Shear, bond bearing,camber & deflection in prestressed concrete

WELCOME

Dept. of Civil Engineering

AHSANULLAH UNIVERSITY OF SCIENCE & TECHNOLOGY

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Course No: CE-416

Course Name: PRESTRESS CONCRETE DESIGN SESSIONAL

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PRESENTATION ON

Shear, Bond, Bearing, Camber,

DeflectionIn Prestressed

Concrete

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Presentated ByName Student ID

Md. Zahidul Islam 10.01.03.142

Shaikh Mahfuzur Rahman 10.01.03.143

Rifath Ara Rimi 10.01.03.145

MD. Jahirul Islam 10.01.03.146

MD. Rakibul Islam 10.01.03.148

Md. Neshar Ahmed 10.01.03.151

Raiyan Fardous Ratul 10.01.03.153

Md. Shahadat Hossain 10.01.03.154

Md. Ridwan-Ur-Rahman 09.02.03.109

Group : 4

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SHEARIN

PRESTRESSEDCONCRETE

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What Is Shear Force

Shear forces are unaligned forces pushing one part of a body in one direction, and another part the body in the opposite direction. Shear force acting on a substance in a direction perpendicular to the extension of the substance.

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Shear Mechanism

In a simply supported rectangular beam, self weight & super imposed loads act downward, reaction acts upward. Resultants of all these vertical forces generates vertical shear in a member.

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ShearNormal Concrete Vs Pre-stressed Concrete

• Comparatively smaller sectioned member needed for load carrying, so less self weight i.e. less shear.

RCC BEAM

Prestressed Concrete Member

D1

D2

D1>D2 i.e. for same load carrying 8

ShearNormal Concrete Vs Pre-stressed Concrete

• Sagged tendon in most case provide additional shear but opposite direction.

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ShearNormal Concrete Vs Pre-stressed Concrete• Prestressing prevents the occurrence of shrinkage

cracks which could conceivably destroy the shear resistance.

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Modes of Failure in Prestressed

Beam11

Flexure-Compression (FC):Flexure compression failures are the result of having a beam with higher shear strength than flexural strength. Failure occurs at the point of maximum flexural stress where the compressive strain exceeds its capacity.

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Flexure-Shear FailureA flexure-shear failure, is the result of a crack which begins as a flexural crack, but as shear increases, the crack begins to “turn over” and incline towards the loading point. Failure finally occurs when the concrete separates and the two planes of concrete slide past one another. This mode of failure is common in beams which do not contain web reinforcement.

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Shear-Compression FailureShear compression failures, shown in Figure, typically occur in beams which contain adequate web reinforcement. In this mode, the crack propagates through the section until it begins to penetrate the compression zone. This crack causes a redistribution of compressive forces in the compression zone onto a smaller area. When the compressive strength is exceeded, a shear compression failure occurs. This type of failure is common in deep beams, where arch action is prevalent. The compressive strut caused by arch action prevents a diagonal tension crack from propagating into the compression zone.

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Web-shear FailureBefore a section cracks from flexure, it is possible to exceed the tensile strength of the concrete at the point of maximum shear stress. This mode is primarily observed in sections with thin webs. Failure occurs at the location of peak shear stress, as shown in Figure. While, the mechanics of this failure are identical to flexure-shear, failure is brittle and occurs with little or no warning.

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Factors Influencing Shear Strength• Axial Force: Shear failures are commonly due to tensile failure of the concrete.

Axial compression can delay the onset of critical tension in the section, axial tension can hasten the failure. Compression, such as provided by an axial force or prestressing tendons, provides an increase in shear strength.

• Tensile Strength of Concrete: As the tensile strength of the concrete is increased, there is a corresponding increase in the shear strength of the section.

• Longitudinal Reinforcement Ratio: Low amount of steel may result in wider flexural cracks, resulting in reduced dowel action and aggregate interlock.

• Shear Span-to-Depth Ratio: High values of require a larger compression zone, raising the amount of shear which can be transferred by the uncracked concrete shear transfer mechanism, thus increasing shear strength

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Shear Carrying of Concrete & Tendon on Different Tendon Profile

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Some Important Notes about Shear in Prestressed Concrete

• Prestressed beam never fail under direct shear or punching shear. They fail as a result of tensile stress produced by shear.

