Civil Engineering Reference
In-Depth Information
Figure 5.6
Arrangement of tendons
calculation. However, it is not worth carrying out such a recalculation until a more
accurate fi gure for the force in the cables has been found.
5.14 Cable zone
Clearly the bending moment varies along the length of the beam and reduces to zero
at its ends. Once the prestress force has been defi ned at mid-span, it is necessary to
calculate the safe eccentricity for the prestress centroid at all points along the beam.
This safe eccentricity is defi ned such that the specifi ed minimum stresses on the extreme
fi bres are not exceeded under any combination of dead and live loads. The safe zone
for the centroid of the cables is called the cable zone.
If the prestress centroid is at the upper limit of the cable zone, under maximum
applied bending moments, the stress on the bottom fi bre of the beam will be equal to
the minimum allowable stress. When the prestress centroid is at the lower limit of the
zone, the stress on the top fi bre will be at the permitted minimum.
The cable zone at the mid-span section of the beam may be found by considering
the moments given in Table 5.1. First, assume that the cable force is the theoretically
defi ned value of 9.38 MN, and the limiting stresses are zero on both extreme fi bres.
Under the effect of the maximum moment of 16.51 MNm the centre of pressure must
