InfraLens
HomeIS CodesIRCHandbookDesign RulesPMCQA/QCBIMGATE PrepArticlesToolsAbout Join Channel
Join
HomeIS CodesIRCHandbookDesign RulesPMCQA/QCBIMGATE PrepArticlesToolsAbout Join WhatsApp Channel
InfraLens
HomeIS CodesIRCHandbookDesign RulesPMCQA/QCBIMGATE PrepArticlesToolsAbout Join Channel
Join
HomeIS CodesIRCHandbookDesign RulesPMCQA/QCBIMGATE PrepArticlesToolsAbout Join WhatsApp Channel
IRC 6 : 2017

Standard Specifications and Code of Practice for Road Bridges — Loads and Load Combinations

AASHTO LRFD Bridge Design Specifications · EN 1991-2
CurrentEssentialCode of PracticeTransportation · Bridges and Bridge Engineering
PDFGoogleIRC Portal
Link points to Internet Archive / others. Not hosted by InfraLens. Details
Summary

IRC 6 is the loading standard for ALL road bridges in India — defines what loads a bridge must carry. Class 70R (70 tonne wheeled/tracked vehicle) is the standard design load for NH bridges. Class AA is the older heavy load class. Every bridge designer in India uses IRC 6 daily.

Specifies loads, forces, and their combinations for design of road bridges including dead load, live load (Class AA, A, 70R), wind, seismic, temperature, and impact factors.

