IRC 6 : 2017Standard Specifications and Code of Practice for Road Bridges — Loads and Load Combinations

IRC 6:2017 specifies standard loads and stresses for the design of highway bridges in India. It is mandatory for any road bridge, flyover, culvert, or road-over-bridge designed for MoRTH, NHAI, state PWD, or municipal authorities. Private bridges (estate roads, industrial access) typically follow IRC 6 as de facto standard.

You reference IRC 6 whenever: - Designing a new highway bridge, flyover, or grade separation - Designing box culverts, slab culverts, or pipe culverts on roads - Analysing existing bridges for load capacity / rating (for permit overload vehicles) - Bridge retrofit or widening design - Calculating earthquake loads on bridges (supplements IS 1893 Part 3 for bridges)

Pair with: - IRC 112:2020 — concrete bridges design (RCC and prestressed concrete) - IRC 24:2010 — steel bridges design - IRC 22:2015 — composite construction (steel-concrete) for bridges - IRC 78:2014 — bridge foundations - IS 1893 Part 3:2014 — seismic design of bridges (Indian Standard, used alongside IRC 6 provisions)

IRC 6:2017 defines standardized design vehicle loads for bridge design:

Class AA — heaviest, for military and special vehicle crossings: - Tracked vehicle: 700 kN on two tracks 3.6 m apart, 6.0 m long - Wheeled vehicle: 500 kN on 4 wheels, 2.75 m transverse × 6.4 m longitudinal - Used for: major highways, bridges on military supply routes, container corridors

Class 70R — modern heavy vehicle (added in 1964, revised in later editions): - Tracked: 700 kN on tracks - Wheeled: 1,000 kN total on 7 axles, total length 15.44 m - Bogie load: 400 kN on 4 wheels in a bogie configuration - Used for: National Highways, state highways carrying regular heavy freight

Class A — standard Indian heavy vehicle: - 27 tonne total (270 kN) distributed across 7 axles over 18.8 m total length - Wheel loads ranging 17-114 kN - Used for: all bridges on roads with regular truck traffic

Class B — lighter version of Class A: - 17 tonne (170 kN) total - Used for: rural roads, private estate roads, light-traffic situations

Bridge loading selection matrix: - National Highways (NH): Design for Class 70R or Class AA (whichever gives higher response) - State Highways (SH), Major District Roads (MDR): Design for Class 70R and Class A, check both - Other District Roads (ODR), Village Roads: Design for Class A minimum (B if very rural) - Urban arterials, flyovers: Class 70R and Class A

Multi-lane multiplier: For bridges with > 1 lane, apply reduction per Table 2 of IRC 6 (0.9 for 2 lanes, 0.8 for 3 lanes, 0.75 for 4+ lanes) — because simultaneous maximum loading on all lanes is statistically improbable.

Bridge: 30 m span single-span RCC T-beam bridge, 2-lane, carriageway 7.5 m wide, NH corridor in Zone III.

Step 1 — Dead load: Deck slab: 25 kN/m³ × 0.225 m × 7.5 m = 42.2 kN/m run (of bridge length) T-beams (2 beams): 2 × 25 × 1.8 × 0.3 = 27 kN/m run Footpath (both sides): 2 × 1.5 × 0.1 × 25 = 7.5 kN/m run Total DL: 76.7 kN/m run

Total DL over span: 76.7 × 30 = 2,301 kN Max BM from DL: wL²/8 = 76.7 × 30²/8 = 8,629 kN·m

Step 2 — Live load (Class 70R wheeled, 2-lane with reduction): Class 70R 7-axle vehicle, total 1,000 kN over 15.44 m. For 30 m simple span, position the vehicle to maximize mid-span BM. Most adverse position: middle axles centered at mid-span. Max BM from 1 vehicle of Class 70R ≈ 5,560 kN·m (from IRC 6 worked examples table or detailed calculation using influence line)

With 2-lane multiplier 0.9: M_LL = 5,560 × 0.9 = 5,004 kN·m (one lane carries full 70R, other reduced)

Actually IRC 6 recommends checking two scenarios: (a) one lane fully loaded + other lane empty, (b) both lanes loaded at 0.9× factor. Use whichever gives higher BM.

For this case, scenario (a) governs for unsymmetric sections; scenario (b) for symmetric. With 7.5 m carriageway, use 2 lanes × 0.9 = M_LL = 10,008 kN·m (both lanes combined).