• In some rare instance the transverse component of prestress increases the shear in concrete.

• By following load balancing approach, it is theoretically possible to design a beam with no shear in concrete under a given condition of loading.

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Development of Shear Cracking

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Steps of Shear Design

For a Simply Supported Beam Section with UDL loading

• Step -1: Calculate the moment of inertia of the section.

• Step -2: Calculate Support reaction.• Step -3: Calculate Moment at desire beam section

from x distance from support. • Step -4: Calculate ‘a’ and then the eccentricity of

tendon at desire (x) distance from support i.e. ex 20

For Flexural Shear Crack

• Calculate• Calculate• Calculate Flexural Cracking Moment

• Calculation of cracking flexural shear

• Calculation of Nominal flexural shear

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For Web Shear Crack• Calculate • Calculation of Nominal web shear

• Calculate ultimate load • Calculate factored shear at a section x distance from support

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Shear Reinforcement Spacing

Smallest spacing among S1, S2, S3 should be chosen as stirrup spacing.

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End of topic Shear in

Prestressed Concrete

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“BOND” in Prestressed

Concrete

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Definition

Interlocking between two properties e.g. pre-stressed tendon and concrete.

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Main Types of Internal Prestressed Concrete

• Pre-Tension Concrete: Pre-stressing steel is tension stressed prior to the placement of the concrete and unloaded after concrete has harden to required strength.

• Bonded post-tensioned concrete: Unstressed pre-stressing steel is placed with in the concrete and then tension stressed after concrete has harden to required strength

• Un-bonded post-tensioned concrete: Differs from bonded post-tensioning by providing the pre-stressing steel permanent freedom of movement relative to the concrete.

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Transfer of Prestressing Force:Bond between concrete and prestressing steel.

Bearing at end anchorages.

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Existence of Bond in Prestressed

concrete

1.Pre-Tension Concrete

2.Bonded post-tensioned concrete

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“Bond ” effects in Prestressed concreteBond exists on two different basis:

1. Pre-tensioning system

Used as a means of transferring the prestressing force of tendon to the

concrete section.

2. Post-tensioning system

In this, bond is necessary for two purposes,

-Protection against corrosion

-Increase in ultimate strength

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Bond effect in Pre-tensioned construction

1.It is furnished by two factors,

-Reduction in area of cross section of steel

-Adhesive property

2.The phenomenon of recovery of lateral contraction develops a wedge

action at the end of the cable by which prestressing force is transferred.

3.This property was discussed detail by Hoyer and is called “HOYER EFFECT”.

4.Transverse reinforcement has to be provided to resist tensile force.

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Bond mechanisms in the prestressing concrete :

1) Adhesion between concrete and steel 2) Mechanical bond at the concrete and steel interface 3) Friction in presence of transverse compression.

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Hoyer EffectAfter stretching the tendon, the diameter reduces from the original value due to the Poisson’s effect. When the prestress is transferred after the hardening of concrete, the ends of the tendon sink in concrete. The prestress at the ends of the tendon is zero. The diameter of the tendon regains its original value towards the end over the transmission length. The change of diameter from the original value (at the end) to the reduced value (after the transmission length), creates a wedge effect in concrete. This helps in the transfer of prestress from the tendon to the concrete. This is known as the “Hoyer effect”.

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Development length(Ld):

The development length (Ld) is the sum of the transmission length (Lt) and the bond length (Lb).

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Transmission length: The bond needed to transmit the complete prestressing force is called

transmission length(Lt). The stress in the tendon is zero at the ends of the members. It increases

over the transmission length to the effective prestress (fpe) under service loads and remains practically constant beyond it.

Fig : Variation of prestress in tendon along transmission length 35

Factors that influence the transmission length:

1) Type of tendon ¾ wire, strand or bar

2) Size of tendon 3) Stress in tendon 4) Surface deformations of the tendon

¾ Plain, indented, twisted or deformed 5) Strength of concrete at transfer 6) Pace of cutting of tendons

¾ Abrupt flame cutting or slow release of jack 7) Presence of confining reinforcement 8) Effect of creep 9) Compaction of concrete 10) Amount of concrete cover.

