Prestressed Concrete

PRIST UNIVERSITY (Estd. u/s 3 of UGC Act, 1956) Vallam, Thanjavur -613403 _________________________________________________________________

M.TECH (STRUCTURAL ENGINEERING)

QUESTION BANK Course Details: Course Code & Title

: 13255E16A –PRESTRESSED CONCRETE DESIGN

Regulation

: 2013 Regulation

Nature of the Programme

: M.Tech (Structural Engineering)

H.O.D

Staff-In-Charge

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LIST OF ELECTIVES SEMESTER I 13255E16A

HARD CORE ELECTIVE I PRESTRESSED CONCRETE DESIGN

L T P C 3 1 0 4

AIM and OBJECTIVES: This course introduces students to the fundamental principles of pre-stressed concrete behavior and design, So that they can act effectively to optimize existing forms of construction and apply fundamental concepts with confidence in unusual and challenging situations. UNIT I INTRODUCTION 10 Difference between reinforced and pre-stressed concrete – Principles of prestressing – Methods and systems of pre-stressing – Principles of electro thermal pre-stressing and chemical pre-stressing – Classification of pre-stressed concrete structures – Materials – High strength concrete and High strength steel – Stressstrain diagram - Losses in pre- stress. UNIT II LIMIT STATE DESIGN 8 Design of prismatic pre-stressed concrete members for bending at working loads –Check for ultimate load stage (Limit State Design). UNIT III BEHAVIOR OF BEAMS 8 Simple cable profiles – Calculation of deflections – Design of beams for shear and torsion at working and ultimate loads. Design of Anchorage zone by Guyon’s method – Concept of Magnel’s method – IS 1343 recommendations. UNIT IV COMPOSITE BEAMS 10 Composite pre-stressed concrete beams – Design procedure – Calculation of stresses at important stages both for propped and un propped constructions – Shrinkage stresses. UNIT V BEHAVIOR OF CONTINUOUS BEAMS 8 Statically indeterminate structures – Concept of concordant cable and linear transformations – Sketching of pressure lines for continuous beams – Circular pre stressing. REFERENCES: 1. Leonhardt.F., Prestressed Concrete, Design and Construction, Wilhelm Ernst and Shon, Berlin, 1964. 2. Freyssinet, Prestressed Concrete 3. Krishna Raju, N.Advanced Reinforced Concrete Design, CBS Publishers and Distributors, 1986. 4. Rajagopal.N, (2005), Prestressed Concrete, Second Edition, Narosa Publishing House. 5. Dayarathnam P,(2004), Prestressed Concrete Structures,S.ChandPublishers.

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UNIT-I PART-A 1) 2) 3) 4) 5) 6)

Define Prestress. List out the system of prestressing. What is meant by pre tension and post tension? What are the advantages of prestress concrete? What are the materials used in prestressed concrete? Write down the shrinkage strain coefficient recommended in IS code for pre tension and post tension. 7) What are the types of loss of prestress? 8) Write the creep coefficient θ for three values of age of loading? 9) Define creep. 10) Define creep coefficient. 11) List the desirable properties of grout. 12) Define the term tendon and anchorage. PART-B 1.Explain the system of prestressing. 2.What are the materials used in prestressed concrete.Briefly explain? 3.Explain the losses of prestressing. 4 Consider the concrete beam of rectangular c/s of 150mm wide &300mm depth ,prestressed by 4 high tensile wires of 5mm dia stressed due to 1200N/mm2.The wires are located @ eccentricity of 80mm. The stresses developed @ soffit of beam will be examined by normal concrete & Equivalent c/s. 5. A rectangular concrete beam 250*600mm is prestressed by means of 4 nos of 14mm dia high tensile bars located @ 200 mm from the soffit of the beam. If the effective stresses in the wire is 700N/mm2. What is the Max.B.M that can be applied to the c/s with out causing tension @the soffit of the beam. 6. A concrete beam 100mm wide &300mm depth is prestressed by straight wires carrying an initial forces are 150 KN at an eccentricity of 50mm. The modulus of elasticity of steel &concrete are 210&35 N/mm2 respectively. Estimate the percentage of losses of stress is steel due to elastic deformation of steel. If the area of steel is 188mm2.

