Question Paper Cod L040 ME. DEGREE EXAMINATION, MAY/JUNE 2017. Elective Structural Engineering ST 7008 — PRE-STRESSED CONCRETE (Regulations 2013) Maximum : 100 marks Answer ALL questions. Use IS : 1348 and IS ; 456 is permitted: PART A — (10x 2 = 20 marks) ‘Why mild steel is not suitable for pre-stressing? Define “Pre-tensioning”. What is meant by partial pre-stressing? Sketch the reinforcement pattern in the anchorage zone of a post tensioned beam. Mention the modes of failure of a pre-stressed concrete beam in bending. What is meant by “Concordant cable"? What type of pre-stressing is preferred in columns and why? Mention any two principle of circular pre-stressing. Sketch any two forms of shear connectors used in pre-stressed concrete composite beams, Sketch the stress distribution diagram in a propped pre-stressed concrete composite section.1. 13, o) &) @ ) PART B— (6 x 13 = 65 marks) ‘A rectangular concrete beam of cross section 200 x 300 mm is pre- stressed by means of 14 wires of 5 mm diameter located at 65 mm from the bottom of the beam and 3 wires of 5 mm diameters, 25 mm from the top. Assuming the pre-stress in the steel is 850 Nimmé, calculate the stresses at the extreme fibres of the mid span section when the beam is, supporting its own weight over a span of 7 m. Or ‘A pre-stressed concrete pile 300 mm diameter contains 50 per tensioned wires of each of 3 mm diameters. The wires are initially tensioned with a total force of 300 KN. Calculate the final stress in concrete and the percentage loss of stress in stec! after all losses. Take Es= 210 KNimm?, Ec = 32 KN/mm*, Relaxation of steel stress = 5% of initial stress. Shrinkage of concrete in 10 days = 0.0002. Design for flexure a pre-stressed concrete beam of I ~ section to carry a superimposed load of 10 KN/mm over a span. of 25m. Assume 20% pre stress loss, The thickness of the flanges and the web may be taken as 150 mm. Maximum permissible compression in concrete is 12 Nimm?. ‘Take width of the flange as 0.30 times the over all death. Or A pre-stressed concrete beam of rectangular section 200 x 400 mm is ‘subjected to a shear force of 500 KN at the support under working loads. "The effective pre-stress in the tendon is 1500 KN and the shape is parabolic with an eccentricity of 200 mm at the centre. The simply supported beam has a span of 20 m Design the shear reinforcement and MAO grade concrete is used. ‘A two-span continuous beam ABC (AB = BC = 8m) is of rectangular section, 300 mm wide and 600 mm deep. The beam is pre-stressed by a parabolic cable, concentric at end supports and having an eccentricity of 100 mm towards the soffit of the beam at the centre of B spans and 200 mm towards the top of the beam at mid support B. The effective force in the cable is 500 KN. @ Show that the cable is concordant (i) Locate the pressure line in the beam when it supports an imposed load of 5.50 KN/m is addition to its self weight. Or Design a pre-stressed concrete beam continuous over two equal spans of 10 m to support live loads of 25 KN each at the centre of spans. The loads may be applied independently or jointly. Permissible stresses being zero in tension and 15 N/mm? in compression. Loss ratio = 0.80. Determine a concordant profile and show it on an elevation of the beam. Allowing for a minimum cover of 100 mm, sketch a suitable transformed profile to reduce the slope of the tondons at the central support to a minimum. Check the limit state of serviceability and collapse. 2 61048ua @ ‘A pre-strossed concrete pipe of 1.2 m diameter, having a core thickness of 80 mm is required to withstand a service pressure intensity of 1.30 N/mm?, Estimate the pitch of 5 mm diameter high tensile wire winding it the initial stress is limited to 1000 N/mm? Permissible stresses in concrete being 12.50 N/mm? in. compression and zero in tension. The loss ratio is 0.80. If the direct tensile strength of concrete is 2.50 N/mm?, estimate the load factor against cracking. Or A cylindrical pre-stressed concrete water tank of internal diameter 20 m is required to store water over @ depth of 10 m. The permissible compressive stress in concrete at transfer is 14 N/mm?and the minimum compressive stress under working pressure is 1 N/mm®, The load ratio is 0.80. Wires of 6 mm diameter with initial pre-stress of 1000. N/mm? are available for circumferential winding and Feryssinet cables made tp of 12 wires of 8 mm diameter stressed to 1200 N/mm? are to be used for vertical pro-stressing. Design the tank walls assuming the base as fixed. ‘The cube strength of concrete is 60 N/mm. A rectangular pre-tensioned concrete beam has a breadth of 200 mm and depth of 300 mm, and the pre-etress after all-loseos have occurred is 14 Nim? at the soffit and zero the top. The beam is incorporated in a composite ‘T” beam by casting top flange of breadth 300 mm and depth of 90 mm, Calculate the maximum uniformly distributed live load that can be supported un a simply supported epan of 4.50 m without tensile stresses occurring. (@) _ if the slab is externally supported while casting and Gi) if the pre-tensioned beam supports the weight of the slab while casting. Or Explain: @ Shrinkage stresses in composite construction. Gi) Transverse pre-stressing of bridge decks. PART C — (1x 15 = 15 marks) Consider a water tank of 1 lakh litre capacity to be constructed in your place. Design the wall. Assume suitable data if necessary. Or Explain the design of pre-stressed tension and compression member in detail.