QA/QC Templates for IRC 6
Site-ready checklists & test reports with clause references
✅
Bridge Foundation Checklist
checklist
ExcelPDF
✅
Bridge Substructure Checklist
checklist
ExcelPDF
✅
Bridge Superstructure Checklist
checklist
ExcelPDF
Key Values
Class 70R wheeledTotal 1000 kN on 7 axles
Class 70R tracked700 kN on two tracks
Class AA wheeled400 kN on 4 axles
Practical Notes
! Class 70R is the standard design vehicle for all NH/SH bridges — must be checked for both wheeled and tracked configurations.
! Impact factor decreases with span length — for long spans (>50m) it's negligible.
! The 2017 edition aligns seismic provisions with IS 1893:2016 — check zone factor and importance factor.
! Load combinations follow Limit State Method (ULS and SLS) — the older Working Stress Method is being phased out.
! For urban flyovers, also check for crowd loading (5 kPa) on footpaths per IRC 6.
! Always verify unit weights of locally sourced construction materials as they can vary significantly from standard values.
! When designing for Class AA loading, consider the possibility of military vehicular traffic, especially on national highways and strategic routes.
! The impact factor calculation is sensitive to span length; ensure accurate span measurements are used to avoid over or underestimation of dynamic effects.
! For bridges subjected to significant wind exposure, conduct a detailed wind tunnel study or refer to advanced wind load guidelines, especially for high piers or unusual bridge geometries.
! Seismic design for bridges must consider site-specific seismic zone factors and soil conditions as per relevant IS codes.
! Temperature stresses are critical for long span bridges and integral abutments; account for the full range of expected temperature variations.
! The load combinations specified in the code are essential for designing a safe and economical structure; do not omit any relevant combinations.
! When dealing with bearings, consider not only vertical loads but also horizontal loads from braking, acceleration, and seismic events.
! Scour depth calculations for foundation design are paramount, especially in flood-prone areas; ensure conservative estimates are used.
! For bridges carrying utility pipes or services, ensure adequate clearance and support provisions are made, considering live load and temperature movements.
! The 'dead load' for bridges often includes future wearing coat and utility loads which must be estimated realistically.
! Pay close attention to eccentric loading conditions, especially when deck is not symmetrically loaded or when dealing with lateral forces.
! For bridges on curves, the centrifugal force calculation needs to be carefully applied, considering the design speed and radius of the curve.
! The live load distribution factors can significantly impact the internal forces in girders; these should be derived based on current IRC recommendations or verified through analysis.
! Ensure that the design speed chosen for live load calculations aligns with the projected traffic speeds and the road classification (NH, SH, MDR, etc.).
Cross-Referenced Codes
IRC 112:2020Code of Practice for Design of Reinforced Con...
→
IRC 78:2014Standard Specifications and Code of Practice ...
→
IS 5:2019Colours for Ready Mixed Paints and Enamels
→
IRC 24:2010Standard Specifications and Code of Practice ...
→
IS 1893:2016Criteria for Earthquake Resistant Design of S...
→
IS 875:1987Design Loads (Other than Earthquake) for Buil...
→
bridge loadsClass AAClass 70Rlive loadbridge designload combinationIRC
Code-Specific Templates for IRC 6
✅
Bridge Foundation Checklist
checklist
Excel / PDF
✅
Bridge Substructure Checklist
checklist
Excel / PDF
✅
Bridge Superstructure Checklist
checklist
Excel / PDF
✅
Bearing & Expansion Joint Checklist
checklist
Excel / PDF
📝
Bridge Method Statement
form
Excel / PDF
📐
Bridge Works ITP
plan
Excel / PDF
📋
Bridge Construction Register
register
Excel / PDF
📊
Proof Load Test Report
test-report
Excel / PDF
📊
Deflection Measurement Report
test-report
Excel / PDF
📊
Bearing Test Report
test-report
Excel / PDF
✅
Setting Out Checklist
checklist
Excel / PDF
Similar International Standards
AASHTO LRFD Bridge Design SpecificationsAASHTO (US)
HighCurrent
Section 3: Loads and Load Factors
Both define bridge live loads and combinations. AASHTO uses HL-93 loading; IRC uses Class 70R/AA/A.
EN 1991-2:2003CEN (EU)
HighCurrent
Eurocode 1: Actions on structures — Part 2: Traffic loads on bridges
Both specify traffic loads for bridge design. EN uses Load Model LM1-LM4.
Key Differences
≠IRC: Class 70R (1000 kN). AASHTO: HL-93 (design truck 325 kN + lane load). EN: LM1 (tandem 600 kN + UDL). Different vehicle models.
≠IRC uses span-based formula. AASHTO uses 33% for truck, 0% for lane. EN uses fixed factors.
Key Similarities
≈All three use Limit State Design with partial safety factors for load combinations.
Parameter Comparison
ParameterIS ValueInternationalSource
Heavy design vehicleClass 70R: 1000 kNHL-93 truck: 325 kNAASHTO LRFD
⚠ Verify details from original standards before use
Quick Reference Values
Class 70R wheeledTotal 1000 kN on 7 axles
Class 70R tracked700 kN on two tracks
Class AA wheeled400 kN on 4 axles
Class A train554 kN train of vehicles
Impact factor (RCC, 10m span)~15%
Impact factor (steel, 10m span)~25%
Seismic zone factorSame as IS 1893
Unit weight of concrete (reinforced)25 kN/m³
Unit weight of concrete (plain)24 kN/m³
Unit weight of steel78.5 kN/m³
Unit weight of brick masonry19 kN/m³
Unit weight of stone masonry26 kN/m³
Maximum spacing of bearings (expansion)30 m
Design speed for Class AA loading30 km/h
Design speed for Class A loading40 km/h
Design speed for 70R loading30 km/h
Impact factor (I) for general spansI = 1.5 / (L+30)
Lateral wind pressure (at 10m height)1.5 kN/m²
Effective width of web for bending in steel girdersTwice the depth of the girder or 12 times the flange width, whichever is less
Minimum embedment depth for abutments1.5 m below scour level
Maximum allowable deflection for spans up to 30mSpan/500
Coefficient of friction for bearings (typical roller)0.02
Minimum clearance over deck for ventilation shafts5.0 m
Vertical acceleration factor for utility structures1.5
Horizontal acceleration factor for utility structures1.0
Allowable shear stress in concrete (plain)0.2 N/mm²
Allowable shear stress in concrete (reinforced)0.5 N/mm²
Key Formulas
Impact factor (RCC) = 4.5/(6+L) for L≤12m
Impact factor (steel) = 9/(13.5+L)
where L = loaded length of span in metres
Impact Factor, I = 1.5 / (L + 30) for L < 30 m
Impact Factor, I = 4.5 / (L + 30) for L > 30 m
Wind Load per unit area, Pw = P x Kz x Vz² (kN/m²)
Seismic Force, F_EQ = (W x A/g) x (Iw x Z x Ie x S_d(t))
Temperature Stress, σ_T = α * ΔT * E
Equivalent UDL for single axle load = 1.8 * P (for impact)
Key Tables
Table 1 — Class 70R wheeled vehicle loading
Table 2 — Class AA tracked and wheeled loading
Table 3 — Class A train of vehicles
Table 4 — Impact factor for different bridge types
