PCS-1a

1Prestressed Concrete Structures

d lModule 1

Introduction, Prestressing Systems and Material Properties

Prepared by:Amlan K Sengupta

Devdas Menon

Indian Institute of Technology Madras

Reference Books

1) Krishna Raju, N., Prestressed Concrete, Tata McGraw-

References

1) Krishna Raju, N., Prestressed Concrete, Tata McGrawHill Publishing Company Ltd.

2Reference Books

2) Rajagopalan, N., Prestressed Concrete, Narosa

References

2) Rajagopalan, N., Prestressed Concrete, NarosaPublishing House.

Reference Books

3) Collins, M. P. and Mitchell, D., Prestressed Concrete

References

3) Collins, M. P. and Mitchell, D., Prestressed Concrete Structures, Prentice Hall Inc.

3Reference Books

4) Lin, T.Y. and Burns, N. H., Design of Prestressed

References

4) Lin, T.Y. and Burns, N. H., Design of PrestressedConcrete Structures, John Wiley and Sons.

Reference Books

5) Naaman, A. E., Prestressed Concrete Analysis and

References

5) Naaman, A. E., Prestressed Concrete Analysis and Design: Fundamentals, Techno Press 3000.

4Reference Books

6) Nawy, E. G., Prestressed Concrete A Fundamental

References

6) Nawy, E. G., Prestressed Concrete A Fundamental Approach, Prentice Hall Inc.

Reference Books

7) Nilson, A., Design of Prestressed Concrete, John Wiley

References

7) Nilson, A., Design of Prestressed Concrete, John Wiley and Sons.

5Code

IS:1343 - 2012, Prestressed Concrete Code of Practice ,

References

Bureau of Indian Standards.

Allied Codes

IRC:112 - 2011, Code of Practice for Concrete Road Bridges, Indian Roads Congress.

IRS Concrete Bridge Code: 1997, Indian Railway Standard Code of Practice for Plain, Reinforced and PrestressedConcrete for General Bridge Construction, Ministry of Railways.

International Codes

ACI 318M 11 Building Code Requirements for Structural

References

ACI 318M-11, Building Code Requirements for Structural Concrete and Commentary, American Concrete Institute.

BS 8110 : Part 1 : 1985, Structural Use of Concrete : Part 1 Code of Practice for Design and Construction, British Standard Institution.

EN 1992 (EC2), Eurocode 2: Design of Concrete Structures

HandbookPrecast / Prestressed Concrete Institute, PCI Design Handbook

61) Introduction, Prestressing Systems and Material Properties

Topics

2) Losses in Prestress

3) Analysis of Members

4) Design of Members

5) Analysis and Design for Shear and Torsion5) Analysis and Design for Shear and Torsion

6) Calculations of Deflection and Crack-width

Topics

7) Transmission of Prestress

8) Cantilever and Continuous Beams

9) Special Topics

7Introduction

Module 1-a (1st Hour)

Basic ConceptEarly Attempts of PrestressingBrief HistoryDevelopment of Building Materials

Basic Concept

8Introduction

Basic Concept

What is prestressing ?

Prestressing is the application of an initial load on a structure, to enable it to counteract the stresses arising from subsequent loads during its service period.

Examples of Prestressing Before Development of Prestressed Concrete

Introduction

Prestressed Concrete

The concept of prestressing existed before the applications in concrete. Two such examples are given here.

9Examples of Prestressing Before Development of Prestressed Concrete

Introduction

Force-fitting of metal bands on wooden barrelsIt induces a state of initial hoop compression, to counteract the hoop tension caused by filling of liquid in the barrels.

Metal bands

Fig. 1a-1 Force-fitting of metal bands on wooden barrels

Examples of Prestressing Before Development of Prestressed Concrete

Introduction

Pre-tensioning the spokes in a bicycle wheelThe pre-tension is to such an extent that there will always be a residual tension in the spoke.

Fig. 1a-2 Pre-tensioning the spokes in a bicycle wheel

Spokes

chandra s sHighlight


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Basic Concept

Introduction

Concrete in which effective internal stresses are induced (usually, by means of tensioned steel) before the structure is loaded, to counteract the stresses resulting from the applied service loads.

