Lecture Lecture 1 1 Introduction Introduction Dr. Hazim Dwairi - - PDF document

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

1

The Hashem ite University Departm ent of Civil Engineering

Lecture Lecture 1 1 – – Introduction Introduction

Dr Hazim Dwairi Dr Hazim Dwairi

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

Historical Background Historical Background

• First patent of prestressed concrete was in

First patent of prestressed concrete was in 1872 1872 by P H Jackson at San Francisco by P H Jackson at San Francisco he used a tie he used a tie by P.H. Jackson at San Francisco by P.H. Jackson at San Francisco -

• he used a tie

he used a tie rod (Prestressed) to construct a beam from rod (Prestressed) to construct a beam from individual concrete block. individual concrete block.

• Early attempts of prestressing failed due to the

Early attempts of prestressing failed due to the loss of prestressing force with time loss of prestressing force with time – – a better a better understanding of losses was needed in addition understanding of losses was needed in addition

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

understanding of losses was needed, in addition understanding of losses was needed, in addition to, high strength steel. to, high strength steel.

• In

In 1928 1928, E. , E. Freyssinet Freyssinet of France started modern

• f France started modern

development of prestressed concrete. development of prestressed concrete.

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

2

Everyday Life Exam ples Everyday Life Exam ples

• Force

Force-

• fitting of metal

fitting of metal bands on wooden bands on wooden bands on wooden bands on wooden barrels barrels P i i h i i h

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

• Pre

Pre-

• tensioning the

tensioning the spokes in a bicycle spokes in a bicycle wheel wheel

Definition Definition

• Prestressed is a form of concrete in which

Prestressed is a form of concrete in which internal stresses are introduced by means of a internal stresses are introduced by means of a high strength pre high strength pre strained reinforcement strained reinforcement high strength pre high strength pre-strained reinforcement. strained reinforcement. Prestressing relies on bond and/or bearing Prestressing relies on bond and/or bearing mechanisms to achieve stress transfer to mechanisms to achieve stress transfer to concrete. concrete.

• Prestressing force induce internal actions of

Prestressing force induce internal actions of such magnitude and distribution to counteract such magnitude and distribution to counteract

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

such magnitude and distribution to counteract such magnitude and distribution to counteract the external loading. In Prestressed concrete the external loading. In Prestressed concrete members, steel is in tension and concrete is in members, steel is in tension and concrete is in compression, even before the application of any compression, even before the application of any external loading. external loading.

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

3

Reasons for Prestressing Reasons for Prestressing

• Prestressed concrete has been developed to

Prestressed concrete has been developed to

• vercome some of the limitations of Reinforced
• vercome some of the limitations of Reinforced

Concrete namely: Concrete namely: Concrete, namely: Concrete, namely:

In flexure of reinforced concrete member, concrete is

In flexure of reinforced concrete member, concrete is cracked and functions only to hold the reinforced bars cracked and functions only to hold the reinforced bars in place and protect them from carrion, thereby, in place and protect them from carrion, thereby, adding excess weight without additional strength. adding excess weight without additional strength.

Cracking lowers the moment of inertia of the section,

Cracking lowers the moment of inertia of the section, th b i i d fl ti P t i li i t th b i i d fl ti P t i li i t

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

thereby increasing deflection. Prestressing eliminates thereby increasing deflection. Prestressing eliminates cracks. cracks.

make use of the high strength in tension of

make use of the high strength in tension of prestressing steel strands which is prestressing steel strands which is 270 270Ksi = Ksi =1860 1860MPa MPa (four to five times of that of conventional steel). (four to five times of that of conventional steel).

Reasons for Prestressing Reasons for Prestressing

Eliminate cracking at service loading conditions.

Eliminate cracking at service loading conditions.

Improve shear and

Improve shear and torsional torsional strengths. strengths.

Add protection to the steel.

Add protection to the steel.

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

4

Full & Partial Prestressing Full & Partial Prestressing

• Full prestressing: sufficient precompression to

Full prestressing: sufficient precompression to ensure “crack free” at full design load. ensure “crack free” at full design load. Freyssinet Freyssinet g y 1930 1930. .

