Loadbearing structures and detailed design - Concrete Structures for Wind Turbines - page 151

Therefore, they should be arranged in such a way that the necessary prestress can be
applied for the final condition as well as interim conditions during construction.
Individual tendons can also be prestressed at intermediate points if required for the
stability of an interim condition during construction. The tendons are normally
connected with screw couplers. The edge distances of the tendon end plates must
be very accurately maintained in order to prevent prestressing forces causing spalling at
the edges of the shell.
4.7.4.6 Design of openings
Vertical direction
The inclusion of openings means that the heavily loaded concrete is missing as a
component of the cross-section in the flexural compression zone (Figure 4.22). Door
openings at the base of the tower are particularly critical. Therefore, the widths of
openings should be kept as small as possible (e.g. L w ¼ 0.72m).
The door reveals require additional reinforcing steel (not prestressing tendons, which
would increase the compression!). The centroid axis is displaced because of the opening.
The eccentricity moments due to the concentric dead loads and the prestressing increase
Fig. 4.22 Vertical reinforcement at an opening in the cross-section of a precast concrete element