This document provides an outline for lectures on prestressed concrete, including basic concepts, materials, flexural analysis, design considerations, shear/torsion, loss of prestress over time, composite beams, and deflections. Key points covered include how prestressing controls cracking by applying compressive stresses to concrete before service loads; common prestressing methods of pre-tensioning and post-tensioning; estimating stresses in uncracked concrete beams using elastic theory; and accounting for various load stages in analysis and design.
The document provides information on methods of prestressing in concrete, including pretensioning and post-tensioning. It discusses:
- Pretensioning involves stressing steel tendons before the concrete is cast around them.
- Post-tensioning involves stressing steel tendons after the concrete has cured using jacks, then grouting the voids.
- Both methods put the concrete in compression and increase its strength and durability compared to conventional reinforced concrete.
The document provides information on methods of prestressing concrete, including pretensioning and post-tensioning. It discusses:
- Pretensioning involves stressing steel tendons before the concrete is cast around them.
- Post-tensioning involves stressing steel tendons after the concrete has cured using jacks, then grouting the voids.
- Both methods put the concrete in compression and increase its strength and durability compared to conventional reinforced concrete.
This document discusses methods of prestressing concrete, including pretensioning and post-tensioning. Pretensioning involves stressing steel tendons before concrete is poured around them. Post-tensioning involves stressing steel tendons inserted into voids in cured concrete using jacks. Both methods put the concrete in compression and improve its tensile strength. Common applications include building floors/roofs, bridges, and parking structures.
This document discusses different methods of prestressing concrete, including pretensioning and post-tensioning. Pretensioning involves stressing steel tendons before placing concrete around them, while post-tensioning involves stressing tendons after the concrete has cured using hydraulic jacks. Post-tensioning allows for longer spans, thinner slabs, and more architectural freedom compared to conventional reinforced concrete or pretensioned concrete. Common applications of post-tensioning include parking structures, bridges, and building floors and roofs.
This document provides information about prestressed concrete, specifically focusing on post-tensioning methods. It defines post-tensioning as a method of reinforcing concrete with high-strength steel strands called tendons. After the concrete cures, the tendons are tensioned using hydraulic jacks and wedged into place to transfer pressure to the concrete. There are benefits to post-tensioning like allowing longer spans, thinner structures, and reduced cracking compared to conventional reinforced concrete. The document discusses bonded and unbonded post-tensioning methods and provides examples of applications like buildings, bridges, and parking structures.
This document discusses prestressed concrete, which uses steel that is tensioned to put concrete in compression and increase its strength. There are two main types: pre-tensioned concrete, where steel is tensioned before the concrete is poured; and post-tensioned concrete, where steel is tensioned after the concrete has hardened. Post-tensioned concrete can be bonded or unbonded. Prestressed concrete allows for longer spans, thinner sections, and increased strength over traditional reinforced concrete. It has applications in buildings, bridges, parking structures, and other structures.
This document discusses prestressed concrete and defines key terms like pretensioning and post-tensioning. Pretensioning involves stretching steel tendons before concrete is poured, while post-tensioning stretches steel inserted into hardened concrete. The document covers advantages of prestressing like reduced cracking and member sizes. It also discusses design considerations like prestress losses from shrinkage, creep, and relaxation. Both pretensioning and post-tensioning methods are outlined, along with tendon types like bars, wires, and strands.
This document provides an overview of pre-stressed and precast concrete. It discusses basic concepts like pre-stressing, uses of pre-stressed concrete, materials used including high-strength concrete and steel, and methods of prestressing like pre-tensioning and post-tensioning. It also covers topics like tendon profiles, advantages and disadvantages of pre-stressed concrete, losses in prestressing, types of prestressing steel, properties of prestressing steel, and use of non-prestressed reinforcement. The document is submitted by 5 students and contains 15 chapters with information on concepts, introduction, early introduction, uses, the basic idea, methods, profiles, advantages, disadvantages, losses, materials, types of
The document provides information on methods of prestressing in concrete, including pretensioning and post-tensioning. It discusses:
- Pretensioning involves stressing steel tendons before the concrete is cast around them.
- Post-tensioning involves stressing steel tendons after the concrete has cured using jacks, then grouting the voids.
- Both methods put the concrete in compression and increase its strength and durability compared to conventional reinforced concrete.
The document provides information on methods of prestressing concrete, including pretensioning and post-tensioning. It discusses:
- Pretensioning involves stressing steel tendons before the concrete is cast around them.
- Post-tensioning involves stressing steel tendons after the concrete has cured using jacks, then grouting the voids.
- Both methods put the concrete in compression and increase its strength and durability compared to conventional reinforced concrete.
This document discusses methods of prestressing concrete, including pretensioning and post-tensioning. Pretensioning involves stressing steel tendons before concrete is poured around them. Post-tensioning involves stressing steel tendons inserted into voids in cured concrete using jacks. Both methods put the concrete in compression and improve its tensile strength. Common applications include building floors/roofs, bridges, and parking structures.
