IRC Code: Guidelines for Design and Construction of Precast Pre-Tensioned Girders for Bridges (IRC:SP:71-2018)

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Precast pre-tensioned girders play a crucial role in modern bridge construction, offering superior strength, cost-efficiency, and rapid installation. The IRC:SP:71-2018 provides comprehensive guidelines for designing and constructing these girders, ensuring structural integrity and longevity. These guidelines align with IRC:112, incorporating the latest advancements in limit state design principles.

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Scope of IRC:SP:71-2018

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The guidelines in IRC:SP:71-2018 cover the design, materials, construction, and quality assurance for precast pre-tensioned girders used in bridges. These girders are utilized in simply supported, continuous, and integral bridge superstructures, enhancing durability and performance.

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Design Requirements for Precast Pre-Tensioned Girders

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1. Basis of Design

• Designs should comply with IRC:112 and adopt the Limit State Method (LSM).
• The design should consider Ultimate Limit State (ULS) and Serviceability Limit State (SLS).
• Load combinations should follow IRC:6.

2. Materials and Strength Requirements

• Concrete: Minimum M40 grade.
• Prestressing Steel: High-tensile strands conforming to IS:14268.
• Minimum Strength at Transfer: 80% of fck.
• Cement Type: OPC, PPC, or other suitable types as per IS:456.

3. Prestressing Losses

• Elastic shortening: As per IRC:112, dependent on modulus of elasticity of concrete and prestressing steel.
• Shrinkage and creep losses: Accounted based on loading sequence and material properties.
• Relaxation losses: Considered per Tables 6.2 and 6.3 of IRC:112.

4. Minimum Dimensions

• Top Flange Thickness: 100 mm
• Bottom Flange Thickness: 150 mm
• Web Thickness: 150 mm
• Deck Slab Thickness (where applicable): 150 mm

5. Reinforcement and Cover

• Minimum longitudinal reinforcement: 0.15% for M45 concrete and 0.18% for higher grades.
• Cover to prestressing tendons should follow IRC:112 provisions.
• Shear connectors must be designed as per IRC:112 Clause 10.3.4.

6. Transmission and Development Length of Tendons

• Transmission length is calculated as per IRC:112 Clause 15.3.2.2.
• Development length should be 40 times the nominal strand diameter beyond critical stress points.

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Construction Requirements

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1. Precasting Process

• Use automated batching plants with moisture control systems.
• Employ high-quality formwork ensuring leakproof joints.
• Ensure compaction using needle and form vibrators.
• Follow green cutting or surface retarders for proper bonding at construction joints.

2. Pretensioning and Detensioning

• Pretensioning should be done using hydraulic jacks, ensuring uniform elongation.
• Debonding of strands should be done using HDPE tubes, avoiding PVC tubes.
• Gradual detensioning should be performed to prevent sudden stress variations.

3. Handling and Transportation

• Lifting points must be as per approved drawings.
• Transport only after achieving 28-day concrete strength.
• Ensure stable arrangement to prevent toppling.

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Quality Control and Testing

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• Compression strength tests must be conducted on additional cubes to verify concrete strength at various stages.
• Surface preparation should be done using hacking or surface retarders.
• Regular inspection and calibration of equipment must be ensured.

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Conclusion

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The IRC:SP:71-2018 guidelines provide a structured approach to designing and constructing precast pre-tensioned girders for bridges. By adhering to these specifications, engineers can optimize material usage, ensure durability, and enhance overall bridge performance.

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