Wind Energy

(a) (b)
450
Load [kN]
400
350
300
250
200
150
100
50
0
without shear keys
shear key height = 1.25mm
h/s = 0.06
shear key height = 0.3mm
h/s = 0.01
strain [%]
58 High-cycle Fatigue 311
Strain
Stiffness
0.0 1.0 2.0 3.0 4.0 5.0 6.0 0 5.000 10.000 15.000 20.000
Pile-displacement [mm] Number of cycles
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
16.000
14.000
12.000
10.000
Fig. 58.2. (a) Static load–displacement curves for different Grouted Joints specimens
(b) Strain and stiffness development of Grouted Joints in fatigue tests
when reaching the ultimate load the pile is gradually pushed through the
grouted connection with considerable load-bearing capacity remaining. One
aspect that should be noted, is the deformation at the ultimate load. Related
to the grouted length of 90 mm, a pile displacement of about 2 mm equals
a deformation of over 2%. In view of alternating tension and compression
stresses in Grouted Joints for Tripod foundations, this might have detrimental
effects on the fatigue strength as well as the deviations at the top of the tower.
In this case, it might be sensible to apply the first maximum load for design
purposes.
Comparative calculations according to the regulations of the American
Petroleum Institute [1] and Det Norske Veritas [2] show that all of the tests
are conservative compared to the American Petroleum Institute, but do not
reach the characteristic values given by Det Norske Veritas.
Fatigue tests have supported that the load redistributes when reaching the
first maximum or a comparable strain as displayed in Fig. 58.2b. At this point
the strain increases from about 0.5% to 1.2% during a few load cycles. The
following gradual increase corresponds to the second part of the static curve
in Fig 58.2a. Regarding the stiffness development of Grouted Joints in fatigue
tests the typical decrease in stiffness can be observed up to the first maximum
load. At this point the stiffness recovers and nearly reaches its initial value,
before gradually decreasing again.
58.4 Conclusions
In this paper the performance of UHPC in Grouted Joints subjected to static
and fatigue loading is discussed. It is described (Fig 58.1), that the fatigue
strength of UHPC in high-cycle fatigue seems to be lower compared to normalstrength
concrete. Regarding UHPC in Grouted Joints nearly bilinear load–
deflection curves for specimens with shear keys in uniaxial compression are
shown, which represent two different load-bearing mechanisms and a very
ductile failure. The redistribution of the load at the first maximum described
for static tests can clearly be recognized in fatigue tests as well. Furthermore
8.000
6.000
stiffness [MPa]