SKP Engineering College

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1 SKP Engineering College Tiruvannamalai A Course Material on Prestressed Concrete Structures By P. Selvarasan Assistant Professor Civil Engineering Department Computer Science Engineering Department 1 Internet Programming

2 Quality Certificate This is to Certify that the Electronic Study Material Subject Code: CE 6702 Subject Name: Prestressed Concrete Structures Year/Sem: IV/VII Being prepared by me and it meets the knowledge requirement of the University curriculum. Signature of the Author Name: P. Selvarasan Designation: Assistant Professor This is to certify that the course material being prepared by Mr.P.Selvarasan is of the adequate quality. He has referred more than five books and one among them is from abroad author. Signature of HD Name: Mr. A. Saravanan Seal: Signature of the Principal Name: Dr.V.Subramania Bharathi Seal: Computer Science Engineering Department 2 Internet Programming

3 CE6702 PRESTRESSED CONCRETE STRUCTURES L T P C OBJECTIVES: To introduce the need for prestressing as well as the methods, types and advantages of prestressing to the students. Students will be introduced to the design of prestressed concrete structures subjected to flexure and shear. UNIT I INTRODUCTION THEORY AND BEHAVIOUR 9 Basic concepts Advantages Materials required Systems and methods of prestressing Analysis of sections Stress concept Strength concept Load balancing concept Effect of loading on the tensile stresses in tendons Effect of tendon profile on deflections Factors influencing deflections Calculation of deflections Short term and long term deflections - Losses of prestress Estimation of crack width. UNIT II DESIGN FOR FLEXURE AND SHEAR 9 Basic assumptions for calculating flexural stresses Permissible stresses in steel and concrete as per I.S.1343 Code Design of sections of Type I and Type II posttensioned and pre-tensioned beams Check for strength limit based on I.S Code Layout of cables in post-tensioned beams Location of wires in pre-tensioned beams Design for shear based on I.S Code. UNIT III DEFLECTION AND DESIGN OF ANCHORAGE ZONE 9 Factors influencing deflections Short term deflections of uncracked members Prediction of long term deflections due to creep and shrinkage Check for serviceability limit state of deflection. Determination of anchorage zone stresses in post-tensioned beams by Magnel s method, Guyon s method and IS1343 code design of anchorage zone reinforcement Check for transfer bond length in pre-tensioned beams. UNIT IV COMPOSITE BEAMS AND CONTINUOUS BEAMS 9 Analysis and design of composite beams Methods of achieving continuity in continuous beams Analysis for secondary moments Concordant cable and linear transformation Calculation of stresses Principles of design. UNIT V MISCELLANEOUS STRUCTURES 9 Design of tension and compression members Tanks, pipes and poles Partial prestressing Definition, methods of achieving partial prestressing, merits and demerits of partial prestressing. TOTAL: 45 PERIODS Computer Science Engineering Department 3 Internet Programming

4 TEXTBOOKS: 1. Krishna Raju N., "Prestressed concrete", 5th Edition, Tata McGraw Hill Company, New Delhi, Pandit.G.S. and Gupta.S.P., "Prestressed Concrete", CBS Publishers and Distributers Pvt. Ltd, Computer Science Engineering Department 4 Internet Programming

5 CONTENTS S.No Particulars Page 1 Unit I 6 2 Unit II 25 3 Unit III 58 4 Unit IV 80 5 Unit V 112 Computer Science Engineering Department 5 Internet Programming

6 Unit 1 Introduction Theory And Behavior Part A 1. List out the advantages of prestressed concrete. (AUC Nov/Dec 2011 & 2012) In case of fully prestressed member, which are free from tensile stresses under working loads, the cross section is more efficiently utilized when compared with a reinforced concrete section which is cracked under working loads. 2. What is meant by pretensioned and post tensioned concrete? (AUC Nov/Dec 2010 & 2011) Pre tensioning: A method of Pre stressing concrete in which the tendons are tensioned before the concrete is placed. In this method, the prestress is imparted to concrete by bond between steel and concrete. Post tensioning: A method of pre stressing concrete by tensioning the tendons against hardened concrete. In this method, the prestress is imparted to concrete by bearing. 3. Why is high tensile steel needed for prestressed concrete construction? (AUC Nov/Dec 2012) High strength concrete is necessary for prestress concrete as the material offers highly resistance in tension, shear bond and bearing. In the zone of anchorage the bearing stresses being hired; high strength concrete is invariably preferred to minimizing the cost. High strength concrete is less liable to shrinkage cracks and has lighter modulus of elasticity and smaller ultimate creep strain resulting in a smaller loss of prestress in steel. The use of high strength concrete results in a reduction in a cross sectional dimensions of prestress concrete structural element with a reduced dead weight of the material longer span become technically and economically practicable. Tensile strength of high tensile steel is in the range of 1400 to 2000 N/mm 2 and if initially stress upto 1400 N/mm 2 their will be still large stress in the high tensile reinforcement Computer Science Engineering Department 6 Internet Programming

7 after making deduction for loss of prestress. Therefore high tensile steel is made for prestress concrete. 4. What are the various methods of prestressing? Pre-tensioning Post-tensioning 5. What are the systems of prestressing? (AUC May/June 2013) Pre-tensioning system Post-tensioning system 6. List the loss of prestress. (AUC Nov/Dec 2010 & 2013) Nature of losses of prestress. Loss due to elastic deformation of concrete. Loss due to shrinkage of concrete. Loss due to creep of concrete. Loss due to relaxation of stress in steel. Loss of stress due to friction. Loss due to anchorage slip. What are the classifications of prestressed concrete structures? (AUC 7. Nov/Dec 2013) Externally or internally prestressed Pretensioning and post tensioning End-Anchored or Non-End Anchored Tendons Bonded or unbonded tendons Precast, cast-in-place, composite construction Partial or full prestressing 8. Define load balancing concept. (AUC Apr/May 2011 & 2012) It is possible to select cable profiles in a prestressed concrete member such that the traverse component of the cable force balances the given type of external loads. This can be readily illustrated by considering the free body of concrete with Computer Science Engineering Department 7 Internet Programming

