1 Prepared By:Mr.A.Sathiyamoorthy, M.E., AP/Civil

Transcription

1 UNIVERSITY QUESTIONS PART A UNIT 1: INTRODUCTION THEORY AND BEHAVIOUR 1. List the loss of prestress. 2. Define axial prestressing. 3. What is the need for the use of high strength concrete and tensile steel in prestressed concrete? 4. Why high strength steel is essential for prestressed concrete? R-08-MAY&NOV List down the factors that influence the deflection of prestressed concrete members. R-08-MAY&NOV What do you understand by unbounded tendon? R-08-MAY&NOV List out the advantages of prestressed concrete. 8. What is concordant prestressing? 9. What are the classifications of prestressed concrete structures? R-08 MAY Define bonded and non-bonded prestressing concrete. R-08 MAY What is pressure or thrust line? R-08 MAY List out the losses of pretensioned and post tensioned prestress. R-08 MAY What is meant by pretensioned and post tensioned concrete? R-08 MAY Define load balancing concept. R-08 MAY Define pre tensioning and post tensioning. R-08 NOV Define creep strain. R-08 NOV State the advantages of pre stressing concrete. R-08 NOV Name the two materials in prestressing. R-08 NOV Draw the various profiles of the tendons. R-08 NOV Define short term and long term deflection. R-08 NOV Distinguish between concentric and eccentric prestressing. R-08 NOV Explain why steel with a low yield stress is not used in prestressed construction. UNIT 2: DESIGN FOR FLEXURE AND SHEAR 1. What are the different types of flexural failure modes observed in PSC beams? 2. What is strain compatibility method? 3. How will you classify a structure as Type II or class 2 structure? 4. How to calculate ultimate shear strength of uncracked section in flexure as per IS 1343? 5. List the types of flexural failure. 6. Write the design procedure for strain compatibility method. 7. List the types of shear cracks. 8. List out the assumptions on the compatibility of strain. R-08 MAY Define shear stress and principal stress. R-08 MAY List the types of shear cracks. R-08 MAY Define cracking load. R-08 MAY What is effective reinforcement ratio? R-08 MAY Draw two layouts of post tensioned beam. R-08 MAY Draw two layouts of pre tensioned beam. R-08 NOV

2 15. What are the ways of improving the shear resistance a prestressed concrete beam? R-08 NOV-2014 UNIT 3: DEFLECTION AND DESIGN OF ANCHORAGE ZONE 1. Define the term end block. 2. What is meant by bursting force? 3. Why control of deflection is very essential? 4. Mention the functions of end block. 5. Define anchorage. 6. Sketch the loop reinforcement, hairpin bars in end blocks. 7. State any two function of end block. R-08 MAY Discuss limiting zone for prestressing force. R-08 MAY State Mohr s theorems. R-08 MAY Define transmission length. R-08 MAY What is the zone of transmission in end block of prestressed concrete structure? R-08 MAY Enumerate effect of tendon profile on deflection? R-08 NOV Enumerate stress distribution in end block. R-08 NOV Methods to study the stress distribution in end block. R-08 NOV Draw the stress distribution diagram for single anchor plate and double anchor plate in end block. UNIT 4: COMPOSITE BEAMS AND CONTINUOUS BEAMS 1. How to achieve compositeness between precast and cast in situ part? 2. Distinguish between propped and unpropped construction methods. 3. What are the advantages of composite construction in PSC? 4. What is unpropped construction in composite PSC construction? 5. Define propped construction. R-08-MAY&NOV How to achieve compositeness between precast and cast in situ part? R-08-MAY&NOV List the advantages of composite prestressed concrete construction. 8. Define modular ratio. 9. State the advantages of composite construction. R-08 MAY What is meant by composite construction of prestressed and in situ concrete? R-08 MAY Define the term reduction factor. R-08 MAY Define unpropped construction. R-08 NOV What is meant by shear connectors? R-08 NOV What are the disadvantage of prestressed continuous beams? R-08 NOV What are the various methods of achieving continuity in continuous beam? 2

