Last reviewed 16 Sept 2026 · Facts as of 16 Sept 2026 · 13 min read
Components of a bridge
| Part | Components |
|---|---|
| Superstructure | Deck slab, girders/beams/trusses/arches/cables, wearing coat, footpaths, kerbs, railings/crash barriers, expansion joints |
| Bearings | Transfer loads from superstructure to substructure while permitting movements and rotations |
| Substructure | Piers (intermediate supports), abutments (end supports that also retain approach fill), wing walls and return walls (retain earth of approaches), pier and abutment caps |
| Foundations | Open (spread) footings, well (caisson) foundations, pile foundations, raft |
| Approaches | Approach embankments/viaducts, approach slabs, guard stones, river training and protection works |
Classification of bridges
| Basis | Types |
|---|---|
| Function | Highway (road) bridge, railway bridge, road-cum-rail bridge, foot bridge, aqueduct, pipeline bridge, flyover/grade separator, road overbridge (ROB) and road underbridge (RUB) at rail crossings, viaducts |
| Material | Timber, masonry (stone/brick arches), steel, reinforced concrete (RCC), prestressed concrete (PSC), composite (steel girders with concrete deck) |
| Structural form | Slab bridges, beam/girder bridges (T-beam, box girder, plate girder), truss bridges, arch bridges, rigid frame bridges, balanced cantilever bridges, cable-stayed bridges, suspension bridges, movable bridges (bascule, swing, vertical lift) |
| Span (IRC terminology) | Culvert — linear waterway up to 6 m; minor bridge — total length up to 60 m; major bridge — total length more than 60 m; long-span bridges with very large individual spans |
| Position of deck | Deck bridge (deck on top of the main structure), through bridge (deck at the bottom level of the structure), semi-through bridge |
| Water level (HFL) | High-level bridges (deck above HFL with clearance), submersible bridges/causeways (designed to be overtopped by floods) |
| Loading class | Designed for IRC Class 70R, Class AA, Class A, Class B loads |
| Permanence | Permanent, temporary (e.g. Bailey bridges, pontoon/floating bridges) |
Structural forms — key features
- Slab bridges — solid or voided slabs for short spans.
- T-beam and box girder bridges — common RCC/PSC bridges for medium spans; box girders have high torsional stiffness (curved bridges, flyovers).
- Truss bridges — steel members in triangulated frames — railway bridges and medium to long spans.
- Arch bridges — carry loads mainly in compression; need strong abutments to resist thrust.
- Balanced cantilever bridges — built segmentally outward from piers.
- Cable-stayed bridges — deck supported by inclined cables from towers; efficient for medium to long spans.
- Suspension bridges — deck hung from main cables anchored at ends; longest spans.
Site selection and investigations
Ideal site: a straight reach with a narrow, well-defined channel and stable, high banks; firm foundation strata at reasonable depth; square crossing (flow perpendicular to the bridge); absence of whirls and cross currents; minimum training works; good approaches without costly embankments; proximity to the planned alignment; availability of materials and access.
Investigations: topographic survey (catchment and site plan, cross-sections upstream and downstream), hydrological data (rainfall, flood records, HFL, LWL, velocity, discharge), geotechnical investigations (boreholes, soil and rock properties, scour characteristics), navigational requirements, seismic zone, traffic and road/rail data.
Hydraulic design
Design discharge
Estimated by empirical formulas (Dickens, Ryves, Inglis), the rational method (small catchments), area–velocity method from flood marks (slope–area method with Manning's formula), unit hydrograph method, and flood frequency analysis. IRC:5 and IRC:SP:13 guide the design discharge and return periods; the maximum of several estimates is normally checked.
Linear waterway
- For alluvial streams, the regime waterway (Lacey's) (m; in m³/s) is taken as the effective linear waterway.
- For quasi-alluvial/rocky streams, the waterway is fixed by the natural stream width at HFL; contraction is limited to control afflux and scour.
- Effective linear waterway = total width between abutments minus the effective obstruction of piers.
Afflux
Afflux is the rise in water level upstream of a bridge due to contraction of the waterway.
= afflux (m); = velocity of approach (m/s); = unobstructed natural waterway area at the site (m²); = contracted waterway area at the bridge (m²).
Excessive afflux can flood upstream areas, cause high velocities through the bridge and deep scour.
Scour depth (IRC:78)
= mean depth of scour below HFL (m); = design discharge per metre width of effective linear waterway (m³/s/m); = silt factor ( = weighted mean diameter of bed particles in mm).
Maximum scour depth for design of foundations:
- Piers:
- Abutments: with approach embankment retained (or where scour can occur all round)
Foundations are taken sufficiently below the maximum scour level (with minimum grip lengths prescribed in IRC:78) and are protected by aprons, flooring or pitching where needed.
Clearance and freeboard
- Vertical clearance — between HFL (with afflux) and the lowest point of the superstructure, to pass floating debris; increases with discharge (IRC:5 values).
- Freeboard — the height of the formation level of approaches/guide bunds above HFL.
- Navigational clearance — for navigable waterways, as required by the waterway authority.
Economic span
For a bridge of a given total length with equal spans, the total cost is minimum when the cost of superstructure of one span ≈ cost of substructure (one pier with its foundation).
If the superstructure cost of a span varies as and the cost of a pier with foundation is (independent of span), the economic span is:
Deep or costly foundations favour longer spans; shallow foundations and cheap piers favour shorter spans.