Environmental Engineering Reference
In-Depth Information
rapidly as the height of the tower increases. At the same time, the internal forces due to
external loads also increase.
This means that the compressive stresses due to dead loads, the flexural compressive
stresses due to wind and also the compressive stresses caused by the concentric
prestressing accumulate in the cross-sections and joints of the lower segments, whereas
the maximum compression capacity given by the concrete strength is limited. It is for
this reason that concretes of strength class B55, even spun concrete of class B65 to DIN
4228 [15], were the types most commonly used in the past.
The introduction of the DAfStb guideline for high-strength concrete, and as a
consequence the new DIN 1045-1 [33], means that even higher strengths, for
example C 70/85, are being increasingly used for the segments of precast concrete
towers. Further developments in the direction of ultra-high-performance concretes
(UHPC) are to be expected. One way of ensuring that UHPC mixes comply with
requirements regarding sufficient robustness and ductility is to add steel fibres to
the mix.
There is considerable room for errors in the design of and workmanship at joints
between segments. The danger when applying the grout and subsequently position-
ing a segment is that the grout can seep into the ducts for the prestressing tendons if
the seals have not been properly fitted. On the other hand, any gaps in the grouting at
the joints can lead to grout being forced out through the joints when pressure-
grouting the ducts.
Such problems are specific to the use of grouted post-tensioned tendons. The alterna-
tive is therefore external prestressing with tendons within the interior of the tower shaft,
a method that has already been used for a number of wind turbines. However, the
special conditions described in Sections 4.7.1, 4.7.5 and 4.8.3 must be taken into
account.
Another option is to prestress the segments individually and anchor the tendons in
annular structural steel flanges at the horizontal joints. Such annular flanges extend into
the interior of the tower shaft in such a way that the segments can be connected on site
with high-strength friction-grip (HSFG) bolts. This approach results in a hybrid
structure with individual shaft elements of high-strength prestressed concrete and
steel connections between those elements. A further possibility with this form of
construction is being able to offset the vertical joints between half-shell elements,
something that was not possible with the reinforced filled joints used up to now.
Current developments are therefore characterised by innovations on the materials side
(HPC, UHPC, fibre-reinforced concrete) on the one hand and by robust design
principles (external prestressing, modern connection methods) on the other.
5.4 Offshore substructures in concrete
The offshore wind farms planned for the North Sea and Baltic Sea are up to 40 km
from the coast, where the depth of the water varies from 30 to 45m depending on