• Partial prestressing: precompression is not

Partial prestressing: precompression is not enough to prevent cracks under full design load. enough to prevent cracks under full design load. Thus, the member will normally contain some Thus, the member will normally contain some ti l t l b ti l t l b

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

conventional steel bars. conventional steel bars.

• In many cases, partial prestressing improves the

In many cases, partial prestressing improves the structural performance and is commonly used. structural performance and is commonly used.

Reinforced Concrete Mem bers Reinforced Concrete Mem bers

c C h d T jd As 1 “jd” almost constant under increasing load

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

• 1. “jd” almost constant under increasing load
• 2. “T & C” increase proportionally to applied load

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

5

Prestressed Concrete Mem bers Prestressed Concrete Mem bers

h ct C h a1 C T Aps cb C a2 No Load Dead Load M =T a1 = C a1 Dead + Live M =T a2 = C a2

Prestressed Concrete Prestressed Concrete Dr.

• Dr. Hazim

Hazim Dwairi Dwairi The Hashemite University The Hashemite University

• 1. Internal lever arm “a” increases under the applied load.
• 2. T&C remain virtually constant under working load

conditions

Plain Concrete Plain Concrete

ft b h 2

6

t r t

bh f M M = =

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

' 2 2 '

0167 . ) 617 . )( 1 . ( 6

c c

f bh bh f = =

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

6

Reinforced Concrete (Service Reinforced Concrete (Service lim it state) lim it state)

h 0.9h

C

(0.9-0.12) h =0.78h

T

b h 0.78h

= = 081 ) 36 . )( )( 45 . ( 2 1

' 2 ' c

f bh b f C

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

⎩ ⎨ ⎧ = = = = moment L.L to due 0465 . moment D.L to due 0617 . 0632 . arg 78 . 3 0632 . ) 78 . ( 081 .

' 2 ' 2 ' 2 2 2 ' 2 2 c c c c c

f bh f bh f bh M er l times is M f bh h C M f bh

Prestressed Prestressed concrete (working concrete (working lim it state) lim it state)

C C

0.9h 0.56h

'

45 .

c

f

T C

PS+DL+SI PS+DL+SI+LL 0.23h

'

45 .

c

f 0.66h 0.56h

) 56 )( 225 ( ) 56 . (

' 3 =

h bhf h C M DL d i i h 225 . ) 45 . ( 5 .

' '

= =

c c

bhf bh f C

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

element) concrete Plain ( M 5 . 7 element) ( 2 ) 126 . ( ) 56 . )( 225 . (

1 2 ' 2

= = = = RC M f bh h bhf

c c

0.56h a : LL DL ed superimpos with 0.23h a : DL ed superimpos with = + =

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

7

Advantages of Prestressing Advantages of Prestressing

• Section remains uncracked under service loads

Section remains uncracked under service loads

Increase in durability by reducing steel corrosion

Increase in durability by reducing steel corrosion y y g y y g

Full section is utilized

Full section is utilized

Higher moment of inertia (higher stiffness)

Higher moment of inertia (higher stiffness)

Less deformations (improved serviceability).

Less deformations (improved serviceability).

Increase in shear capacity.

Increase in shear capacity.

Suitable for use in pressure vessels, liquid retaining

Suitable for use in pressure vessels, liquid retaining

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

structures. structures.

Improved performance (resilience) under dynamic

Improved performance (resilience) under dynamic and fatigue loading. and fatigue loading.

Advantages of Prestressing Advantages of Prestressing

• High span

High span-

• to

to-

• depth ratios

depth ratios

Reduction in self weight

Reduction in self weight g

More aesthetic appeal due to slender sections.

More aesthetic appeal due to slender sections.

More economical sections.

More economical sections.

• Larger spans possible with prestressing (bridges,

Larger spans possible with prestressing (bridges, buildings with large column buildings with large column-

• free spaces)

free spaces)

• Typical values of span

Typical values of span-

• to

to-

• depth ratios in slabs are given

depth ratios in slabs are given b l b l

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

below: below: Non-prestressed Slab 28:1 Prestressed Slab 45:1

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

8

Advantages of Prestressing Advantages of Prestressing

• Suitable for precast construction

Suitable for precast construction

Rapid construction

Rapid construction

Rapid construction

Rapid construction

Better quality control.

