**4. Laboratory experiment**

#### **4.1. Equipment, specimen and conditions**

Since it is impossible to verify the failure mode and fatigue life of the target structure, laboratory experiment using the 1/12 scale model was prepared. In order to generate high cycle load with reliable stability, a bi-axial shaking table driven by magnetic force was employed for this study (see **Figure 12**). Tri-axial cyclic loading machine controlled hydraulic jack is not suitable for high cycle loading, even though the load generated by hydraulic jack can be higher than inertial force generated by a shaking table.

The overview of specimen is shown in **Figure 13**. This is basically designed as a 1/12 scale model of target structure shown in **Figure 1**. However, the reinforcement ratio in both pedestal and footing is higher than the real structure because it is impossible to simply scale down the diameter of reinforcing bars. The weight of blades and nacelle was 52.5 kgf that reflects the weight of steels set on the top of tower. The tower was made of STK400 steel pipe, and this model was reused for all foundation specimens. The tower model and foundation specimens were tight by eight M8 anchor bolts. The footing was fixed by four bolts at each corner of footing to shaking table.

Accelerometers were set at the top of tower and on top surface of footing to capture horizontal motions in two axes. Laser displacement meters and LVDTs were set for the top of tower and shaking table, respectively to compare the values with obtained from accelerometers. Strains of surfaces of pedestal concrete and surfaces of steel tower were measured by strain gauges. Furthermore, mold-type strain gauges were embedded beside eight anchor bolts to capture the trend of cracking inside concrete.

bolts. All the specimens except G-D-E were examined by 5 Hz sine wave vibrating with different amplitudes. Furthermore, considering the change of main axis of tower vibration in time domain (see **Figure 3**), vibrating of X and Y axis in turn was tried for three cases, that is, N-D-500, N-D-400 and G-D-900. The other hand, G-D-E is the case using X and Y axis actuator simultaneously for reproducing ground acceleration record that was obtained in this area

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Since it is impossible to observe cracking inside concrete during experiment, the criteria of fatigue failure of these specimens were defined as 20 mm of relative horizontal displacement at the top of tower. It is large enough because 20 mm of the model is almost equivalent to 240 mm for the real structure. Another reason is that the specimens dangerously resonate when the relative displacement exceed 20 mm [19]. Because the natural frequency of specimens which is usually 6.5–7.6 Hz before loading had gradually decreased with increase of

In the case of N-S-500, sudden fracture of an anchor bolt occurred, then loading was stopped. It failed but the failure mode was different from others. The four surviving cases did not show

The comparison of the development of relative displacement at the top of tower for G-S-900 and G-D-900 was shown in **Figure 14**. It was clear that the G-S-900 under single axis vibration survived longer than G-D-900 under the vibrating of X and Y axis in turn. Interestingly, N-S-500 also survived longer than N-D-500 (see **Figure 15**), even though the failure mode of

any changes in measurable data for a long time, then loading was canceled.

during Great East Japan Earthquake 2011.

**Figure 13.** Photos and drawings of specimen.

N-S-500 was different from N-D-500.

number of load cycle due to fatigue damage of concrete.

**4.2. Results**

The conditions of all specimens are shown in **Table 2**. The laboratory experiment consisted of two series. The first series named "N-" was the prototype. The second series named "G-" installed gypsum between tower and foundation to smooth top surface of concrete specimens as well as monitoring strain gauges attached on the nut to control initial torque of anchor

**Figure 12.** Bi-axial shaking table driven by magnetic power.

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**Figure 13.** Photos and drawings of specimen.

bolts. All the specimens except G-D-E were examined by 5 Hz sine wave vibrating with different amplitudes. Furthermore, considering the change of main axis of tower vibration in time domain (see **Figure 3**), vibrating of X and Y axis in turn was tried for three cases, that is, N-D-500, N-D-400 and G-D-900. The other hand, G-D-E is the case using X and Y axis actuator simultaneously for reproducing ground acceleration record that was obtained in this area during Great East Japan Earthquake 2011.

#### **4.2. Results**

**4. Laboratory experiment**

**4.1. Equipment, specimen and conditions**

112 Stability Control and Reliable Performance of Wind Turbines

inertial force generated by a shaking table.

the trend of cracking inside concrete.

**Figure 12.** Bi-axial shaking table driven by magnetic power.

ing to shaking table.

