IRC SP 67:2005 outlines detailed directives for implementing external and unbonded prestressing tendons in bridge constructions. It covers aspects such as design criteria, material specifications, detailing instructions, and protective methods to ensure the structural soundness and longevity of prestressed concrete bridges conforming to Indian and global standards.
Overview
IRC SP 67:2005 outlines detailed directives for implementing external and unbonded prestressing tendons in bridge constructions. It covers aspects such as design criteria, material specifications, detailing instructions, and protective methods to ensure the structural soundness and longevity of prestressed concrete bridges conforming to Indian and global standards.
Audience
Contents
Structure
IRC SP 67 establishes fundamental principles and specifications for the design, materials, detailing, and safeguarding of prestressing tendons in bridge applications.
Scope: Addresses design parameters, material criteria, detailing practice, and protective measures for prestressing steel.
Cable Vibration Frequency Equation: [ f = \frac{F}{2L \sqrt{m}} ] Where:
Minimum Curvature Radius at Deviators (Table 1):
| Tendon Strand Size | Min Radius (m) |
|---|---|
| 19 strands of 13 mm & 12 strands of 15 mm | 2.5 |
| 31 strands of 13 mm & 19 strands of 15 mm | 3.0 |
| 53 strands of 13 mm & 37 strands of 15 mm | 5.0 |
| Single strands (13, 15, 18 mm diameter) | 40 times nominal strand diameter |
Protection of Prestressing Steel:
Ultimate Moment of Resistance Assumptions:
Referenced Codes: IRC:6, IRC:18, IRC:24, BS 4447-1973, FIB 1993.
This section clarifies the extent of IRC SP 67, focusing on unbonded prestressing applications in bridge structures.
Encompasses design, material selection, detailing, protection, and operational use of unbonded tendon systems.
Suitable for external tendons encased in HDPE or steel ducts, emphasizing leak-proof ducting per Clause 1.1.
Includes design of deviators, anchorages, and related accessories aligned with IRC:24-2001 and IRC:21-2000.
Addresses durability and protection requirements during construction and service life (Clause 7).
Cross references other IRC standards (IRC:6-2000, IRC:18-2000, IRC:21-2000) and international norms like BS 4447-1973 and FIB guidelines.
Adopts a design philosophy treating prestressing partly as load and partly as strength contributor, ensuring verification at both Serviceability and Ultimate Limit States.
Minimum Radius of Curvature at Deviators (Clause 6.3):
| Tendon Size | Minimum Radius (m) |
|---|---|
| 19-13 mm and 12-15 mm | 2.5 |
| 31-13 mm and 19-15 mm | 3.0 |
| 53-13 mm and 37-15 mm | 5.0 |
| Single strands (13, 15, 18 mm) | 40 times nominal diameter |
[ f = \frac{1}{2L} \sqrt{\frac{F}{m}} ]
Guidelines for the use of unbonded prestressing tendons as specified in IRC SP 67.
Sheathing:
Components:
Anchorages and Guide Tubes:
Design Considerations:
Load Combinations:
Sheath Pressure Resistance Summary Table included.
Essential material requirements mandated by IRC SP 67.
Sheathing:
Prestressing Steel Protection:
Deviators and Anchorages:
Minimum Radius of Curvature for Tendons at Deviators referenced from Table 1.
Ultimate Moment of Resistance assumptions detailed, including strain limits and strength reduction factors.
Critical design formulas and tables stipulated in IRC SP 67.
Frequency of Cable Vibration (Clause 1.2): [ f = \frac{1}{2L} \sqrt{\frac{F}{m}} ] where variables denote vibration frequency, support spacing, tension force, and mass per unit length respectively.
Minimum Radius of Curvature at Deviators (Clause 6.3):
| Tendon Strand Size | Minimum Radius (m) |
|---|---|
| 19-13 mm & 12-15 mm | 2.5 |
| 31-13 mm & 19-15 mm | 3.0 |
| 53-13 mm & 37-15 mm | 5.0 |
| Single strands (13,15,18mm) | 40 times nominal diameter |
Ultimate Moment of Resistance (Appendix Summary):
Shear Resistance Guidelines (Clause 5.3.3):
Design of Anchorages and Deviators (Clause 5.3.4):
Essential detailing instructions for prestressed concrete elements according to IRC SP 67.
| Tendon Strand Size | Minimum Radius (m) |
|---|---|
| 19-13 mm & 12-15 mm | 2.5 |
| 31-13 mm & 19-15 mm | 3.0 |
| 53-13 mm & 37-15 mm | 5.0 |
| Single strands (13,15,18mm) | 40 times nominal diameter |
Protection of Prestressing Steel (Clause 7):
Alignment and Anchorage (Clauses 4.4, 6.1):
Ultimate Strength and Section Analysis:
Frequency of Cable Vibration formula reiterated.
Measures to protect prestressing steel, anchorages, and deviators as per IRC SP 67.
Temporary Protection:
Permanent Protection:
Regular inspection and maintenance to ensure longevity.
Curvature Radius Requirements (Table 1) applied to prevent tendon damage.
Sheathing Specifications:
Design and Load Considerations:
Cable Vibration Frequency formula included.
