Prestressed-Concrete-Structures.pdf

S.K.P. Engineering College, Tiruva na namalai

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SKP E gineering College Ti uvan uvanna nama mala laii – 6066 606611 11

Course Material on Prestressed ressed Concrete Concrete Struct Structures ures

By

P. Selvarasan Assistant Professor Civil Engine Engineerin ering g Departm Department ent

Computer Science Engineering

epartment

1

Inter et Programming

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S.K.P. Engineering College, Tiruvannamalai

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Quality Certificate

This is to Certify Certify that the Electronic Electronic Study Material Material

Subject Subject Code: Code: CE 6702 6702 Subject Name: Prestressed Prestressed Concrete Concrete Structures Structures Year/Sem Year/Sem:: IV/VII IV/VII Being prepared by me and it meets the knowledge requirement of the University curriculum.

Signature of the Author Name: P. Selvar Selvarasan asan Designation: Designation: Assistant Professor

This is to certify that the the course material material being prepared prepared by Mr.P.Selvarasan Mr.P.Selvarasan is of the adequate quality. He has referred more than five books and one among them is from abroad abroad author. author.

Signature of HD

Signature of the Principal

Name: Mr M r. A. Saravanan

Name: Dr D r.V.Subramania Bharathi

Seal:

Seal:




























CE6702

PRESTRESSED CONCRETE STRUCTURES

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LTPC3003

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 Bas Basic conc oncepts – Advantag ntages es – Mater ateria ialls requir quire ed – Sy Sys stem tems and met methods of prestressing – Analysis of se sections – Stress concept – Strength concept – Load balanc balancing ing conce concept pt – Effec Effectt of load loading ing on the the tens tensile ile stres stresses ses in tend tendon ons s – Effect Effect of tend tendon on prof profil ile e on defl deflec ecti tion ons s – Fact Factor ors s infl influe uenc ncin ing g defl deflec ecti tion ons s – Calc Calcul ulat atio ion n of defle deflect ctio ions ns – Short Short term term and and long long term term deflec deflectio tions ns - Losses Losses of of prest prestre ress ss – Estima Estimatio tion n of crack width. UNIT II DESIGN FOR FLEXURE AND SHEAR 9 Basic assumptions assumptions for calculating calculating flexural stresses – Permissible Permissible stresses stresses in steel and and concrete concrete as per per I.S.13 I.S.1343 43 Code Code – Design Design of of section sections s of Type Type I and and Type Type II posttensioned and pre-tensioned pre-tensioned beams – Check for strength strength limit limit based based on I.S. 1343 1343 Code – Layout of cables cables in post-tensioned post-tensioned beams – Location of wires wires in pre-tensioned pre-tensioned beams – Design for shear based based on I.S. 1343 1343 Code. UNIT III DEFLECTION AND DESIGN OF ANCHORAGE ZONE 9 Factor Factors s influ influen encin cing g defle deflecti ction ons s – Short Short ter term m defle deflecti ction ons s of unc uncra racke cked d membe members rs – Prediction of of long term deflections deflections due due to creep and and shrinkage shrinkage – Check for serviceab serviceability ility 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 reinforcemen reinforcementt – Check for transfer transfer bond length length in pre-tensione pre-tensioned d beams. UNIT IV COMPOSITE BEAMS AND CONTINUOUS BEAMS 9 Analysis and design of composite beams – Methods of achieving achieving continuity in contin continuou uous s beams beams – Analy Analysi sis s for seco seconda ndary ry momen moments ts – Conco Concord rdant ant cab cable le and and linear linear transfor transformatio mation n – Calculat Calculation ion of stres stresses ses – Principl Principles es of of desig design. n. UNIT V MISCELLANEOUS STRUCTURES 9 Desi Design gn of of tens tensio ion n and and comp compre ress ssio ion n memb member ers s – Tank Tanks, s, pip pipes es and and pol poles es – Part Partia iall prestressing prestressing – Definition, Definition, methods of achieving achieving partial prestressing, prestressing, merits and demerits demerits of partial prestressing.




























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TEXTBOOKS: 1. Krishna Raju N., "Prestressed concrete", 5th Edition, Tata McGraw Hill Company, New Delhi, 2012 2. Pandit.G.S. and Gupta.S.P., "Prestressed Concrete", CBS Publishers and Distributers Pvt. Ltd, 2012.




























