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Chapter: Civil - Prestressed Concrete Structures - Circular Prestressing

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Depth anchorage to the Depth of end block on the distribution of bursting tension

The effect of varying the ratio of depth anchorage to the depth of end block on the distribution of bursting tension

The effect of varying the ratio of depth anchorage to the depth of end block on the distribution of bursting tension.

 

Bursting tensile forces

 

a)The bursting tensile forces in the end blocks, or regions of bonded post-tensioned members, should be assessed on the basis of the tendon jacking load. For unbonded members, the bursting tensile forces should be assessed on the basis of the tendon jacking load or the load in the tendon at the limit state of collapse, whichever is greater ( see Appendix B ).

 

The bursting tensile force, Fbst existing in an individual square end block loaded by a symmetrically placed square anchorage or bearing plate, may be derived from the equation below:


b) The force Fbst will be distributed in a region extending from 0.1 yo to yo from the loaded face of the end block. Reinforcement provided to sustain the bursting tensile force may be assumed to be acting at its design strength (0.87 times characteristic strength of reinforcement) except that the stress should be limited to a value corresponding to a strain of 0.001 when the concrete cover to the reinforcement is less than 50 mm.

 

c) In rectangular end blocks, the bursting tensile forces in the two principal directions should be assessed on the basis of 18.6.2.2. When circular anchorage or bearing plates are used, the side of the equivalent square area should be used.

Where groups of anchorages or bearing plates occur, the end blocks should be divided into a series of symmetrically loaded prisms and each prism treated in the above manner. For designing end blocks having a cross-section different in shape from that of the general cross-section of the beam, reference should be made to specialist literature.

 

d)  Compliance with the requirements of (a), (b) and (c) will generally ensure that bursting tensile forces along the load axis are provided for. Alternative methods of design which make allowance for the tensile strength of the concrete may be used, in which case reference should be made to specialist literature.

 

e)   Consideration should also be given to the spalling tensile stresses that occur in end blocks where the anchorage or bearing plates are highly eccentric; these reach a maximum at the loaded face.

 

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