IRC Bridge Design Trilogy — IRC 6, IRC 21, IRC 78 Explained

A typical Indian road bridge sits on three IRC codes that work as a set: IRC 6:2017 defines the loads, IRC 21:2000 defines the design of the RCC superstructure, and IRC 78:2014 defines the substructure and foundation design. This article maps the boundary between them, explains what each contributes, and shows how a working bridge design flows across the three.

Critical update: IRC 112:2020 is the unified concrete bridge code — it supersedes IRC 21 for new bridge superstructure design (limit-state-based, replacing IRC 21's working-stress approach). IRC 21 is still cited for legacy retrofit and small bridges. We cover both here. For a deeper IRC 112 walkthrough, see our IRC 112 Explained guide.

The Three-Code Map

Code	Covers	What you produce
IRC 6:2017	Loads + load combinations on bridges — dead, live (Class AA/A/B), wind, seismic, temperature, impact, braking, centrifugal, footpath, hydrodynamic	Loading diagrams, governing live-load case, factored design loads
IRC 21:2000 → IRC 112:2020	RCC superstructure design — slab, girder, deck slab, RCC voided slab, T-beam & box girder	Member sizes, reinforcement, prestressing, crack-width check, deflection
IRC 78:2014	Substructure — pier / abutment design + foundations (open foundation, well foundation, pile foundation)	Pier dimensions, pier reinforcement, foundation type + size, capacity check

Step 1 — Loads per IRC 6:2017

IRC 6 lays out the load classes that determine every other downstream design number:

Class AA loading — heaviest. Tracked vehicle 700 kN or wheeled 400 kN. For National Highways, major State Highways, and military routes. IRC 6 Cl. 204.
Class A loading — standard. 27t truck train. Most district + state roads. The default unless the project is tagged for AA.
Class B loading — light. For temporary structures, low-traffic rural roads, low-importance bridges.
Class 70R — heavy single vehicle (70 ton wheel/tracked). Often the governing live load for short spans on AA routes. IRC 6 Annex A.

For each load class, IRC 6 gives detailed wheel-loading patterns, impact factors (varies with span and load class), and load combinations. Key load combinations (Cl. 202):

DL + LL ± wind
DL + LL ± seismic (per IRC SP 114 for new bridges, or IS 1893 historically — see also the IRC Bridge Seismic Map)
DL + LL + temperature + braking

Step 2 — Superstructure Design

If using IRC 21:2000 (legacy / small bridges)

IRC 21 uses working-stress method. Concrete grade typically M20-M40, steel Fe 415 or Fe 500. Key clauses:

Cl. 304 — permissible stresses (concrete: 7-13 N/mm² in flexure, depending on grade)
Cl. 305 — modular ratio (m = 10 for M20, 8 for M30+)
Cl. 306 — cover requirements (40 mm minimum for slab decks, 45-50 mm for soffit of slabs)
Cl. 307 — crack-width control limits

If using IRC 112:2020 (current — all new bridges)

Limit-state-based, aligned with EN 1992-2. M30 minimum concrete grade. Two limit states (ULS + SLS) with separate factor sets. Deflection + crack-width separately verified. The big payoff: more efficient sections compared to IRC 21's conservative WSM. See our IRC 112 walkthrough for the full procedure.

Step 3 — Substructure per IRC 78:2014

IRC 78 covers pier + abutment design, plus foundation type selection. Foundation categories:

Open foundation — spread footing on rock or stiff soil. IRC 78 Cl. 706. Used where SBC ≥ 200 kN/m² and scour depth is manageable.
Pile foundation — driven or cast-in-situ piles, single or grouped. IRC 78 Cl. 709. Most river bridges with deep alluvium use bored cast-in-situ RCC piles 800-1500 mm diameter. Backed by the testing regime in IS 2911 Part 4 — see our Pile Foundation guide.
Well foundation — open caisson sunk to bearing stratum. IRC 78 Cl. 708 + Cl. 710. Classic for major rivers like Ganga, Brahmaputra. Diameter 6-12 m typical.

Foundation depth is set by the scour depth (IRC 78 Cl. 703 + Annex 5) — calculated by Lacey's formula: d_sm = 1.34 × (D_b² / K_sf)^(1/3), where D_b is the discharge intensity. Foundation goes 2× scour depth below HFL. For sites with cyclone-induced rainfall, the design discharge is much higher — check the Cyclone Map.

