IRC Code: Guidelines for Design and Construction of Pretensioned Girder of Bridges (IRC:SP:71-2006)

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Pretensioned girders are widely used in modern bridge construction due to their ability to provide superior strength, durability, and cost efficiency. The IRC:SP:71-2006 lays down specific guidelines for the design and construction of precast pretensioned girders in bridges. This article provides a detailed overview of the guidelines, ensuring better understanding and compliance for engineers and construction professionals.

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Scope of IRC:SP:71-2006

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The guidelines in IRC:SP:71-2006 cover design and construction aspects of precast pretensioned girders used in bridges. These girders are an essential component of bridge superstructures, allowing for enhanced durability and load-bearing capacity. The document provides specifications for materials, stress limits, prestressing losses, quality assurance, and structural detailing.

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Design Requirements for Pretensioned Girders

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1. Material Properties

• Concrete for pretensioned girders must be of at least M40 grade.
• The minimum strength at transfer of prestress should be 0.8 fck or 35 MPa, whichever is lower.
• High-performance concrete (HPC) and steam curing can be used for achieving higher early strength.

2. Prestressing Losses

Losses in prestress occur due to several factors, including elastic shortening, creep, shrinkage, and relaxation of prestressing tendons.

• Elastic shortening loss: Calculated as per the modulus of elasticity of the prestressing steel and concrete.
• Relaxation loss: Dependent on the type of prestressing steel used.
• Shrinkage and creep: Need to be accounted for based on site conditions and material properties.

3. Minimum Dimensions of Girders

• Top flange thickness: 100 mm
• Bottom flange thickness: 150 mm
• Web thickness: 150 mm
• Deck slab thickness (if applicable): 150 mm

4. Reinforcement and Cover Requirements

• Adequate bonded reinforcement must be provided to account for tensile stresses.
• Cover to prestressing tendons should follow IRC:21 provisions.
• Stirrup reinforcement should be spaced closely near supports to handle high shear forces.

5. Transmission and Development Length of Tendons

• Transmission length (Lt) = 240 × strand diameter / √fck (for 7-wire strands as per IS:14268).
• Development length must be at least 40 times the nominal strand diameter beyond critical stress points.

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Construction Requirements

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1. Precasting of Girders

• Fully automated, computer-controlled batching plants should be used.
• Proper formwork and shuttering must be in place to achieve dimensional accuracy.
• Adequate curing measures should be adopted to prevent shrinkage cracks.

2. Pretensioning Process

• Single pull or multi-pull jacks can be used for stressing tendons.
• Stressing must be done symmetrically to avoid distortion.
• Reference marks should be established for measuring elongation of strands.

3. Debonding of Strands

• HDPE tubes should be used for debonding (PVC tubes are not recommended).
• Ends of the debonding tubes should be sealed with epoxy putty to prevent ingress of cement slurry.

4. Handling and Transportation

• Lifting points must be as per approved drawings.
• Adequate precautions should be taken to prevent damage to the girders during transit.
• Girders should only be transported after achieving 28-day strength.

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Quality Control and Testing

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• Stringent quality assurance must be maintained as per IRC:SP-47.
• Compression tests should be conducted on additional cubes to verify concrete strength at various stages.
• Surface preparation should be done using green cutting or surface retarders for effective bonding.
• Load tests can be performed if specified in the contract.

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Conclusion

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The IRC:SP:71-2006 guidelines provide a structured approach to designing and constructing precast pretensioned girders for bridges. These guidelines help ensure structural integrity, safety, and longevity of bridge superstructures. By following these provisions, engineers can optimize material usage, reduce construction time, and enhance overall bridge performance.

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