IRC 6:2017 — Standard Specifications and Code of Practice for Road Bridges, Section II: Loads and Stresses
The Indian standard governing all loads on road bridges: live load classes (70R, AA, A, B), impact factors, wind, seismic, temperature, and load combinations for both working stress and limit state design.
Scope
IRC 6:2017, "Standard Specifications and Code of Practice for Road Bridges, Section II: Loads and Stresses," is the authoritative standard for determining all loads on road bridges in India. Published by the Indian Roads Congress, it covers dead load, live load (IRC Classes 70R, AA, A, and B), impact (dynamic) effects, longitudinal forces (tractive/braking), centrifugal forces, wind loads, seismic loads, water current forces, buoyancy, earth pressure, temperature effects, erection loads, and collision loads. The 2017 edition incorporates limit state design in alignment with IRC 112 (Limit State Design of Concrete Bridges) while retaining working stress provisions for existing practice. The standard applies to all types of bridges: girder, box girder, cable-stayed, suspension, arch, and culverts.
Purpose
IRC 6 establishes uniform loading criteria for the design of road bridges across India to ensure structural safety and serviceability. By standardising vehicle loads, impact factors, and environmental loads, it provides a consistent basis for bridge design regardless of location or designer. The standard accounts for Indian traffic conditions, vehicle characteristics, and environmental factors including monsoon wind and seismic zones. It also provides the load factors and combinations necessary for both working stress (Cl. 203) and limit state (Cl. 204) design approaches, allowing engineers to use either methodology.
Engineering Applications
IRC 6 applies to all road bridge design projects in India: national highways, state highways, district roads, and rural roads. Live load classes are selected based on the roadway classification: Class 70R for National Highways and expressways, Class A for all permanent bridges (mandatory minimum for any public road), and Class B for temporary bridges and low-volume rural roads. Class AA is used for heavy industrial corridors. The standard is used for girder and slab bridge design (dead load + live load + impact), bearing and expansion joint selection (longitudinal and temperature forces), pier and abutment design (earth pressure + water current + live load surcharge), foundation design (all applicable loads), and seismic design in Zones II through V as per the seismic coefficient method. Wind loads are computed for the maximum gust speed at deck level, considering the bridge type (through-type vs. deck-type) for wind area.
Design Philosophy
IRC 6 provides two parallel design philosophies. Working stress design (WSD) uses the elastic method with permissible stresses and no explicit load factors (Cl. 203 loads are service-level). Limit state design (LSD) per IRC 112 uses partial safety factors for loads and materials. For LSD, load factors are specified in Cl. 204 for the following limit states: Limit State of Strength (LS1), Limit State of Serviceability (LS2), and Limit State of Fatigue (LS3). The live load models are calibrated to represent the actual worst-case traffic loading on Indian roads. The Class 70R vehicle (350 kN tracked or 400 kN wheeled) represents the heaviest military and commercial vehicle expected on national routes. The impact factor formula I = 4.5/(6+L) accounts for the dynamic amplification of static loads, decreasing with increasing span length L as the dynamic response becomes less critical for longer spans.
Important Requirements
All bridges must be designed for at least Class A loading. The critical load case must be determined by considering all classes applicable to the road type. For bridges on National Highways, 70R loading governs for spans up to 90 m. For multiple lanes, the number of design lanes equals the carriageway width divided by the lane width (3.5 m for 70R, 2.5 m for Class A). Live load reduction factors per Cl. 205 apply for multi-lane loading: two lanes 100%, three lanes 90%, four lanes 75% for 70R loading. Impact factor I = 4.5/(6+L) for 70R tracked/Class A (Cl. 208), and I = 9/(13.5+L) for 70R wheeled. Longitudinal forces (Cl. 210) = 20% of the live load for Class A and 20% of the live load for 70R. Wind load per Cl. 209 uses basic wind speed Vb from IS 875 (Part 3), modified for bridge height and terrain. Seismic loads per Cl. 219 use zone factors from 0.10 (Zone II) to 0.36 (Zone V).
Key Parameters
The following table summarises IRC 6 live load classes and their key axle configurations:
| Load Class | Type | Total Weight (kN) | Axle Configuration | Contact Area | Typical Use |
|---|---|---|---|---|---|
| 70R (Tracked) | Tracked vehicle | 350 | Single track, 4.57 m length | 800 x 150 mm | NH, expressways |
| 70R (Wheeled) | 4-axle truck | 400 | 4 axles x 100 kN each | 250 x 250 mm (each wheel) | NH, expressways |
| Class AA | Heavy industrial | 700 (tracked) / 400 (wheeled) | Single track or 4-axle | Per 70R | Industrial corridors |
| Class A | Standard truck train | 554 | 3-axle + 3-axle train, 114 kN max axle | 250 x 250 mm (each wheel) | All permanent bridges |
| Class B | Light vehicle | 143 | 2-axle + 2-axle train, 57 kN max axle | 200 x 200 mm (each wheel) | Temporary / rural roads |
Key formulas from IRC 6 include:
Impact Factor (70R wheeled): I = 9 / (13.5 + L)
where L = loaded length in metres
Longitudinal Force = 0.20 x Live Load (all classes)
Centrifugal Force = (0.0079 x V²) / R (as fraction of live load)
where V = speed in km/h, R = radius in metres
Wind Load per unit area: F = 0.5 ρ V² CD G
where ρ = air density, V = gust wind speed, CD = drag coefficient
Note: The impact factor for 70R wheeled vehicles is higher than for tracked vehicles at all spans because the wheeled vehicle has less tyre damping and greater dynamic amplification. For culverts with span ≤ 3 m, the impact factor reaches a maximum of 0.5 (50% dynamic allowance). The loaded length L for computing impact factor is the span length for simply supported members and the length of the loaded portion for continuous members.