This document discusses different methods of prestressing concrete, including pretensioning and post-tensioning. Pretensioning involves stressing steel tendons before placing concrete around them, while post-tensioning involves stressing tendons after the concrete has cured using hydraulic jacks. Post-tensioning allows for longer spans, thinner slabs, and more architectural freedom compared to conventional reinforced concrete or pretensioned concrete. Common applications of post-tensioning include parking structures, bridges, and building floors and roofs.
This document provides information about prestressed concrete, specifically focusing on post-tensioning methods. It defines post-tensioning as a method of reinforcing concrete with high-strength steel strands called tendons. After the concrete cures, the tendons are tensioned using hydraulic jacks and wedged into place to transfer pressure to the concrete. There are benefits to post-tensioning like allowing longer spans, thinner structures, and reduced cracking compared to conventional reinforced concrete. The document discusses bonded and unbonded post-tensioning methods and provides examples of applications like buildings, bridges, and parking structures.
This document discusses prestressed concrete, which uses steel that is tensioned to put concrete in compression and increase its strength. There are two main types: pre-tensioned concrete, where steel is tensioned before the concrete is poured; and post-tensioned concrete, where steel is tensioned after the concrete has hardened. Post-tensioned concrete can be bonded or unbonded. Prestressed concrete allows for longer spans, thinner sections, and increased strength over traditional reinforced concrete. It has applications in buildings, bridges, parking structures, and other structures.
This document discusses prestressed concrete and defines key terms like pretensioning and post-tensioning. Pretensioning involves stretching steel tendons before concrete is poured, while post-tensioning stretches steel inserted into hardened concrete. The document covers advantages of prestressing like reduced cracking and member sizes. It also discusses design considerations like prestress losses from shrinkage, creep, and relaxation. Both pretensioning and post-tensioning methods are outlined, along with tendon types like bars, wires, and strands.
This document provides an overview of pre-stressed and precast concrete. It discusses basic concepts like pre-stressing, uses of pre-stressed concrete, materials used including high-strength concrete and steel, and methods of prestressing like pre-tensioning and post-tensioning. It also covers topics like tendon profiles, advantages and disadvantages of pre-stressed concrete, losses in prestressing, types of prestressing steel, properties of prestressing steel, and use of non-prestressed reinforcement. The document is submitted by 5 students and contains 15 chapters with information on concepts, introduction, early introduction, uses, the basic idea, methods, profiles, advantages, disadvantages, losses, materials, types of
Prestressed concrete is a combination of steel and concrete that uses compressive stresses applied during construction to oppose tensile stresses that occur in use. There are three main types: pre-tensioned concrete uses steel tendons tensioned before concrete is placed; bonded post-tensioned concrete uses unstressed steel placed then tensioned after curing; and unbonded post-tensioned concrete provides freedom of movement between steel and concrete. Pre-tensioned concrete requires molds that can resist internal forces and calculations to account for losses over time. Prestressed concrete provides benefits like reduced cracking and corrosion, higher strength, and more economical construction for bridges compared to steel.
This document summarizes a presentation on prestressed concrete. It begins with an introduction to prestressed concrete and how it overcomes weaknesses in concrete in tension. It then describes the principles of prestressing by inducing compressive stresses with high-strength tendons before loads are applied. The document compares reinforced concrete with prestressed concrete and describes the methods of pre-tensioning and post-tensioning. It provides examples of prestressed concrete structures like beams, bridges and discusses advantages like reduced size and increased spans as well as disadvantages like higher material costs.
Prestressed concrete ,post tensioning ,pre tensioning, where normal concrete can not be used and need of more strength is required this type of concrete are used. Metal bars are replaced by the tendoms which are generally used to create tension in concrete. So because of that beam bends in upward direction and when load is applied it come in normal conditon.
Pre-stressed concrete uses tensioned steel cables or rods to put concrete members under compression and increase their strength. It allows for longer spans than reinforced concrete. There are three methods: pre-tensioned concrete uses tensioned tendons before pouring concrete; bonded post-tensioned concrete uses tendons tensioned after pouring; unbonded post-tensioned concrete uses individually coated tendons without bonding to the concrete. Prestressed concrete has advantages like less cracking and material efficiency but also disadvantages like higher costs.
Comparison of reinforced concrete and prestressed concreteSpice Shuvo
ย
This document compares reinforced concrete and prestressed concrete. Reinforced concrete uses steel reinforcement embedded in concrete to increase its tensile strength. Prestressed concrete applies compression to concrete before loading to counteract tensile stresses when in use. For construction, reinforced concrete requires steel bars and formwork while prestressed concrete uses steel tendons stressed after the concrete reaches strength. Prestressed concrete allows for thinner sections, reduced self-weight, and less deflection compared to reinforced concrete. However, it requires higher quality materials and specialized equipment. In summary, the document outlines the key differences in material composition and behavior between the two composite concrete materials.
This document discusses prestressed concrete, which uses tensioned steel cables or bars to put concrete members into compression and increase their strength. It describes three main methods: pre-tensioned concrete where the steel is tensioned before the concrete is cast; bonded post-tensioned concrete where steel is tensioned after casting to compress the concrete; and unbonded post-tensioned concrete where greased steel is used to allow individual adjustment. Applications include buildings, bridges, nuclear reactors and earthquake resistant structures. Advantages are lower costs, thinner members, and increased spans.