Basic Concept

Introduction

Concretes tensile strength is only 8-14% of its compressive strength. Cracks develop at early stages of loading in flexural

members (beams, slabs).

To prevent such crack, compressive forces can be suitably applied in the perpendicular direction.

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Basic Concept

P t i h th b di h d

Introduction

Prestressing enhances the bending, shear and torsional capacities of the flexural members.

In pipes and liquid storage tanks, the hoop tensile stresses can be effectively counteracted by circular prestressing.

Introduction

Basic Concept

Prestressing of structures was introduced in late nineteenth century. The following sketch explains the application of prestress.

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Early Attempts of Prestressing

Introduction

Place and stretch mild steel rods, prior to concreting

Fig. 1a-3 Prestressing of concrete beams by mild steel rods

Release the tension and cut the rods after concreting


Introduction

After hardening of concrete, release the tension in the rods. The rods will try to regain their original length, but this is prevented by the surrounding concrete to which the steel is bonded. Thus, the concrete is now effectively in a state of pre-compression. It is capable of counteracting tensile stress, such as arising from the load shown in the f ll i k t hfollowing sketch.

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IntroductionEarly Attempts of Prestressing

Fig. 1a-4 A prestressed beam under an external load

IntroductionEarly Attempts of Prestressing

The early attempts of prestressing were not completely successful. It was observed that the effect of prestress reduced with time.

The load resisting capacities of the members were limited. Under sustained loads, the members were found to fail.

Why?


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Introduction

Concrete shrinks with time. Moreover under sustained load, the strain in concrete increases with time. This is known as creep strain. The reduction in length due to creep and shrinkage is also applicable to the embedded steel, resulting in significant loss in the tensile strainthe tensile strain.


Introduction

In the early applications, the strength of the mild steel and the strain during prestressing were less. TThe residual strain and hence, the residual prestress was only about 10% of initial value. The following sketches explain the phenomena.


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Introduction


Original length of steel rod (L1)

Fig. 1a-5 Beam before applying prestress

Original length of concrete beam (L2)

Reduced length of concrete beam (L )

Introduction


Reduced length of concrete beam (L3)

Fig 1a 6 Beam at transfer of prestressFig. 1a-6 Beam at transfer of prestress

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Final length of prestressed beam (L4)


Introduction

Final length of prestressed beam (L4)

Fig. 1a-7 Beam after long-term losses of prestressFig. 1a 7 Beam after long term losses of prestress

Introduction


R id l t i i t l i i l t il t i i t lResidual strain in steel = original tensile strain in steel compressive strains corresponding to short-termand long-term losses.

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Introduction


Original tensile strain in steel = (L2 L1)/L1Compressive strain due to elastic shortening of beam (short-term loss in prestress) = (L2 L3)/L1Compressive strain due to creep and shrinkage (long-term losses in prestress) = (L3 L4)/L1

Residual strain in steel = (L4 L1)/L1

The maximum original tensile strain in mild steel

Introduction


The maximum original tensile strain in mild steel = Allowable stress / elastic modulus = 140 MPa / 2105 MPa = 0.0007

The total loss in strain due to elastic shortening, g,creep and shrinkage was also close to 0.0007. Thus the residual strain was negligible.

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Introduction

What was the solution to increase residual strain and the effective prestress?

Adopt high strength steel with much higher original strain. This leads to the scope of high prestressingforceforce.

Adopt high strength concrete to withstand the high prestressing force.

Brief History

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Brief History

Introduction

Before the development of prestressed concrete, two significant developments of reinforced concrete are the invention of Portland cement and introduction of steel in concrete.

1824 Aspdin J (England)1824 Aspdin, J., (England)Obtained a patent for the manufacture of Portland cement.

Brief History

Introduction

1857 Monier, J., (France)Introduced steel wires in concrete to make flower pots, pipes, arches and slabs.

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Brief History

Introduction

The following events were significant in the development of prestressed concrete.

1886 Jackson, P. H., (USA)Introduced the concept of tighteningsteel tie rods in artificial stone andsteel tie rods in artificial stone and concrete arches.

