Explanatory Handbook to IRC:22-2015 Standard Specifications and Code of Practice for Road Bridges, Section VI-Composite Construction

Scope of IRC SP 120-2018 (Composite Steel Concrete Structures)

• Covers design, detailing, and construction of composite steel-concrete structures for bridges.
• Includes fatigue design, shear connectors, composite columns, filler beam decks, precast slabs on steel beams.
• Applicable for bridges under IRC loading and relevant Indian conditions.

Key Sections (from Preamble Table):

Important Notes:

• Terminology & Symbols: Clause 2.2 defines terms used throughout.
• Design Basis: Composite action between steel and concrete, mechanical shear connectors.
• Fatigue: Detailed fatigue strength calculations and exemptions.
• Shear Connectors: Spacing, strength, detailing, and partial shear connection.
• Composite Columns: Axial, bending, local buckling, and shear checks.
• Appendices: Provide formulas for moment resistance, plastic neutral axis location, and material properties.

Typical Formula for Shear Connector Design (C.6.3):

[ V_{u} = 0.29 \times f_{u} \times A_{sc} ]

Where:

• (V_u) = design shear strength of connector
• (f_u) = ultimate tensile strength of steel
• (A_{sc}) = cross-sectional area of shear connector

flowchart TD
    A[Composite Steel-Concrete Bridge] --> B[Fatigue Design]
    A --> C[Shear Connectors]
    A --> D[Composite Columns]
    A --> E[Filler Beam Decks]
    A --> F[Precast Slabs]
    B --> B1[Fatigue Strength Calculation]
    C --> C1[Spacing & Strength]
    D --> D1[Axial Compression & Bending]
    E --> E

IRC SP 120 - Terminology, Definitions & Symbols

While IRC SP 120 does not provide a dedicated clause for Terminology & Definitions explicitly, Clause 2 and C.2.2 cover key terms related to Steel-Concrete Composite Bridge Girders and Limit State Design.

Key Terminology & Symbols (Typical for Composite Bridge Design)

Typical Definitions

• Composite Section: A structural element made of steel and concrete acting together.
• Shear Connector: Devices that transfer shear between steel and concrete.
• Limit State Design: Design approach ensuring safety and serviceability by considering ultimate and serviceability limit states.

Important Notes

• Use Limit State Design principles for safety.
• Refer Clause 2 for detailed terminology related to composite girders.
• Symbols and terms align with IS 456 and IS 800 standards.

flowchart LR
    A[Steel Section] --> B[Composite Section]
    C[Concrete Deck] --> B
    B --> D[Shear Connectors]
    D --> E[Shear Transfer]

For detailed formulas and tables, refer to the respective chapters (e.g., Chapter 6 for Shear Connectors).

IRC SP 120-2018 — Major Design Provisions Summary

Key Chapters & Clauses Overview:

Important Design Provisions & Formulas:

1. Fatigue Design (C.5.2 - C.5.4)

• Fatigue strength calculation based on stress range and detail category.
• Use S-N curves and Miner’s rule for fatigue damage accumulation.

2. Shear Connectors (Chapter 6)

• Design Strength:
  [ P_u = 0.5 \times f_u \times A_{sc} ]
  where ( f_u ) = ultimate tensile strength of connector, ( A_{sc} ) = cross-sectional area.
• Spacing: Minimum and maximum spacing limits to avoid premature failure and ensure composite action.

3. Composite Columns (Chapter 7)

• Axial Load Resistance:
  [ P_{Rd} = \phi_c A_c f_{cd} + \phi_s A_s f_{yd} ]
  where ( A_c, A_s ) = areas of concrete and steel, ( f_{cd}, f_{yd} ) = design strengths, (\phi) = reduction factors.
• Combined Compression & Bending: Use interaction curves or formulas given in C.7.8.