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The bond length:

Fig : Variation of prestress in tendon at ultimate

The bond length (Lb) is the minimum length over which, the stress in the

tendons can increase from the effective prestress(fpe) to ultimate

prestress(fpu) at critical location. The expression of the bond length isderived as,

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The bond length depends on the following factors:

1) Surface condition of the tendon

2) Size of tendon

3) Stress in tendon

4) Depth of concrete below tendon38

End zone reinforcementThe prestress and the Hoyer effect cause transverse tensile stress (σ t). This is largest during the transfer of prestress.To resist the splitting of concrete, transverse reinforcement need to be provided at each end of a member along the transmission length. This reinforcement is known as “End zone reinforcement’’.The minimum amount of end zone reinforcement is given as,

h = total depth of the section M= moment at the horizontal plane at the level of CGC due to the compressive stress block above CGC fs = allowable stress in end zone reinforcement

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Bond in Post-tensioned construction Effect of bond in post-tensioned construction has two distinct

purposes;

1.Protection against stress corrosion

-Moisture enters into duct

-Cause corrosion to high tension steel

-Rusting reduces effective area of steel

-This causes splitting of wires called stress corrosion

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2.Increase in ultimate strength

● In bonded construction

-Crack at the critical section does not affect the strain in

steel

-Because of this, the compressive area is not reduced

considerably

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Process– Concrete is casted around a curved

duct (usually corrugated), to allow room for the Tendon to be inserted.

– After the concrete has hardened the tendons are pulled in tension and then wedged.

– The duct is then injected with grout

There are 2 layers of bonding media

in post-tensioned construct

-Bond between the steel and the

sheath or duct

-Bond between the sheath and

the concrete42

End of this topic

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Bearing or Bearing plate

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Definition

A bearing plate is a specially-designed metal plate used to spread the force of a load-bearing wall or column out over a larger area

Fig: Bearing plates45

The end zone (or end block) of a post-tensioned member is a flared region which is subjected to high stress from the bearing plate next to the anchorage block. It needs special design of transverse reinforcement. The design considerations are bursting force and bearing stress.

Some Important things to know

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Behavior of the local zone• The behavior of the local zone is influenced by the anchorage

device and the additional confining spiral reinforcement

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The transverse tensile stress is known as splitting tensile stress. The resultant of the tensile stress in a transverse direction is known as the bursting force(Fbst). Compared to pre-tensioned members, the transverse tensile stress in post-tensioned members is much higher.

Behavior of the local zone (Contd.)

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For calculating bursting force (Fbst) an individual square end zone loaded bearing plate.

Calculating bursting force

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End Zone Reinforcement• The amount of end zone reinforcement in each direction (Ast)

can be calculated from the following equation.

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• The bearing stress in the local zone should be limited to the following allowable bearing stress (fbr,all)

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Dispersion of bearing stress in concrete

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Manufacturing of an end block specimen

Fabrication of end zone reinforcement Anchorage block and guide

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Manufacturing of an end block specimen (Contd.)

End zone reinforcement with guide and duct End block after casting

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End of this topic

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Camber & Deflection

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Camber

Camber is the upward deflection in the beam after release of the prestressing strands due to the eccentricity of the force in the strands. The camber of the beam is usually the largest contribution to hunch.

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Factors of camber

The ability to predict camber accurately is critical for the design and constructions . However, this is a complex task, since the camber is dependent on many variables, some of which are interdependent and change over time. Four of the most significant variables are the properties of the concrete , 1. creep of the concrete, 2. concrete temperature 3. the magnitude 4. location of the prestress

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Deflection

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Definition In general, Deflection is the degree to which a structural

element is displaced under a load.

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Types of Deflection

Short-term deflection occurs immediately upon the application of a load.

Long-term deflection takes into account the long-term shrinkage and creep movements.

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Causes of Deflection in PSC Beams

Due to external loads

Due to prestress force

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Tendon ProfileThe deflection due to prestress depends on the profile of the c.g.s. line

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Methods of Calculation

Double Integration Method

Moment Area Method

Conjugate Beam Method

Principle of Virtual Load

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Calculations of the Short-term Deflection

The usual loading which should be investigated in calculating deflections are:

Prestress plus dead loadPrestress plus maximum service loadPrestress plus minimum service load

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THANK YOU

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ANY QUESTION

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