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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




























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Unit 1 Introdu Introductio ction n – Theory Theory And And Behavio Behavior r Part – A 1. List List out out the the adva advant ntag ages es of pres prestr tres esse sed d conc concre rete te.. (AUC (AUC Nov/ Nov/De Dec c 201 2011 1 & 201 2012) 2) In case of fully prestressed prestressed member, which are free from tensile tensile stresses under working 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 pretensioned and and post tensioned tensioned concrete? concrete? (AUC Nov/Dec 2010 & 2011) of Pre stressing stressing concrete concrete in which the tendons are tensioned tensioned Pre tensioning: A method of before the concrete is placed. In this method, the prestress is imparted to concrete by bond between steel steel and concre concrete. te. stressing concrete by tensioning tensioning the the tendons tendons against against Post tensioning: A method of pre stressing hardened concrete. In this method, the prestress is imparted to concrete by bearing. 3. Why is high high tensile tensile steel needed needed for prestresse prestressed d concrete concrete construction? 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 being hired; high strength concrete is invariably 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 strength concrete results 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 strength of high tensile steel steel is in the range of 1400 to 2000 2000 N/mm 2 and if initially stress upto upto 1400 1400 N/mm their will will be still large large stress stress in the high tensile tensile reinforcement reinforcement






























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after making deduction for loss of prestress. Therefore high tensile steel is made for prestress concrete. 4. What What are the vario various us method methods s of prestre prestressin ssing? g? 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 of stress due to friction. Loss due to anchorage slip.

7.

What are the classifications of prestressed concrete structures? (AUC Nov/Dec 2013) Externally or internally prestressed Pretensioning and post tensioning End-Anchored or Non-End Anchored Tendons






























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the tendon tendon replaced replaced by by forces forces acting acting on beam beam 9. What What are are the the fact factor ors s infl influe uenc ncin ing g defl deflec ecti tion ons? s? Length of the deflection field Spacing between the deflection plate Difference of potential between the plates Accelerating Accelerating voltage of the second anode.

(AUC (AUC Apr/ Apr/M May 2011 2011))

10. What are the sources of prestress force? Mechanical Hydraulic Electrical Chemical

(AUC Ap Apr/May 2012)

11. Define kern distance. (AUC Apr/May 2010) Kern is the core area of of the section in which if the load load applied tension 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






























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to this group. of construction construction in which which the tendons are not Non-bonded prestressing: A method of bonded to the surrounding surrounding concrete. The tendons 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 counteracted to a desire degree degree in reinforced concrete concrete member the prestress prestress is commonly introduced by tensioning the steel reinforcement. 17. Define anchorage. A device generally generally used to enable the the tendon tendon to impart impart and maintain maintain prestress prestress to to the concrete is called anchorage. anchorage. E.g. Fressinet, BBRV systems, 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 early strength strength is required required to enable quicker quicker application application of prestress; A larger elastic modulus is needed to reduce the shortening of the member; A mix that reduces creep of the






























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Post-tensioned ribbed slab; This is “glued segmental” construction; 20.Define Magnel diagram. A Magnel Diagram Diagram is a plot of the the four lines lines associated associated with with the limits limits on stress. stress. As can 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.






























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Part – B 1. A rectangular rectangular prestress prestressed ed beam 150 mm wide wide and 300 300 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 load counteracts counteracts the the bending bending effect effect of the the prestressing prestressing force (neglecting self weight of beam) and b) If the pressure pressure line line passes through through the the upper upper kern of the the section section under under the action of the external load, self weight and prestress. (AUC Nov/Dec 2011, Apr/May 2010) Solution:
































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2. Describe the effect of loading on the tensile stresses in tendons. (AUC Nov/Dec 2011)































S.K.P. Engineering College, Tiruva na namalai

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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 location of the thrust line in the beam beam and plot its position position at quarter quarter and central span section. (AUC Apr/May 2012) (AUC May/June 2013, Nov/Dec 2013) Solution:


































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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)


































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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 10–4. (AUC Apr/May 2011)


































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10 Explain the factors influencing deflections?

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(AUC Apr/Ma y




































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cracking and partially for the ultimate limit state of collapse which can result in




































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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. 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 High load load and and mom momen entt carr carryi ying ng capaci capacity ty.. Stand Standard ardiza izati tion on in desi design gn for for mass mass pro produc ductio tion. n. Excelle Excellent nt durab durabilit ility y under under adverse adverse environ environment mental al condi condition tions. s. Crack Crack free free characte characterist ristics ics under under handli handling ng and driving. driving. Resist Resistanc ance e to to tens tensile ile loads loads due to uplift uplift.. Combined load moment capacity.




























