Structural hollow sections — square (SHS), rectangular (RHS), and circular (CHS) — are some of the most efficient steel profiles available. Their closed geometry gives exceptional torsional stiffness and a clean aesthetic that open sections cannot match. This guide covers their engineering behaviour, typical applications, key design considerations, and connection challenges.
The Three Types
SHS — Square Hollow
Equal width and depth. Equal stiffness in both axes. Ideal for columns and axially loaded members. Very efficient for biaxial bending. EN 10210 / EN 10219.
RHS — Rectangular Hollow
Different width and depth. Higher bending resistance about the major axis. Used as beams, lintels, and chord members of trusses. Wide face can accept direct bolt connections.
CHS — Circular Hollow
Equal resistance in all planes. Best aerodynamic and hydrodynamic profile. Optimal for columns with no preferential buckling axis. Common in trusses, masts, and offshore structures.
Manufacturing Standards
There are two key European production standards for structural hollow sections:
- EN 10210 — Hot-finished structural hollow sections. The hot-forming process relieves residual stresses and improves corner geometry. These sections can be used up to Class 1 under EN 1993-1-1.
- EN 10219 — Cold-formed structural hollow sections. The cold-forming process introduces residual stresses and less favourable corner radii. Tighter section classification limits may apply and strength reduction at corners must be checked.
Key difference: Always confirm whether the specification requires EN 10210 (hot-finished) or EN 10219 (cold-formed). This affects section classification, fatigue resistance, and which design tables apply. For critical or fatigue-loaded applications, EN 10210 is generally required.
Why Use Hollow Sections?
Torsional Efficiency
The defining advantage of hollow sections is their closed cross-section which makes them far superior in torsion. The torsional constant It of a closed section is given by Bredt’s formula:
Where Am is the area enclosed by the median line of the cross-section walls. For a CHS this simplifies to It = πd3t/4. Compare: an IPE 200 has It = 7.02 cm4, while an SHS 150×150×5 has It = 582 cm4 — over 80 times greater — at a similar weight.
Corrosion Protection
Hollow sections have a smaller external surface area per unit length than equivalent-weight open sections. More importantly, if the tube ends are sealed, internal surfaces require no corrosion protection at all. This is a major maintenance and lifecycle cost advantage in aggressive environments.
Aesthetics
The clean, flat external faces of SHS/RHS and the smooth curve of CHS make them popular for architecturally exposed steelwork. There are no protruding flanges, no re-entrant corners to trap dirt, and the sections are easily painted or powder-coated.
Section Properties — Comparison
| Property | SHS 150×150×8 | RHS 200×100×8 | CHS 168.3×8 | IPE 200 |
|---|---|---|---|---|
| Weight G (kg/m) | 35.5 | 37.0 | 32.0 | 22.4 |
| Area A (cm²) | 45.3 | 46.7 | 40.8 | 28.5 |
| Iy (cm4) | 836 | 1523 | 1168 | 1940 |
| Wpl,y (cm³) | 133 | 187 | 166 | 221 |
| It (cm4) | 1316 | 786 | 2336 | 7.02 |
| iy (cm) | 5.79 | 5.72 | 5.35 | 8.26 |
The table illustrates why hollow sections are chosen for torsion-critical applications — and why IPE beams, while heavier for equivalent bending strength, are preferred for simple beams due to their far higher section modulus per unit weight.
Design Considerations
Section Classification (EN 1993-1-1 Table 5.2 Sheet 3)
For SHS/RHS under bending, the limiting flat width-to-thickness ratios are:
| Class | Flange in compression (c/t ≤) | Web in bending (c/t ≤) |
|---|---|---|
| Class 1 | 33ε | 72ε |
| Class 2 | 38ε | 83ε |
| Class 3 | 42ε | 124ε |
For CHS: Class 1 if d/t ≤ 50ε²; Class 2 if d/t ≤ 70ε²; Class 3 if d/t ≤ 90ε². Higher d/t ratios are Class 4 requiring EN 1993-1-6 shell buckling checks.
Lateral-Torsional Buckling
Hollow sections have a very high torsional stiffness relative to their bending stiffness. As a result, CHS and SHS sections are generally not susceptible to lateral-torsional buckling — the LTB reduction factor χLT = 1.0 under EN 1993-1-1 Clause 6.3.2.1 for sections with Iy/Iz ≤ 2.0 (CHS and SHS satisfy this trivially). RHS members bending about the major axis do require LTB checks if unrestrained, though the susceptibility is much lower than I-sections.
Local Buckling in Compression
For CHS columns under axial compression (EN 1993-1-1 §6.3.1), the cross-section classification determines the buckling curve. CHS sections use curve ‘a’ for hot-finished and ‘c’ for cold-formed. SHS/RHS columns use curve ‘b’ (hot-finished) or ‘c’ (cold-formed). The imperfection factor α is significantly better for hot-finished sections.
Connections to Hollow Sections
Connecting to hollow sections is the most challenging aspect of their use. The most common approaches are:
- Welded joints — T, Y, K, N, X types: Covered by EN 1993-1-8 Chapter 7 with specific resistance formulas for each joint type. Efficiency depends on the chord/brace diameter ratio β and thickness ratio τ = tb/tc.
- End plates: A plate welded to the tube end allows bolted connections. Stiffener plates may be needed if the end plate is loaded in bending.
- Through bolts: For SHS/RHS, a bolt can pass through both walls. This works well for simple shear connections but limits the position flexibility.
- Hollo-Bolt / blind bolts: Proprietary fasteners that expand behind the face when tightened. Used where only one side is accessible. Check manufacturer load capacity data.
- Internal plates / fin plates: A plate slotted into a cut in the tube wall and plug-welded provides moment resistance without external protrusions.
Connection note: Welded tube-to-tube joints in trusses are typically the governing design criterion for hollow section trusses — not the member forces. Design the joint geometry (gap vs. overlap, branch angle, chord utilisation) early in the design process.
Fire Resistance
Hollow sections — particularly CHS — can be filled with concrete or water to enhance fire resistance without external intumescent coating. Concrete-filled CHS columns (CFCHS) are compact, efficient in compression, and can achieve REI 60 to REI 120 without any surface protection. This is a major advantage in applications where exposed steel is preferred aesthetically but fire resistance is required.
Typical Applications
| Section | Common Uses | Key reason |
|---|---|---|
| CHS | Space frames, trusses, columns, masts, offshore jackets, handrails | Equal stiffness all axes; superior torsion; clean aesthetics |
| SHS | Column sections, lattice frames, post-and-beam connections, architectural columns | Square face suits end-plate and bolted connections; biaxial bending |
| RHS | Purlins, lintels, chord members, crane rails, agricultural structures | Higher Wy about major axis; wide flange accepts surface-mounted connections |
Specifying Hollow Sections
When specifying hollow sections, the following information must be stated:
- Section size: e.g. RHS 200×100×8 (height × width × wall thickness, all in mm)
- Manufacturing standard: EN 10210 (hot-finished) or EN 10219 (cold-formed)
- Steel grade: typically S235, S275, or S355 with appropriate sub-grade for toughness
- Delivery condition (M = thermomechanically rolled, often specified for S355M for improved toughness)
- Weld seam position for RHS, if relevant to connection layout
References: EN 10210-1, EN 10219-1. For reference only — verify against current standards.