Diagram illustrating tendon protection and deviator setup.
Key parameters and requirements for prestressing in other structural forms according to IRC SP 67.
Frequency of Cable Vibrations (Clause 1.2): [ f = \frac{1}{2L} \sqrt{\frac{F}{m}} ]
Minimum Radius of Curvature at Deviators (Clause 6.3):
| Tendon Strand Size | Radius of Curvature (m) |
|---|---|
| 19-13 mm & 12-15 mm | 2.5 |
| 31-13 mm & 19-15 mm | 3.0 |
| 53-13 mm & 37-15 mm | 5.0 |
| Single strands (13,15,18mm) | 40 times nominal diameter |
Protection of Prestressing Steel (Clause 7):
Ultimate Moment of Resistance Principles:
Reference documents include IRC:6-2000, IRC:18-2000, IRC:24-2001, BS 4447-1973, and FIB 1993.
Compilation of essential formulas, tables, and standards referenced in IRC SP 67.
Frequency of Vibrations (Clause 1.2): [ f = \frac{1}{2L} \sqrt{\frac{F}{m}} ]
Minimum Radius of Curvature at Deviators (Clause 6.3):
| Tendon Sizes | Minimum Radius (m) |
|---|---|
| 19-13 mm & 12-15 mm | 2.5 |
| 31-13 mm & 19-15 mm | 3.0 |
| 53-13 mm & 37-15 mm | 5.0 |
| Single strands (13,15,18mm) | 40 times nominal diameter |
Protection of Prestressing Steel (Clause 7):
Ultimate Moment of Resistance (Appendix A-1(1)):
Referenced Codes:
Summary diagram of tendon arrangements included.
Detailed procedure and assumptions for determining ultimate moment capacity as per IRC SP 67 and IRC 18-2000.
Assumptions:
Ultimate Moment Principles:
Material Safety Factors:
| Material | Safety Factor |
|---|---|
| Concrete | 1.5 |
| Steel | 1.15 |
Stress-Strain Parameters:
Load Factors (Clause 5.1):
Simplified Analysis Steps:
Frequently Asked
Permissible prestressing steel for external and unbonded tendons includes strands housed within HDPE or metallic steel sheaths featuring smooth internal surfaces. The steel must conform to Indian Standards referenced in IRC codes (e.g., IRC:21-2000). Typically, high-strength, low-relaxation strands or cables made from high tensile steel wires are used. Steel components for deviators, anchorages, and fasteners must comply with IRC:24-2001. Anchorages and tendons are factory-manufactured under strict quality control and must pass dynamic fatigue testing of two million cycles per FIB or BS 4447-1973 standards to ensure durability and performance.
Anchorages, deviators, brackets, and concrete blisters must be designed for the full nominal ultimate tendon capacity as a design load. Materials should meet IRC:24 for steel and IRC:21 for concrete reinforcement, with anchorages preferably replaceable or reusable and factory-manufactured under strict QA/QC. Tendons must be housed in HDPE or metallic sheaths that are fully leak-tight to prevent corrosion. Protective coatings and leak-tight joints are essential. Local concrete zones around anchorages must be reinforced against bursting and spalling according to manufacturer recommendations. Deviators and anchorages should be spaced so that straight tendon segments do not exceed 12 times the beam depth or 12 meters, ensuring proper stress transfer and tendon seating.
Recommended sheathing materials include High Density Polyethylene (HDPE) and metallic steel sheaths, which may be plain or coated (galvanized, epoxy-coated, or lubricated). Sheaths must have smooth internal surfaces to facilitate tendon installation and grouting. All ducts and joints must be fully leak-tight, resisting pressure equal to 1.1 times the sum of the maximum expected gravity head and grouting pressure. Materials and fabrication must comply with relevant Indian or IRC standards such as IRC:18-2000 and IRC:24-2001. Factory certification and acceptance testing, including dynamic fatigue tests as per FIB or BS 4447-1973, are necessary to ensure durability.
The standard treats prestressing loads partly as permanent loads varying between initial prestress and post-loss levels (±20%) and partly as contributors to the load-resisting mechanism. Ultimate load factors applied are 1.0 for determinate structures, while indeterminate structures use 1.2 for unfavorable and 0.9 for favorable load combinations. The tendon’s contribution to flexural capacity is considered additional strength beyond initial prestress, calculated by shifting the stress-strain origin accordingly. Design follows IRC:5-1998, IRC:18-2000, and IRC:21-2000 load combination provisions, supplemented by specific strength models for external and unbonded prestressing.
Prestressing systems must undergo rigorous quality assurance and testing. Sheathing ducts must be HDPE or metallic with smooth interiors and fully leak-tight at 1.1 times the combined gravity head and grouting pressure. Materials must conform to IRC standards for steel, welding, and reinforcement. Anchorages and tendons should be factory-made under strict QA/QC, approved by competent authorities, and tested per FIB or BS 4447-1973 standards. Dynamic fatigue testing requires two million cycles on cable and anchorage assemblies. Independent laboratory certification and sampling of at least three specimens from supply batches are compulsory to verify compliance and ensure durability and reliability.
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