How to Choose the Right Hollow Section Size?
Choosing the right Hollow Section size is not simply a matter of selecting the largest tube that fits. The decision depends on load, span, support conditions, steel grade, wall thickness, and connection details. A square section may suit a compact column, while a rectangular section can provide greater stiffness in one direction. Shape matters.
The scale of steel use is substantial. World Steel Association’s World Steel in Figures 2024 reports 1,892.2 million tonnes of crude steel production in 2023. That figure gives industry context, not a design rule. For sizing, engineers rely on structural standards and verified section properties. AISC 360-22 sets requirements for steel member strength and stability, while CIDECT design guides address structural hollow sections and their connections. Applicable standards vary by project location.
A practical selection starts with the actual design loads and the clear span. Consider a roof beam carrying snow, wind, and the weight of cladding. Its depth, wall thickness, and restraint can affect deflection and buckling differently. Connections matter too; a strong member can still be difficult to fabricate or connect safely. Check section capacity, serviceability, local buckling, and corrosion exposure before choosing a size. Then confirm the result using manufacturer data and the relevant design code. This is where early estimates often need revision. Hollow Section dimensions alone cannot replace a project-specific structural check.
Understand Hollow Section Types and Their Dimensions
Hollow sections come in three common forms, and each suits different design needs. Square hollow sections (SHS) have equal outer sides, making them straightforward to align at corners and joints. Rectangular hollow sections (RHS) have different width and height, which can help when bending is stronger in one direction. Circular hollow sections (CHS) distribute loads evenly around their perimeter and can be useful in columns or exposed frames. Shape matters.
A size is usually stated by its outside dimensions and wall thickness. An RHS marked 100 × 50 × 4 mm, for example, is 100 mm high, 50 mm wide, with a nominal 4 mm wall. The inside opening is not simply the outer size minus twice the wall thickness: corner radii affect it. Check the supplier’s dimensional tables and applicable product standard, since tolerances and available sizes vary.
Compare section properties, not just weight or outside dimensions. For a beam, bending direction and span matter; for a column, load and effective length matter. Connection details matter too. A thick wall may help with a bolted joint, but it does not automatically make the whole member suitable. There is no perfect shortcut. I would verify the design assumptions with a qualified structural professional, especially when loads, supports, or material grades are uncertain.
How to Choose the Right Hollow Section Size? — Understand Hollow Section Types and Their Dimensions
| Hollow Section Type |
Example Nominal Size (mm) |
Outside Dimensions |
Wall Thickness (mm) |
Typical Selection Consideration |
| Square Hollow Section (SHS) |
25 × 25 × 2 |
25 mm × 25 mm |
2 |
Compact framing, light supports, and smaller fabricated assemblies. |
| Square Hollow Section (SHS) |
50 × 50 × 3 |
50 mm × 50 mm |
3 |
A versatile size for general structural and fabrication work. |
| Square Hollow Section (SHS) |
75 × 75 × 4 |
75 mm × 75 mm |
4 |
Consider for posts and frames where strength is needed in both principal directions. |
| Square Hollow Section (SHS) |
100 × 100 × 5 |
100 mm × 100 mm |
5 |
Often considered for heavier columns and support frames; verify capacity by design. |
| Rectangular Hollow Section (RHS) |
50 × 25 × 2 |
50 mm × 25 mm |
2 |
Useful where a slim profile is preferred and bending demand differs by direction. |
| Rectangular Hollow Section (RHS) |
60 × 40 × 2 |
60 mm × 40 mm |
2 |
A common proportion for light frames, rails, and general fabrication. |
| Rectangular Hollow Section (RHS) |
100 × 50 × 3 |
100 mm × 50 mm |
3 |
The deeper dimension can provide greater bending stiffness when oriented appropriately. |
| Rectangular Hollow Section (RHS) |
150 × 100 × 5 |
150 mm × 100 mm |
5 |
Consider for larger beams or frames after checking loads, span, and deflection. |
| Circular Hollow Section (CHS) |
48.3 × 3.2 |
48.3 mm outside diameter |
3.2 |
Round profile suited to tubular frames, bracing, and connections designed for circular sections. |
| Circular Hollow Section (CHS) |
60.3 × 3.2 |
60.3 mm outside diameter |
3.2 |
A frequently used tube size; confirm the applicable product standard and tolerances. |
| Circular Hollow Section (CHS) |
88.9 × 4.0 |
88.9 mm outside diameter |
4 |
Suitable for consideration in larger tubular supports and bracing systems. |
| Circular Hollow Section (CHS) |
114.3 × 4.5 |
114.3 mm outside diameter |
4.5 |
A larger-diameter option for structural applications requiring a circular profile. |
Note: Sizes shown are representative nominal dimensions; availability and dimensional tolerances vary by product standard, grade, and market. Select a section using the design loads, span, support conditions, connection details, corrosion requirements, and applicable structural code. Verify the final section with a qualified design professional.
