IRC 22:2008 is the Indian Standard (IRC) for standard specifications and code of practice for road bridges — composite construction. IRC 22 covers composite steel-concrete bridge design — where steel girders work together with the concrete deck slab. Shear connectors (usually welded studs) transfer horizontal shear between steel and concrete. Economical for medium spans (20-40m).
Design of composite steel-concrete road bridges where steel girders act compositely with RCC deck slab through shear connectors.
Steel-concrete composite construction for road bridges — shear connectors, deck design, partial interaction.
| Reference | Value | Clause |
|---|---|---|
| Scope — composite girder span range— longer spans require IRC 112 / specialist design | Spans up to ~80 m typical | Cl. 600 |
| Concrete grade — minimum deck slab | M30 | Cl. 603.1 |
| Concrete grade — recommended— for severe exposure or pumped decks | M35–M40 | Cl. 603.1 |
| Steel grade — structural members | Fe-410W (E250) min | Cl. 603.2 (IS 2062) |
| Shear connector — typical type | Welded headed studs | Cl. 606.1 |
| Stud diameter — common range | 19, 22, 25 mm | Cl. 606.2 (Table 16) |
| Stud height — minimum | ≥ 4 × stud diameter | Cl. 606.2.1 |
| Stud spacing along beam — minimum | 5 × diameter | Cl. 606.2.3 |
| Stud spacing along beam — maximum | 600 mm or 4 × slab thickness | Cl. 606.2.3 |
| Edge distance for studs | ≥ 25 mm clear from slab edge | Cl. 606.2.3 |
| Effective flange width — interior beam— L = effective span; whichever less | L/4 or beam-spacing | Cl. 604.1 |
| Effective flange width — edge beam | L/12 + (b₁/2) | Cl. 604.1 |
| Modular ratio — short-term loading | 8 (Fe-410, M30) | Cl. 603.4 |
| Modular ratio — long-term (creep) | 16 (= 2 × short-term) | Cl. 603.4 |
| Limit state load combination — DL+LL | 1.35 DL + 1.5 LL | Cl. 605.1 (Table 14) |
| Differential shrinkage strain— for design of locked-in stresses | 200 × 10⁻⁶ | Cl. 603.6 |
| Deflection limit — under live load— for highway bridges | L/800 | Cl. 605.4 |
| Vibration limit — natural frequency | ≥ 5 Hz desirable | Cl. 605.5 |
| Fatigue stress range — Fe-410 stud weld | 60–80 N/mm² | Cl. 606.4 (Annex H) |
| Construction sequence stress check— stresses in steel before deck cures | mandatory | Cl. 605.2 |
IRC 22:2008 is Section VI — Composite Construction of the IRC standard specifications for road bridges. It covers the design of steel-concrete composite bridges — where a concrete deck slab acts structurally with underlying steel girders through shear connectors.
You use IRC 22 for: - Steel I-girder + concrete deck bridges (typical span 20-60 m) - Steel box girder + concrete deck bridges (span 30-120 m) - Steel truss + concrete deck bridges (long spans, railway-adjacent crossings) - Composite column design for bridge piers (rare in India, emerging)
Composite construction gives long spans at lower self-weight than all-RCC, and faster erection than cable-stayed. The penalty is higher material cost (steel is pricier than concrete) and more specialized fabrication.
Pair with: - IRC 24:2010 — steel girder design (the steel portion of composite bridges) - IRC 112:2020 — concrete deck design rules - IRC 6:2017 — loads - IRC 5:2015 — geometry - IS 2062:2011 — structural steel grades - IS 456:2000 — concrete deck material and cover
In a composite bridge, the steel girder and concrete deck are connected by shear studs or channel shear connectors so they deflect together as a single unit. This doubles or triples the effective stiffness of the girder alone.
Key parameters per IRC 22:
Shear connector design (Clause 606): - Stud shear connectors: 19 mm or 22 mm diameter, 100-150 mm long - Channel shear connectors: ISMC 75, 100, or 125 sections welded transversely - Longitudinal spacing: 150-300 mm for intense shear zones (near supports); 400-600 mm for low-shear regions - Total horizontal shear transfer = V_h per Clause 606 formula
Effective width of concrete flange (Clause 603): - For simply-supported span L: b_eff = L/8 (each side of girder web centreline), total ≤ girder spacing - For continuous span near interior supports: reduced by ~30% (Clause 603.3)
Design approach: IRC 22:2008 uses working stress method for stress check at service + ultimate strength check for collapse. IRC 22 is being updated to align with LSM, but 2008 edition remains current.