This document provides information on a syllabus for a course on prestressed concrete. It outlines the course objectives which are to understand the principles, necessity, techniques, losses, and analysis and design of prestressed concrete members. The course outcomes are for students to acquire knowledge on the evolution of prestressing, prestressing techniques, and skills in analyzing and designing prestressed structural elements per code provisions. The syllabus then outlines 5 units that will be covered which include introduction, methods and systems, losses of prestress, flexure, shear, transfer of prestress, composite beams, and deflections. Relevant textbooks and codes are also listed.
The document summarizes the planning, analysis, and design of a prestressed concrete bridge. It includes the design of various components like the deck slab, beams, piers, footings, and pile foundations. The bridge is a single span of 30 meters made of M40-M45 grade concrete and high strength steel tendons. The design considers aspects like dead and live loads, shear forces, bending moments, reinforcement requirements, and stress limits to construct the different elements of the prestressed concrete bridge according to code specifications.
This document discusses prestressed concrete and provides details on:
- The definition and principle of prestressing concrete by applying compression prior to external loads
- Common prestressing methods like hydraulic, mechanical, electrical, and chemical prestressing
- Tests conducted on prestressed concrete components like post-tensioned splices and cast-in-place splices
- Advantages of prestressed concrete like reduced materials and increased strength
- Applications in bridges, buildings, water tanks, and more
- A case study on widening the Harrods Creek Arch Bridge using prestressed concrete
The document provides information about precast concrete, including:
- Precast concrete is concrete that is cast off-site in a controlled environment using reusable molds. Elements can be joined to form structures.
- Products include buildings, walls, slabs, columns. Elements are poured into molds, cured, then transported and installed.
- History of precast concrete dates back to Rome. Examples given include the Sydney Opera House and buildings by Richard Meier.
- Advantages include reduced construction time, quality control, and earthquake resistance. Disadvantages include high costs for small projects and difficulty altering cast-in services.
Post-tensioning is a method of reinforcing (strengthening) concrete or other materials with high-strength steel strands or bars, typically referred to as tendons. Post-tensioning applications include office and apartment buildings, parking structures, slabs-on-ground, bridges, sports stadiums, rock and soil anchors, and water-tanks.
>>>Published by Post-Tensioning Institute
Reinforced cement concrete (RCC) is a composite material made of cement concrete reinforced with steel bars. Some key points:
- Franรงois Coignet built the first reinforced concrete structure, a four story house in Paris in 1853.
- RCC is used in the construction of columns, beams, footings, slabs, dams, water tanks, tunnels, bridges, walls and towers due to its high strength and durability.
- The steel reinforcement provides tensile strength, while the concrete primarily resists compressive forces and protects the steel from corrosion. Together they form a very strong, stable structural material.
Prestressed concrete uses high-strength steel tendons to place concrete in compression before loading. This counters the tensile stresses from loads and allows for longer spans and lighter structures. There are two main methods: pretensioning, where tendons are tensioned before concrete is poured, and post-tensioning, where tendons are tensioned after concrete cures. Prestressed concrete has advantages like reduced cracking, increased load capacity, and smaller deflections under loads.
Pre stressed concrete- modular construction technologyAnjith Augustine
ย
This document provides an overview of pre-stressed concrete, including its history, types (pre-tensioning and post-tensioning), materials, applications, advantages, and tensioning devices. Some key points include: pre-stressed concrete was developed in the 1930s-1940s and the first pre-stressed concrete bridge was built in India in 1948; it uses high-strength steel tendons to put concrete under compression and improve its tensile strength; common applications include bridges, buildings, and other structures; and advantages are increased strength, reduced cracking, and lighter/thinner designs.
Pre-stressed concrete was a major innovation that replaced conventional reinforced concrete, allowing for longer spans, higher impact resistance, and greater load capacity without tensile stresses. It involves casting concrete around high-strength steel that is placed under compression before use to counteract tensile stresses when in service. There are two main types: pre-tensioning applies tension before casting, while post-tensioning does so after casting, using ducts to hold the steel. Pre-stressed concrete enables more efficient structures through factory casting and reduced material needs.
This document discusses prestressed concrete, including:
- The basic concepts of prestressing including using metal bands, pre-tensioned spokes, and introducing stresses to counteract external loads.
- Design concepts like losses in prestressing structures from elastic shortening, creep, shrinkage, relaxation, friction, and anchorage slip.
- Provisions for prestressing in the Indian Road Congress Bridge Code and Indian Standard Code.
- Construction aspects like casting of girders, post-tensioning work, and load testing of structures.
Here are the key steps in concrete frame construction:
1. Excavation and foundation work - This involves excavating the land and laying the foundation system such as raft or pile foundations.
2. Erection of formwork - Formwork is erected to give shape to the concrete elements like columns, beams, slabs, etc. It is designed to bear the pressure of wet concrete.
3. Reinforcement cage - Steel reinforcement bars are cut, bent and assembled into cages and placed accurately in position in the formwork.
