Knowing the graph of tendon force versus tendon ordinate, the extension of tendon during stressing can be determined by calculating the area under force profile.
The calculated
extension between the faces of anchors is usually compared with that measured
directly on site during stressing. The purpose of this is to verify the
tensioning of tendon is satisfactory.
Adjustment should
be made based on the pull-in of wedges at the dead end of tendon (typically 2mm
to 4mm) and extension of strands between the measuring point behind the
stressing jack and the face of anchor.
The samples of
strands batch used in the project are collected and tested to obtain its
modulus of elasticity, Et. Short length strands samples are used, and they are
held between fixed jaws in rig during testing.
When longer
strands are used, the effective Et value may be reduced due to the effect of
spiral outer wires. The phenomenon where the strands are usually twisted around
each other during installation and straighten during stressing also contributes
to the reduction of Et.
When conducting
analysis, effective Et is obtained by reducing the experimental Et value by up
to 5%.
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