Step 3 — Impact factor (dynamic allowance): IRC 6 Clause 211.2: For Class 70R, impact factor = 4.5 / (6 + L) = 4.5 / (6 + 30) = 0.125 (12.5%) Live load + impact: M_LL+I = 10,008 × 1.125 = 11,259 kN·m

Step 4 — Load combination for serviceability: SLS total BM: M_DL + M_LL+I = 8,629 + 11,259 = 19,888 kN·m

Step 5 — Ultimate limit state (per IRC 112 for design): Partial safety factors: γ_DL = 1.35, γ_LL+I = 1.5 (bridge-specific) M_u = 1.35 × 8,629 + 1.5 × 11,259 = 11,649 + 16,889 = 28,538 kN·m

Step 6 — Check against Class AA (tracked) to see if it governs: Class AA tracked, 700 kN on 6 m length of two tracks: M_AA_tracked ≈ 4,900 kN·m for 30 m span This is less than Class 70R wheeled case (5,560 kN·m per lane). Class 70R governs.

Design basis: Bridge designed for Class 70R wheeled + Class A (as alternate on secondary lane configurations). Seismic analysis per IS 1893 Part 3:2014 (not covered here but required in DBR). Wind analysis per IRC 6 Clause 209 (different from building wind of IS 875 Part 3 — includes longitudinal wind on bridge railing).

1. Using IS 875 Part 3 building wind instead of IRC 6 bridge wind. Bridge wind loads per IRC 6 Clause 209 include wind on bridge railings and edge members — different from building wind. Using IS 875 alone under-designs bridge lateral stability. Always use IRC 6 for bridges.

2. Missing longitudinal forces from braking. IRC 6 Clause 211.3 requires longitudinal braking force = 20% of vehicle weight on one lane for short bridges (<20 m), reducing to 10% for longer bridges. This creates tension in deck, torsion in piers, and horizontal reactions at abutments. Often overlooked.

3. Wrong impact factor for span range. Impact factor formulas in Clause 211.2 are span-dependent. Designers often apply a single 25% impact factor universally; IRC 6 actually gives values from ~10% (long spans >40 m) to ~25% (short spans <5 m). Using uniform 25% is conservative but adds cost.

4. Ignoring pedestrian live load on footpaths. IRC 6 Clause 206 specifies 5 kN/m² on footpaths up to 3 m wide. For larger footpaths (bridge approaches on urban flyovers), reduce per Clause 206.4. This footpath LL is separate from vehicular LL — add it for design of deck slab, footpath beams, and parapet.

5. Assuming only one vehicle on the bridge at a time. IRC 6 requires you to consider multiple vehicles on different lanes, not just one Class 70R on one lane. For 4+ lane bridges, the governing case may be 2-3 Class A vehicles on adjacent lanes, not one Class 70R. Check all plausible vehicle combinations.

• IRC 112:2020 — concrete bridge design; load combinations from IRC 6
• IRC 24:2010 — steel bridge design
• IRC 22:2015 — composite (steel-concrete) bridges
• IRC 78:2014 — bridge foundations and substructures
• IRC SP 37 — guidelines for load testing of bridges
• IS 1893 Part 3:2014 — seismic design of bridges and retaining walls
• IS 875 Parts 1-5 — general building loads; IRC 6 supersedes for bridges but parts may be referenced
• IRC SP 13 — guidelines for culverts
• MoRTH Specifications — for construction tolerances and acceptance

IRC 6:2017 is the most-referenced IRC code. Revised from the 2014 edition with significant updates: explicit seismic provisions, refined wind loads, updated multi-lane factors, and new load combinations aligned with Eurocode framework.

Indian bridge loading reality: - NH projects (Golden Quadrilateral, Bharatmala, etc.) universally specify Class 70R and Class AA for all major bridges. Design is usually Class 70R-governed. - State highways design for Class 70R and Class A — state PWD practice varies by state - Urban flyovers often have additional over-design for future heavier traffic (Class AA + 20% buffer is common) - Rural bridges on ODRs and VRs design for Class B in many states; some state PWDs now require Class A as minimum

Overload vehicles: Despite Class 70R being the design maximum, actual trucks on Indian roads can exceed this — bulk cement carriers, container trailers, heavy machinery transport. Many bridges fail gradually from repeated overloading even if individual overloads are within elastic range. IRC recommends periodic load rating (per IRC SP 37) every 10-15 years for old bridges to assess capacity for current traffic.

Seismic on bridges: For seismic zones III-V, IRC 6 + IS 1893 Part 3 govern. Bridge-specific concerns — isolation bearings, movement joints, pier-abutment continuity — are substantially different from building seismic design. Specialist bridge engineers typically do this; don't treat it as a variant of building design.

Upcoming revision: IRC working group has circulated proposed IRC 6:2025 draft including climate-change wind load adjustments, updated seismic hazard maps for bridges, and load factors aligned with IRC 112:2020. Published around 2025-2026.