4. Concreting - Concrete is poured, compacted and finished after placing the reinforcement cages in position.
5. Curing - After concreting, the concrete elements
Reinforced concrete is a composite material consisting of concrete and steel reinforcement. Franรงois Coignet built the first iron reinforced concrete structure in 1853. Reinforced concrete uses the strengths of both materials - concrete is strong in compression and steel is strong in tension. It is used widely in construction for buildings, bridges, tunnels and other structures due to its high strength and durability.
Better Builder Magazine brings together premium product manufactures and leading builders to create better differentiated homes and buildings that use less energy, save water and reduce our impact on the environment. The magazine is published four times a year.
Prestressed concrete is a combination of steel and concrete that uses compressive stresses applied during construction to oppose tensile stresses that occur in use. There are three main types: pre-tensioned concrete uses steel tendons tensioned before concrete is placed; bonded post-tensioned concrete uses unstressed steel placed then tensioned after curing; and unbonded post-tensioned concrete provides freedom of movement between steel and concrete. Pre-tensioned concrete requires molds that can resist internal forces and calculations to account for losses over time. Prestressed concrete provides benefits like reduced cracking and corrosion, higher strength, and more economical construction for bridges compared to steel.
This document summarizes a presentation on prestressed concrete. It begins with an introduction to prestressed concrete and how it overcomes weaknesses in concrete in tension. It then describes the principles of prestressing by inducing compressive stresses with high-strength tendons before loads are applied. The document compares reinforced concrete with prestressed concrete and describes the methods of pre-tensioning and post-tensioning. It provides examples of prestressed concrete structures like beams, bridges and discusses advantages like reduced size and increased spans as well as disadvantages like higher material costs.
Prestressed concrete ,post tensioning ,pre tensioning, where normal concrete can not be used and need of more strength is required this type of concrete are used. Metal bars are replaced by the tendoms which are generally used to create tension in concrete. So because of that beam bends in upward direction and when load is applied it come in normal conditon.
Pre-stressed concrete uses tensioned steel cables or rods to put concrete members under compression and increase their strength. It allows for longer spans than reinforced concrete. There are three methods: pre-tensioned concrete uses tensioned tendons before pouring concrete; bonded post-tensioned concrete uses tendons tensioned after pouring; unbonded post-tensioned concrete uses individually coated tendons without bonding to the concrete. Prestressed concrete has advantages like less cracking and material efficiency but also disadvantages like higher costs.
Comparison of reinforced concrete and prestressed concreteSpice Shuvo
ย
This document compares reinforced concrete and prestressed concrete. Reinforced concrete uses steel reinforcement embedded in concrete to increase its tensile strength. Prestressed concrete applies compression to concrete before loading to counteract tensile stresses when in use. For construction, reinforced concrete requires steel bars and formwork while prestressed concrete uses steel tendons stressed after the concrete reaches strength. Prestressed concrete allows for thinner sections, reduced self-weight, and less deflection compared to reinforced concrete. However, it requires higher quality materials and specialized equipment. In summary, the document outlines the key differences in material composition and behavior between the two composite concrete materials.
This document discusses prestressed concrete, which uses tensioned steel cables or bars to put concrete members into compression and increase their strength. It describes three main methods: pre-tensioned concrete where the steel is tensioned before the concrete is cast; bonded post-tensioned concrete where steel is tensioned after casting to compress the concrete; and unbonded post-tensioned concrete where greased steel is used to allow individual adjustment. Applications include buildings, bridges, nuclear reactors and earthquake resistant structures. Advantages are lower costs, thinner members, and increased spans.
This document provides information on a syllabus for a course on prestressed concrete. It outlines the course objectives which are to understand the principles, necessity, techniques, losses, and analysis and design of prestressed concrete members. The course outcomes are for students to acquire knowledge on the evolution of prestressing, prestressing techniques, and skills in analyzing and designing prestressed structural elements per code provisions. The syllabus then outlines 5 units that will be covered which include introduction, methods and systems, losses of prestress, flexure, shear, transfer of prestress, composite beams, and deflections. Relevant textbooks and codes are also listed.
The document summarizes the planning, analysis, and design of a prestressed concrete bridge. It includes the design of various components like the deck slab, beams, piers, footings, and pile foundations. The bridge is a single span of 30 meters made of M40-M45 grade concrete and high strength steel tendons. The design considers aspects like dead and live loads, shear forces, bending moments, reinforcement requirements, and stress limits to construct the different elements of the prestressed concrete bridge according to code specifications.
This document discusses prestressed concrete and provides details on:
- The definition and principle of prestressing concrete by applying compression prior to external loads
- Common prestressing methods like hydraulic, mechanical, electrical, and chemical prestressing
- Tests conducted on prestressed concrete components like post-tensioned splices and cast-in-place splices
- Advantages of prestressed concrete like reduced materials and increased strength
- Applications in bridges, buildings, water tanks, and more
- A case study on widening the Harrods Creek Arch Bridge using prestressed concrete
The document provides information about precast concrete, including:
- Precast concrete is concrete that is cast off-site in a controlled environment using reusable molds. Elements can be joined to form structures.