Practical Engineering Notes
For maximum bending moment in longitudinal girders, the most critical transverse placement of 70R tracked vehicle is with the track at 1.2 m from the kerb face. For Class A loading, two trains of vehicles (one in each direction) are considered simultaneously, with each train consisting of a driving vehicle (3-axle, 114 kN rear axle) followed by a trailer (3-axle, 68 kN rear axle). The critical longitudinal position for maximum moment is typically with the heaviest axle at the section of interest. For hogging moment at supports of continuous bridges, the designer must consider alternate span loading with live load on adjacent spans only. Temperature effects (Cl. 212) consider a range of ±20°C for steel bridges and ±10°C for concrete bridges for the Indian climate. The differential temperature gradient for concrete box girders is considered per IRC 112.
Field Tip: When designing for 70R wheeled loading, the maximum shear at supports often occurs with the first axle (100 kN) positioned at the support face, while the maximum moment occurs with the heaviest axle group (axles 3 and 4 at 2.0 m spacing) centred at mid-span. Always check both tracked and wheeled 70R configurations — wheeled often governs for shear in short spans (15-30 m), while tracked governs for moment in medium spans (30-60 m).
Typical Workflow
An IRC 6 bridge loading workflow: identify the road classification and select applicable load classes. Determine carriageway width and number of design lanes. Position live loads transversely for maximum eccentricity/eccentric loading on longitudinal girders. Position live loads longitudinally for maximum bending moment and shear at critical sections using influence lines. Apply impact factor I based on loaded length. Combine dead load + superimposed dead load + live load (with impact) for service conditions. Apply load factors per Cl. 204 for limit state design. Check wind load: determine basic wind speed Vb, apply risk factor k1, terrain factor k2, and topography factor k3 to obtain design wind speed Vz. Compute wind force per unit area and apply to exposed area of bridge superstructure and live load on deck. Check seismic load per Cl. 219: determine zone factor Z, importance factor I, response reduction factor R, and compute design horizontal seismic coefficient Ah = Z I Sa / (2Rg). Combine loads per the applicable limit state.
Common Mistakes
Warning: Common errors include using the wrong impact factor formula for the load type (using tracked formula for wheeled loading or vice versa), not reducing multi-lane live loads per Cl. 205, neglecting the minimum clearance distance from kerb for transverse positioning, and failing to check both hogging and sagging moment envelopes for continuous bridges. For seismic loading, using the wrong zone factor or forgetting to apply the importance factor for critical bridges (I = 1.5 for bridges on lifeline routes) are frequent issues. Also common: neglecting temperature gradient effects in concrete box girders and not verifying the buoyancy check for bridges in water courses.
Best Practices
Use influence line diagrams for accurate positioning of live loads at critical sections. Develop complete load envelopes (max/min moment, shear, axial force) for all critical sections. For continuous bridges, run at least five live load cases: alternate span loading for hogging moment, all spans loaded for sagging moment, and individual span loading for maximum shear. Check load combinations for both working stress and limit state design. Document all live load positions assumed in the analysis. For multi-cell box girder bridges, verify the transverse distribution using either the grillage analogy or refined finite element analysis, not just simple lever rule distribution. Use the Bending Moment Calculator for rapid verification of critical sections under 70R loading.
Limitations
IRC 6 does not cover fatigue design in detail (referenced to IRC 112 and IRC 24). It does not address bridge-specific wind dynamic effects such as vortex shedding or flutter for long-span bridges (wind tunnel testing is required per IRC 6 commentary). The standard does not provide provisions for soil-structure interaction in seismic analysis. Collision loads on bridge piers from vehicular impact are simplified and may not reflect site-specific risk. For very long-span bridges (spans over 300 m), specific dynamic analysis and site-specific wind and seismic studies are recommended beyond the code provisions. The seismic coefficient method in Cl. 219 is applicable only for regular bridges up to 30 m height; taller or irregular bridges require dynamic analysis.
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References
- IRC 6:2017. Standard Specifications and Code of Practice for Road Bridges, Section II: Loads and Stresses. Indian Roads Congress, 2017.
- IRC 112:2011. Code of Practice for Concrete Road Bridges. Indian Roads Congress, 2011.
- IRC 5:2015. Standard Specifications and Code of Practice for Road Bridges, Section I: General Features of Design. IRC, 2015.
- IS 875 (Part 3):2015. Code of Practice for Design Loads for Buildings and Structures, Part 3: Wind Loads. BIS, 2015.
- Victor, D.J. Essentials of Bridge Engineering. 6th ed., Oxford & IBH, 2011.