*PED061*

PED – 061

I Semester M.E. (PSC) (civil) Degree Examination, January 2015 (2K8 Scheme) 2K8PC101 (PSC/Str) : PRE-STRESSED CONCRETE STRUCTURES Time : 3 Hours

Max. Marks : 100

Instructions : 1) Answer any five full questions. 2) Use of IS -1343 and IS - 456 is permitted. 3) Assume any missing data suitably. 1. A prestressed concrete beam of section 150mm × 300mm is used over an effective span of 8 m to support a concentrated load of 40 kN and the self-weight of the beam. The beam is prestressed by an inclined cable carrying a force of 500 kN and located at an eccentricity of 80 mm below the neutral axis at the center of the span and concentric at the supports. a) Determine the location of Thrust line in the beam, plot its position at quarter and mid span. b) Plot the stresses at various stages at mid-span. c) Compare the results using Force concept of analysis.

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2. A Pre-Tensioned beam 250mm × 360mm is prestressed by 10 wires of 8 mm diameter, initially stressed to 1000 N/mm2 . The C.G. of steel wires is located at 105 mm from the soffit. Determine the maximum stress in concrete immediately after transfer, allowing elastic shortening of concrete only at the level of C.G. of steel. If however, the concrete is subjected to additional shortening due to creep and shrinkage and the steel is subjected to relaxation of stress of 5% find the % loss of stress in steel wires. Take Es = 210 kN/mm2, Ec = 36.85 kN/mm2, Creep 20 Coefficient ( φ ) = 1.60 and Total residual shrinkage strain = 3 × 10–4. 3. A simply supported prestressed concrete beam having a rectangular cross section 150mm × 450mm spans over 10 m. It is prestressed by a parabolic cable with an eccentricity of 60 mm at mid span and concentric at supports. The cable consists of 7 wires of 7 mm diameter and carries an intial prestress of 1100 N/mm2. If the expected loss of prestress is 18% and the modulus of rupture is 4 N/mm2. Determine the central deflection of the beam under: a) Self-weight b) Self-weight + Prestress + Live load of 2.75 kN/m c) Cracking Load and d) 1.4 times the working load.

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*PED061*

PED – 061

4. A simply supported beam of 10 m is subjected to a live load of 16 kN/m. Design a rectangular section, post tensioned Type-I beam for the following data by Limit state method; f ck = 35 N /mm2, f p = 1600 N/mm 2 and fci = 30 N/mm 2. The compressive stresses are likely to increase in service. 20 5. a) Write notes on Concordant Cable profile.

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b) A continuous beam has two spans each of 10 m. It is prestressed with a parabolic cable having zero eccentricity at the supports and a maximum eccentricity of 200 mm at mid – section of both the spans. The cross section of the beam is 200 mm × 800mm, if the effective Prestressing force is 300 kN, determine secondary moment and the resultant moment developed at (B). Sketch the pressure line due to prestress alone and suggest a concordant cable profile. 15 6. a) Write a brief note on shear cracks.

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b) A post tensioned bonded prestressed concrete beam of T – Section has a flange width of 1500 mm and 200 mm thick, the thickness of the web is 300 mm. The area of high tensile steel is 5000 mm2, located at an effective depth of 1800 mm. If the characteristic strength of concrete and steel are 40 N/mm2 and 1600 N/mm2, calculate the flexural strength of the section. 15 7. A composite T-girder of span 5 m is made up of a pre-tensioned rib 100 mm × 200 mm, with an in-situ cast slab, 400 mm wide and 40 mm thick. The rib is prestressed by a straight cable having an eccentricity of 33.33 mm and carrying an initial force of 150 kN. The loss of prestress may be assumed to be 15%. Check the composite beam for the limit state of deflection if it supports an imposed load of 3.2 kN/m for a) propped construction and b) un-propped construction. Assume E = 35 kN/mm2 for both pre-cast and cast-in-situ elements. 8. Explain any four of the following ; a) Immediate losses and Time-bound losses in prestress. b) Load Balancing Concept. c) Safe zone for cable line. d) End anchorage stresses in post tensioned members. e) Types of flexural failure. ———————

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