3 UNIT 5: MISCELLANEOUS STRUCTURES 1. What is the stress induced in concrete due to circular prestressing? 2. What are the design criteria for prestressed concrete pipes? 3. List the different types of prestressing adopted for the walls of a water tank. 4. Define circular prestressing. 5. Define partial prestressing. R-08-MAY&NOV How are the tanks classified based on the joint? R-08-MAY&NOV What are the different types of joints used between the walls and floor slab of the prestressed concrete tank? 8. Write the merits and demerits of partial prestressing. 9. Define vertical prestressing. R-08 MAY Principle of circular prestressing. R-08 MAY Write an expression for vertical moment. R-08 MAY Write a short note on tank floor. R-08 MAY Define longitudinal prestressing. R-08 NOV What is the main function of longitudinal prestressing? R-08 NOV What is circumferential prestressing? R-08 NOV Differentiate between prestressed cylindrical and non cylindrical pipes. R-08 NOV State the advantages of partially prestressing. R-08 NOV Define degree of prestressing. R-08 NOV What are needs of prestressing compression member? 20. Sketch the arrangement of tendons and anchorages in circular prestressing of concrete pipes. PART B UNIT 1: INTRODUCTION THEORY AND BEHAVIOUR 1. A rectangular prestressed beam 150mm wide and 300mm deep is used over an effective span of 10m. The cable with zero eccentricity at the supports and linearly varying to 50mm at the centre carries an effective prestressing force of 500kN. Find the magnitude of the concentrated load located at the centre of the span for the following conditions at the centre of span section: A) If the load counteracts the bending effect of the prestressing force(neglecting self weight of beam) and B) If the pressure line passes through the upper kern of the section under the action of the external load, self weight and prestress A prestressed concrete beam, 200mm wide and 300mm deep is used over an effective span of 6m to support an imposed load of 4kN/m. The density of concrete is 24kN/m 3. Find the magnitude of the eccentric prestressing force located at 100mm from the bottom of the beam which would nullify the bottom fibre stress due to loading. 3. A concrete beam with a rectangular section 120mm wide and 300mm deep, is stressed by a straight cable carrying an effective force of 200kN. The span of the beam is 6m. The cable is straight with a uniform eccentricity of 50mm. If the beam has an uniformly distributed load of 6kN/m. E c=38kn/mm 2. Estimate the deflection at centre of span for the following case: a) Prestress + self weight of the beam b) Prestress + self weight of the beam + live load. 3

4 4 BHARATHIDASAN ENGINEERING COLLEGE 4. A pretensioned beam 200mmX300mm is prestressed by 10 wires each of 7mm diameter, initially stressed to 1200 MPa with their centroids located 100mm from the soffit. Estimate the final percentage loss of stress due to elastic deformation, creep, shrinkage and relaxation. Assume relaxation of steel stress = 60MPa, E s=210gpa, E c=36.9gpa, creep coefficient = 1.6 and residual shrinkage strain = 3x A PSC beam supports an imposed load of 3kN/mm over a simply supported span of 10m. the beam has I section with an overall depth of 450mm. the thickness of flange and web are 75mm and 100mm respectively. The flange width of the beam is 200mm. It is constructed with a concrete having unit weight of 24kN/m 3. The beam is prestressed with an effective prestressing force of 350kN at a suitable eccentricity such that the resultant stress at the soffit of the beam at mid span is zero. Find the eccentricity required for the force. Also calculate the stresses at the top of the section. 6. A PSC beam of 250mm wide and 400mm deep and 12m span is prestressed with 10 wires of 7mm dia located at a constant eccentricity of 75mm and carrying an initial stress of 1200N/mm 2. The modulus of elasticity of steel and concrete are 210kN/mm 2 and 38kN/mm 2 respectively. The relaxation of stress in steel is assumed as 6% of initial stress. Take creep coefficient as 1.8 and slip at anchorage as 1mm. The shrinkage of concrete is 350x10-6 for pretensioning and 160x10-6 for post tensioning. Take the frictional coefficient for wave effect as per m. Calculate the % of loss of stress if a) The beam is pretensioned b) The beam is post tensioned. 