Define the Loads and Structural Requirements
Before choosing a hollow section, define what it must carry and how it will be supported. List permanent loads, such as cladding or equipment, and variable loads, such as people, stored materials, wind, or snow. Include the load direction and any offset from the section’s center. Small eccentricities matter.
Next, record the clear span, support conditions, connection details, and acceptable deflection. A section may resist the expected load yet still bend too much for its use. Check strength, stability, and serviceability, including local buckling and overall member buckling. Consider corrosion exposure and whether the section can be inspected. These details are easy to miss early; assumptions about support stiffness, in particular, may need revision. Use applicable design standards, and have a qualified structural engineer verify final sizing.
Tips: Make a simple load schedule before comparing sizes. Note units and assumptions beside every value. If equipment may be added later, include that possibility rather than relying on a rough guess.
Choose a Profile Shape and Preliminary Size
Start with the member’s job, not the section catalogue. A square hollow section suits columns and frames where connections meet from several directions. Rectangular sections can offer greater stiffness in one bending direction, useful for beams with a consistent load path. Round sections work well when loads or connections come from changing directions. Shape matters.
For a preliminary size, note the span, support conditions, expected loads, and available connection space. Then compare outside dimensions and wall thickness; two profiles with the same height may behave quite differently. A deep section may reduce deflection, but it can also complicate a tight joint or add unnecessary weight. Check how the member will be oriented, too. Rectangular sections are not equally stiff on both axes. A quick sketch helps.
Treat catalogue tables as a screening tool, not a final design. Verify capacity, deflection, local buckling, and connection details against the project’s design requirements. Corrosion exposure and fabrication tolerances can affect the practical choice. Small oversight. An early estimate may miss how a gusset plate crowds the joint, or how a thin wall affects welding. Record assumptions beside the proposed size, then have a qualified structural professional review the design before fabrication.
Check Strength, Stability, and Serviceability
How to Choose the Right Hollow Section Size?
Check Strength, Stability, and Serviceability
A hollow section must do more than carry the design load. Check bending, shear, and compression against the relevant material properties and load combinations. A larger outside dimension does not always mean a stronger member; wall thickness and section shape matter too. For example, a long, slender column may buckle before its steel reaches its strength limit. Check the member’s unsupported length, end restraints, and load direction. Connections deserve attention as well. A strong tube can still be weakened by poorly positioned bolts, welds, or holes.
Then assess stability and serviceability under realistic conditions. Consider local buckling in the section walls, overall member buckling, and any torsional effects. Check deflection and vibration against project-specific limits, especially for floors, frames, or supports carrying sensitive equipment. Small movements can be noticeable. Corrosion exposure and fabrication tolerances may also affect the usable wall thickness. A first-pass size is rarely the final answer. It is easy to focus on strength and miss comfort or fit-up. Review calculations, material data, and connection details together, and ask a qualified structural engineer to verify the final selection against applicable design requirements.
Verify Standards, Connections, and Practical Constraints
Check the governing design standard and material grade before comparing hollow section sizes. Standards define dimensions, tolerances, and strength values; requirements can differ by project location. Confirm the specified wall thickness, not just the nominal size. Mill tolerances may leave less material than a drawing suggests. Small detail. It matters.
Connections can control the choice. A section that meets the member calculations may leave too little room for bolts, welds, or an end plate. Check bolt edge distances, tool access, and how forces enter the section walls. Thin walls may need stiffeners or internal diaphragms to resist local distortion. Ask the structural engineer to review these details with the actual connection design, not a simplified sketch. I have seen this step pushed late, and changes then affect more than one component.
Practical limits matter, too. Confirm local stock lengths, transport capacity, lifting points, and cutting equipment before fixing the size. Consider corrosion protection and whether hollow ends need drainage or sealing for the intended environment. A larger section is not automatically safer or easier to build; it can add weight and complicate handling. Record any assumptions about loads, support conditions, and fabrication tolerances. If a key dimension is uncertain, pause and verify it rather than treating the catalogue value as exact.
