Unit 1 anchorage systems

anchorage devices

1. PRETSTRESSED CONCRETE STRUCTURES UNIT – I INTRODUCTION – THEORY AND BEHAVIOUR

2. Pre tensioning system Post tensioning system Systems of prestressing

3. Pre tensioning systems

4. Hoyers long line method

5. Post tensioning system

6. Stages of post tensioning

7. The various types of devices used for tensioning steel are grouped under four principal catagories . Mechanical Hydraulic Electrical or Thermal Chemical Tensioning devices

8. Mechanical devices Hydraulic devices It includes weights with or without lever transmission. Geared transmission in conjunction with pulley blocks. Screw jacks with or without gear drives. In simple it is wire binding machines. Hydraulic jacks are the simplest means of producing large prestressed forces or extensively used as tensioning devices. This systems are used for the ranges of 5 to 100tonnes Large hydraulic jacks for forces in the range of 200 to 600tonnes have also been developed by baur-leonhardt .

9. Electrical or Thermal devices Chemical devices It have been used successfully in 1958 for tensioning of steel wires and deformed bars. The steel wires are electrically heated and anchored before placing of concrete in the moulds. It is also called as thermo-electric prestressing . Expanding cements are used and the degree of expansion is controlled by varying curing conditions. Since the expansive action of cement by sitting in restrained it induce tensile forces in tendon and compressive forces in concrete.

10. Freyssinet anchorage system Gifford Udall systems Lee-McCall systems Magnel blaton system BBRV system Baur Leonhardt system Post tensioning anchorages

11. Freyssinet anchorage system

12. Freyssinet anchorage system This system is widely used in Europe and India It consists of a cylinder with a conical interior through which the high tensile wires pass and against the walls of which the wedged by a conical plug lined longitudinally with grooves to house the wires. The main advantage of this system is that a large number of wires or strands can be simultaneously tensioned using double acting hydraulic jacks.

13. Gifford Udall systems

14. It is developed in UK. It consists of steel split cone and cylindrical female cone anchorages to house the high tensile wires bearing against steel plates. Each wire is tensioned separately and anchored by forcing a sleeve wedge into a cylindrical grip resting against a bearing plate. The ducts are generally formed by metal sheaths cast into the concrete member. Gifford Udall systems

15. Lee-McCall systems

16. In this method, the tendons comprise high tensile bars of diameter varying from 12 to 40mm which are threaded at the ends. After tensioning, each bar is anchored by screwing a nut and washer tightly against the end plates. In this system the forces are transmitted by the bearing at the end blocks. While the system eliminates the loss of stress due to anchorage slip, it has a disadvantage in that curved tendons cannot be used. Lee-McCall systems

17. Magnel blaton system

18. This system adopts metallic sandwich plates, flat wedges, and a distribution plate for anchoring the wires. Each sandwich plate can house up to four pairs of wires. The distribution plate may be cast into the member at the desired location. The number of wires in the magnel cable varies from 2 to 64. Magnel blaton system

19. BBRV System

20. This system is well suited for transmitting large forces. A BBRV tendon consists of several parallel lengths of high tensile wires, with each end terminating in cold formed button head with a machined anchorage fixture. In the case of tendons formed by strands, they are anchored to the machined fixture by split cone sleeves. BBRV System