What Is a Hollow Section?
A hollow section is a closed-profile steel product with a hollow core, supplied as round, square or rectangular tubing and used wherever a light, stiff and torsion-resistant member is required. Because the steel sits at the outer surface of the profile, a hollow section delivers a high radius of gyration and a high second moment of area per unit of weight. That combination is why hollow sections are widely specified for bridges, building frames, columns, machine bases and load-bearing supports. Carbon steel is the most common base material, while stainless steel and aluminium alloy hollow sections are chosen when corrosion resistance or weight saving dominates the design brief.
Main Hollow Section Families
Three shapes cover almost all demand, and each one has a different balance of stiffness, jointing effort and cost.
| Family | Shape | Typical use |
|---|---|---|
| CHS | Circular hollow section | Columns, poles, pipework, cable ducts |
| SHS | Square hollow section | Trusses, frames, architectural columns |
| RHS | Rectangular hollow section | Beams, purlins, chassis, machine bases |
A circular profile offers the best torsional behaviour and the lowest drag, a square profile is the easiest to connect in two directions, and a rectangular profile puts the greater depth where the bending moment demands it.
Typical Range and Governing Standards
Base material: carbon steel, stainless steel or aluminium alloy
Wall thickness: 0.5 mm to 20 mm, depending on size and process
Outer diameter or side length: 6 mm to 200 mm
Length: random length or fixed length cut to order
Surface: plain, oiled, hot-dip galvanized or painted
Cold formed welded hollow sections are normally supplied to EN 10219-1, with grades such as S275J0H and S355J2H. Hot finished hollow sections follow EN 10210-1. In North American practice, ASTM A500 covers cold-formed welded and seamless carbon steel structural tubing, with Grade B and Grade C the most frequently ordered. Cold formed hollow sections for the Chinese market are commonly produced to GB/T 6728. Wall thickness tolerance, corner radius and mass per metre must always be checked against the standard that governs the ordered size, because the three systems are not interchangeable without a grade mapping.
Structural Advantages and Applications
The closed shape gives balanced stiffness about both axes, so a hollow section resists buckling and torsion more efficiently than an open profile of the same mass. It also presents a smooth, paint-friendly surface with no hidden cavities, which lowers maintenance cost in exposed or humid service.
Building structures: columns, roof trusses, space frames and industrial plant supports that must carry load at low self weight.
Transportation pipelines: conveying gas, water, slurry and chemical media, where the closed wall provides pressure containment and good sealing.
Machinery manufacturing: vehicle frames, chassis members, crane booms, ship and aircraft sub-assemblies built from welded tube.
Infrastructure services: cable support structures, ventilation and exhaust ducts, and agricultural irrigation lines feeding sprinkler or drip networks.
How to Specify a Hollow Section
Standard and steel grade, written exactly as the standard defines it
Shape, outside dimension and wall thickness, with the tolerance class
Manufacturing route: cold formed welded, hot finished or seamless
Length, end finish and whether ends are square cut, bevelled or capped
Surface condition, protective coating and inspection or test certificate requirement
Ordering on outside dimension alone is a common source of rework; the wall thickness drives the section properties, so both figures belong on the same line of the specification.
FAQ
Q: What is the difference between a hollow section and a pipe?
A pipe is dimensioned by its nominal bore and is rated for pressure service, while a hollow section is dimensioned by outside size and wall thickness and is rated for structural load. The same mill may produce both, but they are ordered against different standards.
Q: Which hollow section shape is strongest?
For a given mass and outside dimension, the circular hollow section has the best torsional stiffness, while square and rectangular sections are easier to weld and to fit with flat connection plates.
Q: What wall thickness range is available?
Cold formed hollow sections are commonly produced from 0.5 mm up to 20 mm wall, with the exact upper limit set by the outside dimension and the mill process.
Q: Can hollow sections be galvanized?
Yes. A closed profile must be vented and drained before hot-dip galvanizing so that the zinc can reach the internal surface and no trapped air or moisture remains.
Q: Is a hollow section the same as structural tubing?
The two terms describe the same closed-profile product. Structural tubing is the North American wording, usually tied to ASTM A500, while hollow section is the European wording used with EN 10219 and EN 10210.
Q: How is a hollow section grade selected?
Selection follows the design yield strength, the lowest service temperature and whether impact testing is required, then the equivalent grade is taken from the applicable EN, ASTM or GB standard.