Hollow Sections vs I-Beams: When Each Wins

Every steel beam is a bet about how the load will try to deform it. An I-beam (wide-flange) concentrates material in two flanges far from the neutral axis — brilliant for bending about its strong axis, where that lever arm does the most work. A hollow section wraps the same steel into a closed loop: a square (SHS), rectangle (RHS), or circle (CHS).

The consequence is geometric, not metallurgical. An open section is efficient in one plane and exposed everywhere else — it is prone to lateral-torsional buckling and weak in twist. A closed section distributes stiffness around its whole perimeter, so it resists torsion and weak-axis buckling far better. The classic rule of thumb: hollow sections win wherever loads twist, where columns can buckle either way, or where the member is simply on display.

The decisive number is the St. Venant torsion constant, J — the geometric term in torsional stiffness GJ, analogous to EI in bending. For a thin-walled closed tube, J follows Bredt's formula and scales with the square of the enclosed area: shear flows continuously around the loop, so the whole cross-section fights the twist.

An open I-section has no closed loop. Its walls instead warp, and its St. Venant J is tiny — engineering references note the torsion constant of an RHS is roughly an order of magnitude higher than a comparable I-section. That is why a tube carries a sign cantilever or an eccentric crane load with almost no twist, while an I-beam in the same role needs bracing or stiffeners. The flip side: for pure strong-axis bending, the I-beam's deep flanges are usually the more material-efficient shape.

When you select an HSS6x6x1/4 or a RHS 150x100x6 in any tool, you are reading a standardized table — and those tables encode real manufacturing rules. In North America, hollow sections are governed by ASTM A500 (cold-formed) and the newer ASTM A1085, with the AISC Shapes Database publishing the listed properties.

A crucial subtlety lives in that table: AISC computes section properties from a design wall thickness equal to 0.93x the nominal for ASTM A500 tubing, because A500 allows a minus-10% tolerance on wall thickness. Per AISC 360-16 Section B4.2, calculations involving HSS wall thickness use 0.93 times the nominal. So an HSS6x6x1/4 — nominally 0.250 in — is calculated on roughly a 0.233 in wall. Ignore that factor and your capacity is optimistic by several percent. ASTM A1085 tightened the wall tolerance to minus-5%, which lets designers use the full nominal wall.

Europe organizes the same family by how the tube was made. EN 10210 covers hot-finished structural hollow sections, while EN 10219 covers cold-formed welded sections shaped at ambient temperature. They are not interchangeable on paper.

• Corner radii: EN 10210 hot-finished sections have larger, more generous corners; EN 10219 cold-formed corners are tighter — which changes the cross-sectional area and properties printed in the table.
• Residual stress & ductility: hot-finishing relieves internal stress and is favored for fatigue and harsh environments; cold-forming work-hardens the corners, raising local yield strength.
• Tolerances: cold-formed EN 10219 typically holds tighter outside-dimension tolerances, which is why it dominates cost-driven fabrication.

The takeaway for a designer: the letters CHS/SHS/RHS describe the shape, but the EN number behind them describes the steel — and the properties table is built from that distinction, not just the outside dimensions.

Tubes are ancient; rectangular structural tubes are not. Circular hollow sections grew out of 19th-century tube-making, but the rectangular hollow section was not developed until 1952, by Stewarts & Lloyds in the United Kingdom — the Glasgow-headquartered tube maker itself formed by a 1903 merger of A. & J. Stewart & Menzies and Lloyd & Lloyd. RHS is, in structural terms, a post-war profile; it only became widely available in the early 1970s.

That youth is why an entire research body exists to back it. CIDECT — the international association of structural-hollow-section manufacturers — was founded in 1962 and has since funded 200+ research projects and a celebrated series of Design Guides (CHS joints, RHS joints, fire, concrete-filled columns, fatigue). Those guides are the source code behind the hollow-section joint rules that later flowed into Eurocode 3, AISC 360 and other national codes.

Use a hollow section when the member twists, when a column must resist buckling in both directions, when the steel is architecturally exposed, or when you want clean lines and lower drag. Stick with an I-beam for heavy strong-axis bending, long-span floor beams, and anywhere you rely on cheap bolted connections — because joining tubes from a single side, with no internal access, is the genuine cost penalty of going hollow.

Whichever you choose, the decision is only as good as the table behind it: the right design wall thickness, the right EN or ASTM family, the correct torsion and warping constants. CalcSteel is a browser-native structural app (React/TypeScript front end, Python finite-element backend) with 1,140+ steel profiles — RHS, SHS, CHS and I-sections alike — and code checks for NBR 8800, AISC 360, Eurocode 3 and IS 800. It is free to start, with Pro at US$24/month billed annually. The honest pitch: it won't make the geometry choice for you, but it will run the buckling, torsion and connection checks on the standardized numbers so you can compare a tube against a beam in minutes. Try it in the editor.