11-06-2013, 12:04 PM
SEMESTER EXAMINATIONS FOR PRE STRESSED CONCRETE
PRE STRESSED CONCRETE.pdf (Size: 39.18 KB / Downloads: 33)
Answer any five questions
All questions carry equal marks
1. A pretensioned beam 250mm wide and 300mm deep is prestressed by 12 wires each of 7mm diameter initially stresses to 1200N/mm2 with their centroids located 100mm from the soffit. Estimate the final percentage loss of stress due to elastic deformation, creep, shrinkage and relaxation using IS:1343-80 code and the following data:
Relaxation of steel stress = 90N/mm2
Es=210kN/mm2 Ec=35kN/mm2
Creep coefficient (= 1.6 )Φ
Residual shrinkage strain = 3 ×10-4
2.a) How do you compute the loss of stress in steel due to curvature and wobble effect?
b) What is anchorage slip? How do you compute the loss of stress due to anchorage slip?
c) What are the factors influencing the loss of stress due to creep of concrete?
3. An unsymmetrical I-section has an overall depth of 200mm. The top flange width and depth are equal to 1200 and 300mm respectively, and the bottom flange width and depth are equal to 750 and 200mm respectively. The thickness of the web is 300mm. The tendons having a cross-sectional area of 7000 mm2 are located 200mm from the soffit. If the ultimate compressive strength of concrete and the tensile strength of steel are 42 and 1750 N/mm2 respectively and the tendons are effectively bonded to concrete, estimate the flexural strength of the section. (Adopt IS:1343 provisions).
4.a) A concrete beam is prestressed by a parabolic cable having an eccentricity of e1 towards the soffit at centre of span and an eccentricity of e2 towards the top near support sections. Find the ratio these eccentricities for zero deflection at the centre of span due to prestress only.
b) Outline the various factors influencing the effective moment of inertia of cracked concrete sections.
5.a) Outline the factors influencing the ultimate shear resistance of prestressed concrete sections with flexure-shear cracks.
b) Discuss the various methods of predicting long-term deflection of uncracked prestressed concrete members.