Worked Mini-Example — 30 m Span Two-Lane Bridge

Highway bridge, 2-lane carriageway, 30 m simply supported span on alluvial site (river crossing).

IRC 6 — loads. Class AA wheeled 400 kN (single vehicle on bridge), impact factor 0.255 (span 30 m). Lane DL = 13 kN/m self-weight of deck. Effective design wheel load with impact = 502 kN.
IRC 112 — superstructure. 30 m T-beam + slab deck. Two main girders, each 1.8 m deep × 0.4 m web. M40 concrete, Fe 500D steel. Each girder takes ~50% of total load via distribution analysis (Courbon's method or grillage). Designed for ULS bending + shear; SLS deflection limit L/800 = 37.5 mm.
IRC 78 — substructure + foundation. Two RCC piers 5 m wide × 1.5 m thick, on bored cast-in-situ piles 1.0 m diameter × 18 m long, group of 6 per pier. Pile capacity verified by static formula + IS 2911 Part 4 load test. Foundation depth = HFL − scour − pile embedment in good bearing stratum.

Each step's output feeds the next. The bridge engineer's workflow is fundamentally: IRC 6 hands forces to IRC 21/112, which hands reactions to IRC 78, which checks the soil.

Common Pitfalls (Boundary Mistakes)

Using IS 1893 for bridge seismic instead of IRC SP 114. Building-code seismic ≠ bridge seismic. Bridges have different ductility behaviour, importance factors (typically 1.5 for major bridges), and response reduction. The IRC Bridge Seismic Map is distinct from the building seismic map.
Mixing IRC 21 WSM with IRC 112 LSM in the same design. Pick one — usually IRC 112 for any new bridge over 30 m span. Mixing leads to inconsistent factor application.
Ignoring scour for short-span bridges. Even small RCC slab bridges on flood-prone seasonal streams need IRC 78 Annex 5 scour check. Failure is sudden when missed.
Class A vs Class AA on State Highways. Specification varies — some SH stretches are AA, others A. Confirm against the project's IRC SP 84 / SP 73 traffic study before locking the live load.

Related InfraLens Resources

FAQ

Is IRC 21 still used after IRC 112?

IRC 21 is officially superseded by IRC 112:2020 for new bridge superstructure design. However, IRC 21 remains useful for: legacy retrofit assessment, small culverts and minor bridges where IRC 112 may be over-detailed, and matching the design basis of existing structures. Always confirm the project's contract or DPR specifies which code applies.

When does Class AA loading apply?

Per IRC 6 Cl. 204, Class AA applies on National Highways, important State Highways, expressways, and routes carrying heavy military traffic. The project authority specifies the load class in the DPR / contract. For two-lane highway bridges on NH, both Class A train AND Class AA wheeled/tracked must be checked — whichever governs.

Difference between IRC 6 seismic and IRC SP 114?

IRC 6 has the Annex on seismic with simplified provisions. IRC SP 114:2018 is the dedicated, detailed seismic-design guideline for road bridges — more sophisticated treatment of ductility, base isolation, performance-based design. For any major bridge in seismic zone IV/V, IRC SP 114 is mandatory. See the IRC Bridge Seismic Map.

Open / pile / well — how to pick the foundation type?

Decision tree per IRC 78: (1) Rock or stiff soil with SBC ≥ 200 kN/m² at shallow depth + scour manageable → open foundation. (2) Deep soft layers with hard stratum within 30 m → pile foundation. (3) Major river with very deep alluvium (50+ m) → well foundation. Cost typically: open < pile < well. Pile is the default for most modern bridges on alluvial sites.

How is bridge live load different from building live load?

Building live load (per IS 875 Part 2) is a uniform pressure (e.g. 4 kN/m² for offices). Bridge live load (per IRC 6) is a vehicular convoy — Class A train (27t truck + trailer), Class AA single heavy vehicle, or 70R tracked. The position is variable; the engineer places it for worst-case bending and shear via influence lines. Modern grillage software does this automatically.

Summary

An Indian road bridge sits on three pillars: IRC 6 → IRC 21/112 → IRC 78. Loads come from IRC 6, the superstructure design from IRC 21 (legacy) or IRC 112 (current), and the substructure + foundation from IRC 78. Add IRC SP 114 for seismic in higher zones. The 95% of bridge design work is competently navigating these four documents and applying them in the right order. For deeper dives on each, follow the linked InfraLens guides above.