Hollow sections close the loop that open sections leave dangling: the material wraps right around the centroid, so it resists bending and torsion in every direction with no weak axis to design around. The choice between square (SHS), rectangular (RHS) and circular (CHS) is rarely about strength alone - it is about how the load behaves and how the member connects.
When each one wins
SHS is the all-rounder. With equal stiffness about both axes it is ideal for columns, bracing and any member that sees load from more than one direction. A column carrying axial load with no dominant bending axis is the textbook SHS case - see the SHS reference tables for the full size range.
RHS trades that symmetry for efficiency under directional bending. When the moment is clearly about one axis - a beam, an edge member, a transom - the deeper dimension puts material where it earns its keep, giving a higher section modulus per kilogram than an SHS of the same mass. Browse the RHS sizes to compare.
CHS is the torsion and aesthetics specialist. A closed circle has by far the highest torsional constant of the three, so it dominates where twisting governs - curved members, members loaded eccentrically, exposed architectural steel. It also sheds wind cleanly. The price is connections: welding a brace to a round face needs profiled (saddle) cuts, which costs fabrication time. See the CHS reference.
The connection cost nobody prices early
Strength comparisons are easy; the real decision often turns on detailing. A bolted end plate or a flat gusset lands cleanly on the flat face of an SHS or RHS. On a CHS, every transverse connection is a curved interface - more cutting, more weld preparation, more cost. If your frame has many node connections, a flat-faced section can be cheaper even when a CHS is lighter.
A quick decision path
- Load from multiple directions / a column? → SHS.
- Bending dominated about one axis / a beam? → RHS, deep dimension in the plane of bending.
- Torsion governs, or it is exposed and must look clean? → CHS, and budget for the connections.
- Tie-break on cost? → count the connections. Flat faces (SHS/RHS) usually fabricate cheaper.
Once you have the shape, size it on real numbers: open any section in the steel catalogue to pull dimensions, mass and properties, then run an EC3 capacity check on the exact size.