8 the tendon replaced by forces acting on beam 9. What are the factors influencing deflections? (AUC Apr/May 2011) Length of the deflection field Spacing between the deflection plate Difference of potential between the plates Accelerating voltage of the second anode. 10. What are the sources of prestress force? (AUC Apr/May 2012) Mechanical Hydraulic Electrical Chemical 11. Define kern distance. (AUC Apr/May 2010) Kern is the core area of the section in which if the load applied tension will not be induced in the section Kt = Zb / A, Kb = Zt / A, If the load applied at K t compressive stress will be the maximum at the top most fiber and zero stress will be at the bottom most fiber. If the load applied at K b compressive stress will be the maximum at the bottom most fiber and zero stress will be at the top most fiber. 12. What is Relaxation of steel? When a high tensile steel wire is stretch and maintained at a constant strain the initially force in the wire does not remain constant but decrease with time. The decrease of stress in steel at constant strain is termed relaxation of steel. 13. What is concordant prestressing? Pre stressing of members in which the cable follow a concordant profile. In case of statically indeterminate structures. It does not cause any changes in support reaction. 14. Define bonded and non bonded prestressing concrete. Bonded prestressing: Concrete in which prestress is imparted to concrete through bond between the tendons and surrounding concrete. Pre tensioned members belong Computer Science Engineering Department 8 Internet Programming

9 to this group. Non-bonded prestressing: A method of construction in which the tendons are not bonded to the surrounding concrete. The tendons may be placed in ducts formed in the concrete members or they may be placed outside the concrete section. 15.Define axial prestressing. Members in which the entire cross-section of concrete has a uniform compressive prestress. In this type of prestressing, the centroid, of the tendons coincides with that of the concrete section. 16. Define prestressed concrete. It is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from external loads (or) counteracted to a desire degree in reinforced concrete member the prestress is commonly introduced by tensioning the steel reinforcement. 17. Define anchorage. A device generally used to enable the tendon to impart and maintain prestress to the concrete is called anchorage. E.g. Fressinet, BBRV systems, etc., 18. What are the main factors for concrete used in PSC? Ordinary Portland cement-based concrete is used but strength usually greater than 50 N/mm 2 ; A high early strength is required to enable quicker application of prestress; A larger elastic modulus is needed to reduce the shortening of the member; A mix that reduces creep of the concrete to minimize losses of prestress; 19.What are the uses of prestressed concrete? Railway Sleepers; Communications poles; Pre-tensioned precast hollow core slabs; Pre-tensioned Precast Double T units - for very long spans (e.g., 16 m span for car parks); Pre-tensioned precast inverted T beam for short-span bridges; Computer Science Engineering Department 9 Internet Programming

10 Post-tensioned ribbed slab; This is glued segmental construction; 20.Define Magnel diagram. A Magnel Diagram is a plot of the four lines associated with the limits on stress. As can be seen, when these four equations are plotted, a feasible region is found in which points of 1 P and e simultaneously satisfy all four equations. Any such point then satisfies all four stress limits. Computer Science Engineering Department 10 Internet Programming

11 Part B 1. 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. (AUC Nov/Dec 2011, Apr/May 2010) Solution: Computer Science Engineering Department 11 Internet Programming

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13 2. Describe the effect of loading on the tensile stresses in tendons. (AUC Nov/Dec 2011) Computer Science Engineering Department 13 Internet Programming

14 3. A concrete beam of 10 m span, 100 mm wide and 300 mm deep is prestressed by 3 cables. The area of each cable is 200 mm2 and the initial stress in the cable is 1200 N/mm2. Cable 1 is parabolic with an eccentricity of 50 mm above the centroid at the supports and 50 mm below at the centre of span. Cable 2 is also parabolic with zero eccentricity at supports and 50 mm below the centroid at the centre of span. Cable 3 is straight with uniform eccentricity of 50 mm below the centroid. If the cables are tensioned from one end only. Estimate the percentage loss of stress in each cable due to friction. Assume μ = 0.35 and k = per m. (AUC Nov/Dec 2011) Solution: Computer Science Engineering Department 14 Internet Programming

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16 4. 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 beam and plot its position at quarter and central span section. (AUC Apr/May 2012) (AUC May/June 2013, Nov/Dec 2013) Solution: Computer Science Engineering Department 16 Internet Programming

17 5. A rectangular concrete beam 360 mm deep and 200 mm wide is prestressed by means of fifteen 5 mm diameter wires located 65 mm from the bottom of the beam and three 5 mm wires, located 25 mm from the top of the beam. If the wires are initially tensioned to a stress of 840 N/mm2, calculate the percentage loss of stress due to elastic deformation of concrete only. ES = 210 kn/mm2 and EC = 31.5 kn/mm2. (AUC Nov/Dec 2013) Computer Science Engineering Department 17 Internet Programming

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19 6. 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/m3. 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. (AUC Apr/May 2011) 7. A pretensioned beam 200 mm x 300 mm is prestressed by 10 wires each of 7 mm diameter, initially stressed to 1200 MPa with their centroids located 100 mm from the soffit. Estimate the final percentage loss of stress due to elastic Computer Science Engineering Department 19 Internet Programming

20 deformation, creep, shrinkage and relaxation. Assume relaxation of steel stress = 60 MPa, ES = 210 GPa, EC = 36.9 GPa, creep coefficient = 1.6 and residual shrinkage strain = 3 x (AUC Apr/May 2011) Computer Science Engineering Department 20 Internet Programming

21 8. A concrete beam with a rectangular section, 120 mm wide and 300 mm deep, is stressed by a straight cable carrying an effective force of 200 kn. The span of the beam is 6 m. The cable is straight with a uniform eccentricity of 50 mm. if the beam has an uniformly distributed load of 6 kn/m. EC = 38 kn/mm2. Estimate the deflection at the centre of span for the following cases: i. Prestress + self weight of the beam ii. Prestress + self weight + live load. (AUC Nov/Dec 2010, May/Jun 2013, Apr/May 2010) Solution: Computer Science Engineering Department 21 Internet Programming

22 9. A simply supported PSC beam of cross section 100 mm wide and 250 mm deep is loaded with a uniformly distributed load of magnitude 1.2 kn/m on a span of 8 m. Obtain the stress distribution at mid span by stress and strength concept, if the prestressing force is 250 kn applied eccentrically all along with its centre of gravity at 40 mm. Assuming the density of concrete as 24 kn/m3. (AUC Nov/Dec 2012) Solution: Computer Science Engineering Department 22 Internet Programming

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24 10. Explain the factors influencing deflections? (AUC Apr/May 2012) 11. What is the necessity of using supplementary or untensioned reinforcement in prestressed concrete members? (AUC Apr/May 2012) Reinforcement in prestressed members not tensioned with respect to the surrounding concrete before the application of loads. These are generally used in partially prestressed members. The untensioned reinforcement is required in the cross section of a prestressed member for various reasons such as to resist the differential shrinkage, temperature effects and handling stresses. Hence this reinforcement can cater for the serviceability requirements such as control of Computer Science Engineering Department 24 Internet Programming