7. Explain the systems and methods of prestressing with neat sketches. R-08-MAY&NOV A PSC beam of span 8m having a rectangular section of 150mmX300mm. The team is prestressed by a parabolic cable having an eccentricity of 75mm below the centroidal axis at the center of the span and an eccentricity of 25mm above the cenroidal axis at the support sections. The initial force in the cable is 350kN. The beam supports three concentrated loads of loads of 10kN each at intervals of 2m. E c=38kn/mm 2. a) Neglecting losses of prestress, estimate the short term deflection due to (prestress + self weight) b) Allowing for 20% loss in prestress, estimate long term deflection under(prestress + self weight + live load) assume creep co-efficient as R-08-MAY&NOV A prestressed concrete beam of section 120 mm wide by 300 mm deep is used over an effective span of 6 m to support a uniformly distributed load of 4 kn/m, which includes the self-weight of the beam. The beam is prestressed by a straight cable carrying a force of 180 kn and located at eccentricity of 50 mm. Determine the location of the thrust line in the beamand plot its position at quarter and central span section. 10. A rectangular prestressed beam 150 mm wide and 300 mm deep is used over an effective span of 10 m. The cable with zero eccentricity at the supports and linearly varying to 50 mm at the centre carries an effective prestressing force of 500 kn. Find the magnitude of the concentrated load located at the centre of the span for the following conditions at the centre of span section: a) If the load counteracts the bending effect of the prestressing force (neglecting self-weight of beam) and b) If the pressure line passes through the upper kern of the section under the action of the external load, self-weight and prestress. 11. A prestressed concrete beam, 200 mm wide and 300 mm deep is used over an effective span of 6 m to support an imposed load of 4 kn/m. the density of concrete is 24 kn/m 3. Find the magnitude of the eccentric prestressing force located at 100 mm from the bottom of the beam which would nullify the bottom fibre stress due to loading. And also find the magnitude of the

5 concentric prestressing force necessary for zero fibre stress at the soffit when the beam is fully loaded. R-08 MAY A prestressed concrete beam, 200mmX300mm, is prestressed with wires(area = 320 mm 2 ) located at a constant eccentricity of 50mm and carrying an initial stress of 1000 N/mm 2. The span of the beam is 10m. Calculate the percentage loss of stress in wires if (a) the beam is pretensioned, and (b) the beam is post tensioned, using the data: E s= 210 KN/mm 2, E c= 45 KN/mm 2, relaxation of steel stress o 5% of the initial stress, φ = 1.6, anchorage slip = 1mm, shrinkage of concrete = 300X10-6 for pre & 200X10-6 for post tensioning. & frictional coefficient for wave effect = /m. book 13. A prestressed concrete beam of section 150mm wide by 350mm deep is used over an effective span of 8m to support a uniformly distributed load of 6KN/m, which includes the self weight of the beam. The beam is prestressed by a straight cable carrying a force of 200KN and located at an eccentricity of 50mm. Determine the location of thrust-line in the beam and plot its position at quarter and central span section. R-08 MAY A rectangular concrete beam 100mm wide by 250mm deep spanning over 8m is prestressed by a straight cable carrying an effective prestressing force of 250kN located at an eccentricity of 40mm. The beam supports a live load of 1.2kN/m. a) Calculate the resultant stress distribution for the centre-of-span cross-section of the beam assuming the density of concrete as 24kN/m 3. Book R-08 MAY-2013 b) Find the magnitude of the prestressing force with an eccentricity of 40mm which can balance the stresses due to dead and live loads at the soffit of the centre span section. 15. A rectangular concrete beam of cross section 30cm deep and 20cm wide is prestressed by means of 15 wires of 5mm diameter located 6.5cm from the bottom of the beam and 3 wires of diameter of 5mm, 2.5 cm from the top. Assuming the prestress in the steel as 840N/mm 2, calculate the stresses at the extreme fibres of the mid span section when the beam is supporting its own weight over a span of 6m. If a uniformly distributed live load of 6kN/m is imposed, evaluate the maximum working stress in concrete. The density of concrete is 24kN/m 3. R-08 NOV An unsymmetrical I-section beam is used to support an imposed load of 2kN/m over a span of 8m. The sectional details are top flange, 300mm wide and 60mm thick; bottom flange, 100mm wide and 60mm thick; thickness of the web=80mm; overall depth of the beam=400mm. At the centre of the span, the effective prestressing force of 100kN is located at 50mm from the soffit of the beam. Estimate the stresses at the centre-of-span section of the