Fig. 1a-8 Steel tie rods in arches

Brief History

Introduction

1888 Doehring, C. E. W., (Germany) Manufactured concrete slabs and smallbeams with embedded tensioned steel.

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Brief History

Introduction

1908 Stainer, C. R., (USA) Recognised losses due to shrinkage and creep, and suggested retightening the rods to recover lost prestress.

1923 Emperger, F., (Austria) Developed a method of winding and pre-tensioning high tensile steel wires around concrete pipes.

Brief History

Introduction

1924 Hewett, W. H., (USA)Introduced hoop-stressed horizontal reinforcement around walls of concrete tanks through the use of turnbuckles.

Thousands of liquid storage tanks and concrete pipes were built in the two decades to follow.

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Brief History

Introduction

1925 Dill, R. H., (USA) Used high strength unbonded steel rods. The rods were tensioned and anchored after hardening of the concrete.

1926 Eugene Freyssinet (France)Used high tensile steel wires, with ultimate strength as high as 1725 MPa and yield stress over 1240 MPa.I 1939 h d l d i l d fIn 1939, he developed conical wedges for end anchorages for post-tensioning and developed double-acting jacks.

He is often referred to as the Father of Prestressed concrete.

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Brief History

1938 H E (G )

Introduction

1938 Hoyer, E., (Germany) Developed long line pre-tensioning method.

1940 Magnel, G., (Belgium) Developed an anchoring system forDeveloped an anchoring system for post-tensioning, using flat wedges.

Brief History

During the Second World War, applications of

Introduction

During the Second World War, applications of prestressed and precast concrete increased rapidly. The names of a few persons involved in developing prestressed concrete are mentioned.

Guyon, Y., (France) built numerous prestressedconcrete bridges in western and central Europe.

Abeles, P. W., (England) introduced the concept of partial prestressing.

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Brief History

Leonhardt F (Germany) Mikhailor V (Russia) and

Introduction

Leonhardt, F., (Germany), Mikhailor, V., (Russia) and Lin, T. Y., (USA) are famous in the field of prestressedconcrete.

The International Federation for Prestressing (FIP), a professional organisation in Europe was established in 1952. The Precast/Prestressed Concrete Institute(PCI) was established in USA in 1954.

Brief History

Prestressed concrete was started to be used in

Introduction

Prestressed concrete was started to be used in building frames, parking structures, stadiums, railway sleepers, transmission line poles and other types of structures and elements.

In India, the applications of prestressed concrete diversified over the years. The first prestressed

t b id b ilt i 1948 d th A R ilconcrete bridge was built in 1948 under the Assam Rail Link Project. Among bridges, the Pamban Road Bridgeat Rameshwaram, Tamilnadu, remains a classic example of the use of prestressed concrete girders.

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Fig 1-a 9 Pamban Road Bridge at Rameshwaram, Tamilnadu

(Courtesy: http://www.ramnad.tn.nic.in)

Development of Building Materials

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Introduction

The development of prestressed concrete can be studied in the perspective of traditional building materials.

In the ancient period, stones and bricks were extensively used. These materials are strong in compression, but gweak in tension.


Introduction

For tension, bamboos and coir ropes were used in bridges. Subsequently iron and steel bars were used to resist tension. These members tend to buckle under compression.

Wood and structural steel members were effective both in tension and compression.

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Introduction

In reinforced concrete, concrete and steel are combined such that concrete resists compression and steel resists tension. This is a passive combination of the two materials.

In prestressed concrete high strength concrete andIn prestressed concrete high strength concrete and high strength steel are combined such that the full section is effective in resisting tension and compression. This is an active combination of the two materials.

Introduction

Compression (C) Tension (T) C and T


Stones, Bricks Bamboo, Ropes Timber

Structural steelSteel bars, wires

Reinforced C t

Passive combination

Concrete

Concrete

PrestressedConcrete

High Strength Steel

High Strength Concrete

Active combination


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Introduction

Summary

Basic ConceptEarly Attempts of PrestressingBrief HistoryDevelopment of Building Materials

Documents

PCS-1a