- Products include buildings, walls, slabs, columns. Elements are poured into molds, cured, then transported and installed.
- History of precast concrete dates back to Rome. Examples given include the Sydney Opera House and buildings by Richard Meier.
- Advantages include reduced construction time, quality control, and earthquake resistance. Disadvantages include high costs for small projects and difficulty altering cast-in services.
Post-tensioning is a method of reinforcing (strengthening) concrete or other materials with high-strength steel strands or bars, typically referred to as tendons. Post-tensioning applications include office and apartment buildings, parking structures, slabs-on-ground, bridges, sports stadiums, rock and soil anchors, and water-tanks.
>>>Published by Post-Tensioning Institute
Reinforced cement concrete (RCC) is a composite material made of cement concrete reinforced with steel bars. Some key points:
- Franรงois Coignet built the first reinforced concrete structure, a four story house in Paris in 1853.
- RCC is used in the construction of columns, beams, footings, slabs, dams, water tanks, tunnels, bridges, walls and towers due to its high strength and durability.
- The steel reinforcement provides tensile strength, while the concrete primarily resists compressive forces and protects the steel from corrosion. Together they form a very strong, stable structural material.
Prestressed concrete uses high-strength steel tendons to place concrete in compression before loading. This counters the tensile stresses from loads and allows for longer spans and lighter structures. There are two main methods: pretensioning, where tendons are tensioned before concrete is poured, and post-tensioning, where tendons are tensioned after concrete cures. Prestressed concrete has advantages like reduced cracking, increased load capacity, and smaller deflections under loads.
Pre stressed concrete- modular construction technologyAnjith Augustine
ย
This document provides an overview of pre-stressed concrete, including its history, types (pre-tensioning and post-tensioning), materials, applications, advantages, and tensioning devices. Some key points include: pre-stressed concrete was developed in the 1930s-1940s and the first pre-stressed concrete bridge was built in India in 1948; it uses high-strength steel tendons to put concrete under compression and improve its tensile strength; common applications include bridges, buildings, and other structures; and advantages are increased strength, reduced cracking, and lighter/thinner designs.
Pre-stressed concrete was a major innovation that replaced conventional reinforced concrete, allowing for longer spans, higher impact resistance, and greater load capacity without tensile stresses. It involves casting concrete around high-strength steel that is placed under compression before use to counteract tensile stresses when in service. There are two main types: pre-tensioning applies tension before casting, while post-tensioning does so after casting, using ducts to hold the steel. Pre-stressed concrete enables more efficient structures through factory casting and reduced material needs.
This document discusses prestressed concrete, including:
- The basic concepts of prestressing including using metal bands, pre-tensioned spokes, and introducing stresses to counteract external loads.
- Design concepts like losses in prestressing structures from elastic shortening, creep, shrinkage, relaxation, friction, and anchorage slip.
- Provisions for prestressing in the Indian Road Congress Bridge Code and Indian Standard Code.
- Construction aspects like casting of girders, post-tensioning work, and load testing of structures.
Here are the key steps in concrete frame construction:
1. Excavation and foundation work - This involves excavating the land and laying the foundation system such as raft or pile foundations.
2. Erection of formwork - Formwork is erected to give shape to the concrete elements like columns, beams, slabs, etc. It is designed to bear the pressure of wet concrete.
3. Reinforcement cage - Steel reinforcement bars are cut, bent and assembled into cages and placed accurately in position in the formwork.
4. Concreting - Concrete is poured, compacted and finished after placing the reinforcement cages in position.
5. Curing - After concreting, the concrete elements
Reinforced concrete is a composite material consisting of concrete and steel reinforcement. Franรงois Coignet built the first iron reinforced concrete structure in 1853. Reinforced concrete uses the strengths of both materials - concrete is strong in compression and steel is strong in tension. It is used widely in construction for buildings, bridges, tunnels and other structures due to its high strength and durability.