4. Filler Beam Decks (Chapter 8)

• Section classification and bending moment calculations per clause C.8.3 and C.8.4

Girder Cross Sections (IRC SP 120 - Clause 4.2 & 603.1.4)

Cross Girder Section Properties (Clause 4.2)

• Top Flange: 400 × 20 mm thick
• Bottom Flange: 400 × 20 mm thick
• Composite Section Properties:

Main Girder Section Properties (Clause 603.1.4)

• Top Flange: 500 × 20 mm
• Bottom Flange: 500 × 20 mm (mid-span), 450 × 20 mm (support)
• Web: 1700 mm height × 12 mm thickness

Key Formulas, Tables & Specs for Actions on Bridges (IRC SP 120)

1. Actions on the Bridge (Clause 1.2 & IRC 6-2017)

• Self-weight of structural elements: Calculated from material densities and cross-sectional dimensions.
• Live loads & impact: As per IRC:6-2017, consider appropriate load factors and dynamic load allowances.
• Load combinations: Use limit state method combining dead, live, wind, seismic, and temperature effects per IRC:22-2015.

2. Material Properties

*fy varies with thickness:

• t < 20 mm → 350 N/mm²
• 20 < t ≤ 40 mm → 330 N/mm²
• t > 40 mm → 320 N/mm²

3. Modular Ratio (Clause 16.5)

[ m = \frac{E_s}{E_c} ]

• (E_s = 200, GPa) (steel)
• (E_c = 33, GPa) (short term), (16.5, GPa) (long term)

4. Girder Cross-Section (Clause 4 & 1.5)

• Deck slab thickness: thick RCC slab with 1.5 m cantilever beyond outer girders.
• Composite action with profiled steel decking.
• Use modular ratio for composite section analysis.

5. Reference Codes for Actions & Design

• IRC:6-2017 —

IRC SP 120 - Global Analysis Key Points

1. Global Analysis Criteria (Clause 8.2 & 6)

• Consider non-uniform transverse loads after concrete hardening.
• Account for deflection differences between adjacent filler beams.
• Simplified analysis with one rigid cross-section allowed if accuracy is acceptable.
• Shrinkage and slip effects between steel and concrete can be ignored since composite action is assumed.
• In transverse bending, steel beams are ignored for resistance calculations.

2. Time-based Effects on Concrete (Clause 124.76)

3. Stress due to Temperature & Restraint (Typical Section)

Summary:

• Use composite action from 3 days onwards.
• Consider deflection compatibility for transverse load distribution.
• Ignore steel contribution in transverse bending.
• Use the stress table for design checks under temperature and restraint effects.

IRC SP 120 - Limit State of Strength (Clause 7.2)

Key Concepts:

• Limit State of Strength ensures that the structure can withstand maximum expected loads without failure.
• It involves checking ultimate loads with appropriate partial safety factors.

General Formula for Ultimate Load:

[ \text{Design Load} = \gamma_f \times \text{Characteristic Load} ]

• (\gamma_f) = Load factor (typically 1.5 for dead and live loads)

Design Strength:

[ \text{Design Strength} = \frac{\text{Characteristic Strength}}{\gamma_m} ]

• (\gamma_m) = Material partial safety factor (e.g., 1.5 for concrete, 1.15 for steel)

Typical Checks:

• Bending moment: (M_u \leq M_{design})
• Shear force: (V_u \leq V_{design})
• Axial load: (P_u \leq P_{design})

Example Partial Safety Factors (IRC SP 120):

Summary:

• Use factored loads for ultimate design.
• Apply material safety factors to characteristic strengths.
• Check all strength parameters against design values.

flowchart LR
    A[Characteristic Load] --> B[Apply Load Factor \(\gamma_f\)]
    B --> C[Design Load]
    D[Characteristic Strength] --> E[Apply Material Factor \(\gamma_m\)]
    E --> F[Design Strength]
    C --> G[Compare with Design Strength]
    F --> G
    G --> H{Safe?}
    H -->|Yes| I[Design OK]
    H -->|No| J[Modify Design]

For detailed formulas and tables, refer to Clause 7.2 of IRC SP 120 and relevant IS codes for materials.