25 cracking and partially for the ultimate limit state of collapse which can result in considerable reductions in the costlier high tensile steel. The saving in prestressing steel contributes to an overall saving in cost of the structure. Fully prestressed members are prone to excessive upward deflections especially in bridge structures where dead loads form a major portion of the total service loads and these deflections may increase with time due to the effect of creep. It is also well established that fully prestressed members due to their higher rigidity have a lower energy absorption capacity in comparison with partially prestressed members which exhibit a ductile behaviour. There is no disadvantage in allowing tensile stress of about 5 N/mm 2 in a concrete where the strains are appropriately guided. High tensile steel and mild steel have been used as untensioned reinforcement. The present practice is to use high yield strength deformed bars which are considerably cheaper than prestressing steel and at the same time have higher yield strength and better crack control. Computer Science Engineering Department 25 Internet Programming

26 Unit 2 Design For Flexure And Shear Part A (AUC May/June 2013, Nov/Dec 1. Define partial prestressing. 2011) The degree of prestress applied to concrete in which tensile stresses to a limited degree are permitted in concrete under working load. In this case, in addition to tensioned steel, a considerable proportion of untensioned reinforcement is generally used to limit the width of cracks developed under service load. 2. Mention any two functions of end blocks. (AUC May/June 2013, Nov/Dec 2013) Provide Lateral (horizontal) stability from wind and other horizontal (Racking) loads. Provide additional vertical load capacity for the ends of the joists from point loads above. 3. Define anchorage zone. (AUC Nov/Dec 2011) Prestressed concrete contains tendons which are typically stressed to about 1000 MPa. These tendons need to be anchored at their ends in order to transfer (compressive) force to the concrete. The zone of region is called Anchorage zone. 4. How can PSC beam be considered to carry its own(auc Nov/Dec 2012) Computer Science Engineering Department 26 Internet Programming

27 weight? By providing an external initial stress (the prestress) which compresses the beam. Now they can only separate if the tensile stress induced by the self weight of the beam is greater than the compressive prestress introduced. 5. Mention the advantages of partial prestressing. (AUC Nov/Dec 2012 & 2013) Limited tensile stresses are permitted in concrete under service loads with controls on the maximum width of cracks and depending upon the type of prestressing and environmental condition. Untensioned reinforcement is required in the cross-section of a prestressed member for various reasons, such as to resist the differential shrinkage, temperature effects and handling stresses. Hence this reinforcement can cater for the serviceability requirements, such as control of cracking, and partially for the ultimate limit state of collapse which can result in considerable reduction in the costlier high tensile steel. Saving in the cost of overall structure. 6. Write any two assumptions on the compatibility of strains.(auc Apr/May 2012) 2. The stress distribution in the compression zone of concrete can be defined by means of coefficients applied to the characteristic compressive strength and the average compressive strength stress and the position of the centre of compression can be assessed. 3. The distribution of concrete strain is linear (plane sections normal to axis remain plane after bending). 4. The resistance of concrete in tension is neglected. Computer Science Engineering Department 27 Internet Programming

28 value. The maximum compressive strain in concrete at failure reaches a particular 7. What is effective reinforcement ratio? (AUC Apr/May 2012) Ratio of effective area of reinforcement to the effective area of concrete at any section of a structural member is known as effective reinforcement ratio. 8. At initial stage what forces are considered in prestressed concrete design? (AUC Apr/May 2011) Prestressing force is considered in prestressed concrete design at initial stage. 9. Why anchorage zone has to be given special attention in design? (AUC Apr/May 2011) Because the main reinforcement in the anchorage zone should be designed to withstand the bursting tension, which is determined by the traverse stress distribution on the critical axis, usually coinciding with the line of action of the largest individual force. 10. Draw a sketch showing the stress distribution in end block by double anchor plate. (AUC Apr/May 2010) 11. Explain conventional failure of an over reinforced prestressed concrete beam. Computer Science Engineering Department 28 Internet Programming

29 (AUC Apr/May 2010) An Over reinforced members fail by the sudden crushing of concrete. The failure being reinforced members fail by the sudden crushing of concrete. The failure being characterized by small deflections and narrow cracks, the area of steel being comparatively large, the stresses developed in steel at failure of the member may not reach the tensile strength. 12. What is meant by end block in a post tensioned member? (AUC Nov/Dec 2010) The zone between the end of the beam and the section where only longitudinal stress exists is generally referred to as the anchorage zone or end block. 13. List any two applications of partial prestressing. (AUC Nov/Dec 2010) Used in large diameter concrete pipes Used in railway sleepers Water tanks Precast concrete piles to counter tensile stress during transport and erection. Used in bridges construction 14. Define degree of prestressing. A measure of the magnitude of the prestressing force related to the resultant stress occurring in the structural member at working load. 15. Define Bursting tension. The effect of transverse tensile stress is to develop a zone of bursting tension in a direction perpendicular to the anchorage force resulting in horizontal cracking Computer Science Engineering Department 29 Internet Programming

30 16. Define Proof stress. The tensile stress in steel which produces a residual strain of 0.2 percent of the original gauge length on unloading. 17. Define cracking load. The load on the structural element corresponding to the first visible crack. 18. Define Debonding. Prevention of bond between the steel wire and the surrounding concrete. 19. Write formula for Moment of resistance in BIS code. Mu = A pb A ps (d - d n ) 20. What are the types of flexural failure? Fracture of steel in tension Failure of under-reinforced section Failure of over-reinforced section Other modes of failure Computer Science Engineering Department 30 Internet Programming

31 Part B 1. Explain in detail the concept of strain compatibility method. (AUC Nov/Dec 2011) Computer Science Engineering Department 31 Internet Programming

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33 2. Discuss briefly about the stress distribution in end block. (AUC Nov/Dec 2011) Computer Science Engineering Department 33 Internet Programming

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36 3. Discuss in detail about limit state design criteria for prestressed concrete members. (AUC Nov/Dec 2011) 4. Explain the inadequacies of the elastic and ultimate load methods in the design of prestressed concrete members. Discuss on the philosophy of limit state Computer Science Engineering Department 36 Internet Programming

37 design explaining various limit states. (AUC Apr/May 2011) Inadequacies of the elastic and ultimate load methods: Computer Science Engineering Department 37 Internet Programming