Table 5 — Load combination factors for ULS and SLS
Table 1 — Unit Weights of Materials
Table 2 — Standard Live Loads for Bridges (Class AA)
Table 3 — Standard Live Loads for Bridges (Class A)
Table 4 — Standard Live Loads for Bridges (70R)
Table 5 — Impact Factor (I)
Table 6 — Coefficients for Wind Load
Table 7 — Load Combinations (for Ultimate Limit State)
Key Clauses
Cl. 201 — Dead load calculation
Cl. 202 — Live loads: Class 70R, Class AA, Class A
Cl. 203 — Impact factor for live load
Cl. 204 — Wind load on bridges
Cl. 205 — Horizontal forces (braking, centrifugal)
Cl. 206 — Seismic forces (aligned with IS 1893)
Cl. 207 — Temperature effects
Cl. 208 — Load combinations (Limit State Method)
Cl. 101 — Dead Loads
Cl. 103 — Live Loads
Cl. 104 — Standard Moving Loads for Bridges
Cl. 105 — Loads due to Wind
Cl. 106 — Loads due to Earthquake
Cl. 107 — Temperature Stresses
Cl. 109 — Impact or Dynamic Allowance
Cl. 110 — Centrifugal Force
Cl. 200 — Load Combinations for Bridges
What is Class 70R loading?+
The standard heavy vehicle for bridge design in India — a 70 tonne vehicle. Comes in two configurations: wheeled (7 axles, 1000 kN total) and tracked (two tracks, 700 kN total). Both must be checked. All NH/SH bridges must be designed for at least Class 70R.
What is the impact factor for bridges?+
A multiplier on live load to account for dynamic effects (vehicles bouncing, braking). For RCC bridges: 4.5/(6+L) for spans ≤12m. For steel: 9/(13.5+L). Impact decreases with longer spans. For a 10m RCC span: impact = 4.5/16 = 28%.
What is the primary difference between Class AA, Class A, and 70R loading in IRC 6:2017?+
Class AA loading represents a standard heavy military tracked vehicle, suitable for general bridges. Class A loading represents a standard wheeled vehicle, generally lighter than Class AA. 70R loading represents a standard heavy wheeled truck with a specific axle configuration, often used for specific traffic scenarios or as an alternative to Class A. The choice depends on the anticipated traffic and the importance of the bridge.
How is the impact factor calculated and why is it important?+
The impact factor (or dynamic allowance) accounts for the dynamic effect of moving loads on the bridge structure. It's calculated using formulas that relate to the span length, typically increasing for shorter spans and decreasing for longer ones. This factor is crucial because it increases the forces experienced by bridge components, leading to more conservative and safer designs.
What are the key considerations for wind load design according to IRC 6:2017?+
Wind load design involves determining the wind pressure based on the bridge's location, height, and the prevailing wind speeds. IRC 6:2017 provides guidelines for calculating wind pressure on different bridge elements, considering factors like wind speed squared, exposure coefficients, and the shape of the bridge components. It's particularly important for long-span bridges and those in high wind-prone regions.
When are seismic loads considered in bridge design?+
Seismic loads are considered for bridges located in earthquake-prone zones. IRC 6:2017, in conjunction with IS 1893, specifies seismic design parameters such as the seismic zone factor, importance factor, and soil type. The design ensures that the bridge can withstand seismic forces without catastrophic failure, even if some damage occurs.
What is the significance of load combinations in bridge design?+
Load combinations represent different realistic scenarios of loads acting on the bridge simultaneously. IRC 6:2017 provides specific combinations of dead load, live load, wind, seismic, temperature, etc., with appropriate load factors. Designing for these combinations ensures the bridge can safely withstand the most critical loading scenarios it's likely to encounter throughout its service life.
Can I use Class AA loading for all bridges under NHAI jurisdiction?+
While Class AA loading is a standard for many bridges, the specific loading requirement for NHAI projects is determined by the type and importance of the bridge, as well as anticipated traffic. Often, Class AA is used, but specific project requirements or endorsements from MoRTH might dictate otherwise. It's crucial to refer to the project's specific design brief and any special conditions.
What is the effective span for impact factor calculation?+
The effective span for impact factor calculation is typically taken as the clear span plus half the bearing depth at each end, or the centerline-to-centerline span of the girders. The specific definition can vary slightly depending on the bridge type and how the bearings are designed. Always refer to the relevant clauses for precise determination.
How does temperature variation affect bridge design?+
Temperature variations cause expansion and contraction of bridge materials, leading to internal stresses and deformations. For bridges with significant temperature ranges and longer spans, these stresses can be substantial. Design must accommodate these movements through expansion joints, bearings, and by ensuring that structural elements can withstand the resulting forces.
What is the role of 'dead load' in bridge design beyond the structure's self-weight?+
Dead load in bridge design includes not only the self-weight of structural elements like girders, deck slab, and piers but also the weight of the wearing coat, parapets, footpaths, utility services (pipes, cables), and any other permanent fixtures. Accurately estimating these additional dead loads is vital for an accurate structural analysis.
Are there any specific load requirements for bridges under the PMGSY program?+
Yes, bridges designed under the Pradhan Mantri Gram Sadak Yojana (PMGSY) often have specific load requirements, typically based on anticipated traffic in rural areas. While IRC 6 provides the framework, the implementing agencies and specific guidelines for PMGSY might specify particular load types or combinations to be adopted, often aiming for economy and practicality for rural roads.
What are the implications of an incorrect impact factor on a bridge's design life?+
Using an incorrect impact factor can lead to underestimation or overestimation of stresses. Underestimating the impact factor can result in fatigue damage over time, premature cracking, and a reduced design life. Overestimating it might lead to an overly conservative (and thus uneconomical) design with thicker sections or stronger materials than necessary.
How should eccentric loading be handled in bridge design?+
Eccentric loading occurs when the applied load is not symmetrical with respect to the bridge's axis of bending. This can lead to combined bending and torsional stresses. IRC 6:2017 requires careful analysis of such cases, often by resolving the eccentric load into a concentric load and a moment. Proper consideration of these effects is crucial for preventing premature failure, especially in deck slab and girder design.