Key Specifications & Formulas for Filler Beam Decks (IRC SP 120, Chapter 8)

• Girder Type: Can be simply supported or continuous due to steel flanges taking tensile bending stresses top & bottom.

• Beam Type: Rolled or welded beams allowed; skew angle ≤ 30° for uniform stress distribution.

• Depth Limits: 250 mm ≤ Depth ≤ 1100 mm (balance deflection & dead load).

• Web Spacing: Restricted to ensure uniform load distribution; exact spacing depends on design but must maintain uniformity.

• Clear Distance Between Steel Flange Edges: Minimum 150 mm for proper concrete pouring.

• Concrete Cover: Minimum cover specified to prevent corrosion and abrasion (typically 25-40 mm depending on exposure).

• Surface Preparation: Steel beams must be de-scaled for bonding; no additional shear connectors needed.

Typical Design Checks

• Flexural Strength:
  [ M_u \leq \phi M_n ] where (M_n) = nominal moment capacity of composite section.

• Deflection Limits:
  [ \delta \leq \frac{L}{250} \quad \text{(for live load)} ]

• Spacing of Filler Beams:
  Maintain web-to-web spacing for uniform load transfer; typical spacing ~ 1.0 to 1.5 m depending on load and girder depth.

Summary Table

graph LR
A[Filler Beam Deck] --> B[Steel Beams (Rolled/Welded)]
A --> C[Concrete Deck]
B --> D[Steel Flanges (Top & Bottom)]
C --> E[Concrete Cover & Pouring]
B --> F[De-scaled Surface for Bonding]

This ensures composite action without shear connectors, uniform load distribution, and durability.

Design of Stiffeners (IRC SP 120)

1. Types of Stiffeners:

• Intermediate Web Stiffener: Prevents web buckling, provided along main and cross girders.
• Load Carrying Stiffener: At bearing, cross girder, and jack locations to transfer loads.

2. Key Design Checks:

A) Intermediate Web Stiffener:

• Check stiffener force: [ F = \frac{V - V_a}{\gamma_m} \leq F_{or} ]
  • If ( V < V_{on} ), no stiffener force exists.

B) Load Bearing Stiffener:

• Shear capacity check of end panel (Clause 509.5, IRC:24-2010).
• Modular ratio ( m = \frac{E_s}{E_c} = \frac{200000}{33000} = 6.06 < 7.5 ).
• Adjusted modular ratios for creep:
  • Permanent loads: ( m_p = \frac{m}{K_c} = \frac{7.5}{0.5} = 15.0 )
  • Transient loads: ( m_t = 7.5 )

3. Section Classification (Example at Midspan):

• Effective web width after deducting ineffective width: [ b_{eff} = 1700 - 384 = 1316 \text{ mm} ]

IRC SP 120 does not explicitly provide clauses on shear connectors. However, general practice for design and spacing of shear connectors in composite construction (as per IS 11384 and IS 456) can be applied:

Key Formulas for Shear Connectors

• Shear strength of one stud:
  [ P_u = 0.8 \times A_s \times f_u ]
  where,
  (A_s) = cross-sectional area of stud,
  (f_u) = ultimate tensile strength of stud material.

• Spacing limits:

  • Longitudinal spacing ( \geq 3d ) (d = diameter of stud)
  • Transverse spacing ( \geq 6d )
  • Maximum spacing to ensure composite action typically ( \leq 300 , mm )

Typical Specifications

• Stud diameter: 16 mm or 19 mm
• Stud height: 75 mm to 100 mm
• Minimum edge distance: 50 mm from slab edge

Detailing Tips

• Weld studs properly to flange of steel beam
• Avoid studs near beam ends or supports where shear is low
• Ensure uniform distribution to transfer shear effectively

graph LR
A[Steel Beam Flange] --> B[Shear Connector (Stud)]
B --> C[Concrete Slab]

This ensures composite action by transferring shear forces between steel and concrete. For detailed design, refer to IS 11384 or Eurocode 4.