38 Philosophy of limit state design: 5. The end block of a post tensioned PSC beam 300 mm wide and 300 mm deep is subjected to a concentric anchorage force of 800 kn by a freyssinet anchorage system of area mm2. Design and detail the anchorage reinforcement for the end block. (AUC Nov/Dec & Apr/May 2012 & 2010) Solution: Computer Science Engineering Department 38 Internet Programming

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40 6. A pretensioned T section has a flange 1200 mm wide and 150 mm thick. The width and depth of rib are 300 mm and 1500 mm respectively. The high tensile steel has an area 4700 mm2 and is located at an effective depth of 1600 mm. if the characteristic cube strength of the concrete and the tensile strength of steel are 40 and 1600 N/mm2 respectively; calculate the flexural strength of the T section. (AUC Nov/Dec 2013, Apr/May 2010) Solution: Computer Science Engineering Department 40 Internet Programming

41 7. The end block of a prestressed concrete beam, rectangular in section, is 100 mm wide and 200 mm deep. The prestressing force of 100 kn is transmitted to concrete by a distribution plate, 100 mm wide and 50 mm deep, concentrically located at the ends. Calculate the position and magnitude of the maximum tensile stress on the horizontal section through the center and edge of the anchor plate. Compute the bursting tension on these horizontal planes. (AUC Nov/Dec 2013, May/June 2013) Solution: Computer Science Engineering Department 41 Internet Programming

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45 8. A prestressed concrete beam of effective span 16 m is of rectangular section 400 mm wide and 1200 mm deep. A tendon consists of 3300 mm2 of strands of characteristic strength 1700 N/mm2 with an effective prestress of 910 N/mm2. The strands are located 870 mm from the top face of the beam. If fcu = 60 N/mm2, estimate the flexural strength of the section as per British code provisions for the following cases. i. Bonded tendons ii. Unbounded tendons (AUC May/June 2013) (AUC Apr/May 2012) Solution: Computer Science Engineering Department 45 Internet Programming

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47 9. What is meant by partial prestressing? Discuss the advantages and disadvantages when partial prestressing is done. (AUC Nov/Dec 2010) Partial prestressing: The degree of prestress applied to concrete in which tensile stresses to a limited degree are permitted in concrete under working load. In this case, in addition to tensioned steel, a considerable proportion of untensioned reinforcement is generally used to limit the width of cracks developed under service load. Advantages: Limited tensile stresses are permitted in concrete under service loads with controls on the maximum width of cracks and depending upon the type of prestressing and environmental condition. Untensioned reinforcement is required in the cross-section of a prestressed member for various reasons, such as to resist the differential shrinkage, temperature effects and handling stresses Hence this reinforcement can cater for the serviceability requirements, such as control of cracking, and partially for the ultimate limit state of collapse which can result in considerable reduction in the costlier high tensile steel. Saving in the cost of overall structure. Disadvantages: The excessive upward deflections, especially in bridge structure where dead loads from a major portion of the total service loads, and these deflections may increase with time of creep. Computer Science Engineering Department 47 Internet Programming

48 10.The end block of a post tensioned beam is 80 mm wide and 160 mm deep. A prestressing wire 7 mm in diameter stressed to 1200 N/mm 2 has to be anchored against the end block at the centre. The anchorage plate is 50 mm by 50 mm. the wire bears on the plate through a female cone of 20 mm diameter. Given the permissible stress in concrete at transfer, f ci as 20 N/mm 2 and the permissible shear in steel as 94.5 N/mm 2. Determine the thickness of the anchorage plate. (AUC Nov/Dec 2010) Solution: Computer Science Engineering Department 48 Internet Programming

49 11. The end block of a prestressed beam 200 mm wide and 300 mm deep has two freyssinet anchorages (100 mm diameter) with their centres at 75 mm from top and bottom of the beam. The force transmitted by each anchorage being 200 kn, estimate the maximum tensile stress and the bursting tension developed. (AUC Nov/Dec 2010) Solution: 12. The end block of a post tensioned prestressed member is 550 mm wide and 550 mm deep. Four cables each made up of 7 wires of 12 mm diameter strands and carrying a force of 1000 kn are anchored by plate anchorages, 150 mm by 150 mm located with their centres at 125 mm from the edges of the end block. The cable duct is of 50 mm diameter. The 28 day cube strength of concrete fcu is 45 N/mm2. The cube strength of concrete at transfer fci is 25 N/mm2. Permissible bearing stresses behind anchorages should confirm with IS: The characteristic yield stress in mild steel anchorage reinforcement is 260 N/mm2. Design suitable anchorages for the end block. (AUC Apr/May 2011) Computer Science Engineering Department 49 Internet Programming

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51 13. A post tensioned bonded prestressed concrete beam of rectangular cross section, 400 mm wide by 550 mm deep, is subjected to a service load bending moment of knm, torsional moment of 46.6kNm and shear force of 66.6 kn. The section has an effective prestressing force, determined from service load requirements of magnitude 500kN at an eccentricity of 150 mm, provided by 5numbers of 12.5 mm stress relieved strands of cross sectional area 506 mm2 with an ultimate tensile strength of 1820 N/mm2. If the cube strength of concrete is 40 N/mm2, design a suitable longitudinal and transverse reinforcements in the beam using IS: code recommendations based on the skew bending approach. (AUC Nov/Dec 2012) Solution: Computer Science Engineering Department 51 Internet Programming

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58 Unit 3 Deflection And Design Of Anchorage Zone Part A 1. What are the functions of water stopper (water bar) in water tank construction? (AUC May/June 2013) The base slab is subdivided by joints which are sealed by water stops. The reinforcement in the slab should be well distributed to control the cracking of the slab due to shrinkage and temperature. 2. Differentiate prestressed cylinder and non-cylinder pipe. (AUC May/June 2013) Prestressed cylinder pipe: It is developed by the Lock Joint Company. A welded cylinder of 16 gauge steel is lined with concrete inside and steel pipe wrapped with a highly stressed wire. Tubular fasteners are used for the splices and for end fixing of the wire and pipe is finished with a coating of rich mortar. It is suitable upto 1.2 m diameter. Prestressed non-cylinder pipe: It is developed by Lewiston Pipe Corporation. At first concrete is cast over a tensioned longitudinal reinforcement. A concrete pipes after curing are circumferentially stressed by means of a spiral wire wound under tension and protected by a coat of mortar. The main function of longitudinal prestress is to prevent cracking in concrete during circumferential winding and cracking due to the bending stresses developed during the handling and installation of pipes 3. Define circular prestressing. (AUC Nov/Dec 2011, 2012, 2013) When the prestressed members are curved in the direction of prestressing, the prestressing is called prestressing. For example, circumferential prestressing in pipes, tanks, silos, containment structures and similar s a type of circular prestressing Computer Science Engineering Department 58 Internet Programming

