MIDTERM Flashcards
Can be found in the same way as that of a reinforced concrete
Ultimate Moment of Resistance
The steel is strained in order to produced the prestressed force
Ultimate Moment of Resistance
The steel strain should be taken as that due to bending plus that due to prestress force after loss
Ultimate Moment of Resistance
Formula for Ultimate Moment of Resistance
Mu = fpb · Aps · (d-dn)
For a rectangular beam, or a flanged beam in which the flange thickness is not less than
0.9x
If d>0.9x
dn is?
0.45x
What are the several causes of contraction of steel wires effecting the reduction (loss) in the prestressed concrete beam
- Pre-tensioning
- Post-tensioning
- Loss due to elastic deformation
It undergoes a gradual reduction with time from the stages of transfer due to various causes
Initial prestressing
The force which is used to stretch the wire to the required length must be available all the time as prestressing force is the steel is to be prevented from?
Contracting
Can be seen during the process of anchoring (loss in prestress)
Pretensioning
The stressed tendon tends to slip before the full grip is established, thus losing some of its imposed strain (induced stress)
Loss due to anchorage draw-in
From the time the tendons are anchored until transfer of prestressing force to the concrete, the tendons are held between the two abutments at a constant length
Pretensioning
The stretch tendons during this time interval will lose some of its induced stress due to phenomenon known as
Relaxation of Steel
As soon as the tendons are cut, the stretched tendons tends to go back to their original state, but are prevented from doing so by the interfacial bond developed between
The concrete and the tendons
The tendons are located inside ducts, and the hydraulic jacks held directly against the member
Post-tensioning
Can be seen during the stressing operation of tendons (loss in prestress)
Post-tensioning
During stressing operation, the tendons tend to get straightened and slide against duct, thus resulting in the development of
Frictional Resistance
The stress in the tendon at a distance away from the jacking end will be smaller than that indicated by the pressure gauge mounted on the jack known as
loss due to friction
The loss of prestress due to deformation of concrete depends on the modular ratio and the average stress in concrete at the level of steel
Loss due to elastic deformation
Prestress in concrete at the level of steel symbol
fc
Modulus of elasticity of steel symbol
Es
Modulus of elasticity of conrete symbol
Ec
Modular Ratio
Le
Formula of
Strain in concrete at the level of steel
fc / Ec
Formula of
Stress in steel corresponding to this strain
(fc / Ec) · Es
Formula of
Elastic Deformation of Concrete
Le · fc
Formula pf
Loss of stress in steel
Le · fc
Results in shortening of tensioned wires and hence contributes to the loss of stress
Loss due to shrinkage
Is influenced by the type of cement, aggregate and the method of curing used
Shrinkage of concrete
Use of high strength concrete with low water cement ratio results in ______ and consequent loss of prestress
Reduction in shrinkage
Progressive loss of water from concrete is its primary cause
Drying shrinkage
Is higher at the surface of the member
The rate of shrinkage
____ between the interior surface of large member may result in strain gradients leading to surface cracking
The differential shrinkage
Is essential to prevent cracks due to shrinkage in prestress members
Propoer curing
Moist curing is restored in order to prevent shrinkage until the time of transfer
Pretensioned members
The total residual shrinkage strain will be larger in pretensioned members after transfer of stress is a recommendation made by code
IS:1343
Total residual shrinkage symbol
εcs
The loss of stress in steel due to creep of concrete can be estimated if the magnitude of ultimate creep strain or creep-coefficient is known
Loss due to creep
