Loadbearing structures and detailed design - Concrete Structures for Wind Turbines - page 142

Prestressing steel:

f p0.1d ¼ f p0.1k / g S or f p0.1dA ¼ f p0.1k / g SA

7. Checking the level of prestress.
The design value of the 0.1% proportionality limit for the prestressing steel may not be
exceeded if the stability of the loadbearing structure in the deformed state is not to be
endangered (see also Section 3.1):

e pd e p0 : 1d ¼ E p f p0 : 1k =g S

g P according to Table 4.3 also apply for the

On the other hand, the partial safety factors

detailed design.
This results in limited reserves of stress in the prestressing steel at the ultimate limit
state. With a favourable effect of the prestress, this can lead to the prestressing steel not
being able to be fully mobilised for flexural tension action effects:

For example : St 1500 = 1770 : s pm 0 ¼ 0 : 55 f pk ¼ 973 : 5 MPa

s p ; Rd ¼ 1500 = 1 : 15 ¼ 1304 MPa

Maximum reserves for flexural tension with unbonded tendons:

Ds p0d Ds p0k g P ; inf ¼ðs p ; Rd s pm0 Þg P ; inf ¼ð 1304 973 : 5 Þ 0 : 83 ¼ 274 MPa

However, this flexural tension reserve is irrelevant when the stress increase in the
prestressing steel Ds ptd at the ultimate limit state has been ignored right from the very start.
The situation is more favourable when using grouted post-tensioned tendons; the values
in the above example change as follows:
Maximum reserves for flexural tension with grouted post-tensioned tendons:

Ds p0d s p ; Rd s pm0 ¼ 1304 973 : 5 ¼ 330 : 5 MPa

This difference in stress can be exploited at the ultimate limit state in order to transfer
the bending moments across the joints in a prestressed concrete tower. The goal should
be to achieve a flexural tension reserve in the region of the design value of the
reinforcing steel. To do this, however, the level of prestress must be reduced as follows:
s pm 0 s p ; Rd f yd ¼ 1304 435 ¼ 869 MPa ð¼ 0

:

49 f pk Þ

With a favourable effect of the prestress, that is for verifying decompression, a relatively
low level of prestress is then available: The realistic assumption of a 25% loss in
prestress with concentric prestressing would then result in

s p 1 k ; inf ¼ 0 : 90 s pm 1 ¼ 0 : 90 0 : 75 s pm 0 ¼ 0 : 90 0 : 75 869 ¼ 586 MPa

However, the flexural tension reserve then increases noticeably. In the case of grouted
post-tensioned tendons, the result is then
Ds p 1 d s p ; Rd s pm 1 ¼ 1304 586 ¼ 718 MPa ð> f yd Þ