Material specifications: - Steel: IS 2062 E350 or E450 (higher-grade common for bridges) - Concrete: M40 minimum for deck slab - Shear studs: IS 1786 Fe 500D or equivalent ASTM A29 - Welding: IS 9595 or AWS D1.5 for bridge welding
Typical design sequence for a simple-span composite I-girder bridge (40 m span):
Step 1 — Geometry: deck width per IRC 5, number of girders (typically 4-6 for 2-lane), girder spacing 2.5-4 m, girder depth 1.8-2.5 m.
Step 2 — Loads: DL + SDL + LL (Class 70R + Class A) + impact + seismic + temperature. Per IRC 6:2017.
Step 3 — Section analysis at key locations: - Midspan (max sagging moment) — bottom flange tension, top flange compression, concrete compression - Quarter-span (intermediate) — shear check - Support (max shear, hogging for continuous) — web buckling, shear capacity
Step 4 — Concrete deck flange effective width per Clause 603.
Step 5 — Iterative design: steel section + reinforcement in deck + shear connector layout + stiffener design.
Step 6 — Deflection check: L/600 under live load + impact per IRC 22 Clause 604.
Step 7 — Fatigue check on steel per Clause 605 — critical for bridges with heavy truck traffic.
Step 8 — Construction stage analysis: - Before concrete cures: steel girder alone carries dead load of steel + deck - After concrete cures: composite section carries SDL + LL - Construction sequence (propped vs unpropped) significantly affects final stresses
1. Inadequate shear connector design. Shear connectors transfer horizontal force between concrete and steel. Under-designed connectors lead to slip (composite action breaks down) — bridge loses its stiffness advantage. Design per Clause 606 with 15% reserve factor.
2. Missing construction-stage check. Unpropped construction: steel girder alone carries ~60-70% of eventual service load before concrete hardens. Check that bare steel can handle this. Propped construction reduces this but adds falsework cost.
3. Wrong effective width at supports. For continuous bridges, the deck's effective width REDUCES near interior supports (hogging moment region) per Clause 603.3. Using midspan effective width uniformly over-estimates section capacity near supports.
4. Fatigue not checked. Steel bridges see cyclic loading (every truck passing = one cycle). Fatigue-critical zones: web-flange welds, shear connector welds, stiffener welds. Missing fatigue check has led to early distress in 20-30 year old Indian composite bridges.
5. Cover and corrosion for deck steel. The deck slab's reinforcement has exposure similar to RCC bridges. Apply IRC 112:2020 cover requirements per exposure class — IRC 22 doesn't explicitly specify these in detail.
6. Ignoring thermal effects on composite section. Steel and concrete have different thermal coefficients. In summer, the steel girder heats faster than concrete, causing curvature and deflection. Per Clause 607, include temperature gradient (typically 16°C ΔT between top and bottom) in load cases.
IRC 22:2008 is current but a 2025+ revision is expected to align with LSM (matching IRC 112) and Eurocode 4.
Indian composite bridge adoption: - NHAI and state highway projects increasingly prefer composite for spans 30-80 m. Faster construction vs RCC (prefabricated steel girders installed first, concrete deck cast in place) and lower cost than pure steel. - Mumbai Trans-Harbour Link, Bandra-Worli Sea Link approaches, Delhi Metro elevated sections — all use composite bridge construction - Technical knowledge is concentrated in a few specialist design consultancies (Tata Consulting Engineers, AECOM India, Intercontinental Consultants). State PWDs often lack in-house composite bridge expertise; many composite projects are NHAI-direct or major-consultancy-designed.
Supply chain reality: - Steel fabrication shops for bridge girders: ~30 in India with IRC 22/AWS D1.5 certification. Larsen & Toubro, Tata BlueScope, JSW Steel, Shri Shakti Traders dominate. - Lead time: 4-8 months for custom fabricated girders. Plan procurement early. - Transportation limit: girders over 30 m segment length need special road permits.
When to choose composite over all-RCC or all-steel: - Spans 25-60 m: composite is usually cheapest - Spans < 25 m: RCC (IRC 112) is cheaper and simpler - Spans > 80 m: cable-stayed, truss, or segmental PSC (IRC 112) - Cost-sensitive with long construction time: RCC - Time-sensitive (need fast erection): composite or steel
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