59 4. What are the design criteria for prestressed concrete tanks? (AUC Nov/Dec 2011) It is to resist the hoop tension and moments developed are based on the considerations of desirable load factors against cracking and collapse. It is desirable to have at least a minimum load factor of 1.2 against cracking and 2 against ultimate collapse as per IS code. It is desirable to have at least a minimum load factor of 1.25 against cracking and 2.5 against ultimate collapse as per BS code. The principal compressive stress in concrete should not exceed one-third of the characteristic cube strength. When the tank is full, there should be a residual compressive stress of at least 0.7 N/mm 2. When the tank is empty, the allowable tensile stress at any point is limited to 1 N/mm 2. The maximum flexural stress in the tank walls should be assumed to be numerically equal to 0.3 times the hoop compression. 5. What are the design criteria for prestressed concrete pipes?(auc Nov/Dec 2012) Circumferential prestressing, winding with or without longitudinal prestressing. Handling stresses with or without longitudinal prestressing. Condition in which a pipe is supported by saddles at extreme points with full water load but zero hydrostatic pressure. Full working pressure conforming to the limit state of serviceability. The first crack stage corresponding to the limit state of local damage. 6. How are the tanks classified based on the joint? (AUC Nov/Dec 2013) Tank wall with fixed base. Tank wall with hinged base. Tank wall with sliding base. 7. Define two stage constructions. (AUC Apr/May 2012) In the first the concrete is cast over a tensioned longitudinal reinforcement. In the second stage the concrete pipes after curing are circumferentially stressed by means of a spiral Computer Science Engineering Department 59 Internet Programming

60 wire wound under tension and protected by a coat of mortar. 8. Write any two general failures of prestressed concrete tanks. (AUC Apr/May 2012) Deformation of the pre-cast concrete units during construction. Manufacturing inaccuracies led to out of tolerance units being delivered to the site under investigation. It May have affected the ability to achieve a good seal. 9. What is the stress induced in concrete due to circular prestressing? (AUC Apr/May 2010) The circumferential hoop compression stress is induced in concrete by prestressing counterbalances the hoop tension developed due to the internal fluid pressure. 10. Explain the effect of prestressing force in concrete poles. (AUC Apr/May 2010) It should be reduced in proportion to the cross section by the techniques of debonding or dead ending or looping some of the tendons at mid height. 11. Write the various types of loadings that act on prestressed concrete poles. (AUC Nov/Dec 2010) Bending due to wind load on the cable and on the exposed face. Combined bending and torsion due to eccentric snapping of wires. Maximum torsion due to skew snapping of wires. Bending due to failure of all the wires on one side of the pole. Handing and erection stresses. 12. What are the advantages of prestressing water tanks? (AUC Apr/May 2011) Water storage tanks of large capacity are invariably made of prestressed concrete. Square tanks are used for storage in congested urban and industrial sites where land space is a major constraint. This shape is considerable reduction in the thickness of concrete shell. The efficiency of the shell action of the concrete is combined with the prestressing at the edges Computer Science Engineering Department 60 Internet Programming

61 13. How are sleepers prestressed? (AUC Apr/May 2011) Two block sleepers Longitudinal sleepers Beam type single piece prestressed concrete sleepers. 14. Mention the importance of shrinkage in composite construction? The time dependent behavior of composite prestressed concrete beams depends upon the presence of differential shrinkage and creep of the concretes of web and deck, in addition to other parameters, such as relaxation of steel, presence of untensioned steel, and compression steel etc. 15. What are the different types of joints used between the slabs of prestressed concrete tanks? Movement joint Expansion joint Construction Joint Temporary Open Joints. 16. What are the advantages of partially prestressed concrete poles? Resistance to corrosion in humid and temperature climate and to erosion in desert areas. Easy handling due to less weight than other poles. Easily installed in drilled holes in ground with or without concrete fill. Lighter because of reduced cross section when compared with reinforced concrete poles. Fire resisting, particularly grassing and pushing fire near ground line. 17. What are the types of prestressed concrete pipes? Monolyte construction Two stage construction 18. Distinguish between non-cylinder and cylinder pipes. Non-cylinder pipes: The design principles are used for determining the minimum thickness of concrete required and the pitch of circumferential wire winding on the pipe. Cylinder pipes: Computer Science Engineering Department 61 Internet Programming

62 The design principles of cylinder pipes are similar to those of the non-cylinder pipes except that the required thickness of concrete is computed by considering the equivalent area of the light gauge steel pipe embedded in the concrete. 19. Define the losses of prestress. Due to elastic deformation of concrete during circumferential wire winding, there is a loss of prestress which depends upon the modular ratio and the reinforcement ratio. 20. What are the advantages of prestressed concrete piles? High load and moment carrying capacity. Standardization in design for mass production. Excellent durability under adverse environmental conditions. Crack free characteristics under handling and driving. Resistance to tensile loads due to uplift. Combined load moment capacity. Computer Science Engineering Department 62 Internet Programming

63 Part B 1. Design a cylindrical prestressed concrete water tank to suit the following data: Capacity of tank = x 10 6 liters. Maximum compressive stress in concrete at transfer not to exceed 13 N/mm 2 (compression). Minimum compressive stress under working load to be 1 N/mm 2. The prestress is to be provided by circumferential winding of 7 mm dia with an initial stress of 1000 N/mm 2 and by vertical cables of 12 wires of 8 mm diameter which are stressed to 1200 N/mm 2. Loss ratio = The cube strength of concrete is 40 N/mm 2.Design the walls of the tank and details of circumferential wire winding and vertical cables for the following joint condition at the base: elastomeric pads (assume coefficient of friction as 0.5). (AUC May/June 2013, Nov/Dec 2013, Apr/May 2011) Solution: Computer Science Engineering Department 63 Internet Programming