beam for the following load condition: a) Prestress + self-weight b) Prestress + self-weight + live load. R-08 NOV A psc beam of section 200mm wide by 300mm deep is used over an effective span of 6m to support an imposed load of 4kN/m. The density of concrete is 24kN/m 3. At the centre-of-span section of the beam, find the magnitude of a) the concentric prestressing force necessary for zero fibres stress at the soffit when the beam is fully loaded b) the eccentricity prestressing force located 100mm from the bottom of the beam which would nullify the bottom fibres stresses due to loading. R-08 NOV A psc beam with a rectangular section 120mm wide by 300mm deep supports a udl of 4kN/m, which includes the self-weight of the beam. The effective span of the beam is 6m. The beam is concentrically prestressed by a cable carrying a force of 180kN. Locate the position of the pressure line in the beam. R-08 NOV A psc beam of section 120mm wide by 300mm deep is used over an effective span of 6m to support a udl of 4kN/m, which includes the self-weight of the beam. The beam is prestressed by a 5

6 straight cable carrying a force of 180kN and located at an eccentricity of 50mm. Determine the location of the thrust line in the beam and plot its position at quarter and central span section. 20. A rectangular concrete beam 250mm wide by 300mm deep is prestressed by a force of 540kN at a constant eccentricity of 60mm. The beam supports a concentrated load of 68kN at the centre of a span of 3m. Determine the location of the pressure line at the centre, quarter span and support sections of the beam. Neglect the self-weight of the beam. R-08 NOV A rectangular concrete beam 300mm wide and 800mm deep supports two concentrated loads of 20kN each at the third point of a span of 9m. a) suggest a suitable cable profile. If the eccentricity of the cable profile is 100mm for the middle third portion of the beam, calculate the prestressing force required to balance the bending effect of the concentrated loads(neglect the self-weight of the beam). b) for the same cable profile, find the effective force in the cable if the resultant stress due to seflweight, imposed loads and prestressing force is zero at the bottom fibre of the mid-span section. Assume D c=24kn/m 3. UNIT 2: DESIGN FOR FLEXURE AND SHEAR 6 1. A pretension T-section has a flange 1200mm wide and 150mm thick. The width and depth of rib are 300mm and 1500mm respectively. The high tensile steel has an area 4700mm 2 and is located at an effective depth of 1600mm. If the characteristic cube strength of the concrete and the tensile strength of steel are 40N/mm 2 and 1600N/mm 2 respectively, calculate the flexural strength of the T- section. Ex Explain the various methods of flexural failure encountered in prestressed concrete member. 3. Design a simply supported type I prestressed beam with M T=435kNm (including an estimated M SW=55kNm). The height of the beam is restricted to 920mm. The prestress at transfer f po= 1035N/mm 2 at transfer and 11.0 N/mm 2 at service. The properties of the prestressing strands are given below: a) Type of prestressing tendon 7 wire strand b) Nominal diameter = 12.8mm c) Nominal area = 99.3 mm A post tensioned prestressed beam of rectangular section 300mm wide is to be designed for an imposed load of 14kN/m over a span of 10m. The stress in concrete must not exceed 17N/mm 2 in compression and 1.4N/mm 2 in tension at any time. The loss of prestress may be assumed as 18%. Calculate a) The minimum possible depth of the beam b) The minimum prestressing force required for the given section c) The minimum eccentricity for the above prestressing force. 5. Design a post tensioned girder for a span of 22m to support a live load of 6kN/m. The M 50 grade mix is used for construction with a permissible compressive stress of 35N/mm 2, and a tensile stress of 1.7N/mm 2. The permissible stress in concrete shall not exceed 17.5N/mm 2 and 16.5N/mm 2 in compression and 1.15N/mm 2 and zero in tension for both transfer and working loads respectively. Take the modulus of elasticity of concrete as 38kN/mm 2. The loss of prestress at transfer is15%. High tensile strength wires of 8mm dia and having a characteristic tensile strength of 1600N/mm 2 should be used for prestressing the member. The modulus of elasticity of wires is 210kN/mm 2. Design the beam as a class 1 structure and carryout the flexural and shear check alone.