Similar to 1-Basics of Prestressed Concrete.pdforgs (20)
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1-Basics of Prestressed Concrete.pdforgs
1. PRESTRESSED CONCRETE
Basic concepts & Flexural Analysis
Dr. Qasim Shaukat Khan
Associate Professor
Civil Engineering Department
UET Lahore
Email: qasimkhan@uet.edu.pk
1
2. 1. Basic Concepts [Introduction, Stress Control by
Prestressing, Partial Prestressing, Prestressing Methods,
Changes in Prestress Force]
2. Materials [Introduction to High Strength Steel, Types of
Prestressing Steel, Stress-Strain Properties of Steel, Steel
Relaxation, Types of Concrete, Concrete in Uniaxial
Compression and Tension, Time dependent Deformation
of Concrete]
3. Flexural Analysis [Partial loss of Prestress Force, Elastic
Flexural Stresses in Uncracked Beams, Allowable
Flexural Stresses, Cracking Load, Flexural Strength
Analysis and ACI Design Equations, Partial Prestressing,
Elastic Flexural Stress after Cracking and Strength of
Partially Prestressed Beams]
LECTURE OUTLINE
2
3. 4. Flexural Design [Basis of Design, Flexural Design based on
Allowable Stresses, Shape Selection and Flexural Efficiency,
Load Balancing, Flexural design Based on Partial
Prestressing , Flexural Crack Control]
5. Shear and Torsion [Shear and Diagonal Tension in
Uncracked Beams, Diagonal Cracking Shear, Web
Reinforcement for Shear, Shear Design Criteria based on
ACI Criteria, Torsion in Concrete Structures, Torsion Design
of Prestressed Concrete]
6. Partial Loss of Prestress Force [Detailed Estimation of
Losses, Losses due to Friction, Anchorage Slip, Elastic
Shortening of Concrete, Creep and Shrinkage in Concrete,
Relaxation of Steel]
LECTURE OUTLINE
3
4. 7. Composite Beams [Types of Composite Construction, Load
Stages, Section Properties, Elastic Flexural Stresses, Flexural
Strength, Horizontal Shear Transfer, Shear and Diagonal
Tension]
8. Deflections [Basis for Calculations, Approximate Method
for Deflection Calculation, Deflection of Partially Prestressed
Beams, Allowable Deflections]
LECTURE OUTLINE
4
5. REFERENCES
Siddiqi, Z.A. (2016) Concrete Structures, Third Edition
(Part II), Help Publishers
Loo, Y-C and Chowdhury SH (2013) Reinforced and
Prestressed Concrete, Second edition, Cambridge Univ
Press.
Warner, R.F., Rangan, B.V., Hall, A.S. and
Faulkes, K.A. (1998) Concrete Structures, Addison
Wesley Longman
Gilbert, R.I. & Mickleborough, N.C. (1990) Design of
Prestressed Concrete, 1st Edn, Unwin Hyman.
Nilson, A. H. (1987) Design of Prestressed Concrete, Second
Edition, John Wiley & Sons
5
6. HISTORY
EUGENE FREYSINNET
A French engineer pioneered the
use of prestressed concrete in the
1930โs.
YVES GUYON, a student of
Freysinnet once summarized the
importance of the method saying:
โThere is probably no structural
problem to which prestress cannot
provide a solution, and often a
revolutionary one.โ
6
7. Reinforced concrete is one of the most widely used
structural materials in construction.
Due to the low tensile strength of concrete, steel bars
are introduced to carry all internal tensile forces.
Consider a simple reinforced concrete beam shown below:
w
REINFORCED CONCRETE
linear stresses
๏ณS
๏ณC
C
T
w
BEAM UNDER SERVICE LOADING
Section
Reinforcing bars
7
8. The external loads cause tension in the bottom fibers
which may lead to cracking, as shown on previous slide.
Most reinforced concrete beams are cracked due to Service
Loading.
Cracked cross-sections resist the applied moment by a
compressive force in the Concrete, C and a tensile force in
the Steel,T.
Tension reinforcement does not eliminate cracking and thus
does not prevent a loss of stiffness which cracking creates.
REINFORCED CONCRETE
8
9. PRESTRESSED CONCRETE
PRESTRESSED CONCRETE is a particular form of
reinforced concrete, which involves the application of an initial
compressive load (Pre-loading before the application of
Service Loads) on a structure to reduce or eliminate the
internal tensile forces / stresses and there by control or
eliminate cracking.
The compressive force is imposed and sustained by highly
tensioned steel reinforcement reacting on the concrete.
The concept of Prestressing of concrete is to introduce
sufficient axial precompression in beams so all tension in the
concrete was eliminated in the member at service load.
9
10. PRESTRESSED CONCRETE
A prestressed concrete beam section is considerably stiffer than
the equivalent cracked reinforced section.
Prestressing may also impose internal forces which
counterbalance external loads and may reduce or eliminate
deflection.
By varying the compressive prestress, the number and width of
cracks can be limited to the desired degree or zero deflection.
Full prestressing offers the possibility of complete elimination
of cracks at full service load, however, this results in large
camber.
Partial prestressing results in significant economy by reducing
the amount of prestressed reinforcement with some flexural
cracking within permissible limits at service loads.
10
11. METHODS OF PRESTRESSING
Prestressing is applied to a concrete member by highly tensioned
steel reinforcement (wire, strand, or bar) reacting on the concrete.
The high strength steel is most often tensioned using hydraulic
jacks. The tensioning operation may occur before or after the
concrete is cast and results in two classification:
(i) PRE - TENSIONED
Pretensioned prestressed concrete members are produced by
stretching the tendons between external anchorages before the
concrete is placed.
As fresh concrete hardens, it bonds to the steel. After the concrete
has attained the desired strength, the jacketing force is released,
and the force is transferred by bond from steel to concrete.
(ii) POST - TENSIONED
In Post tensioned prestressed concrete members, the tendons are
stressed after the concrete has hardened and achieved sufficient
strength, by jacketing against the concrete member itself.
11
13. PRE-TENSIONED
CONCRETE
The prestressing tendons are initially tensioned between
fixed abutments and anchored.
1
Formwork is constructed and the concrete is cast around the
highly stressed tendons and curved.
2
,
As the highly stressed steel attempts to contract the
concrete is compressed. Prestress is developed via bond
between the steel and concrete.