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65 2. Design a non cylinder prestressed concrete pipe of 600 mm internal diameter to withstand a working hydrostatic pressure of 1.05 N/mm2, using a 2.5 mm high tensile wire stressed to 1000 N/mm2 at transfer. Permissible maximum and minimum stresses in concrete at transfer and service loads are 14 and 0.7 N/mm2. The loss ratio is 0.8. calculate also the test pressure required to produce a tensile stress of 0.7 N/mm2 in concrete when applied immediately after tensioning and also the winding stress in steel if ES = 28 kn/mm2 and EC = 35 kn/mm2. (AUC May/June 2013) Solution: 3. Discuss in detail about the design procedure for prestressed concrete tanks. (AUC Nov/Dec 2011& 2012, Apr/May 2010) Computer Science Engineering Department 65 Internet Programming

66 Solution: 4. A non cylinder prestressed concrete pipe of internal diameter 1000 mm and thickness of concrete shell 75 mm is required to convey water at a working pressure of 1.5 N/mm2. The length of each pipe is 6 m. the maximum direct compressive stresses in concrete are 15 and 2 N/mm2. The loss ratio is 0.8. i. Design the circumferential wire winding using 5 mm diameter wires stressed to 1000 N/mm2. ii. Design the longitudinal prestressing using 7 mm wires tensioned to 1000 N/mm2. The maximum permissible tensile stress under the critical transient loading (wire wrapping at spigot end) should not exceed, where fci is the cube strength of concrete at transfer = 40 N/mm2. ci f 8.0 iii. Check for safety against longitudinal stresses that develop, considering the pipe as a hollow circular beam as per IS: 784 provisions. (AUC Nov/Dec 2011& 2012) Solution: Computer Science Engineering Department 66 Internet Programming

67 Computer Science Engineering Department 67 Internet Programming

68 5. A prestressed concrete pipe of 1.2 m diameter, having a core thickness of 75 mm is required to withstand a service pressure intensity of 1.2 N/mm2. Estimate the pitch of 5 mm diameter high tensile wire winding if the initial stress is limited to 1000 N/mm2. Permissible stresses in concrete being 12 N/mm2 in compression and zero in tension. The loss ratio is 0.8, if the direct tensile strength of concrete is 2.5 N/mm2, estimate load factor against cracking. (AUC Nov/Dec 2013) Computer Science Engineering Department 68 Internet Programming

69 Computer Science Engineering Department 69 Internet Programming

70 6. Explain the general features of prestressed concrete tanks. (AUC Apr/May 2012) Computer Science Engineering Department 70 Internet Programming

71 7. Explain the junctions of tank wall and base slab with neat sketch. (AUC Apr/May 2012) Computer Science Engineering Department 71 Internet Programming

72 Computer Science Engineering Department 72 Internet Programming

73 8. A cylindrical prestressed concrete water tank of internal diameter 30 m is required to store water over a depth of 7.5 m. The permissible compressive stress in concrete at transfer is 13 N/mm2 and the minimum compressive stress under working pressure is 1 N/mm2, the loss ratio is 0.75, Wires of 5 mm dia with an initial stress of 1000 N/mm2 are available for circumferential winding and freyssinet cables made up of 12 wires of 8 mm dia stressed to 1200 N/mm2 are to be used for vertical prestressing. Design the tank walls assuming the base as fixed. The cube strength of concrete is 40 N/mm2. For the thickness of wall is 150 mm. (AUC Apr/May 2012) Solution: Computer Science Engineering Department 73 Internet Programming

74 Computer Science Engineering Department 74 Internet Programming

75 Computer Science Engineering Department 75 Internet Programming

76 9. What are the advantages of prestressed concrete poles and piles? Computer Science Engineering Department 76 Internet Programming

77 (AUC Apr/May 2010) Advantages of prestressed concrete poles: Resistance to corrosion in humid and temperature climate and to erosion in desert areas. Easy handling due to less weight than other poles. Easily installed in drilled holes in ground with or without concrete fill. Lighter because of reduced cross section when compared with reinforced concrete poles. Fire resisting, particularly grassing and pushing fire near ground line. Freeze-thaw resistance in cold regions. Clean and neat in appearance and requiring negligible maintenance for a number of years, thus ideally suited for urban installations. Have increased crack resistance, rigidity and can resist dynamic loads better than reinforced concrete poles. Advantages of prestressed concrete piles: High load and moment carrying capacity. Standardization in design for mass production. Excellent durability under adverse environmental conditions. Crack free characteristics under handling and driving. Resistance to tensile loads due to uplift. Combined load moment capacity. Good resistance to hand driving loads and penetration into hard strata. Piles can be lengthened by splicing. Ease of handling, transporting and driving. Overall economy in production and installation. Adaptability to both developed and developing countries in tropical, sub arctic and desert regions. Use of solid and hollow cross sectional configurations to suit design requirements. Ease of connections with pile caps to form pier, trestle and jetty bents to support bridge or wharf decks. Effective use of fender piling to resist the kinetic energy of ship impact. Particularly advantageous for deep foundations to carry heavy loads in weak soils. Computer Science Engineering Department 77 Internet Programming

78 10. Briefly explain the design of prestressed concrete poles. Give some sketches suitable for prestressed concrete poles. (AUC Apr/May 2010 & 2011) Computer Science Engineering Department 78 Internet Programming

79 Cross section of prestressed concrete poles: Computer Science Engineering Department 79 Internet Programming

80 Unit 4 Composite Beams And Continuous Beams Part A 1. Define propped construction. (AUC May/June 2013, Nov/Dec 2013) The dead load stress developed in the precast prestressed units can be minimized by propping them while casting the concrete in situ. This method of construction is termed as propped construction. 2. How to achieve compositeness between precast and cast in situ part and show the sketches? (AUC May/June 2013, Nov/Dec 2013) The composite action between the two components is achieved by roughening the surface of the prestressed unit on to which the concrete is cast in situ, thus giving a better frictional resistance or by stirrups protruding from the prestressed unit into the added concrete or by castellations on the surface of the prestressed unit adjoining the concrete which is cast in situ. 3. What is meant by composite construction of prestressed and in situ concrete? (AUC Nov/Dec & Apr/May 2011) In a composite construction, precast prestressed members are used in conjunction with the concrete cast in situ, so that the members behave as monolithic unit under service loads. The high strength prestressed units are used in the tension zone while the concrete, which is the cast in situ of relatively lower compressive strength is used in the compression zone of the composite members. 4. How deflections in composite members are computed? (AUC Nov/Dec 2011) In the case of composite members, deflections are computed by taking into account the different stages of loading as well as the differences in the modulus of elasticity of concrete in the precast prestressed unit and the in situ cast element. Computer Science Engineering Department 80 Internet Programming

