What are structural steel sections?
Structural steel sections are steel members rolled or formed in the mill to a fixed, standardised cross-section. By concentrating the material where the stresses are highest — in the flanges, far from the neutral axis — a section carries far more per kilogram of steel than a solid bar. That principle makes sections the basis of virtually every steel frame.
The common sections
| Section | Shape | Typical use |
|---|---|---|
| UB / IPE | I-shape, narrow flanges | Floor and roof beams; the standard bending member |
| UC / HEA, HEB, HEM | H-shape, wide flanges | Columns and heavily loaded beams; HEB is heavier than HEA |
| Channel (PFC / UNP) | U-shape | Edge beams, stair strings, trimming around openings |
| Angle | Equal or unequal leg | Connections, bearings, masonry support angles |
| Hollow section (SHS/RHS/CHS) | Square, rectangular, circular | Columns and frames; good in torsion, attractive when exposed |
| Tee and flat bar | T-shape, flat | Small supports and connections |
In addition there is cold-formed steel: thin sheet formed into C, Z or top-hat sections, used for purlins, side rails and light framing.
Steel grade
Structural sections are normally supplied in S235 or S355 to EN 10025. The number is the characteristic yield strength in N/mm²: S355 carries more for the same cross-section, which often allows a slimmer member. The suffix (JR, J0, J2 and so on) describes the notch toughness, and therefore the behaviour at low temperature.
Practical points
- Deflection often governs, not strength. For a domestic floor beam a deflection limit in the order of span/250 to span/360 is usually applied, and that limit normally governs over strength.
- Bearings. A steel beam on masonry needs a spreader plate or padstone; local bearing pressure under the flange is otherwise too high.
- Fire resistance. Unprotected steel loses strength quickly as temperature rises. Where fire resistance is required, intumescent coating, boarding or encasement is needed.
- Corrosion. Outdoors or in damp spaces: hot-dip galvanising or a closed coating system. Watch for condensation at cold bridges.
- Ordering. Always specify section, grade, length and the fabrication required (holes, gusset plates, mitres); drilling a beam once installed is laborious.
Always have the section sized by a structural engineer. A beam replacing a loadbearing wall requires checks on bearing, restraint and the foundation below as well as the section size.
Related terms
- Beam
- Steel frame
- Rolled beam
- Cold-formed steel
- Deflection
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.