Principles of prestressing pdf

It is used for large span structures like bridges. The disadvantages of post tensioning method are that it is costly as compared to pre tensioning method because of use of sheathing. In this system a cable of rectangular section is provided, which contains layers of wires 5 mm to 8 mm diameter.

The wires are arranged with four wires per layer up to sixteen layers deep for a cable of sixty four wires. The wires in the same layer and the wires in adjacent layers are separated with a clearance of 4 mm.

The grills do not offer any appreciable frictional resistance to the wires which can be moved relative to each other during the tensioning process. This is earliest method in this method the wires are stressed and anchored one by one in separate cylinder using small wedging grips called Udall grips. Each grip consists of two half cones.

The bearing plate bears against a thrust ring which is cast into the concrete. The duct end is encircled by a helix. Anchorages are supplied to suit the cables of 2, 4, 6 and 12 wires. In this system also the wires are tensioned individually. The anchorages consist of a single piece collet sleeve wedging in a conical hole.

A steel truncated guide leads each wire from the cable to the anchorage point along a gentle curvature. In addition to the guide a central block is also provided to anchor the central wires. This is the anchor system where the wires are tensioned individually.

As many wires as may be required not exceeding 12 are used in a circular cable. The size of the duct depends on the number of wires — mm diameter for 8 wires - cable and 50 mm diameter for 12 wires cables. An interval of mm spacers are provided to keep the wires separated and to prevent the outer wires from touching the sheathing. This not only reduces the friction but also allows satisfactory grouting.

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Prestressing of concrete beams by mild steel rods 8. A prestressed beam under an external load 9. Variation of length in a prestressed beam Linearly prestressed railway sleepers Forms of Prestressing Steel Wires - single unit made of steel.

Strands — 2,3 or 7 wires wound to form a prestressing strand. Tendon - group of strands or wires wound to form a prestressing tendon. Cross-section of a typical tendon Cable - group of tendons form a prestressing cable.

Bars - A tendon can be made up of a single steel bar. The diameter of a bar is much larger than that of a wire. Forms of reinforcing and prestressing steel High span-to-depth ratios bridges, buildings with large column-free spaces Span-to-depth ratios in slabs Typical precast members Pre-tensioned electric poles Post-tensioning of a box girder Applying tension to the tendons Casting of concrete Cutting of the tendons.

Prestressing bed, end abutment and mould Chuck assembly for anchoring tendons Harping of tendons Hold-down anchor for harping of tendons Tensioning of tendons Anchoring the tendon at the stretching end Casting bed, mould and duct Fine sand - used to avoid segregation. Desirable properties of grout 1 Fluidity 2 Minimum bleeding and segregation 3 Low shrinkage 4 Adequate strength after hardening 5 No detrimental compounds 6 Durable.

Various losses in prestress. Total views 5, On Slideshare 0. From embeds 0.