Test: Prestress Concrete - 1 - Civil Engineering (CE) MCQ

Maximum central deflection,

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The concordant cable profile does not produce secondary moments and there will be no reactions at the supports due to prestressing action.

In the Freyssinet system anchorage consists of cylinder of good quality concrete and is provided with corrugations on the outside, it has a central conical hole and is provided with heavy top reinforcement.
In Gifford-Udall system, the wire are-stressed and anchored one by one in a separate cylinder using small wedding grip called Udall grips. Each grip consists of two split cones.
In Lee Me Call system anchoring of bars is done by screwing special threaded nuts. The nuts bear against a distribution plate provided at the end of the beam.
In the Magnel Blaton system, the wires are anchored by wedging, two at a time into sandwich plates. The sandwich plates are provided with two wedge shaped grooves on its two faces. The wires are taken into each groove and tightened. Then a steel wedge is driven between the tightened wires to anchor them against the plate.

In Freyssinet system, high tension steel wires 5 mm to 8 mm diameter about 12 in number are arranged to form a group into a cable with a spiral spring inside.
In the Gifford Udall system the wires are stressed and anchored one by one in a separate cylinder using small wedging grips called Udall grips.
In the Lee-Mc Call system, high tensile alloy steel bars are used as the prestressing tends.
In the Magnel Blaton system cable of rectangular section is provided. The wires are arranged with four wires per layer. The geometric pattern of the wires is maintained in the same form throughout the length of the cable by providing grills or spacers at regular intervals.

Loss of stress due to shrinkage and creep
= 200 x 10³ x 0.0008 = 160 MPa 
Effective stress left = 200 - 160 = 40 MPa

As per IS: 1343-1980 (code of practice for prestressed concrete), if finishes are to be applied to the prestressed concrete members, the total upward deflection should not exceed span/300, unless uniformity of camber between adjacent units can be insured.

As per clause 18.5.1 of IS: 1343-1980, at the time of initial tensioning, the maximum tensile stress immediately behind the anchorages shall not exceed 80% of the ultimate tensile strength of the wire or bar or strand.