Structural steel comes in a handful of standardized cross-sectional shapes, and each one exists because its shape is efficient for a specific structural job. Rather than designing a unique section for every member, engineers select from a catalogue of pre-defined profiles, manufactured to tight tolerances by hot rolling or cold forming. Here’s what each shape is good at, and why.
I-shaped sections (I-beams, H-beams, wide flanges)
I-shaped sections — also called H-shapes or wide flanges — are the workhorse of structural steel. They’re efficient in bending because they place most of their material far from the neutral axis, exactly where bending stress is highest. I-beams and H-beams look similar but differ in proportion: H-beams typically have equal web and flange thickness, producing a heavier, more substantial cross-section, while I-beam flanges often taper slightly from root to edge. Both are the default choice for floor beams and frameworks where bending governs the design — bridges, towers, and heavy-duty structures all lean on this shape.
Channels (C-sections)
A structural channel has a web with flanges sticking out on only one side — unlike I-beams, which have flanges on both sides. This asymmetry means a channel’s bending axis isn’t centered across the flange width, so a load applied evenly across the top tends to twist the section away from the web. That’s a real factor to design around, which is why channels see less use as primary beams than symmetrical I-shapes. Where they excel: framing in warehouses, purlins, and vehicle frames, helped by their easy cut-to-length fabrication and good strength-to-weight ratio.
Angles (L-sections)
An angle consists of two flanges joined at a right angle, forming an “L”. Angles are simple and economical, making them the natural choice where structural efficiency matters less than simplicity of connection — bracing, shelving, small reinforcements, and as the chord or web members of lighter trusses.
Hollow Structural Sections (HSS): RHS, SHS, CHS
Hollow Structural Sections have a tubular cross-section — rectangular (RHS), square (SHS), or circular (CHS). In the UK these are called by those direct names; “HSS” is the US term, sometimes loosely (and incorrectly) called “hollow structural steel.” Their closed shape distributes material evenly around the centroid, giving HSS superior torsional resistance and buckling resistance compared to open shapes like I-beams. That makes them especially efficient for columns and members loaded in compression or twisting, and they’re popular wherever weight reduction matters without sacrificing strength — architectural facades, scaffolding, pipelines, and welded frames carrying multi-directional loads.
How the choice actually gets made
Section selection follows the member’s structural role, not personal preference:
- Floor beams (bending governs): I-shaped sections (UB/W/IPE depending on the standard you follow), chosen to maximise bending stiffness per kilogram of steel — typically starting from the lightest section that satisfies deflection limits, then checking moment and shear capacity.
- Columns (buckling governs): I-shapes optimized for columns (UC/HEA/HEB), or SHS/CHS, chosen to maximise the minimum radius of gyration. For short, stocky columns almost any shape works; for tall, slender columns, hollow sections are usually most efficient.
- Truss chords: I-beams for heavy trusses; hollow sections for lighter ones, balancing axial capacity with some bending.
- Truss web members and bracing (axial/tension only): angles for economy, hollow sections for compression efficiency, or flat bars and rods for pure tension members.
One more category: plate girders
For spans or loads beyond what standard rolled sections can handle, fabricators weld individual steel plates together into plate girders — custom I-shaped or H-shaped members built up rather than rolled as one piece. These are common in bridges and large industrial buildings where standard catalogue sections simply aren’t deep or strong enough.
Understanding what each shape is naturally good at is the foundation for everything that follows in steel design — including the next post, which moves from selecting a section to detailing how sections actually connect to one another.
Part of our Steel Design & Detailing series. Next: what is steel detailing — shop, GA, and erection drawings.