CHS, SHS or RHS? Choosing the Right Hollow Section

Circular (CHS), square (SHS) and rectangular (RHS) hollow sections belong to the same structural family. Their closed, tubular form gives them the qualities open profiles lack: outstanding torsional stiffness, good resistance to buckling in every direction, no exposed inner surfaces to paint, and a clean appearance that architects consistently ask for.

Within the family, however, the three shapes behave quite differently. The choice between them affects material efficiency, fabrication cost, connection detailing, corrosion protection and even wind loading. Picking the right one early, before the connections are designed, is one of the cheapest optimisations available on a steel project.

Closed sections resist torsion through a continuous shear flow running around the entire perimeter of the wall. This makes their torsion constant I_t dramatically higher than that of open profiles of comparable weight: an I-beam twists easily, a tube does not.

Among the three shapes, CHS is the theoretical optimum: every part of the wall lies at the same distance from the centre, and the shear flow follows an unbroken circular path with no corners to disturb it. An SHS comes close behind, losing only a little efficiency at the corners. An RHS is still excellent by any open-section standard, but the more elongated the rectangle, the further it falls from the circular ideal.

Practical rule: for torsion-dominated members (spandrel beams supporting eccentric façades, curved beams, monorail supports, sign gantries), start with CHS, accept SHS where flat faces simplify the connections, and check RHS carefully if the loading twists it.

When bending clearly dominates about one axis, RHS is the efficient choice. Orienting the long sides vertically places most of the material far from the strong axis, giving the best section modulus per unit mass of the three shapes. Typical wins: floor beams, transfer members, crane runway girders, façade transoms and portal rafters. The penalty is a weaker minor axis: irrelevant where the member is laterally restrained, important where it is not. Hollow sections as a family are far less prone to lateral-torsional buckling than open profiles, which lets RHS beams run long unrestrained lengths that would cripple an equivalent I-section.

For columns and members loaded biaxially, symmetry pays. An SHS has the same stiffness and radius of gyration about both principal axes, so no direction is wasted: the classic choice for building columns, truss chords and space-frame members. A CHS goes one step further with identical properties in every direction, making it ideal for pure axial load, long slender braces and members whose load direction is uncertain. In practice SHS often edges out CHS for columns simply because its flat faces make beam connections easier.

Fabrication is where the shapes diverge most in cost. The flat faces of SHS and RHS accept straight saw cuts, fitted end plates, fin plates and gussets with ordinary shop tooling. Bolting to a face is straightforward with blind bolts or threaded inserts, and truss branches land on a flat chord face with simple mitre cuts. For fabricators without tube-cutting machinery, SHS and RHS are usually the economical answer.

Joining CHS to CHS requires a saddle cut (a curved profile machined into the branch so it wraps the chord), followed by a weld that changes angle continuously around the intersection. Historically this made tubular trusses expensive; modern CNC plasma and laser tube cutters have largely removed the premium for fabricators who own them, but the capability is not universal, so ask your fabricator before committing to an all-CHS truss. Whatever the shape, joint capacity is usually governed by chord face deformation rather than by the weld, so check it as part of member selection.

Exposed structure is where CHS earns its architectural reputation: no edges, no preferred viewing angle, and a slender look at any size. SHS and RHS read as crisper and more rectilinear, aligning naturally with façades and glazing grids, a stylistic choice rather than a ranking.

Durability differences are real. The flat top face of an SHS or RHS laid horizontally can pond water and collect debris, creating a corrosion trap under coatings; a CHS sheds water and dirt from every surface. Whatever the shape, seal the ends or provide drainage so water cannot accumulate inside: a sealed internal void needs no coating, because the trapped air is quickly exhausted of oxygen. If members are to be hot-dip galvanized, coordinate vent and drain holes with the galvanizer early.

In wind, the circle wins again: a CHS has a markedly lower drag than a sharp-edged rectangular shape of similar width, which is why masts, towers, and exposed lattice structures are overwhelmingly tubular. On tall or wind-sensitive structures this reduction feeds directly back into lighter members and foundations.

Each shape is supplied in two product families. Hot-finished sections have low residual stresses, uniform material properties, tight corner geometry and better toughness: preferred for fatigue-loaded, dynamically loaded and low-temperature structures. Cold-formed sections (CHS, SHS, RHS) carry higher residual stresses and larger corner radii, are assigned a less favourable buckling curve in design, and restrict welding within the cold-worked corner zones, but they are typically cheaper and more readily stocked, which is why they dominate everyday construction.

A quick decision checklist:

1. Torsion governs? CHS first, SHS second.
2. One-way bending governs? RHS, long sides vertical.
3. Column or biaxial loading? SHS by default; CHS for pure axial or uncertain load direction.
4. Many bolted or plated connections? Favour the flat faces of SHS/RHS, or confirm CNC tube-cutting capacity for CHS.
5. Exposed to weather or wind? CHS for drag and drainage; detail SHS/RHS to avoid ponding.
6. Fatigue, dynamics or low temperature? Specify hot-finished.
7. Budget and programme tight? Check local stock: cold-formed availability often decides.