Since it is impossible to verify the failure mode and fatigue life of the target structure, laboratory experiment using the 1/12 scale model was prepared. In order to generate high cycle load with reliable stability, a bi-axial shaking table driven by magnetic force was employed for this study (see **Figure 12**). Tri-axial cyclic loading machine controlled hydraulic jack is not suitable for high cycle loading, even though the load generated by hydraulic jack can be higher than

The overview of specimen is shown in **Figure 13**. This is basically designed as a 1/12 scale model of target structure shown in **Figure 1**. However, the reinforcement ratio in both pedestal and footing is higher than the real structure because it is impossible to simply scale down the diameter of reinforcing bars. The weight of blades and nacelle was 52.5 kgf that reflects the weight of steels set on the top of tower. The tower was made of STK400 steel pipe, and this model was reused for all foundation specimens. The tower model and foundation specimens were tight by eight M8 anchor bolts. The footing was fixed by four bolts at each corner of foot-

Accelerometers were set at the top of tower and on top surface of footing to capture horizontal motions in two axes. Laser displacement meters and LVDTs were set for the top of tower and shaking table, respectively to compare the values with obtained from accelerometers. Strains of surfaces of pedestal concrete and surfaces of steel tower were measured by strain gauges. Furthermore, mold-type strain gauges were embedded beside eight anchor bolts to capture

The conditions of all specimens are shown in **Table 2**. The laboratory experiment consisted of two series. The first series named "N-" was the prototype. The second series named "G-" installed gypsum between tower and foundation to smooth top surface of concrete specimens as well as monitoring strain gauges attached on the nut to control initial torque of anchor

Since it is impossible to observe cracking inside concrete during experiment, the criteria of fatigue failure of these specimens were defined as 20 mm of relative horizontal displacement at the top of tower. It is large enough because 20 mm of the model is almost equivalent to 240 mm for the real structure. Another reason is that the specimens dangerously resonate when the relative displacement exceed 20 mm [19]. Because the natural frequency of specimens which is usually 6.5–7.6 Hz before loading had gradually decreased with increase of number of load cycle due to fatigue damage of concrete.

In the case of N-S-500, sudden fracture of an anchor bolt occurred, then loading was stopped. It failed but the failure mode was different from others. The four surviving cases did not show any changes in measurable data for a long time, then loading was canceled.

The comparison of the development of relative displacement at the top of tower for G-S-900 and G-D-900 was shown in **Figure 14**. It was clear that the G-S-900 under single axis vibration survived longer than G-D-900 under the vibrating of X and Y axis in turn. Interestingly, N-S-500 also survived longer than N-D-500 (see **Figure 15**), even though the failure mode of N-S-500 was different from N-D-500.


**Figure 15.** The development of relative displacement at the top of tower (N-S and N-D).

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**Figure 16.** Vertical strain inside concrete for G-S-900 and G-D-900.

**Table 2.** The conditions of all specimens.

The vertical strains inside concrete measured by embedded strain gauges were shown in **Figure 16**. The vibrating direction of G-S-900 is always along X axis. Thus, the amplitude of IS1 and IS5 located in the orthogonal direction was significantly smaller than others. The strains measured in G-D-900 vibrated in both directions demonstrated relatively larger amplitude than G-S-900 in all locations. This suggested that the cracking in wider area possibly accelerated the fatigue damage and led shorter fatigue life.

In order to observe internal cracks, concrete parts were cut in two directions after the loading (see **Figure 17**). The expected cracks from the tip of anchor plate could not be seen clearly for G-S-900 and other specimens under single axis loading, except N-S-700 that failed rapidly in

**Figure 14.** The development of relative displacement at the top of tower (G-S and G-D).

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**Figure 15.** The development of relative displacement at the top of tower (N-S and N-D).

**Figure 16.** Vertical strain inside concrete for G-S-900 and G-D-900.

**Figure 14.** The development of relative displacement at the top of tower (G-S and G-D).

accelerated the fatigue damage and led shorter fatigue life.

**Case Acc.** 

**amplitude (gal)**

N-D-500 500 X or Y X 70,050

114 Stability Control and Reliable Performance of Wind Turbines

N-D-400 400 X or Y X 51,300

G-D-900 700–900 X or Y X 13,500

**Table 2.** The conditions of all specimens.

The vertical strains inside concrete measured by embedded strain gauges were shown in **Figure 16**. The vibrating direction of G-S-900 is always along X axis. Thus, the amplitude of IS1 and IS5 located in the orthogonal direction was significantly smaller than others. The strains measured in G-D-900 vibrated in both directions demonstrated relatively larger amplitude than G-S-900 in all locations. This suggested that the cracking in wider area possibly

**Direction Cycle Fail/survive Gypsum &** 

N-S-700 700 X 1000 Fail — 27.9 N-S-500 500 X 188,900 Fail\* — 27.9

Y 64,950

Y 51,300

Y 13,220

N-S-400 400 X 102,300 Survive — 27.9

G-S-900 700–900 X 51,000 Fail G, T 27.6

G-S-600 600 X 201,300 Survive G 27.6 G-S-900C 900 X 79,800 Fail G, T 27.6 G-D-EQ Seismic wave X and Y 5minx10 Survive G, T 27.6

