CODE REFERENCE

IRC 6 — Bridge Loads

Loads and load combinations for road bridges

Also calledirc 6irc-6bridge loadsvehicle loadsclass a loading

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CODES

Definition

IRC 6:2017 — 'Standard Specifications and Code of Practice for Road Bridges - Section II: Loads and Stresses' is the foundational IRC code governing design loads and stresses for road bridges in India. The current edition (2017) replaced IRC 6:2010 with major updates: vehicle classes (Class A, B, AA, 70R), seismic loads per IS 1893 Part 3:2014, wind loads per IS 875 Part 3:2015, and impact factors. IRC 6 is cross-referenced by IRC 24:2010 (steel bridges), IRC 112:2020 (concrete bridges), and IRC 78:2014 (foundations).

Key IRC 6:2017 provisions: (1) Vehicle live loads — Class A (35 t single load), Class B (62 t single load), Class AA (70 t single load), Class 70R (special wheel arrangement); applied as longitudinal sequences on the bridge with appropriate spacing. (2) Impact factor — 25-50% applied on live load to account for dynamic effect; depends on span and surface condition. (3) Footway loads — 5 kN/m² for footways. (4) Pedestrian loads — variable based on use. (5) Wind loads per IS 875 Part 3 — basic wind speed by region; design wind pressure on vertical and horizontal members. (6) Seismic loads per IS 1893 Part 3:2014 — base shear computation similar to buildings but with bridge-specific factors. (7) Temperature gradient — variation across bridge cross-section. (8) Dead load — concrete + steel + bituminous wearing course (40-75 mm typical for highway bridges).

Design approach per IRC 6:2017: (a) Compute combined loads with appropriate combinations: dead + live + impact (basic), dead + live + wind + impact, dead + live + seismic, etc. (b) Select governing combination at each section. (c) Apply load combinations per IRC 6 Cl. 5 (working stress method or limit state method). The 2017 revision introduced explicit limit state method for road bridges, replacing the older WSM-only approach. For a typical 25 m simple-span bridge: design moment from Class A live load with impact factor 1.30 = 1850 kNm; from Class 70R = 2200 kNm; governing combination determines design. Major Indian projects: Atal Setu (MTHL), Bandra-Worli Sea Link, Kolkata Bridge, all national-highway bridges follow IRC 6.

Where used

All road bridge design in India — primary load code
Highway flyover and overpass design
Railway-road grade separation bridges
Pedestrian-bridge over highways
Bridge maintenance and load rating

Acceptance / threshold

Per IRC 6:2017 + IRC 24/112 + IS 875 + IS 1893 Part 3: load combinations per Cl. 5; design forces at each section; cross-references to material codes (IS 456, IS 800).

Site example

Site reality: a Maharashtra state-highway bridge originally designed for IRC 6:2010 was retrofit-checked for IRC 6:2017 — the 2017 edition's slightly higher Class 70R impact factor required minor reinforcement increase in critical girders. ₹4.8 lakh reinforcement upgrade cost; bridge rated for full IRC 6:2017 capacity. Always use the latest IRC code edition; older editions may be inadequate for modern vehicle loads.

Frequently asked

What is IRC 6?

IRC 6:2017 — 'Standard Specifications and Code of Practice for Road Bridges - Section II: Loads and Stresses' is the foundational IRC code for design loads on road bridges in India. Specifies vehicle classes (A, B, AA, 70R), impact factors, dead load, footway, wind, seismic, and temperature loads. Cross-referenced by IRC 24:2010 (steel), IRC 112:2020 (concrete).

What are IRC vehicle classes?

Class A (35 t single load): standard truck, used for most secondary roads and light highways. Class B (62 t): heavier truck, used for major highways and freight corridors. Class AA (70 t): single-axle heavy load, used for express highways and ports. Class 70R: special multi-axle heavy load for industrial corridors. Bridges are designed for the worst combination; multiple vehicles in sequence with appropriate spacing per IRC 6 Cl. 5.

What is impact factor in IRC 6?

Impact factor is a multiplier on live load accounting for the dynamic effect of moving vehicles on bridges. Per IRC 6:2017: 25-50% increase in live load depending on bridge span and surface condition. Shorter spans have higher impact (50% for 5 m span); longer spans have lower (25% for 25+ m span). Includes effects of bumps, surface roughness, and acceleration/braking. Applied to live load only, not dead load.

Related code reference terms

IS 456 — Plain & Reinforced Concrete

Master code for concrete design in India

IS 800 — Steel Structural Design

Master code for steel structural design

IS 875 — Design Loads

Design loads code: Part 1 dead, Part 2 live, Part 3 wind, Part 4 snow, Part 5 special

IS 1893 — Earthquake Resistant Design

Earthquake resistant design code

IS 1786 — TMT Steel Bars

High Strength Deformed (HSD) Steel Bars specification

IS 10262 — Concrete Mix Design