Durability Considerations as per IRC SP 120 (Clause C.1.6.4)

Key Points:

• Corrosion Protection:

  • Use zinc-based primers after shop fabrication for effective corrosion resistance.
  • Protect primer coating during transportation and erection.
  • Avoid painting friction grip bolt surfaces to ensure friction development.

• Touch-up:

  • Minor scratches after erection should be touched up with the same primer.

• Paint System:

  • Intermediate coat: Micaceous Iron Oxide based paints (best for humid/acidic conditions).
  • Final coat: Polymer-based acrylic or high-build chlorinated rubber paint for enhanced weather resistance.

• Dry Film Thickness (DFT):

  Coat Type	DFT (µm)
  Primer	20 - 25
  Intermediate coat	75 - 85
  Final coat	~75

• Environmental Adaptation:

  • Thickness and paint type depend on exposure conditions and required protection duration.

Summary Table for Paint Thickness

flowchart TD
    A[Steel Fabrication] --> B[Apply Zinc-based Primer (20-25µm)]
    B --> C[Transportation & Erection (Protect Primer)]
    C --> D{Friction Grip Bolts?}
    D -- Yes --> E[Leave Bolt Areas Unpainted]
    D -- No --> F[Full Painting]
    E --> G[Touch-up Minor Scratches]
    F --> G
    G --> H[Apply Intermediate Coat (Micaceous Iron Oxide, 75-85µm)]
    H --> I[Apply Final Coat (Polymer/Acrylic, ~75µm)]
    I --> J[Enhanced Corrosion Resistance & Durability]

Note: Proper painting and maintenance significantly extend steel structure life by minimizing corrosion-related thickness loss.

IRC SP 120-2018: Construction and Erection (Clause C.9.7)

This clause provides guidelines to ensure safe and efficient construction and erection of precast slabs on steel beams.

Key Specifications:

• Precast slabs must be properly aligned and supported during erection to avoid undue stresses.
• Temporary supports/shoring should be designed to carry construction loads safely.
• Handling and lifting of precast elements must follow manufacturer and code recommendations to prevent damage.
• Ensure proper sequencing to maintain structural stability.
• Joints and connections should be checked during erection for tightness and alignment.

Important Considerations:

• Follow Clause C.9.4 to C.9.6 for joint detailing and structural connections.
• Use appropriate testing methods (Clause C.9.8) post-erection to verify integrity.
• Adhere to fire resistance (Clause C.9.9) and maintenance (Clause C.9.10) guidelines.

General Formula for Temporary Support Design:

[ P_{support} \geq \text{Max Construction Load} + \text{Safety Factor} ]

Summary Table: Construction Checks

flowchart TD
    A[Precast Slab Fabrication] --> B[Transportation]
    B --> C[Site Handling & Storage]
    C --> D[Positioning & Alignment]
    D --> E[Temporary Support Installation]
    E --> F[Jointing & Connections]
    F --> G[Inspection & Testing]
    G --> H[Final Erection Completion]

For detailed design values and procedures, refer to IRC:SP:120 Clauses C.9.4 to C.9.10.

IRC SP 120: Design for Fatigue Limit (Clause 7.3)

Key Points on Fatigue Limit Design:

• Fatigue Limit State addresses failure due to repeated stress cycles below ultimate strength.
• Fatigue strength reduces with increasing number of load cycles (S-N curve concept).

Fatigue Strength Calculation (Typical Approach):

[ \sigma_f = \frac{f_{ut}}{N^{m}} ]

Where:

• (\sigma_f) = Fatigue strength at (N) cycles
• (f_{ut}) = Ultimate tensile strength of material
• (N) = Number of cycles
• (m) = Material constant (usually 3 to 5 for steel)

Design Specifications:

• Use S-N curves (Stress vs. Number of cycles) for material fatigue data.
• Apply safety factors on fatigue strength.
• Consider stress concentration factors for welded/jointed parts.
• Limit stress range (\Delta \sigma) to below fatigue limit for infinite life.

