IS: 1343-1980 limits the minimum characteristic strength of prestressed concrete for post-tensioned work and pretension work as
A simply supported post-tensioned prestressed concrete beam of span L is prestressed by a straight tendon at a uniform eccentricity ‘o’ below the centroidal axis. If the magnitude of prestressing force is P and flexural rigidity of beam is EI, the maximum central deflection of the beam is
Maximum central deflection,
Concordant cable profile is
The concordant cable profile does not produce secondary moments and there will be no reactions at the supports due to prestressing action.
The stress block in concrete for an estimate of ultimate strength in flexure of a prestressed beam
In the Limit state design of prestressed concrete structures, the strain distribution is assumed to be
Match List-I (Post-tensioning system) with List-ll (Type of anchorage) and select the correct answer using the codes given below the lists:
D. Magnel Blaton
1. Flat steel wedges in sandwich plates
2. High strength nuts
3. Split conical wedges
4. Conical serrated concrete wedges
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.
Match List-I (Post-tensioning system) with List-ll (Arrangement of tendons in the duct) and select the correct answer using the codes given below the lists:
1. Single bars
2. Wires evenly spaced by perforated spacers
3. Horizontal rows of four wires spaced by metal grills
4. Wires spaced by helical wire core in annular Spacer
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.
An ordinary mild steel bar has been prestressed to a working stress of 200 MPa. Young’s modulus of steel is 200 GPa, Permanent negative strain due to shrinkage and creep is 0.0008. How much is the effective stress left in steel?
Loss of stress due to shrinkage and creep
= 200 x 103 x 0.0008 = 160 MPa
Effective stress left = 200 - 160 = 40 MPa
What is the allowable upward deflection in a prestress concrete member under serviceability limit state condition?
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.
At the time of initial tensioning, the maximum tensile stress immediately behind the anchorage should not exceed which one of the following?
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.