IRC 6:2017 Bridge Loads Explained — Class A, Class AA, Class 70R Live Loads
Every Indian highway bridge designed in the last fifty years has had its structural sizing driven by one document: IRC 6 — Standard Specifications and Code of Practice for Road Bridges, Section II — Loads and Load Combinations. The current edition is IRC 6:2017, revised to consolidate amendments and to align load combinations with the limit-state design philosophy that IRC 112 introduced for concrete bridges.
This article walks through what IRC 6:2017 actually specifies — the dead loads, live-load classifications (Class A, Class AA, Class 70R, Class B), impact factor, wind, seismic, temperature, and how they combine into the load cases that drive superstructure and substructure design. If you are reading a bridge design report or preparing one yourself, this is the language you need to speak fluently.
1. The load categories IRC 6 covers
IRC 6:2017 Cl. 202 enumerates the loads and forces to be considered in bridge design:
- Dead load (DL) — self-weight of structural components.
- Superimposed Dead Load (SIDL) — wearing coat, kerbs, railings, crash barriers, services, utilities.
- Live load (LL) — vehicular loading (Classes A, AA, 70R, B) and pedestrian.
- Impact factor (IF) — dynamic amplification of live load.
- Wind load (W) — lateral wind on superstructure, substructure, vehicles on deck.
- Seismic load (E) — computed per IRC SP 114:2018 for bridges.
- Longitudinal forces — tractive effort of vehicles, braking force.
- Centrifugal force — for bridges on curves.
- Temperature effects (T) — uniform temperature and differential temperature gradient.
- Earth pressure (EP) — on abutments, retaining walls.
- Water current / buoyancy / hydrodynamic forces — for substructure in water.
- Erection loads — construction-stage forces.
- Fatigue load — for components with fatigue-sensitive details (welded steel, prestress anchorages).
Of these, the live loads get the most attention because they are uniquely Indian — Class A and Class 70R are not used anywhere else in the world. If you have designed bridges under AASHTO HL-93 or Eurocode LM1, you will find the Indian loading heavier in absolute weights but configured differently in geometry.
2. Live Load — Class A (the workhorse)
Class A is the default vehicular load for bridges on all roads in India except minor village roads. It represents a typical commercial truck + trailer train. Each Class A "train" is an 8-axle configuration:
- 2 front axles of 27 kN each (wheel load 13.5 kN)
- 4 rear axles of 114 kN each (wheel load 57 kN)
- 2 trailer axles of 68 kN each (wheel load 34 kN)
- Total per train: approximately 554 kN, configured over 18.8 m
Wheel spacing transverse is 1.8 m centre-to-centre (1.8 m gauge). Wheel contact area is prescribed for bending / shear analysis and punching checks on slab decks. Multiple Class A trains are placed on the deck with a minimum spacing of 18.5 m nose-to-nose between trains.
Class A produces the governing moment and shear for most 10–40 m span girder decks carrying normal national-highway traffic.
3. Live Load — Class 70R (for NH and SH)
Class 70R is the heavier loading used for bridges on National Highways and most State Highways. It represents a large military-style tracked or wheeled vehicle. IRC 6 gives three configurations:
- 70R Tracked — 700 kN on two tracks each 0.84 m wide × 4.57 m long, 2.06 m gauge.
- 70R Wheeled — 1000 kN on 7 axles (single-vehicle), max axle 170 kN, spread over 15.4 m.
- 70R Boggie — 400 kN on 2 closely-spaced axles, used for checking local slab effects.
The designer must check which configuration gives the worst effect for each influence line. For a short-span slab bridge, 70R Boggie often governs punching. For a 25 m simply-supported girder, 70R Tracked usually governs midspan moment. For longer continuous spans, 70R Wheeled tends to win.
4. Live Load — Class AA
Class AA is the heaviest military loading specified for bridges on "specified military transport corridors" and for all bridges within a certain radius of defence installations. In practice, Class AA is prescribed by the project authority at RFP stage for specific alignments.
- Class AA Tracked — 700 kN on two tracks (similar geometry to 70R Tracked but heavier contact pressure).
- Class AA Wheeled — 400 kN single axle, 0.30 × 0.15 m contact area per wheel.
For most civilian NH projects, Class AA is not required if 70R is specified — they are not additive, only alternatives for the heaviest configuration.
5. Live Load — Class B
Class B is the lightest standard loading, used for village roads and bridges carrying low commercial traffic. Configuration is similar to Class A but with reduced axle weights (front 20 kN each, rear 68 kN each). Bridges on PMGSY alignments and district roads with AADT under 2,500 commercial vehicles per day may be designed to Class B — but the project authority's specification governs.