81 5. What do you mean by unpropped construction? (AUC Nov/Dec 2012) If the precast units are not propped while placing the in situ concrete, stresses are developed in the unit due to the self weight of the member and the dead weight of the in situ concrete. This method of construction is referred to as unpropped construction. 6. What are the forces considered in the calculation of deflection of prestressed concrete beams? Prestressing force Self weight of the beam Dead load of the concrete Live load acting on the concrete 7. What are the roles played by shear connectors in composite construction? (AUC Apr/May 2010) It is generally assumed that the natural bond at the interface contributes a part of the required shear resistance depending upon the strength of the in situ cast concrete and the roughness of the precast element. Any extra shear resistance over and above this should be provided by shear connectors. 8. What are the advantages in using precast prestressed units? (AUC Apr/May 2011, Nov/Dec 2010 & 2012) Saving in the cost of steel in a composite member compared with a reinforced or prestressed concrete member. Sizes of precast prestressed units can be reduced due to the effect of composite action. Low ratio of size of the precast unit to that of the whole composite member. Composite members are ideally suited for construction bridge decks without the disruption of normal traffic. Computer Science Engineering Department 81 Internet Programming

82 9. Name the loadings to be considered for computing initial deflection. (AUC Nov/Dec 2010) Prestress Self weight of the beam Weight of the in situ cast concrete 10. How do you compute the shrinkage and resultant stresses in composite member? (AUC Nov/Dec 2012) The magnitude of differential shrinkage is influenced by the composition of concrete and the environmental conditions to which the composite member is exposed. In the absence of exact data, a general value of 100 micro strains is provided for computing shrinkage stresses. 11. Distinguish between propped and unpropped construction methods. (AUC Nov/Dec 2012) S.No Propped construction Unpropped construction 1 The dead load stress developed in the precast prestressed units can be minimized by propping them while casting the concrete in situ. This method of construction is termed as propped construction. If the precast units are not propped while placing them in situ concrete, stresses are developed in the unit due to the self weight of the member and the dead weight of the in situ concrete. This method of construction is referred to as unpropped construction. 2 If the pretensioned beam supports If the slab is externally supported while Computer Science Engineering Department 82 Internet Programming

83 the weight of the slab while casting. casting. 12. What are the assumptions made in stresses developed due to differential shrinkage? The shrinkage is uniform over the in situ part of the section. Effect of creep and increase in modulus of elasticity with age and the component of shrinkage, which is common to both the units are negligible. 13. Name the loadings to be considered for computing deflection if the beam is propped section. Prestress Self weight of the beam Dead weight of the in situ cast concrete Live load of the in situ cast concrete 14.Name the loadings to be considered for computing deflection if the beam is unpropped section. Prestress Self weight of the beam Live load of the in situ cast concrete Computer Science Engineering Department 83 Internet Programming

84 15. Sketch the typical cross section of precast prestressed concrete beam. Computer Science Engineering Department 84 Internet Programming

85 Part B 1. Design a precast prestressed inverted T section to be used in a composite slab of total depth 600 mm and width 300 mm. the composite slab is required to support an imposed load of 16 kn/m 2 over a span of 14 m. the compressive stress in concrete at transfer and the tensile stress under working loads may be assumed to be 20 and 1 N/mm 2 respectively. The loss ratio is Determine the prestressing force required for the section. (AUC May/June 2013, Nov/Dec 2013) Solution: Computer Science Engineering Department 85 Internet Programming

86 2. Explain the term shrinkage stresses in composite beams.(auc May/June 2013) Computer Science Engineering Department 86 Internet Programming

87 Computer Science Engineering Department 87 Internet Programming

88 3. A precast pretensioned beam of rectangular section has a breadth of 100 mm and a depth of 200 mm. the beam with an effective span of 5 m is prestressed by tendons with their centroids coinciding with the bottom kern. The initial force in the tendons is 150 kn. 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/m2. Calculate the resultant stresses developed in the precast and insitu concrete assuming the pretensioned 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. (AUC Nov/Dec 2011) Solution: Computer Science Engineering Department 88 Internet Programming

89 Computer Science Engineering Department 89 Internet Programming

90 Computer Science Engineering Department 90 Internet Programming

91 4. Discuss in detail about the factors which influence flexural strength and shear strength of composite prestressed section. (AUC Nov/Dec 2011) Computer Science Engineering Department 91 Internet Programming

92 Flexural strength of composite section: Shear strength of composite section: Computer Science Engineering Department 92 Internet Programming

93 Computer Science Engineering Department 93 Internet Programming

94 5. A precast PSC beam of rectangular section has a breadth of 100 mm and a depth of 200 mm. The beam with an effective span of 5 m is prestressed by tendons with their centroids coinciding with bottom kern. The initial force in the tendon is 150 kn. The loss ratio = 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/m2. Calculate the resultant stresses developed in the precast and insitu cast concrete assuming the pretensioned beam as unpropped during the casting of the slab. Eprecastpsc = 35 kn/mm2, Einsitu con = 28 kn/mm2. (AUC Nov/Dec 2012) Solution: Computer Science Engineering Department 94 Internet Programming

95 Computer Science Engineering Department 95 Internet Programming

96 6. Write step by step design procedure for composite construction. Computer Science Engineering Department 96 Internet Programming

97 Computer Science Engineering Department 97 Internet Programming

98 7. Explain the advantages of using precast prestressed elements along with insitu concrete. (AUC Apr/May 2010) Computer Science Engineering Department 98 Internet Programming

99 8. Explain different types of composite construction with sketches. (AUC Apr/May 2010, 2011, Nov/Dec 2012) Computer Science Engineering Department 99 Internet Programming

100 Computer Science Engineering Department 100 Internet Programming

101 Computer Science Engineering Department 101 Internet Programming

102 9. A composite beam of rectangular section is made of inverted T-beam having a slab thickness of 150 mm and width of 1000 mm. the rib size in 150 mm x 850 mm. The in situ concrete slab has EC = 30kN/m2 and the thickness of cast in situ slab is 1000 mm. If the differential shrinkage in 100 x 10-6 units, estimate the shrinkage stress developed in the precast and cast in situ units. (AUC Apr/May Computer Science Engineering Department 102 Internet Programming

103 Computer Science Engineering Department 103 Internet Programming

104 10. A composite T - beam is made up of a pretensioned rib 100 mm wide and 200 mm deep and cast in situ slab 400 mm wide and 40 mm thick having a modulus of elasticity of 28kN/mm2. If the differential shrinkage is 100 x 10-6 units, estimate the shrinkage stress developed in the precast and cast in situ units. (AUC Nov/Dec 2010) Solution: Computer Science Engineering Department 104 Internet Programming