7 6. A post-tensioned bridge girder with unbonded tendons is of box section of overall dimensions 1200mm wide by 1800mm deep, with wall thickness of 150mm. The high-tensile steel has an area of 4000mm 2 and is located at an effective depth of 1600mm. The effective prestress in steel after all losses is 1000N/mm 2 and the effective span of the girder is 24m. If f ck=40n/mm 2 and f p=1600n/mm 2, estimate the ultimate flexural strength of the section. 7. A pretensioned, T-section has a flange 1200mm wide and 150mm thick. The width and depth of the rib are 300mm and 1500mm respectively. The high tensile steel has an area of 4700mm 2 and is located at an effective depth of 1600mm. If the characteristics cube strength of the concrete and the tensile strength of steel are 40 N/mm 2 and 1600N/mm 2, respectively, calculate the flexural strength of the T-section. 8. A pre tensioned concrete beam (span=10m) of rectangular section, 120mm wide and 300mm deep, is axially prestressed by a cable carrying an effective force of 180kN. The beam supports a total udl of 5kN/m which includes the self-weight of the member. Compare the magnitude of the principal tension developed in the beam with and without the axial prestress. R-08 MAY A rectangular concrete beam of c/s 150mmX300mm is ss over a span of 8m and is prestressed by means of a symmetric parabolic cable, at a distance of 75mm from the bottom of the beam at amid span and 125mm from the top of the beam at support sections. If the force in the cable is 350kN and E c=38kn/mm 2, calculate a) the deflection at mid span when the beam is supporting its own weight, b) the concentrated load which must be applied at mid span to restore it to the level of supports. R-08 MAY A pretensioned, T-section has a flange which is 300mm wide and 200mm thick. The rib is 150mm wide by 350mm deep. The effective depth of the c/s is 500mm. Given A p=200mm 2, f ck=50n/mm 2 and f p=1600n/mm 2, estimate the ultimate moment capacity of the T-section using the IS codes. R-08 MAY A pretensioned psc beam having a rectangular section, 150mm wide and 350mm deep, has an effective cover of 50mm. If f ck=40n/mm 2, f p=1600n/mm 2, and the area of prestressing steel A p=461mm 2, calculate the ultimate flexural strength of the section using IS:1343 code provisions. 12. A pretensioned, T-section has a flange which is 300mm wide 200mm thick. The rib is 150mm wide by 350mm deep. The effective depth of the cross section is 500mm. Given A p=200mm 2, f ck=50n/mm 2 and f p=1600n/mm 2, estimate the ultimate moment capacity of the T-section using the IS codes. R-08 MAY A post tensioned prestressed concrete Tee beam having a flange width of 1200mm and flange thickness of 200mm, thickness of web being 300mm is prestressed by 2000mm 2 of high-tensile steel located at an effective depth of 1600mm. If f ck=40n/mm 2 and f p=1600n/mm 2, estimate the ultimate flexural strength of the unbonded tee section, assuming span/depth ratio as 20 and f pe=1000n/mm 2. R-08 NOV A psc beam (span=10m) of rectangular section, 120mm wide and 300mm deep, is axially prestressed by a cable carrying an effective force of 180kN. The beam supports a total udl of 5kN/m which includes the self weight of the member. Compare the magnitude of the principal tension developed in the beam with and without the axial prestress. R-08 NOV

