5.4 Temporary tie-down stays
As the cantilevers extend from the towers , they become vulnerable to wind buffeting resulting in possibly large deflections . The considerable energy of wind in the low frequency range corresponds to the natural frequency of the erection configuration . Temporary tie-down stays are installed as the cantilevers extend to stabilise against buffeting . These are located on the opposite sides of the leading cantilever on each tower and also assist in countering the bending moment in the towers caused by the imbalance from the leading segment . These stays are installed when the cantilevers reach 95m and 160m for the Flanking and Central Towers respectively .
5.5 Geometry control
During construction of the bridge , the ‘ stay length ’ method by which the global geometry can be controlled is adopted . It makes use of the fact that the geometry of the prefabricated members of the bridge can be measured prior to erection under well controlled conditions and with high accuracy .
The elevation of the cantilevers can be controlled by adjusting the length of the sides in a triangle comprising the lengths of the stay , the cantilever and the height of the tower .
During segment erection , the length of the cantilever and the height of the tower are monitored and compared to their theoretical values . If these two dimensions differ at the given construction stage , it can be assumed that this deviation too will be present in the final bridge unless a cable length adjustment is made . Stay cable lengths are only corrected for
Figure 24 : Flanking towers temporary stays
deviations which are predicted for the final bridge and not for deviations that are temporary .
Differential weight in opposing cantilevers causes deformation in the tower fan . The tower top deflects in the longitudinal direction . The cantilevers tips deflect vertically .
When the cantilevers are short , deformations of this type are relatively small . If an imbalance of more than 1 % occurs , temporary ballast ( eg concrete blocks or strand coils ) is placed on the lighter cantilever . Where the detected imbalance is considerable , permanent ballast may be required .
Figure 25 : Geometric parameters governing the elevation of the cantilever
Figure 26 : Deformations due to differential weights
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