105 Computer Science Engineering Department 105 Internet Programming

106 11. Explain the precast prestressed concrete stresses at serviceability limit state. (AUC Nov/Dec 2012) Computer Science Engineering Department 106 Internet Programming

107 Computer Science Engineering Department 107 Internet Programming

108 12. Design a composite slab for the bridge deck using a standard invested T- section. The top flange is 250 mm wide and 100 mm thick. The bottom flange is 500 mm wide and 250 mm thick. The web thickness is 100 mm and the overall depth of the inverted T-section is 655 mm. The bridge deck has to support a characteristic imposed load of 50 kn/m2over an effective span of 12 m. Grade 40 concrete is specified for the precast pretensioned T section with a compressive strength at transfer of 36 N/mm2. Concrete of grade 30 is used for the insitu part. Determine the minimum prestress necessary and check for safety under serviceability limit state. Section properties: Area = mm2, position of centroid = 220 mm from the soffit.i = 81.1 x 108 mm4, Zt = 18.7 x 106 mm3, Zb = 37 x 106 mm3. Loss ratio = 0.8, Mmin = 0. (AUC Nov/Dec 2012) Computer Science Engineering Department 108 Internet Programming

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112 Unit 5 Miscellaneous Structures Part A 1. Write the advantages of prestressed concrete bridges. (AUC May/June 2013, Nov/Dec 2011, 2012 & 2013, Apr/May 2012) High strength concrete and high tensile steel, besides being economical make for slender sections which are aesthetically superior. In comparison with steel bridges prestressed concrete bridges require very little maintenance. Prestressed concrete bridges can be designed as class 1 type structures without any tensile stresses under service loads, thus resulting in a crack free structure. 2. Draw a typical cross section of pretensioned prestressed concrete bridgdecks. (AUC May/June 2013, Nov/Dec 2011 & 2012) Computer Science Engineering Department 112 Internet Programming

113 3. What are span range for solid slab and T-beam slab decks? Span ranges For solid slab decks 10 to 20 m For T-beam slab decks 20 to 40 m 4. Draw the cross sectional profile of most commonly used prestressed concrete beams in bridges. (AUC Apr/May 2010) 5. Write a short note on post tensioned bridge decks. (AUC Apr/May 2012) Post tensioned bridge decks are generally adopted for longer spans exceeding 20 m. bridge decks with precast post tensioned girders of either T type or box type in conjunction with a cast in situ slab are commonly adopted for spans exceeding 30 m. 6. Why the deck slab of prestressed concrete bridges is mostly made of non prestressed concrete? The pretensioned girders are provided with straight tendons. The use of seven wire strands has been found to be advantageous in comparison with plain or indented wires. The deflected strands are employed in larger girders. 7. What are the advantages of post tensioning system? (AUC Apr/May 2011) Computer Science Engineering Department 113 Internet Programming

114 Post tensioning is suitable for heavy cast in place members. The waiting period in the casting bed is less. The transfer of prestress is independent of transmission length. 8. Draw neat sketches of two efficient sections used for prestressed concrete bridges. (AUC Apr/May 2011) 9. List any four mechanical prestressing systems adopted for bridges in India. (AUC Nov/Dec 2010) Includes weights with or without lever transmission Geared transmission in conjunction with pulley blocks Screw jacks with or without gear drives Wire winding machines 10. What is meant by kern distance in a prestressed concrete bridge? (AUC Nov/Dec 2010) Kern is the core area of the section in which if the load applied tension will not be induced in the section. K t = Z b / A, K b = Z t / A, If the load applied at Kt compressive stress will be the maximum at the top most fiber and zero stress will be at the bottom most fiber. If the load applied at K b compressive stress will be the maximum at the bottom most fiber and zero stress will be at the top most fiber. 11. What are the disadvantages of post tensioning system? The relative disadvantage of post tensioning as compared to pretensioning is the requirement of anchorage device and grouting equipment. 12. Where the prestressed concrete construction is suited? The prestressed concrete is ideally suited for the construction of medium and long span bridges. Computer Science Engineering Department 114 Internet Programming

115 13. Define pretensioned prestressed concrete bridge decks. Pretensioned prestressed concrete bridge decks generally comprise precast pretensioned units used in conjunction with cast in situ concrete, resulting in composite bridge decks which are ideally suited for small and medium spans in the range of 20 to 30 m. 14. What is the span ranges for different types of slabs and beams? Solid slab decks 10 to 20 m T-beam slab decks 20 to 40 m Single or multicell box girder 30 to 70 m Precast box girders more than 50 m Simply supported, continuous beams 20 to 500 m Composite bridge decks 20 to 30 m Precast prestressed I and T - beam 7 to 36 m Y beams 15 to 30 m Computer Science Engineering Department 115 Internet Programming

116 Part B 1. What are the general aspects of prestressed concrete bridges and its advantages over RC bridges? (AUC May/June 2013, Nov/Dec 2013) Advantages over RC bridge: High strength concrete and high tensile steel, besides being economical make for slender sections which are aesthetically superior. In comparison with steel bridges prestressed concrete bridges require very little maintenance. Prestressed concrete bridges can be designed as class 1 type structures without any tensile stresses under service loads, thus resulting in a crack free structure. Prestressed concrete is ideally suited for composite bridge construction in which precast prestressed girders support the cast in situ slab deck. This type of construction is very popular since it involves minimum disruption of traffic. Post tensioned prestressed concrete finds extensive applications in long span continuous girder bridges of variable cross section. Not only does it make for sleek structures, but it also effects considerable saving in the overall cost of construction. Computer Science Engineering Department 116 Internet Programming

117 2. Explain in detail about design of pretensioned prestressed bridge decks. Computer Science Engineering Department 117 Internet Programming

118 3. Draw neat sketches showing the typical cross sections of pre tensioned PSC bridge decks. (AUC Nov/Dec 2012) Computer Science Engineering Department 118 Internet Programming

119 4. Explain in detail the general aspects of post tensioned concrete bridges decks. (AUC Apr/May 2010, Nov/Dec 2010, 2012 & 2013) Computer Science Engineering Department 119 Internet Programming

120 5. Draw neat sketches showing the typical cross sections of post tensioned PSC bridge decks. (AUC Nov/Dec, Apr/May 2012) Computer Science Engineering Department 120 Internet Programming