8 UNIT 3: DEFLECTION AND DESIGN OF ANCHORAGE ZONE 1. The end block of a post tensioned PSC beam 300mm wide and 300mm deep is subjected to a concentric anchorage force of 800kN by a Freyssinet anchorage system of area 11000mm 2. Design and detail the anchorage reinforcement for the end block. 2. The end block of a prestressed concrete beam, rectangular in shape, 100mm wide and 200mm deep. The prestressing force of 100kN is transmitted to concrete through distribution plate, 100mm wide and 50mm deep, concentrically located at ends. Using, Guyon s method, compute the position and magnitude of maximum tensile stress and bursting tension for the end block with concentric anchor force of 100kN. 3. Estimate the transmission length at the end of a pretensioned beam prestressed by 7mm diameter wires. Assume the cube strength of concrete at transfer as 42N/mm A ssb is having dimensions 200mmX450mm is post tensioned with two cables of each having area of 150mm 2. The first cable is parabolic with an eccentricity of 70mm at mid span and zero at support whereas the second cable is having straight profile with uniform eccentricity of 70mm throughout. The initial prestress applied to each cable is 1100mm 2. The modulus of elasticity of concrete is 40kN/mm 2. The length of beam is 7.5m carry two point loads of 25kN at 1/3 rd of span. Determine a) The instantaneous deflection at the centre of span b) The deflection at the centre of span after two years, assuming 18% loss in prestress and effective modulus of elasticity to be 3/4 th of the short term modulus of elasticity. 5. Briefly explain about the stress distribution in anchorage zone of a post tensioned prestressed member. 6. Write short note on: Magnel s method, Guyon s method; IS code Provisions. 7. The end block of a post tensioned bridge girder is 500mm wide by 1000mm deep. Two cables, each comprising 90 high tensile wires of 7mm dia. Are anchored using square anchor plates of side length 400mm with their centres located at 500mm from the top and bottom of the edges of the beam. The jacking force in each cable is 4000kN. Design a suitable anchorage zone reinforcement using Fe415 grade HYSD bars conforming to IS:1343 provision. R-08-MAY&NOV A psc beam of section 120mmX300mm, spans over 6m. The beam is prestressed by a straight cable carrying an effective force of 200kN at an eccentricity of 50mm. E = 38 KN/mm 2. Compute (a) deflection under (prestress + self weight) & (b) find magnitude of udl which will nullify the deflection due to prestress and self weight A post tensioned roof girder spanning over 30m has an unsymmetrical I section with a second moment of area of section of X 10 6 mm 4 and overall depth of 1300mm. The eccentricity of the group of parabolic cables at the centre of span is 580mm towards the soffit and 170mm towards the top of beam at supports. The cables carry an initial prestressing force of 3200kN. The self weight of the girder is 10.8kN/m and the l.l. on the girder is 9kN/m. E = 38 KN/mm 2. φ = 1.6, the total loss of prestress is 15%. Estimate (a) deflection (prestress + self weight) and (b) resultant maximum long term deflection allowing for loss of prestress & creep of concrete A rectangular concrete beam of cross section 150mm wide and 300mm deep is simply supported over a span of 8m and is prestressed by means of a symmetric parabolic cable, at a distance of 75mm from the bottom of the beam at mid span and 125mm from the top of the beam at support sections. If the force in the cable is 350kN and the modulus of elasticity of concrete is 38kN/mm 2, calculate 6.3 R-08 MAY-2014 a) The deflection at mid-span when the beam is supporting its own weight 8

9 b) The concentrated load which must be applied at mid span to restore it to the level of supports. UNIT 4: COMPOSITE BEAMS AND CONTINUOUS BEAMS 9 1. A precast pretensioned beam of rectangular section has a breadth of 100mm and a depth of 200mm. The beam with an effective span of 5m is prestressed by tendons with their centroids coinciding with the bottom kern. The initial force in the tendons is 150kN. The loss of prestress may be assumed to be 15 percent. The beam is incorporated in a composite T-beam by casting a top flange of breadth 400 mm and thickness 40 mm. If the composite beam supports a live load of 8 KN/m 2.calculate the resultant stresses developed in the precast and insitu concrete assuming the pretensioned beam beam as: a) Unpropped, b) Propped during the casting of the slab. Assume the same modulus of elasticity for concrete in precast beam and insitu cast slab Explain the advantages of using precast prestressed elements along with in-situ concrete. 