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UNIT-II PART-A 1. Define Flexure. 2.List out the type of flexural failures. 3.Write down the formula for flexural strength in unbonded tendons. 4.What are the critical combinations considered in minimum section modulus? 5.Write down the formula for prestressing force and corresponding eccentricity. 6.What is Loss ratio value for pre tensioned beam and post tensioned beam ? 7.Draw typical crack patterns of bonded and unbounded beams. 8.What do you mean by balanced eccentricity? 9.What do you mean by limiting zone? 10.Explain the load balancing concept. PART-B 1.A pre tensioned prestressed concrete beam having a rectangular section 150mm wide and 350mm deep has an effective cover of 50mm. If fck = 40 N/mm2, fp = 1600 N/mm2 and Ap = 461mm2. Calculate the ultimate flexural strength of the section using IS 1343 code provisions. 2. A Pre-tensioned beam of rectangular section 400mm wide by 1000mm overall depth is prestressed by 800mm2 of high tensile steel wires at an eccentricity of 300mm. If fck=40N/mm2 , fp=1600N/mm2. Estimate the ultimate flexural strength of the section as per IS 1343 code provisions. 3. A Pre-tensioned beam of rectangular section 350mm wide by 800mm overall depth is prestressed by 940mm2 of high tensile steel wires at an eccentricity of 300mm. If fck=40N/mm2 , fp=1600N/mm2. Estimate the ultimate flexural strength of the section as per IS 1343 code provisions. 4. A Post-tensioned bonded prestressed concrete beam of un-symmetrical Tee section has a flange width of 1500mm and thickness of flange is 200mm. Thickness of rib is 300mm.The area of high tensile steel is 5000mm2 located @ an effective depth of 1800mm.If the characteristic strength of concrete and steel are 40 &1600N/mm2 respectively. Calculate the flexural strength of the Tee section.

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5. A double Tee section having a flange 1200mm wide and 150mm thick is prestressed by 4700mm2 of high tensile steel located at effective depth of 1600mm. The ribs have a thickness of 150mm each. The cube strength of concrete is 40N/mm2 and tensile strength of steel is 1600N/mm2, determine the flexural strength of double Tee section using IS1343 code provisions. 6. A pre-tensioned Tee section has a flange width of 300mm and thickness of flange is 200mm. The rib is 150mm wide & 350mm depth. The effective depth at which high tensile steel area is 200mm2 . Given fck=50N/mm2 and fp=1600N/mm2.Estimate the flexural strength of the Tee section ..

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UNIT-III PART-A 1) What are the factors influencing in deflection? 2) Define End block. 3) What is meant by transmission length? 4) How the stress developed in end block? 5) What is meant by bursting stress? 6) What are the methods used for design of end block? 7) Draw the arrangements of reinforcement in end block. 8) Distinguish between working and ultimate loads? 9) How will you calculate the deflection in case of prestressed concrete. 10) Difference between bursting tension &spalling tension. PART-B 1. A concrete beam with a rectangular section 100mm wide and 300mm deep is stressed by 3 cables , each carrying an effective force of 240KN. The span of the beam is 10m. The first cable is parabolic with an eccentricity of 50mm below the centroidal axis at the supports. The second cable is parabolic with zero eccentricity at the supports at eccentricity of 50mm at the centre of span. The third cable is straight with a uniform eccentricity of 50mm below the centroidal axis. If the beam supports a udl of 5KN/m and Ec=38N/mm2, Estimate the deflection at the following stages. a) Prestress + self weight of beam,and b) Prestress +self weight +live load 2. A concrete beam with a rectangular section 200mm wide and 300mm deep is stressed by 5 cables , each carrying an effective force of 500KN. The span of the beam is 10m. The first cable is parabolic with an eccentricity of 50mm below the centroidal axis at the supports. The second cable is parabolic with zero eccentricity at the supports at eccentricity of 50mm at the centre of span. The third cable is straight with a uniform eccentricity of 50mm below the centroidal axis. If the beam supports a udl of 5KN/m and Ec=38N/mm2, Estimate the deflection at the following stages. a) Prestress + self weight of beam,and b) Prestress +self weight +live load

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3. A prestressed concrete beam spanning over 8m of rectangular section , 150mm wide and 300mm deep. The beam is prestressed by a parabolic cable having an eccentricity of 75mm below the centroidal axis at the centre of the span and an eccentricity of 25mm above the centroidal axis at the support section. The initial force of the cable is 350KN. The beam supports 3 concentrated loads of 10 KN each at intervals of 2m , Ec=38N/mm2. a) Prestress + self weight of beam,and b) Prestress +self weight +live load 4. A prestressed concrete beam spanning over 10m of rectangular section , 350mm wide and 500mm deep. The beam is prestressed by a parabolic cable having an eccentricity of 50mm below the centroidal axis at the centre of the span and an eccentricity of 25mm above the centroidal axis at the support section. The initial force of the cable is 500KN. The beam supports 4 concentrated loads of 10 KN each at intervals of 2m , Ec=38N/mm2. a) Prestress + self weight of beam,and b) Prestress +self weight +live load 5. A prestressed concrete beam spanning over 12m of rectangular section , 300mm wide and 450mm deep.The beam is prestressed by a parabolic cable having an eccentricity of 75mm below the centroidal axis at the centre of the span and an eccentricity of 25mm above the centroidal axis at the support section. The initial force of the cable is 350KN.The beam supports 5 concentrated loads of 20 KN each at intervals of 2m , Ec=38N/mm2. a) Prestress + self weight of beam,and b) Prestress +self weight +live load 6. Explain the End blocks of prestressed concrete