Better quality control.

Reduced maintenance.

Reduced maintenance.

Suitable for repetitive construction

Suitable for repetitive construction

Multiple use of formwork

Multiple use of formwork

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

Availability of standard shapes.

Availability of standard shapes.

Lim itations of Prestressing Lim itations of Prestressing

• Prestressing needs skilled technology.

Prestressing needs skilled technology. Hence it is not as common as reinforced Hence it is not as common as reinforced Hence, it is not as common as reinforced Hence, it is not as common as reinforced concrete. concrete.

• The use of high strength materials is

The use of high strength materials is costly. costly.

• There is additional cost in auxiliary

There is additional cost in auxiliary

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

There is additional cost in auxiliary There is additional cost in auxiliary equipments. equipments.

• There is need for quality control and

There is need for quality control and inspection. inspection.

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

9

Types of Prestressing Types of Prestressing

Prestressing of concrete can be classified in several Prestressing of concrete can be classified in several

• ways. The following classifications are discussed:
• ways. The following classifications are discussed:

y g y g

• Source of prestressing force

Source of prestressing force: : Mechanical Mechanical: the devices includes weights with or : the devices includes weights with or without lever transmission, pulley blocks, screw jacks without lever transmission, pulley blocks, screw jacks and wire and wire-

• winding machines. This type of prestressing

winding machines. This type of prestressing is adopted for mass scale production. is adopted for mass scale production. Hydraulic Hydraulic: producing large prestressing forces : producing large prestressing forces

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

Hydraulic Hydraulic: producing large prestressing forces. : producing large prestressing forces. Hydraulic jacks used for the tensioning of tendons, Hydraulic jacks used for the tensioning of tendons, Electrical Electrical: the steel wires are electrically heated and : the steel wires are electrically heated and anchored before placing concrete in the molds. anchored before placing concrete in the molds. Chemical Chemical.

Types of Prestressing Types of Prestressing

• External or internal prestressing

External or internal prestressing: location of the : location of the prestressing tendon with respect to the concrete section. prestressing tendon with respect to the concrete section. g

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

(a) External (b) Internal

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

10

Types of Prestressing Types of Prestressing

• Pre

Pre-

• tensioning or post

tensioning or post-

• tensioning

tensioning: based on the : based on the sequence of casting the concrete and applying tension to sequence of casting the concrete and applying tension to g y g g y g the tendons. the tendons. Pretensioning Pretensioning: The tension is applied to the tendons : The tension is applied to the tendons before casting of the concrete. The precompression before casting of the concrete. The precompression is transmitted from steel to concrete through bond is transmitted from steel to concrete through bond

• ver the transmission length near the ends.
• ver the transmission length near the ends.

Post Post-

• tensioning

tensioning: The tension is applied to the : The tension is applied to the

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

g pp pp tendons (located in a duct) after hardening of the tendons (located in a duct) after hardening of the

• concrete. The pre
• concrete. The pre-
• compression is transmitted from

compression is transmitted from steel to concrete by the anchorage device (at the end steel to concrete by the anchorage device (at the end blocks) blocks)

Types of Prestressing Types of Prestressing

• Linear or circular prestressing

Linear or circular prestressing: based on the shape of : based on the shape of the member prestressed. the member prestressed.

• Full, limited or partial prestressing

Full, limited or partial prestressing: Based on the : Based on the amount of prestressing force, three types of prestressing amount of prestressing force, three types of prestressing are defined. are defined.

• Uniaxial

Uniaxial, biaxial or multi , biaxial or multi-

• axial prestressing

axial prestressing: based on : based on the directions of prestressing a member. the directions of prestressing a member.

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

Prestressed Concrete Hashemite University

• Dr. Hazim Dwairi

11

Circular Prestressing

Prestressed Concrete Prestressed Concrete

• Dr. Hazim Dwairi
• Dr. Hazim Dwairi

The Hashemite University The Hashemite University

Biaxial Prestressing