3. Write step by step design procedure for composite construction. 4. A precast pretensioned beam of 150mm wide and 300mm deep is prestressed with tendons with their centroids coinciding with the bottom kern. The length of beam is 9m and prestressing force applied to the tendons is 400kN with a loss of prestress of 15%. A cast in situ slab of size 500mmX50mm is constructed over the pretensioned beam to form the composite construction. If the composite beam supports a live load of 3kN/m 2, calculate the resultant stresses developed in the precast and in situ cast concrete by assuming the pretensioned beam as: a) Unpropped b) Propped during the construction. The modulus of elasticity of concrete is 38kN/mm 2 for both precast and a cast-in-situ elements. The unit weight of concrete is 24kN/m A continuous PSC beam ABC(AB=BC=8m) has a uniform rectangular section of width 100mm and depth 250mm. The cable carrying an effective prestressing force of 300kN is parallel to the axis of the beam and located at 75mm from the soffit. Take density of concrete as 24kN/m 3. a) Determine the secondary and resultant moment at central support B b) If the beam supports an imposed load of 1.2kN/m, calculate the resultant stresses at top and bottom of the beam at B c) Locate the resultant line of thrust through beam AB. 6. A precast pretensioned beam of rectangular section has a breadth of 100mm and a depth of 200mm. The beam with an effective span of 5m is prestressed by tendons with their centroids coinciding with the bottom kern. The initial force in the tendons is 150kN. The loss of prestress may be assumed to be 15 percent. The beam is incorporated in a composite T-beam by casting a top flange of breadth 400 mm and thickness 40 mm. If the composite beam supports a live load of 8 KN/m 2.calculate the resultant stresses developed in the precast and insitu concrete assuming the pretensioned beam beam as: a) Unpropped, b) Propped during the casting of the slab. Assume the modulus of elasticity for concrete in precast beam and insitu cast slab are different. Assume E c=35kn/mm A composite T-beam is made up of a prestensioned rib 100mm wide and 200mm deep, and a cast in situ slab 400mm wide and 40mm thick having a modulus of elasticity of 28kN/mm 2. If the

10 differential shrinkage is 100X10-6 units, determine the shrinkage stresses developed in the precast and cast in situ units A composite beam of rectangular section is made up of a pretensioned in inverted T-beam having a slab thickness and width of 150mm and 1000mm, respectively. The rib size is 150mm by 850mm. The cast in situ concrete has a thickness and width of 1000mm with a modulus of elasticity of 30kN/mm 2. If the differential shrinkage is 100x10-6 units, estimate the shrinkage stresses developed in the precast and cast in situ units R-08 MAY Design a continuous prestressed beam of two spans(ab=bc=12m)to support a uniformly distributed live load of 10kN/m. Tensile stresses are not permitted in concrete and the compressive stress in concrete is not to exceed 13N/mm 2. Sketch the details of the cable profile and check for stresses developed at the support and span sections. UNIT 5: MISCELLANEOUS STRUCTURES 1. A non-cylinder PSC pipe of internal diameter 1000mm and thickness of cone shell 75mm in required to convey water at a working pressure of 1.5 N/mm 2. The length of each pipe is 6m. The loss ratio is 0.8 a) Design the circumferential wire winding using 5mm dia wires stretched 1000/mm 2 b) Design the