Last reviewed 30 Sept 2026 · 6 min read
Where steel is used
Steel is strong, light for its strength and can be prefabricated and erected quickly, so it is chosen for long spans, heavy railway loads, sites with restricted access, and repair or replacement bridges (where a short closure is important). Its weaknesses are corrosion and fatigue, which control design and maintenance.
Types of steel bridges
| Type | Span range | Where used |
|---|---|---|
| Rolled beam | up to about 20 m | Small railway spans, footbridges |
| Plate girder | 20–60 m | Railway and highway; the standard Indian Railways steel span |
| Truss (warren, N, K, Pratt) | 40–150 m and more | Long railway and road-cum-rail bridges |
| Box girder | 40–200 m | Curved decks, flyovers, orthotropic decks |
| Arch | 60–300 m | Visual and hydraulic-friendly bridges |
| Cable-supported | Over 150 m | See the long-span note |
Plate girder bridges
A plate girder is a deep I-section built up from a web plate, flange plates and stiffeners.
- Web: resists shear. It is thin and deep, so it is stiffened against buckling with vertical stiffeners (transverse) at intervals and, for deeper webs, longitudinal stiffeners. Bearing stiffeners at supports carry the reaction.
- Flanges: carry the bending moment as a couple of tension and compression forces; the compression flange is braced against lateral-torsional buckling by the deck or the bracing.
- Depth: about span/10 to span/14 for railway girders; economical depth minimises the weight.
- Bracing: cross-bracing between the girders, lateral bracing in the plane of the flanges and portal bracing for through girders.
Railway plate girders are usually through girders with the track carried on cross-girders and stringers between the main girders, to save construction depth. Highway plate girders are deck girders with a reinforced concrete deck slab acting compositely.
Truss bridges
A truss carries load as axial forces in members connected at joints. Top and bottom chords resist the bending moment; web members (diagonals and verticals) resist the shear. A truss is more economical than a plate girder above about 60–70 m because material is concentrated where needed.
- Compression members are governed by buckling, so they use closed or built-up sections; tension members are governed by the net section at connections, so eyebars, plates and angles are common.
- Secondary stresses from rigid joints are considered in the design of the connections.
- Truss bridges carry the track on stringers framed to cross-girders, which transfer the load to the truss joints.
Steel box girders and orthotropic decks
A steel box girder has flanges and webs forming a closed section that resists torsion very well. Its top flange may be an orthotropic deck — a thin steel plate stiffened by longitudinal ribs (troughs) and transverse cross-beams. Orthotropic decks are light and permit long spans, but they are sensitive to fatigue at rib-to-deck and rib-to-cross-beam welds.
Design basics for steel bridge members
- Flexure: section modulus for the working or ultimate bending moment, with the compression flange checked for lateral-torsional buckling.
- Shear: web shear capacity, with buckling; web stiffeners spaced by the code rules.
- Compression members: slenderness ratio, effective length, and the buckling curve.
- Tension members: net area; block shear at end connections.
- Connections: HSFG bolts or welding; fatigue category of every detail.
- Deflection and camber: limit live-load deflection; provide camber for dead-load deflection.
Fatigue. Steel bridges see millions of stress cycles. Cracks start at welds and holes where stress is concentrated. Design uses a detail category and a stress-range–cycles (S–N) curve; the design stress range is the difference between the maximum and minimum stresses in a cycle. Good detailing (avoid abrupt changes, use smooth transitions, avoid weld starts and stops at high stress) is more effective than a heavier section.
Corrosion protection. Painting (zinc-rich primer, epoxy, polyurethane), metallising, hot-dip galvanising or weathering steel in mild environments; details must let water drain, avoiding pockets and sharp crevices.
Composite bridges
A composite bridge uses steel girders and a reinforced concrete slab connected so they act as one unit. Steel works efficiently in tension and the concrete slab in compression, so the section is lighter and stiffer than a steel girder alone — deflection typically drops by 20–30 % and the steel section is smaller.
Composite action
Without connection the slab and girder slip on one another and bend separately. Shear connectors welded to the top flange resist the horizontal shear at the interface and stop the slip, so the two behave as a single flanged beam.
- Effective width of the slab acting as the flange is limited by the span and the girder spacing.
- The section is transformed into an equivalent steel section using the modular ratio ; long-term loads use a larger modular ratio to allow for creep.
- Shrinkage of the slab produces stresses in the composite section and is considered in the design.
Shear connectors
| Type | Description |
|---|---|
| Headed stud | Welded to the flange; the commonest connector |
| Channel connector | Short rolled channel welded with the web facing |
| Angle / bar | Older; less used |
| Perfobond, bonded plates | Special applications |
= vertical shear force, = first moment of the slab area about the neutral axis of the composite section, = second moment of area of the composite section. The connector spacing is chosen so that the connectors' shear capacity per unit length exceeds (with the fatigue range checked for the working load).
Construction sequence
- Steel girders launched and braced.
- Formwork hung or precast planks laid; wet concrete carried by the steel alone (unless the girders are propped).
- Slab hardens; the section becomes composite.
- Superimposed dead and live loads are carried by the composite section.
Because the steel alone carries the wet slab, propping during casting reduces steel stresses and deflections; unpropped construction is faster.
Continuous composite girders
Over interior supports the slab is in tension and may crack. Design uses the reinforcement in the slab (not the concrete) for the hogging moment, and the sections are classified for local buckling; prestressing the slab or sequence casting (pouring the sagging parts first) controls cracking.