Types of Steel Sections Used in Structures

Steel structures are assembled from standard rolled shapes rather than solid bars. A well chosen shape carries the same load as a solid section with a fraction of the steel, because it places material far from the neutral axis where it contributes most to stiffness and strength. Knowing the common types of steel sections, how they are named and where each is used is the first step in steel design to codes such as IS 800 or Eurocode 3.

Types of steel sections at a glance

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I-sections (beams)

An I-section has two flanges joined by a thin web. The flanges resist most of the bending moment and the web resists shear. Bent about the major axis, it is the most efficient common shape for beams, joists, girders and purlins in heavy buildings. Its weakness is low stiffness about the minor axis and low torsional stiffness, so long unbraced beams can fail by lateral torsional buckling. Indian series under IS 808 include ISJB, ISLB, ISMB and ISWB (junior, light, medium and wide flange beams); international equivalents are IPE and UB sections. Parallel flange sections are now widely rolled alongside older taper flange shapes.

H-sections (columns)

H-sections (ISHB, or HE and UC sections internationally) have wide flanges, close to the depth. Their stiffness about both axes is more balanced, so they resist buckling better as columns and also serve as heavy beams and piles.

Channels

A channel (ISMC, ISLC, PFC) has a web and two flanges on one side. It is used for purlins, side rails, stair stringers, lintels and as a component of built-up columns where two channels are laced or battened together. Its shear centre lies outside the section, so a load through the web causes twisting unless the channel is restrained or used in pairs.

Angles

Angles (ISA) are L-shaped and come as equal or unequal leg sections. They are the standard members for trusses, towers, bracing, cleats and connections. Single angles connected by one leg carry eccentric load, which design codes account for.

Tees

Tee sections, usually cut from I-sections, are used as truss chords, stiffeners and in composite floors. They are less common in general building work.

Hollow sections

Circular (CHS), square (SHS) and rectangular (RHS) hollow sections, covered in India by IS 4923 and IS 1161, have high torsional stiffness and equal or near equal stiffness about both axes. They make efficient columns, truss members, space frames, handrails and exposed architectural steelwork, and they are easy to keep clean.

Plates, flats and built-up sections

Plates are welded or bolted into plate girders, box girders and built-up columns when standard rolled sections are not large enough, as in long span bridges.

Hot rolled and cold formed sections

Most of the sections above are hot rolled: steel billets are passed through shaped rolls while red hot, producing thick, compact shapes with some residual stress. Cold formed sections, such as C and Z purlins, lipped channels and light gauge framing, are bent from thin sheet at room temperature. They are light and cheap to transport but have thin walls that buckle locally, so they are designed to separate codes such as IS 801. Hollow sections are made either by hot finishing or by cold forming and welding, and the two kinds have slightly different properties.

Section properties that matter

When choosing a section, the designer uses tabulated properties from the standard or the manufacturer:

• Area A: governs axial capacity, mass per metre and cost.
• Second moment of area I: governs bending stiffness and deflection.
• Elastic and plastic section modulus Z and Zp: govern bending strength (σ = M / Z).
• Radius of gyration r: governs column buckling through the slenderness ratio KL/r.
• Torsion and warping constants: govern lateral torsional buckling and twisting.

For an I-section made of rectangles, the major axis second moment of area is:

$$I_{xx} = \frac{b D^3 - (b - t_w)\,h^3}{12}$$

where b is flange width, D overall depth, tw web thickness and h the clear depth between flanges (root radii ignored).

With Z known, the bending stress for a given moment from the shear force and bending moment diagram follows directly; for example 100 kN·m on this section gives 100 × 10⁶ / 506 × 10³ ≈ 198 MPa, which must be compared with the design strength of the steel grade.

How to choose a steel section

1. Identify the main action: bending, compression, tension, torsion or a combination.
2. Pick the family: I for bending, H or hollow for columns, angles or hollow sections for trusses and bracing.
3. Size for strength and stiffness: use Z for bending, A and r for compression, and I for deflection.
4. Check stability: local buckling of thin plates (section classification), lateral torsional buckling and member buckling.
5. Consider connections, fabrication and availability: a slightly heavier standard section is often cheaper than a rare one.
6. Allow for protection: corrosion protection and fire protection add cost and depend on exposed surface area.

Steel design to IS 800 uses limit state principles very similar to limit state design of RCC, with partial safety factors on loads and materials. The underlying material behaviour is covered in stress and strain.