longitudinal prestressing using 7mm dia wires tensioned to 1000/mm 2. The max permissible tensile stress under the critical transient loading not greater than 0.8 where f ci=40n/mm A PSC circular cylindrical tank is required to store million litres of water. The permissible compressive stress in concrete at transfer should not exceed 13 N/mm 2 & min compressive stress under working pressure should not be less than 1N/mm 2. The loss ratio is HYSD wires of 7mm dia with an initial stress of 1000N/mm 2 are available for winding round the tank. Freyssinet cables of 12 wires of 8mm dia which are stressed to 1200N/mm 2 are available for vertical prestressing. The cube strength of concrete is 40N/mm 2. Design the tank walls Define the partial prestressing. Explain the merits and demerits of partial prestressing. 4. Briefly explain the various steps involved in designing of a prestressed concrete circular pipes. 5. Design a free edge water tank with base hinged of diameter of 36m to store water to a depth of 6m. The permissible compressive stress in concrete at transfer is 15N/mm 2 and minimum compressive stress under working pressure is 1.2N/mm 2. The loss of prestress in 12%. For circumferential winding use 6mm dia wires with an initial prestress of 1200N/mm 2. The 8mm dia Freyessinet cables with initial prestress of 1500N/mm 2 are available for vertical prestressing. M 45 grade concrete is used for the construction. 6. Design a cylindrical prestressed concrete water tank to suit the following data: Capacity of tank=3.5x10 6 liters. Ratio of diameter to height=4. Maximum compressive stress in concrete at transfer not to exceed 14N/mm 2. The prestress is to be provided by circumferential winding of 5mm dia wires and by vertical cables of 12 wires of 7mm dia. The stress in wires at transfer 1000N/mm 2. Loss ratio=0.75. Design the walls of the tank and details of circumferential wire winding and vertical cables for the following joint condition at the base: Sliding base(assume coefficient of friction as 0.5) R-08-MAY&NOV

11 7. A prestressed concrete pipe of 1.2m diameter, having a core thickness of 75mm is required to withstand a service pressure intensity of 1.2N/mm 2. Estimate the pitch of 5mm diameter high tensile wire winding if the initial stress is limited to 1000N/mm 2. Permissible stresses in concrete being 12.0N/mm 2 in compression and zero in tension. The loss ratio is 0.8 if the direct tensile strength of concrete is 2.5N/mm 2, estimate load factor against cracking. R-08-MAY&NOV A psc cylinder pipe is to be designed using a steel cylinder of 1200mm internal diameter and thickness 1.5mm. The service internal hydrostatic pressure in the pipe is 0.8N/mm 2. 4mm diameter high tensile wires initially tensioned to a stress of 1kN/mm 2 are available for circumferential winding. The yield stress of mild steel cylinder is 280N/mm 2. The maximum permissible compressive stress in concrete at transfer is 14N/mm 2 and no tensile stress is permitted under service load conditions. Determine the thickness of the concrete lining and the number of turns circumferential wire winding and the factor of safety against bursting. Assume modular ratio as 6 and loss ratio as 0.8 and f pu=1600n/mm R-08 NOV A cylindrical prestressed concrete water tank of internal diameter 30m is required to store water over a depth of 7.5m The permissible compressive stress in concrete at transfer is 13N/mm 2 and the minimum compressive stress under working pressure is 1N/mm 2. The loss ratio is Wires of 5mm diameter with an initial stress of 1000N/mm 2 are available for circumferential winding and Freyssinet cables made up of 12 wires of 8mm diameter stressed to 1200N/mm 2 are to be used for vertical prestressing. Design the tank walls assuming the base as fixed. The cube strength of concrete is 40N/mm R-08 MAY