The question of which HSS shape to use comes up earlier in design than most people expect. It’s not just a detail selection made after the structural system is established — the shape choice affects how loads are distributed, how connections are made, and how the structure reads architecturally. Getting it right from the beginning avoids the kind of late-stage redesign that nobody on a project wants.
Here’s how the three shape families actually differ in mechanical behavior, and how those differences translate to the structural applications where each shape performs best.
Round HSS: Strength Without a Preferred Direction
A circular cross-section has the same second moment of area about any axis through the centroid. This means a round HSS column has the same bending stiffness in every direction — there’s no strong axis and no weak axis. For a column loaded in pure compression with identical unbraced lengths in both plan directions, this is an efficiency advantage: the section is optimized for the actual loading condition rather than being overdesigned in one direction to meet requirements in the other.
For compression members, the round HSS also has the best radius-of-gyration-to-area ratio of the three shape families, which means it produces higher buckling capacity for a given weight of steel. This is particularly valuable in long diagonal braces, exposed columns, and any member where compression buckling governs rather than yielding.
Round sections are also the most torsionally stiff shape relative to wall thickness. For members subjected to combined bending and torsion — a cantilevered canopy arm, a crane runway girder that receives off-center loads, an element that serves as both beam and brace — the closed circular section develops torsional resistance far more efficiently than the equivalent open section, and better than square or rectangular HSS at the same wall thickness-to-diameter ratio.
The trade-off is connection complexity. Round-to-round welded connections require profile cutting that follows the curved surface geometry. Base plates require specific detailing. Beam-to-round-column bolted connections need careful attention to bolt access.
Square HSS: Bilateral Symmetry with Flat Faces
A square HSS has equal section properties about both principal axes — strong and weak axis bending stiffness are identical. This makes it, like the round section, well-suited to columns with equal unbraced lengths in both directions. But the flat faces of a square section dramatically simplify connection detailing: shear tabs weld flat to the column face, base plates sit flush without rotational complications, and beam-to-column geometry in a moment frame is easier to control.
For structural hollow sections used as columns in building frames, square HSS is often the preferred choice precisely because it combines the bilateral symmetry advantage with the flat-face connection advantage. The AISC Steel Construction Manual provides design tables for HSS column design based on A500 Grade B properties, and the square shape covers the majority of those tables.
Square HSS also stacks and stores efficiently, which matters in fabrication. Members with the same outside dimensions can share the same cuts, the same connection hardware, and the same jig setups. When a project uses multiple identical-size square HSS columns, the fabrication economics are better than for a mix of different round section diameters.
The limitation of square sections is in torsion. While still far superior to open sections for torsional stiffness, the square section’s behavior under torsion is more complex than the round section’s because the corners create stress concentrations. For applications where torsion is the primary design case, round sections are more efficient.
Rectangular HSS: When Axes Aren’t Equal
A rectangular HSS has different section properties about its two principal axes by design. The strong axis — bending about the axis parallel to the longer face — has a higher moment of inertia than the weak axis. This is directly analogous to a W-shape: the section is stiffer and stronger in one direction than the other.
This makes rectangular HSS the appropriate shape for beam applications where loads are primarily in one direction. A roof beam carrying gravity loads bends about one axis only; a rectangular section oriented with the longer dimension vertical is more efficient for that loading case than a square section of the same weight. For girders, transfer beams, and other flexural members, rectangular HSS typically outperforms square HSS of equal weight because more of the material is positioned farther from the neutral axis.
Rectangular sections also appear in lateral bracing where the member’s slenderness ratio matters more in one direction than the other. A diagonal brace in a moment frame may have a much shorter unbraced length in one direction (restrained by the floor diaphragm) than the other (spanning between connection points), and a rectangular section can be oriented to match the asymmetric bracing conditions.
The connection complications of rectangular HSS include the same issues as square — no interior access for bolts, punching shear through the face for welded connections — with the added consideration that the strong and weak faces have different thicknesses in some section sizes, which affects the connection capacity calculation.
The Practical Selection Framework
For columns with equal unbraced lengths in both directions: round or square, selected based on connection preference and architectural intent. Round for exposed architectural applications, square when connection simplicity and fabrication efficiency matter more.
For beams and girders with loads primarily in one direction: rectangular, oriented with the long dimension parallel to the bending axis. Depth-to-width ratio selected to balance bending efficiency against depth constraints imposed by the building system.
For diagonal braces: round for long members where compression governs (best radius of gyration for the weight), rectangular for shorter members where the asymmetric unbraced length condition makes the shape advantage worth the connection detail effort.
For torsion-sensitive applications: round, consistently, regardless of connection complexity.
The shape choice also has procurement implications. Square and rectangular HSS in standard sizes are typically stocked by service centers across North America; round HSS in structural sizes is stocked in a narrower range and may require mill order for less common diameters and wall thicknesses. For projects where schedule is tight, confirming material availability by shape and size before committing to the structural system saves the kind of procurement surprises that cause delays.
