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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