Common sections
Structural steel is rolled or fabricated into standard shapes so that designers can pick a section from a table instead of designing one from scratch. Each shape puts material where a particular job needs it. The I-section, or beam section, concentrates steel in two flanges far from the neutral axis, making it efficient in bending about its major axis. Wide-flange column sections have broader flanges so that they are stiffer about the weak axis too.
Channels have one web and two flanges on the same side. They are used as purlins, side rails, stair stringers and built-up columns made from two channels laced together. Angles, equal or unequal, are the workhorse of trusses, bracing and transmission towers, because they are easy to connect with bolts through one leg. Tees, cut from I-sections, make chords of light trusses.
Hollow sections, circular, square or rectangular tubes, resist torsion and buckling well because their material is spread evenly around the centroid, and they look clean in exposed roofs and footbridges. Plates and flats are cut and welded into plate girders, gussets, base plates and stiffeners. Cold-formed sections, pressed from thin sheet into C, Z and hat shapes, are light and suit secondary members.
In India, dimensions and properties of hot-rolled sections are given in IS 808, and the steel itself is specified in IS 2062. Section tables list mass per metre, area, moments of inertia, section moduli and radii of gyration. Designers compare sections by the property that governs: Z for bending, r for buckling, area for tension.
Pick a beam by Z, a strut by r about its weak axis, and a tie by area.
Built-up I-section: I_x = [B D³ − (B − t_w)(D − 2 t_f)³] / 12Section modulus Z = I / y_max; radius of gyration r = √(I / A)Solid square side a: Z = a³ / 6
Using the major-axis radius of gyration when checking a column that is free to buckle about its weak axis.
Problem. An I-section is 300 mm deep with flanges 150 mm × 12 mm and a web 8 mm thick. Find its area and major-axis section modulus, and compare Z with a solid square bar of the same area.
- Area = 2 × 150 × 12 + (300 − 24) × 8 = 3 600 + 2 208 = 5 808 mm².
- I_x = [150 × 300³ − 142 × 276³] / 12 = (4.050 × 10⁹ − 2.985 × 10⁹) / 12 = 8.87 × 10⁷ mm⁴.
- Z_x = 8.87 × 10⁷ / 150 = 5.91 × 10⁵ mm³.
- Square of the same area: a = √5 808 = 76.2 mm, Z = 76.2³ / 6 = 7.38 × 10⁴ mm³. The I-section has about 8 times the bending modulus for the same weight.
Answer. A = 5 808 mm², Z_x ≈ 5.91 × 10⁵ mm³, about 8 times a solid square of equal area
Quick quiz
1Which section is most efficient in major-axis bending?
The I-section, with material concentrated in the flanges.
2Why are angles common in towers and trusses?
They are easy to bolt through one leg and come in many sizes.
3Which Indian standard gives hot-rolled section dimensions?
IS 808.
- IS 808 (India): Dimensions and properties of hot-rolled steel beam, column, channel and angle sections.
- IS 2062 (India): Hot-rolled structural steel: grades and requirements.
- IS 800 (India): General construction in steel: limit state design of steel members and connections.
Cited by number and clause only. We do not reproduce clause text, tables or figures (BIS Act 2016, s.10(5) and s.11). Check the current edition and amendments before use.
References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- N. Subramanian, Design of Steel Structures, Oxford University Press, latest edition.
- C. G. Salmon, J. E. Johnson, F. A. Malhas, Steel Structures: Design and Behavior, Pearson, 5th edition.
- W. T. Segui, Steel Design, Cengage, latest edition.
Free and official sources
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