3
13
14. PRE-TENSIONED
CONCRETE
ADVANTAGES
โHigher quality control can beachieved.
โLends itself to repetitive construction.
โDecreased construction cycles
โPre-fabrication is advantageous for bridge girders
DISADVANTAGES
โElastic shortening of concrete and creep is high
โHigh losses of prestress result
14
15. POST-TENSIONED
CONCRETE
The three stages of post-tensioned concrete are
shown above.
hollow duct
1. Concrete
cast and
cured uplift forces
TENSILE
FORCE
COMPRESSIVE
FORCE
2. Tendons stressed
and prestress
transferred
dead end
live end
3.Tendons
anchored and
duct grouted
15
16. POST-TENSIONED
CONCRETE
Formwork positioned and hollow duct fixed to desired
profile. Concrete cast and cured.
Tendons usually in place and unstressed.
1
Upon concrete reaching adequate strength, the
tendons are stressed.
2
Tendons are then anchored and the duct is grouted.
3
16
17. POST-TENSIONED
CONCRETE
ADVANTAGES
โMembers can be post-tensioned using relatively light and
portable hydraulic jacks
โAttractive method for segmental construction of large span
bridges
โCan be used for new or existing members using external
tendons
DISADVANTAGES
โUngrouted ducts as used in North America and Europe are
extremely dangerous, particularly during demolition
โExternal tendons generally suffer large time-dependent
losses due to lack of bond between concrete and steel.
17
18. How prestressed concrete is made?
1
It all begins at the
prestressed
concrete plant.
2
This is called a
prestressing strand.
Made of high
strength steel, it
will soon be
embedded in
concrete.
3
The prestressing
strand is stretched
across the casting
bed. Tension will be
applied to the cable
before it's
surrounded by
concrete.
18
19. 4
Of course, cement,
sand, stone, and
water make up
concrete.
5
Special trucks bring
the concrete to the
casting bed where
the pouring begins.
Once the pouring is
complete, a tarp is
placed over the
form and heat is
applied to cure the
cement.
6
How prestressed concrete is made?
19
20. How prestressed concrete is made?
The prestressing
strands are cut and
the concrete form is
removed from the
casting bed.
7 8
The ends are cleaned
and the prestressing
strands are sealed with
a protective coating.
9
The end-product
is shipped to a
building site.
20
24. Precast concrete panels
Precast sandwich wall
panels are economical,
attractive, durable,
energy efficient and
very fast to install.
Buildings are enclosed in
days in any weather which
will considerately speed
up the construction process.
24
25. Post Tensioned (P-T) in Buildings
โช Beams and slabs present good
opportunities for P-T
25
26. Advantages of P-T in Buildings
โช Allows longer spans
โช For spans >7m reduced overall costs
โช Shallower slabs and beams
โ Smaller floor to floorheight
โช Deflection free slabs
โช Waterproof concrete possible
โช Early formwork stripping
โช Less materials handling
โช Reduced CO2 cost for PT concrete structure
26
27. Disadvantages of P-T
โช Specialist contractor required to install
โช High early strength concrete required
โช Ducting and grouting activities
โช More difficult to modify later
โ Not easy to cut openings in P-T slab
โช Anchorage design can be tricky
โช Layout of strands and ducts requires greater planning
and design effort
27
28. FLEXURALANALYSIS
FLEXURALANALYSIS
In flexural analysis, the concrete and steel dimensions, as well as
magnitude and line of action of an effective prestress force are
known.
If loads are known, the resulting stresses are found and compare
with the permissible limits.
Alternatively, if permissible stresses are known, then maximum
loads can be calculated without exceeding the permissible stresses.
FLEXURAL DESIGN
In flexural design, the permissible stresses and material strengths
are known, the loads to be resisted are specified, and Engineer must
determine concrete and steel dimensions as well the magnitude
and line of action of the prestressing force.
28
29. FLEXURALANALYSIS
Both Analysis and Design of Prestressed Concrete may require
the consideration of the following load stages:
1. Initial Prestress, immediately after transfer, when (๐๐) alone
may act on the concrete.
2. Initial Prestress plus self-weight of the member.
3. Initial Prestress plus full Dead Load.
4. Effective Prestress, (๐๐), after losses, plus service loads
consisting of full dead load and expected live loads.
5. Ultimate load, when the expected service loads are increased by
load factors and the member is about to fail.
At and Below, the Service Load, both Concrete and Steel
Stresses are usually within the Elastic Range.
29
30. FLEXURALANALYSIS
PARTIAL LOSS OF PRESTRESS
The Jacking Tension (๐๐), initially applied to the tendon, is
reduced at once to Initial Prestress Force (๐ท๐).
A part of this loss in Jacking Tension occurs due to friction
between a post-tensioned tendon and its encasing duct, even
before the transfer of the prestress force to the concrete. Further
losses occur due to elastic shortening of the concrete and due to
slip at post-tensioning anchorages, which occurs immediately
upon transfer.
Additional losses occur over an extended period because of
concrete shrinkage and creep, and also because of relaxation of
stress in the steel tendon. Consequently, the prestress force is
reduced from (๐ท๐)to its final or effective value (๐ท๐) after all
significant time dependent losses have taken place.
