Environmental Engineering Reference
In-Depth Information
5
Construction of prestressed concrete towers
5.1 Introduction
Prestressed concrete towers for onshore wind turbines are constructed using either
in situ or precast concrete techniques and completed with internal or external
prestressing. Hybrid tower forms have proved to be particularly beneficial. Such
towers consist of a prestressed concrete shaft and a top segment of steel. In recent years
they have turned out to be very economic solutions for turbines of the multi-megawatt
class. The detailed design of prestressed concrete towers for wind turbines was covered
in detail in Chapter 4.
The choice of a suitable tower design is governed by the conditions at the site
(fabrication, transport, erection, etc.) and the number of segments required (precast
concrete construction). Beton-Kalender 2006 [8] contains a comprehensive review of
different designs.
Examples of hybrid tower designs and prestressed towers in precast concrete are
described in this chapter.
The chapter concludes with a section on concrete foundation designs for offshore wind
turbines.
5.2 Hybrid structures of steel and prestressed concrete
Hybrid support structures for wind turbines are characterised by the combination of
various individual loadbearing structures of prestressed concrete and steel which carry
the load together. The tower consists of a concrete shaft surmounted by a separate steel
tower segment that serves as a transition to the nacelle containing the wind turbine
itself.
Exclusively tubular steel towers with heights exceeding 100m and turbine outputs
>
2.0 MWare practically ruled out because of the transport width of the base segments
(
>
4.30m). However, the concrete, reinforcing steel and prestressing steel for concrete
towers can be easily delivered to the site separately for construction in situ.
The hybrid form of construction is frequently the most economic solution for high
towers; for details see [8]. Figure 5.1 shows a hybrid design for a 5 MW wind turbine
and 130m hub height. It consists of a prestressed concrete tower (h
ΒΌ
120m) and a
tubular steel top segment that supports the nacelle with the rotor.
The vibration effects for this loadbearing structure were investigated in Section 4.3.2.
The aerodynamic force coefficient, the equivalent mass for the prestressed concrete
tower and the gust response factor were determined in Section 2.3.1 according to [12]
and used as the basis for calculating the wind loads.
A circular ring beam in combination with a steel adapter is provided at the top of the
prestressed concrete tower for connecting the tubular steel top segment. The principles
for the structural analysis of this structural component can be found in Section 4.10.
