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Chapter: Civil - Prestressed Concrete Structures - Design Concepts

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Prestressed Concrete Structures: End block

Prestressed Concrete Structures: End block
Bursting force: A portion of a pre-stressed member surrounding the anchorage is the end block.

End block

Bursting force

A portion of a pre-stressed member surrounding the anchorage is the end block. Through the length of the end block, pre-stress is transferred from concentrated areas to become linearly distributed fiber stresses at the end of the block. The theoretical length of this block, called the lead length is not more than the height of the beam.

But the stress distribution within this block is rather complicate.


The larger transverse dimension of the end zone is represented as yo. The corresponding dimension of the bearing plate is represented as ypo. For analysis, the end zone is divided into a local zone and a general zone.


The local zone is the region behind the bearing plate and is subjected to high bearing stress and internal stresses. The behavior of the local zone is influenced by the anchorage device and the additional confining spiral reinforcement.

The general zone is the end zone region which is subjected to spalling of concrete. The zone is strengthened by end zone reinforcement.

The transverse stress (?t) at the CGC varies along the length of the end zone. It is compressive for a distance 0.1yo from the end and tensile thereafter, which drops down to zero at a distance yo from the end.

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).


Besides the bursting force there is spalling forces in the general zone.


Fbst for an individual square end zone loaded by a symmetrically placed square bearing plate according to Cl 18.6.2.2 is,


Where, P= pre-stress in the tendon;

po = length of a side of bearing plate;

 yo = transverse dimension of the end zone.

It can be observed that with the increase in size of the bearing plate the bursting force

Fbst reduces.

 

End Zone reinforcement

Transverse reinforcement - end zone reinforcement or anchorage zone reinforcement or bursting link - is provided in each principle direction based on the value of Fbst. The reinforcement is distributed within a length from 0.1yo to yo from an end of the member.

The amount of end zone reinforcement in each direction Ast is:


s The parameter represents the fraction of the transverse dimension covered by the bearing plate.

The stress in the transverse reinforcement, fs = 0.87fy.

When the cover is less than 50 mm, fs = a value corresponding to a strain of 0.001.

The end zone reinforcement is provided in several forms, some of which are proprietary of the construction firms. The forms are closed stirrups, mats or links with loops.


 

Bearing plate & End block

Design the bearing plate and the end zone reinforcement for the following bonded post-tensioned beam. The strength of concrete at transfer is 50 MPa. A pre-stressing force of 1055 kN is applied by a single tendon. There is no eccentricity of the tendon at the ends.





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