per EN 10219-1) is the dominant structural RHS grade worldwide, offering approximately 50% higher yield strength than S235JRH for the same cross-section, enabling significant section size and weight reductions (typically 20–35% less steel weight for equivalent structural performance). The J2H suffix specifies Charpy V-notch impact energy ≥27J at −20°C, making it suitable for cold-climate outdoor structural applications down to approximately −15°C ambient temperature. S355J2H is specified for primary structural members in building frames, industrial structures, bridges, offshore platforms, and transport equipment where structural efficiency and weight minimisation are important. For arctic or sub-arctic applications (design temperatures below −20°C), supplementary impact testing at −40°C or S355NLH / S355MLH grades with guaranteed −50°C impact toughness should be considered. For weight-critical applications (aerospace ground equipment, military vehicles, competitive automotive structures), high-strength grades S420MH and S460MH (hot-finished only, EN 10210) provide further weight savings of 15–20% versus S355J2H at a material cost premium.

Yes, Rectangular Steel Tube (RHS) can be hot-dip galvanized per ISO 1461 / ASTM A123 to provide long-term corrosion protection for outdoor structural applications including highway sign gantries, solar mounting frames, agricultural buildings, outdoor walkway structures, playground equipment, and marine-adjacent structures. However, hot-dip galvanizing of hollow sections including RHS requires specific design provisions to ensure safe processing and satisfactory coating quality. Critical design requirements for galvanizing RHS: (1) Venting and drainage holes — RHS must have vent holes drilled at both ends and at high and low points of the assembled fabrication to allow complete drainage of acid pickle solution and flux during pre-treatment, free ingress and drainage of molten zinc during galvanizing, and safe escape of steam and gases generated when the cold steel enters the 450°C zinc bath. Minimum vent hole diameter should be 12mm or 25% of the largest cross-section dimension, whichever is greater, per ISO 14713-2. Inadequate venting risks explosive steam generation during dipping, trap flux causing bare spots, or hydraulic pressure bursting the tube. (2) Steel chemistry — silicon content between 0.04–0.14% (Sandelin range) produces excessively thick, brittle zinc coatings on RHS. Specify silicon content <0.04% or 0.15–0.25% for predictable coating quality, or use reactive steel grades formulated for galvanizing compatibility. (3) Fabricated assembly preparation — weld spatter, flux residue, paint, grease, and silicone sealant must be completely removed before galvanizing as these cause bare spots. All welds should be continuous (no intermittent welds) to prevent acid trap and post-galvanizing crevice corrosion. (4) Dimensional distortion — thermal stress from the galvanizing process can cause distortion of slender RHS assemblies. Minimum wall thickness of 4mm is recommended for fabricated RHS assemblies to minimise galvanizing distortion. (5) Internal coating — zinc penetrates only a short distance into RHS bore from vent holes; the internal bore of galvanized RHS is typically bare carbon steel and may corrode if moisture is able to enter through inadequately sealed end caps or vent holes after installation.

ASTM A1085 is a relatively new American standard (first published 2013) for cold-formed welded carbon steel hollow structural sections (HSS) that was developed specifically to address several structural engineering concerns with the long-established ASTM A500 standard. Key differences between A1085 and A500: (1) Minimum yield strength — A1085 specifies a single minimum yield strength of 50 ksi (345 MPa) for all HSS shapes and wall thicknesses, eliminating the A500 Grade A (39 ksi / 269 MPa) and Grade B (46 ksi / 317 MPa) grades that created confusion in specifications. This ensures a consistent design basis without the need to differentiate grades. (2) Maximum yield strength limit — A1085 imposes a maximum yield strength of 70 ksi (483 MPa), limiting the yield-to-tensile ratio. This provision, absent in A500, improves connection ductility and prevents connection failures before member yielding in seismic applications — particularly important for special moment frames (SMF) and special concentrically braced frames (SCBF) in seismic design per AISC 341. (3) Wall thickness tolerance — A1085 tightens wall thickness tolerance to ±10% (same as A500) but adds a maximum absolute tolerance of 0.84t for each individual measurement, reducing the variability that affected section property calculations under A500. (4) Charpy V-notch impact testing — A1085 mandates CVN impact testing at −20°F (−29°C) with minimum 25 ft·lbf (34 J) absorbed energy, providing guaranteed low-temperature toughness absent from standard A500. (5) Mass tolerance — A1085 tightens mass (weight) tolerance to ±3.5% versus A500's ±10% for small sections, improving section property consistency. A1085 is now the preferred specification for seismic structural applications and is increasingly specified by structural engineers for all primary structural HSS applications in the United States, commanding a modest price premium (5–10%) over A500 Grade B/C. For international projects, EN 10219 S355J2H is broadly equivalent in yield strength and impact toughness requirements.