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UNIT-IV PART-A 1.What is meant by composite construction? 2.What are the advantages of composite construction? 3.List out the types of composite construction. 4.Define propped and unpropped construction. 5.Draw the stress distribution diagram in propped and unpropped construction. 6.What are the forces acting at an mast? 7.What are shrinkage stresses? PART-B 1. A precast pre tensioned beam of rectangular c/s has a breadth of 100mm & depth of 200mm. The beam with a effective span of 5m is prestressed by tendons with their centroids coinciding the span of 5m.The initial force in the tendon is 150KN. The loss of prestress to be 15%. The is incarbonated in the composite T- beam by casting the flange of breadth 400mm& thickness 40mm. If the composite beam support @ live load of 8KN/m2, calculate the resultant stresses developed in the precast in-situ cast concrete assuming the beam as a) unpropped b) propped during the casting of the slab. 2. A precast pre tensioned beam of rectangular c/s has a breadth of 200mm & depth of 500mm. The beam with a effective span of 5m is prestressed by tendons with their centroids coinciding the span of 5m.The initial force in the tendon is 350KN. The loss of prestress to be 15%. The is incarbonated in the composite T- beam by casting the flange of breadth 600mm& thickness 100mm. If the composite beam support @ live load of 12KN/m2, calculate the resultant stresses developed in the precast in-situ cast concrete assuming the beam as a) unpropped b) propped during the casting of the slab. 3. The c/s of composite beam of T-section having a pretensioned rib 80mm wide & 240mm depth and cast in-situ slab 350mm wide & 80mm thick . The pre tensioned beam reinforced with 8 wires of 5mm dia with an ultimate tensile strength of 1600N/mm2 located 60mm from the soffit of beam. The grade of concrete is M20. Calculate the flexural strength of composite beam.

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4. The c/s of composite beam of T-section having a pretensioned rib 100mm wide & 300mm depth and cast in-situ slab 350mm wide & 100mm thick . The pre tensioned beam reinforced with 8 wires of 6mm dia with an ultimate tensile strength of 1600N/mm2 located 60mm from the soffit of beam. The grade of concrete is M20. Calculate the flexural strength of composite beam. 5. A composite bridge deck of span 12m is made up of precast prestressed symmetrical

I–

section and an in-situ cast slab & precast I-beams are spaced at 750mm centers and the top slab of in-situ concrete is 120mm thick, The c/s details of the precast beam are as follows: Thickness of top and bottom flanges=110mm Width of top and bottom flanges

=200mm

Thickness of web

=75mm

Depth of precast I-beam

=500mm

Self weight of precast concrete

=24KN/m3

Self weight of in-situ concrete

=23.5KN/m3

The form work load is estimated as 0.2KN/m of the span.If the compressive prestress in the beam is 15N/mm2 at the bottom& zero at the top, Calculate the Max.stresses developed in the beam. Imposed load of in-situ cast concrete is 5KN/m2. 6. Explain the types of composite construction.

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UNIT-V PART-A 1.What are the prestressing devices 2.State the Guyon’s Theorem. 3.What are the types of prestressed concrete sleepers? 4.Write down the advantage and disadvantages of continuous prestress concrete beam. 5.Write down the types of prestressed concrete pipes? 6.What is meant by circular prestressing? 7.What are the shapes used for prestressed concrete Tank? 1.

PART-B Design a non-cylindrical prestressed concrete pipe of 600mm internal dia to withstand working hydrostatic pressure 1.05N/mm2 using 2.5mm high tensile wire ,stressed to 1000N/mm2 @ transfer . Permissible Max& Min. stresses in concrete are 14N/mm2 &0.7N/mm2. The loss ratio is 0.8. Calculate the test pressure require to produce the tensile stress 0.7N/mm2. Ec=35N/mm2& Es=210N/mm2

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The prestressed cylinder pipe is to be designed using a steel cylinder of 1000mm internal dia & thickness 1.6mm, the circumferential wire winding of 4mm high tensile wire initially tensioned to a stress of 1000N/mm2, ultimate strength of wire is 1600N/mm2 & yield stress of steel cylinder is 280N/mm2. The Max. permissible stress in concrete @ transfer 14N/mm2. No tensile stresses are permitted under working pressure of 0.8N/mm2. Determine the thickness of concrete lining required, the no .of turn of circumferential wire winding & factor of safety against bursting. Assume modular ratio as 6.

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3. Fig shows a two span continuous beam.Corresponding to the cable profile provided locate the pressure line due to prestress alone. The prestressing force is 1000 kN.

4.Locate the pressure line(C-line) due to the prestress for the following figure.The tendon carries a prestressing force of 1500 kN.

5.Fig shows two proposed P-lines for a prestressed concrete beam which are linear transformations of each other.Show that the C-line in the concrete is the same for both the cable profiles.Assume a prestressing force of 1000 kN.

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6.For the post tensioned beam with a flanged section as shown, the profile of the CGS is parabolic, with no eccentricity at the ends. The live load moment at mid span (MLL) is 648 kNm. The prestress after transfer (Po) is 1600 kN. Assume 15% loss at service. Grade of concrete is M30.The span of the beam is 18 m. Evaluate the following quantities a)Kern levels b)Cracking moment c)Location of pressure line at midspan at transfer and at service. d)The stresses at the top and bottom fibres at transfer and at service. Compare the stresses with the following allowable stresses at transfer and at service. For compression,fc,comp= -18 N/mm2 For tension, fc,tens=1.5 N/mm2.

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