6. How many lanes of live load on a bridge?
IRC 6 Cl. 205 prescribes Reduction Factor for Multiple Lane Loading. For a bridge carrying up to 3 lanes, the designer checks every combination — single lane, two lanes simultaneously, three lanes simultaneously. Simultaneous multi-lane loading has a reduction factor:
| Number of lanes loaded | Reduction Factor |
|---|---|
| 1 | 1.00 |
| 2 | 1.00 |
| 3 | 0.90 |
| 4 | 0.80 |
| 5 or more | 0.75 |
The reduction reflects the statistical reality that all lanes rarely see peak load simultaneously. It is applied to the live load only, not to dead load.
7. Impact Factor — the dynamic amplification
Live loads are applied statically, then amplified for dynamic effect through the impact factor (IF). IRC 6 Cl. 208.2 gives a span-dependent formula:
For Class A and Class B loading: IF = 9 / (L + 13.5) but not exceeding 0.5 — where L is the loaded length in metres.
For Class 70R and Class AA tracked vehicles: IF reduces with span, from 0.25 at short spans down to 0.10 for spans over 45 m. For 70R wheeled: interpolated between these two.
Note that impact factor applies only to the superstructure and its bearings. Substructure (piers, abutments, foundations) is designed with a smaller impact factor (typically 50% of the deck value), since the dynamic effect attenuates before reaching the substructure.
8. Wind load on bridges
IRC 6:2017 Cl. 209 references IS 875 Part 3 for basic wind speeds but overrides the height and shape coefficients for bridge-specific components. Key points:
- Wind acts on the deck (horizontal) and on the vehicles stopped on the deck (computed over a "vehicle-load block" of 2.5 m × 3 m per lane).
- For deep-girder decks or trussed superstructures, the frontal area coefficient Cf is substantially higher than for solid slab decks.
- Gust factor is prescribed for long-span bridges (>100 m) where dynamic wind response may become critical.
- Wind combined with live load uses the "reduced live load" case — only a fraction of full LL is considered simultaneously with peak wind.
9. Load combinations — ULS and SLS
Annex B of IRC 6:2017 gives load combinations aligned with the limit-state approach of IRC 112. Key combinations every designer iterates through:
| Combination | Description | γ on DL | γ on LL + IF | γ on Wind |
|---|---|---|---|---|
| ULS Combination 1 | Basic — DL + LL dominant | 1.35 | 1.50 | – |
| ULS Combination 2 | DL + LL + Wind | 1.35 | 1.50 | 0.90 |
| ULS Combination 3 | DL + Wind dominant | 1.35 | 0.75 | 1.50 |
| ULS Combination 4 | DL + Seismic | 1.35 | 0.20 | – |
| SLS | DL + LL (un-factored) | 1.00 | 1.00 | – |
In practice, Combination 1 governs girder flexural steel design. Combination 2 or 3 may govern lateral systems and bearings. Combination 4 drives substructure design in seismic zones III-V. SLS drives crack width, deflection, and prestress loss checks per IRC 112.
10. What changed from IRC 6:2014 to IRC 6:2017
- Expanded seismic provisions — full cross-reference to IRC SP 114:2018 (which itself was being finalised parallel to the 2017 code revision).
- Clarified load combinations for LSD — earlier editions mixed working-stress combinations with LSD, which caused inconsistency with IRC 112.
- Updated wind provisions — aligned basic wind speed references with IS 875 Part 3:2015.
- Fatigue-load model — new clause covering fatigue-load cycles for bridges carrying heavy commercial traffic.
- Vehicle-load block on wind — the simultaneous LL + Wind combination was made more explicit.
11. FAQ — IRC 6:2017
If my bridge is designed to 70R, do I still need to check Class A?
Yes — Class A can govern certain members even when 70R is heavier in absolute terms, because the two loadings have different geometries. Class A's 1.8 m wheel spacing produces different transverse moments than 70R. Always check the envelope of all applicable live-load classes.
What's the difference between IRC 6 and IRC SP 114?
IRC 6 gives the loads; IRC SP 114 gives the seismic methodology for bridges (response spectrum, importance factors, ductility detailing). Together with IRC 112 they form the three-code stack for concrete bridge design.
Is Class B used for NH bridges?
No — NH bridges are designed to Class A + 70R minimum. Class B is limited to village roads / district roads / PMGSY alignments with low commercial traffic.
How is live load placed on a multi-span bridge?
Live load is positioned to produce the maximum effect at each critical section — using influence lines. For a two-span continuous girder, this means placing the live load on alternate spans to maximise hogging moment at the support and on the relevant span to maximise sagging moment.
Does IRC 6 cover pedestrian bridges?
Yes — Cl. 209 prescribes pedestrian live load of 5 kN/m² for bridges exclusively for pedestrian use, with a reduction for longer spans. This becomes relevant for urban footbridges, metro-integration FOBs, and heritage pedestrian crossings.
Where do I find the braking and tractive-force formula?
IRC 6 Cl. 211 gives braking as 20% of the first two lanes of live load (no impact factor applied), applied 1.2 m above the deck surface. Tractive effort is usually not cumulative with braking in the same direction.