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Part – B 1. Design a cylindrical cylindrical prestressed prestressed concrete concrete water tank tank to suit the following following data: 6 Capacity of tank = 24500 x 10 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 2 circumferential winding of 7 mm dia with an initial stress of 1000 N/mm and by vertical vertical cables of 12 12 wires of 8 mm diameter diameter which are stressed stressed to 1200 2 N/mm . Loss ratio = 0.75. 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:




























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2. Design a non – cylinder cylinder prestressed prestressed concrete concrete pipe pipe of 600 mm internal internal diameter 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 tensioning and also the winding winding stress in steel steel if ES = 28 kN/mm2 and and EC = 35 kN/mm2. (AUC May/June 2013) Solution:

3. Discuss in detail about the design procedure for prestressed concrete tanks.




























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Solution:

4. A non – cylinder prestressed prestressed concrete concrete pipe 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:






























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5. A prestressed concrete pipe of 1.2 m diameter, having a core thickness of 75






























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6. Explain the general features of prestressed concrete tanks. (AUC Apr/May 2012)






























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7. Explain the junctions of tank wall and base slab with neat sketch. (AUC Apr/May 2012)
































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(AUC Apr/May 2010) Advantages of prestressed concrete poles:

• • • • •

Resistan Resistance ce to to corros corrosion ion in humid humid and tempera temperature ture climate climate and to erosion in desert areas. Easy Easy hand handlin ling g due due to les less s weigh weightt than than othe otherr poles poles.. Easily Easily instal installed led in drilled drilled holes holes in groun ground d with with or witho without ut concre concrete te fill. fill. Lighter Lighter because because of reduc reduced ed cros cross s secti section on when when compa compared red with reinforced concrete poles. Fire resis resisting ting,, particu particularl larly y grassin grassing g and pushing pushing fire near near groun ground d line. line.


































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10.Briefly explain the design of prestressed concrete poles. Give some sketches suitable for prestresse prestressed d concrete concrete poles. poles. (AUC Apr/May Apr/May 2010 2010 & 2011) 2011)


































Cross section of prestressed concrete poles:

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Unit 4 Composite Beams And Continuous Beams Part – A 1. Define propped construction.

(AUC May/June 2013, Nov/Dec 2013)

The dead load stress stress developed developed in the precast precast prestressed prestressed units can be minimized by propping them while casting the concrete in situ. This method of construction is termed as propped construction.


































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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 weight of the in situ concrete. concrete. This method method of construction construction is referred referred to as unpropped construction. 6. What are the forces considered in the calculation of deflection of prestressed concrete beams? Prestressing force




































9. Name the loadings to be considered for computing initial deflection.

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the weight of the slab while

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15. Sketch the typical cross section of precast prestressed concrete beam.




































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Part – B




































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loads loads as as recomm recommend ended ed in in IRC – 18. Step 3: Depth of slab and effective span Assuming the the thickness thickness of the the slab at at span for for highway bridge bridge decks and to find the overall thickness of the slab. Step 4: Dead load bending moments To calculate the dead weight of the slab and find the dead load bending moment. Step 5: Live load bending moments Generally the bending bending moment moment due to live load will be maximum for IRC class class AA tracked vehicle. Step 6: Shear due to class AA tracked vehicle For maximum shear force at the support section the IRC class AA tracked vehicle is arranged. Step 7: Check Check for minimum minimum section section modulus Step 8: Minimum prestressing prestressing force Step 9: Eccentricity of cables Step 10: Check for stresses at service loads Step 11: Check for ultimate strength strength (IRC: 18 – 2000) Step Step 12: Check Check for ultimate ultimate shear strength Step 13: Supplementary Supplementary reinforcement reinforcement Step 14: Design of end block reinforcement reinforcement 7. Write the the design design procedure procedure of post post tensioned tensioned PSC T - beam slab bridge bridge deck. deck. (AUC Apr/May 2010, Apr/May 2011) Design procedur procedure e of post tensioned tensioned PSC T - beam slab bridge bridge deck: deck:

Step 1: Data collections Clear span, width of bearing, clear clear width of roadway, footpath, footpath, kerbs, thickness thickness of wearing coat, live load and type of structure. Step 2: Permissible stresses The permissible compressive stresses in concrete at transfer and working loads loads as as recomm recommend ended ed in in IRC – 18. Step 3: Cross section section of deck Step 4: Design of the interior slab panel Step 5: Design of longitudinal girders




























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Reaction factors Dead load from slab per girder Dead load of the main girder Dead load moments and shears in the main girder Live load bending moments in the girder Live load shear forces in girders Design bending moments and shear forces Properties of main girder section Check for minimum section modulus Minimum prestressing force Permissible tendon zone Step 6: Check Check for stresse stresses s Step 7: Check for ultimate flexural strength Failure by yielding of steel Failure by crushing of concrete Step 8: Check for ultimate shear strength Step 9: Supplementary reinforcement • • • • • • • • • • •

• •

Step 10: Design of end block reinforcement Step 11: Cross girders 8. Expla Explain in the the advant advantag ages es of prest prestres ressed sed conc concret rete e brid bridges ges.. 2012) 





(AUC (AUC Apr/M Apr/May ay

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. structure. Prestressed Prestressed concrete is ideally ideally suited for composite bridge 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.




























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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. 

In recent recent years, years, partially partially prestre prestressed ssed concre concrete te (type (type – 3 structure structures) s) has been preferred for bridge construction, because it offers considerable economy in the use of costly high tensile steel in the girder.

Prestressed-Concrete-Structures.pdf

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