Structural Steel I Beam: Shape, Sizes and Design Checks

The structural steel I beam is the most recognisable member in building engineering. Its cross-section, two horizontal flanges joined by a vertical web, puts steel where it does the most work in bending and removes it from where it does little. The result is a beam that carries large loads over long spans for a modest weight of steel, which is why I-sections frame everything from house extensions to transfer structures in high-rise buildings.

This article explains the anatomy of the section, why the shape is efficient (with a worked example), how sections are designated and specified, the checks an engineer makes, and the practical points that matter in Singapore. For the related comparison with wide-flange columns, see our article on H-beam vs I-beam.

Anatomy of an I-beam

Flanges: the top and bottom plates. In a beam sagging under load, the top flange is in compression and the bottom flange in tension. Together they resist most of the bending moment.
Web: the vertical plate joining the flanges. It holds them apart at a fixed distance and carries most of the shear force.
Root radius: the curved fillet where web meets flange in a hot-rolled section, which smooths the stress flow and adds a little area.
Depth and width: overall depth sets bending stiffness about the major axis; flange width sets stiffness about the minor axis and resistance to sideways buckling.

“I-beam” is used loosely for several related families. In British and Singapore practice, Universal Beams (UB) are relatively deep and narrow, intended for bending. Universal Columns (UC) and other H-sections have wider flanges, nearly as wide as the section is deep, making them stiffer about both axes and better suited to columns. European IPE and HE sections follow the same logic. Fabricated plate girders, welded from separate plates, are used where rolled sizes are not deep enough.

Why the I shape is efficient: a worked example

Bending stiffness depends on the second moment of area, I, which grows with the square of each element’s distance from the neutral axis. The moment of inertia article explains the idea; here is the arithmetic for an idealised I-section built from plates, ignoring root radii.

Section: overall depth 300 mm, two flanges 150 mm × 10 mm, web 280 mm × 7 mm.

Area A = 2 × (150 × 10) + 280 × 7 = 3,000 + 1,960 = 4,960 mm², about 39 kg per metre at a density of 7,850 kg/m³.
Each flange: own I = 150 × 10³ / 12 = 12,500 mm⁴; centroid 145 mm from the neutral axis; transfer term = 1,500 × 145² = 31.5 × 10⁶ mm⁴.
Web: I = 7 × 280³ / 12 = 12.8 × 10⁶ mm⁴.
Major-axis I = 2 × (0.0125 + 31.5) + 12.8 ≈ 75.9 × 10⁶ mm⁴.

The flanges, about 60 per cent of the area, provide over 80 per cent of the stiffness, almost entirely through the parallel axis theorem transfer term. A solid square bar of the same 4,960 mm² area (about 70 mm a side) would have I = 70.4⁴/12 ≈ 2.05 × 10⁶ mm⁴, some 37 times less.

The elastic section modulus is W = I / y_max = 75.9 × 10⁶ / 150 ≈ 506 × 10³ mm³. At a yield strength of 355 N/mm², the moment that would first cause yield at the extreme fibres is 355 × 506 × 10³ ≈ 180 × 10⁶ Nmm, or about 180 kNm, before any partial factors or buckling checks.

The same section about its minor axis has I ≈ 2 × (10 × 150³ / 12) + 280 × 7³ / 12 ≈ 5.6 × 10⁶ mm⁴, about 13 times less than the major axis. That asymmetry is the price of efficiency: an I-beam is excellent in one direction and much weaker in the other, which is why lateral restraint matters.

Designations, sizes and grades

When a structural steel I beam is specified, it is identified by a designation that describes its nominal size and weight. A UB designated 305 × 165 × 40, for example, has a serial depth of about 305 mm, a flange width of about 165 mm and a mass of about 40 kg per metre. Actual dimensions differ slightly from the serial size and should always be taken from the section property tables, which also list the area, second moments of area, section moduli and torsional properties for both axes.

Steel grade is specified separately. Common structural grades under the European standard are S275 and S355, where the number is the minimum yield strength in N/mm² for thinner elements; the yield strength reduces slightly for thicker flanges. Our guide to steel grades in construction explains the designations, sub-grades for toughness and how they are chosen.

In Singapore, steel structures are designed to SS EN 1993 (Eurocode 3) and its Singapore National Annex, as listed in BCA’s Approved Document. Steel from sources outside the standard European and equivalent product standards is addressed through BCA’s design guide BC 1 on the use of alternative structural steel materials to Eurocode 3, which sets out how such materials are classified and what reductions, if any, apply.

Design checks for a structural steel I beam

Section classification. Eurocode 3 classifies each section by the slenderness of its flanges and web, which decides whether it can reach full plastic capacity or will buckle locally first.
Bending resistance. Plastic or elastic moment capacity, depending on class, compared with the design moment.
Shear resistance. Mainly carried by the web, with a check on shear buckling for slender webs and on interaction with bending where shear is high.
Lateral-torsional buckling. An unrestrained compression flange can buckle sideways and twist at a moment well below the section’s full capacity. Restraint from a concrete slab, metal decking or secondary beams is often what makes a beam efficient.
Web bearing and buckling. Concentrated loads and reactions can crush or buckle the web locally, sometimes requiring stiffeners.
Deflection and vibration. Serviceability often governs long spans, especially for floors.
Connections. End plates, fin plates, bolts and welds must transfer forces into supporting members, and their design is a significant part of the work.

Where a steel beam supports a concrete slab with shear connectors, composite action greatly increases stiffness and strength, which is covered by SS EN 1994.

Fire, corrosion and installation

Steel loses strength and stiffness at high temperature, so structural steelwork usually needs fire protection to meet the period of fire resistance required by the SCDF Fire Code. Options include intumescent coatings, boards and sprayed protection, or partial encasement in concrete.

Internal steel in air-conditioned buildings needs only modest corrosion protection, but steel exposed to Singapore’s humid outdoor air needs a robust system, often hot-dip galvanising or a multi-coat paint specification, and details that drain water. Steel embedded in or in contact with damp masonry is particularly vulnerable.

In renovation work, inserting a steel beam to replace a wall is common. It needs a properly designed bearing at each end, temporary propping during installation and, in Singapore, design and endorsement by a Professional Engineer where structural elements are altered. Our guide to load-bearing walls explains when this applies.

Frequently asked questions

Why are steel beams shaped like an I?

The I shape places most of the steel in the flanges, far from the neutral axis where it resists bending most effectively, while the thinner web holds the flanges apart and carries shear, giving high stiffness and strength for a given weight of steel.

What is the difference between an I-beam and an H-beam?

An I-beam or Universal Beam is deeper than it is wide and is optimised for bending about one axis, while an H-section or Universal Column has much wider flanges and more similar stiffness about both axes, making it better suited to columns.

How is a structural steel I beam sized?

An engineer calculates the design bending moment, shear and deflection for the span and loads, then selects a section from the tables whose resistance and stiffness are sufficient, checking lateral-torsional buckling, web bearing and connections to Eurocode 3.

What do the numbers in a beam designation mean?

For Universal Beams the three numbers are the nominal serial depth and flange width in millimetres and the mass in kilograms per metre, while actual dimensions and properties are read from the section tables.

Do steel beams need fire protection?

In most buildings they do, because steel loses strength at high temperatures, so beams are protected with intumescent paint, boards or encasement to achieve the fire resistance period required by the Fire Code.

Need a steel beam sized, checked or endorsed for a renovation or new build? Talk to our consultancy or get in touch.