Last reviewed 30 Sept 2026 · 6 min read
What a bridge is
A bridge carries a road, railway, pipeline or canal across an obstacle — a river, valley, another road or a railway. Every bridge does the same three jobs: it spans the gap with a superstructure, it transfers the load to the ground through a substructure and foundations, and it does so safely and durably for its design life (100 years for major highway bridges).
Classification of bridges
| Basis | Types |
|---|---|
| Material | Timber, masonry, plain/reinforced concrete (RCC), prestressed concrete (PSC), steel, composite (steel–concrete) |
| Span | Culvert up to 6 m; minor bridge 6–60 m; major bridge over 60 m (highway classification); long-span bridges are those beyond the reach of ordinary girders — cable-stayed, suspension, arch |
| Function | Highway bridge, railway bridge, road-cum-rail, foot bridge, aqueduct, viaduct, flyover, pipeline bridge |
| Structural action | Girder (beam), slab, box girder, truss, arch, rigid frame, cable-stayed, suspension |
| Level of roadway | Deck bridge (roadway on top of the main members), through bridge (roadway between main members), half-through |
| Position | Fixed, movable (bascule, swing, lift) and floating (pontoon) bridges |
| Life and use | Permanent, temporary (Bailey), submersible |
A viaduct is a long bridge of many spans carrying a road or railway over low ground or valleys (metro viaducts are common). A flyover carries one road over another at a junction. An aqueduct carries a canal or pipeline over a river or road.
Which type suits which span
| Span range (approx.) | Usual choice |
|---|---|
| Up to about 10 m | RCC slab, box culvert |
| 10–25 m | RCC T-beam and slab |
| 25–45 m | PSC I- or T-girder |
| 40–150 m | PSC box girder, steel plate girder, composite girder |
| 100–200 m | Steel truss, arch, extradosed |
| 200–500 m | Cable-stayed |
| Over 500 m | Suspension |
The ranges overlap and vary with site — they are guides, not rules.
Parts of a bridge
- Superstructure — deck slab, girders (or truss/box), wearing coat, kerbs, footpaths, railings, expansion joints, drainage spouts. It spans between supports.
- Bearings — devices between superstructure and substructure that transmit vertical and horizontal loads while allowing controlled rotation and movement.
- Substructure — piers (intermediate supports), abutments (end supports that also retain the approach embankment), wing walls / return walls, pier caps and bed blocks.
- Foundation — open (spread) footing, pile, well (caisson) or raft, carrying the load to competent strata below scour level.
The approach consists of the embankment and its slope protection, and the approach slab that eases the transition from embankment to deck.
Waterway terms
- HFL — High Flood Level: the highest flood level recorded or reliably estimated at the site. Bridge soffit level and freeboard are fixed above it.
- LWL — Low Water Level: the water level in the dry season.
- Afflux: the rise in water level upstream of the bridge caused by the constriction of the waterway by piers and abutments. Afflux increases as the waterway is narrowed and must be kept within limits so that upstream areas are not flooded.
- Linear waterway: the total clear width between abutments minus the width of piers, measured at HFL, through which water flows.
- Vertical clearance (freeboard): the gap between HFL and the underside of the superstructure. It leaves room for floating debris and, where required, navigation.
- Scour: erosion of the river bed and banks by flowing water, deeper near piers, abutments and in bends. Foundations must go below the maximum scour level. Scour is the single most frequent cause of bridge failure.
Lacey's regime relations
Lacey's theory is used for alluvial rivers to estimate the natural width and scour:
= regime wetted perimeter (m); = design flood discharge (m³/s); = silt factor ( = mean particle size in mm); = mean depth of scour below HFL (m) for a river that flows in its regime width. For a river that has been constricted, the normal scour depth is
with = discharge per metre width of the waterway (). The maximum scour depth for design is taken as a multiple of this normal depth — larger in bends and at pier noses, smaller in straight reaches — and the foundation grip length is measured below it.
A bridge is to cross an alluvial river with design flood = 1500 m³/s and silt factor = 1.0.
Regime width: .
Mean scour depth: .
If the waterway is constricted to = 150 m: m³/s per m and — narrowing the waterway increases scour.