Last reviewed 30 Sept 2026 · 6 min read
Why load models matter
A bridge must carry loads of many kinds at the same time, and many of them move. Codes replace real traffic with standard load models that are heavy enough to cover every vehicle that may use the bridge. The Indian Roads Congress code IRC:6 governs highway bridges; the Indian Railways Bridge Rules govern railway bridges.
Classification of loads
| Group | Loads |
|---|---|
| Permanent | Dead load of structure and wearing coat, superimposed dead load (crash barriers, footpaths, utilities), earth pressure, prestress |
| Variable (live) | Vehicle load, impact, footpath load, braking and tractive force, centrifugal force, wind, water current, temperature, buoyancy, seismic |
| Erection / accidental | Construction loads, vehicle collision, ship impact, differential settlement |
IRC live loads (highway bridges)
Highway bridges are designed for standard trains of vehicles:
| Loading | Description |
|---|---|
| IRC Class A | Standard train of axles with a total of about 554 kN (axle loads 27, 27, 114, 114, 68, 68, 68, 68 kN); used on all roads on which permanent bridges are constructed |
| IRC Class B | Lighter train (about 332 kN) for temporary structures and bridges in specified areas |
| IRC Class 70R | Heavy loading: wheeled vehicle of about 1000 kN on seven axles, or a tracked vehicle of 700 kN (70 tonnes); for bridges on national and state highways |
| IRC Class AA | Heavy military-type loading: tracked vehicle of 700 kN or wheeled vehicle of about 400 kN; used on bridges within certain municipal limits, industrial areas and along specified highways |
| Special vehicle (SV) | A very heavy multi-axle vehicle of about 385 tonnes introduced for the heaviest corridors; checked as a single-lane load |
Bridges are designed for Class 70R or Class A, whichever gives the worse effect, or Class AA where specified. The number of lanes loaded and the reduction of intensity for many lanes are given in the code.
Footpath and kerb loads
Footpaths are designed for a pedestrian live load of about 4.0 kN/m², reduced for large loaded areas and long spans, plus the effect of a vehicle mounting the kerb.
Impact (dynamic effect)
A moving load produces greater stresses than the same load applied statically. The static effect is multiplied by an impact factor:
- Class A and B loading: for concrete bridges ; for steel bridges ( = loaded span in metres). Impact falls as the span grows.
- Class AA and 70R: fixed percentages depending on span (larger for short spans), with tracked vehicles treated differently from wheeled ones.
- No impact is added to footpath loads or to the effect on foundations below the ground level in the way it is applied to the superstructure — the code specifies the reductions.
Other variable loads
- Braking (longitudinal) force: 20 % of the first train load plus 10 % of the succeeding train loads, applied at 1.2 m above road surface.
- Centrifugal force on a curved bridge: ( = live load, in km/h, = radius of curve in m).
- Water current force on a pier: kg/m² of the projected area, = velocity in m/s; = shape factor (about 1.5 for a square-ended pier, 0.66 for a circular pier).
- Buoyancy: taken on the submerged volume of the substructure; with foundations in a permeable soil it acts fully.
Wind load depends on the basic wind speed of the region, the height of the deck, the exposed area, and the terrain. It acts on the structure and on the live load, and it is checked together with the girder overturning and bearing uplift.
Temperature — uniform temperature rise or fall (which decides movement and joint size) and temperature gradient through the depth (which produces stresses in concrete decks).
Earth pressure — active pressure on abutments and wing walls, plus a live-load surcharge equivalent to about 1.2 m of earth for vehicles behind the abutment.
Seismic force — considered for bridges in seismic zones through horizontal (and vertical) seismic coefficients or response spectra; see the note on seismic, wind and fatigue effects.