Designer is interested in Initial Prestress (๐ท๐) and the effective
Prestress (๐ท๐).
30
31. FLEXURALANALYSIS
ELASTIC STRESSES
As long as the beam remains uncracked, and both steel and
concrete are stressed only within their elastic ranges, then
concrete stresses can be found using the familiar equations of
mechanics, based on their Linear Elastic behavior up to the
Service loads.
Stresses may also be calculated using Linear Elastic Methods,
even if nominal tension is somewhat in excess of probable value of
Modulus of Rupture. This is because that certain amount of
bonded prestressed reinforcement is provided in the tension zone
to control both cracking and deflection and permits the member
to respond as an uncracked section.
If the member is subjected only to the Initial Prestress Force (๐ท๐),
it has been observed that the compressive resultant acts at the steel
centroid. The concrete stresses (๐๐), at the top face of the member
and (๐๐) at the bottom face of the member can be found by
Superimposing axial and bending effects.
31
32. FLEXURALANALYSIS
ELASTIC STRESSES
If the member is subjected only to the Initial Prestress Force (๐ท๐),
it has been observed that the compressive resultant acts at the steel
centroid. The concrete stresses (๐๐), at the top face of the member
and (๐๐) at the bottom face of the member can be found by
Superimposing axial and bending effects.
๐1 = โ
๐๐
๐ด๐
+
๐๐ผ ๐๐1
๐ผ๐
๐2 = โ
๐๐
๐ด๐
โ
๐๐ผ ๐๐2
๐ผ๐
where e is the tendon eccentricity measured downward from the
concrete centroid, ๐จ๐ is the Area of concrete cross section and ๐ฐ๐ is
the moment of inertia of the concrete cross section, ๐2
is the
radius of gyration ๐2
= ฮค
๐ผ๐ ๐ด๐. These equations can be re-written
in more convenient form as
๐1 = โ
๐๐
๐ด๐
(1 โ
๐๐1
๐2
) ๐2 = โ
๐๐
๐ด๐
(1 +
๐๐2
๐2 )
32
34. FLEXURALANALYSIS
ELASTIC STRESSES
Almost never would the Initial Prestress Force (๐ท๐) can act alone.
In most practical scenarios, with the tendon below the concrete
centroid, the beam will deflect upward because of the bending
moment caused by prestressing. It will then be supported by the
formwork or casting bed essentially at its ends, and the dead load
of the beam itself will cause moments (๐๐) to be superimposed
immediately.
Consequently, at the initial stage, immediately after transfer of
prestress force, the stresses in the concrete at the top and bottom
surfaces are
๐1 = โ
๐๐
๐ด๐
1 โ
๐๐1
๐2
โ
๐๐
๐1
๐2 = โ
๐๐
๐ด๐
1 +
๐๐2
๐2
+
๐๐
๐2
where ๐ด๐ถ is the bending moment resulting from the self-weight
of the member, and ๐1 = ฮค
๐ผ๐ถ ๐ถ1 and ๐2 = ฮค
๐ผ๐ถ ๐ถ2 are the Section
Moduli with respect to the top and bottom surfaces of the beam.
34
36. FLEXURALANALYSIS
ELASTIC STRESSES
Superimposed dead loads in addition to the self weight, may be
placed when the prestress force is still close to its initial value,
that is, before time dependent losses have occurred (Seldom stage).
Superimposed live loads are generally applied sufficiently late for
the greatest part of the loss of prestress to have occurred.
Next stage is, full service load stage, when the effective prestress
(๐๐), acts with the moments resulting from self weight (๐๐ ),
superimposed dead loads (๐๐) and superimposed live loads (๐๐).
The resulting stresses are
where ๐ด๐ = ๐ด๐ถ + ๐ด๐ + ๐ด๐
๐1 = โ
๐๐
๐ด๐
1 โ
๐๐1
๐2 โ
๐๐ก
๐1
๐2 = โ
๐๐
๐ด๐
1 +
๐๐2
๐2 โ
๐๐ก
๐2
36
38. FLEXURALANALYSIS
FLEXURAL STRESSES FOR GIVEN BEAM AND LOADS
The simply supported I-beam carried a uniformly distributed
service dead and service live load totaling 8.02 kN/m over the
12.19 m span, in addition to its own weight. Normal concrete of
density 24 kN/m3 will be used. The beam will be pretensioned
using multiple seven wire strands, eccentricity is constant and
equal to 132 mm. The initial prestress force (๐๐) immediately after
transfer (after elastic shortening loss) is 752 kN. Time dependent
losses due to shrinkage, creep and relaxation are total 15% of the
initial prestress force. Find the concrete flexural stresses at
midspan and support sections under initial and final conditions.
For pretensioned beams using stranded cables, the difference
between sectional properties based on the gross and transformed
section is usually small. Accordingly, all calculations will be
based on the properties of gross concrete section.
38
M.O.I., Ic = 4.99 x 109 mm4; Concrete area, Ac = 114 x 103 mm3
Section Modulus, S1=S2=16.4 x 106 mm3; Radius of gyration r2=
44 x 102 mm2