Typical Fatigue Design Table (Excerpt):

graph LR
A[Repeated Load Cycles] --> B[Stress Range \(\Delta \sigma\)]
B --> C{Is \(\Delta \sigma\) < Fatigue Limit?}
C -- Yes --> D[Infinite Life Design]
C -- No --> E[Finite Life Design]
E --> F[Calculate \(\sigma_f\) using S-N curve]
F --> G[Apply Safety Factors]

Summary: Design for fatigue limit in IRC SP 120 involves limiting stress ranges based on S-N curves, applying safety factors, and ensuring stress concentrations are minimized to prevent fatigue failure.

IRC SP 120-2018: Materials and Properties (Appendix III)

Appendix III details materials and their properties essential for design:

Key Materials:

• Structural Steel:
  • Yield strength (f_y) typically 250 MPa or 415 MPa.
  • Modulus of Elasticity (E) = 2 × 10^5 MPa.
• Concrete:
  • Characteristic compressive strength (f_ck) varies (e.g., M25, M30).
  • Modulus of Elasticity (E_c) ≈ 5000√f_ck MPa.
• Reinforcement Steel:
  • Yield strength (f_y) = 415 MPa or 500 MPa.
  • Modulus of Elasticity (E_s) = 2 × 10^5 MPa.

Typical Properties Table (Excerpt):

Notes:

• Use characteristic strengths for design.
• Material partial safety factors as per IS 456 and IS 800.
• Refer to Appendix III for detailed tables and specific grades.

flowchart TD
    A[Materials] --> B[Structural Steel]
    A --> C[Concrete]
    A --> D[Reinforcement Steel]

    B --> B1[Yield Strength: 250/415 MPa]
    B --> B2[Elastic Modulus: 200 GPa]

    C --> C1[Compressive Strength: 25 MPa (M25)]
    C --> C2[Elastic Modulus: ~25 GPa]

    D --> D1[Yield Strength: 415/500 MPa]
    D --> D2[Elastic Modulus: 200 GPa]

For precise design values, always consult Appendix

IRC SP 120: Worked-Out Examples - Key Points

The Standard Worked-Out Example is detailed in Appendix IV (Page 55) of IRC:SP:120-2018, illustrating design steps for composite steel-concrete bridges.

Key Specifications & Formulas:

• Moment of Resistance (Appendix I):
  For composite sections,
  [ M = \sum F_i \times X_i ] where (F_i) = force in each component, (X_i) = lever arm from neutral axis.

• Plastic Neutral Axis Location (Appendix II):
  Position (h) is critical for composite columns; equations (II.1 to II.7 in IRC:22) define (h) based on section type.

• Buckling Resistance Moment (C.I.5):
  Steel girders during construction resist lateral-torsional buckling per IRC:24.

• Material Properties (Appendix III):
  Refer to IRC:22 Annexure III for steel, concrete grades, welding, and reinforcement specs.

Worked Example Highlights:

• Stepwise design of composite slab on steel beams.
• Calculation of neutral axis, moment capacities.
• Shear and reinforcement checks (Clauses C.8.5, C.8.6).
• Joints and connections per Chapter 9.

flowchart TD
    A[Start: Bridge Description] --> B[Determine Loads]
    B --> C[Select Section & Materials]
    C --> D[Calculate Neutral Axis Position]
    D --> E[Compute Moment of Resistance]
    E --> F[Check Shear & Reinforcement]
    F --> G[Design Joints & Connections]
    G --> H[Verify Buckling Resistance]
    H --> I[Finalize Design & Documentation]

For detailed stepwise calculations, refer Appendix IV of IRC:SP:120-2018.