**torque control**

Fail — 27.9

Survive — 27.9

Fail G, T 27.6

**Compressive stress of concrete (Mpa)**

In order to observe internal cracks, concrete parts were cut in two directions after the loading (see **Figure 17**). The expected cracks from the tip of anchor plate could not be seen clearly for G-S-900 and other specimens under single axis loading, except N-S-700 that failed rapidly in 1000 cycles. Alternatively, unexpected crack at the connection of pedestal and footing was found as pointed by arrows in **Figure 17**. In contrast, obvious cracks were found in both cross sections for G-D-900. On the other hand, the difference was not clear between N-D-500 and N-S-500. These cracks did not always consistent with the internal strains measured by embedded gauge. In other words, it is not easy to capture the occurrence of cracks inside concrete. It should be noted that the strain measured at the bottom of steel tower always remained less than 500 μ, even the model of tower was reused repeatedly.

#### **4.3. Evaluation of fatigue life**

The equivalent numbers of cycle were calculated based on Miner's rule [20] shown in Eq. (4) for G-S-900 and G-D-900 because amplitude of input acceleration was increased step by step for these cases.

*m*

where Neq is the equivalent number of cycle, P<sup>i</sup>

the Neq is 28,378 for G-S-900 and the Neq is 4098 for G-D-900.

**Figure 18.** Normalized amplitude of horizontal load—cycles at failure based on experiment.

ber at Pi

"G-," respectively.

**5. Conclusions**

is the load, P is the standard load, n<sup>i</sup>

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, m is the incline of material stress-number of cycle (S-N) curve for fatigue. The m was

assumed as 17 based on the S-N curve for fatigue of concrete [20]. If we assume P = 900 gal,

Furthermore, the difference of experiment conditions between "N-" series and "G-" series should be considered for evaluation. The ultimate load that can make collapse of structure at one cycle should be different in two series. The ultimate acceleration that is proportional to ultimate inertial force at the top of tower was assumed as 800 gal and 1300 gal for "N-"and

The number of cycles at each normalized amplitudes in this experiment was summarized in **Figure 18**. The approximate curve for single directional vibration is also shown in the figure. According to this figure, the fatigue life of the specimens vibrated in two directions; N-D-500

This chapter shows research of stability of supporting structure of onshore wind turbine foundations based on field measurements, finite element (FE) analysis and laboratory experiment. The vibration characteristics of tower were clarified by field measurement. Then, the damage process of reaching failure was examined by FE models. In addition, limit state of foundation was defined by fatigue limit state of concrete. The space-averaged second invariant of strain was proposed as useful index. Consequently, the stress-number of cycle (S-N) diagram derived from laboratory experiment was shown. That suggested that the fatigue life of the specimens vibrated in two directions tends to be shorter than lives under single directional vibration.

and G-D-900 tend to be shorter than lives under single directional vibration.

is the num-

117

*n*

**Figure 17.** Internal cracks observed after loading for G-S-900 and G-D-900.

**Figure 18.** Normalized amplitude of horizontal load—cycles at failure based on experiment.

where Neq is the equivalent number of cycle, P<sup>i</sup> is the load, P is the standard load, n<sup>i</sup> is the number at Pi , m is the incline of material stress-number of cycle (S-N) curve for fatigue. The m was assumed as 17 based on the S-N curve for fatigue of concrete [20]. If we assume P = 900 gal, the Neq is 28,378 for G-S-900 and the Neq is 4098 for G-D-900.

Furthermore, the difference of experiment conditions between "N-" series and "G-" series should be considered for evaluation. The ultimate load that can make collapse of structure at one cycle should be different in two series. The ultimate acceleration that is proportional to ultimate inertial force at the top of tower was assumed as 800 gal and 1300 gal for "N-"and "G-," respectively.

The number of cycles at each normalized amplitudes in this experiment was summarized in **Figure 18**. The approximate curve for single directional vibration is also shown in the figure. According to this figure, the fatigue life of the specimens vibrated in two directions; N-D-500 and G-D-900 tend to be shorter than lives under single directional vibration.

## **5. Conclusions**

**Figure 17.** Internal cracks observed after loading for G-S-900 and G-D-900.

1000 cycles. Alternatively, unexpected crack at the connection of pedestal and footing was found as pointed by arrows in **Figure 17**. In contrast, obvious cracks were found in both cross sections for G-D-900. On the other hand, the difference was not clear between N-D-500 and N-S-500. These cracks did not always consistent with the internal strains measured by embedded gauge. In other words, it is not easy to capture the occurrence of cracks inside concrete. It should be noted that the strain measured at the bottom of steel tower always remained less

The equivalent numbers of cycle were calculated based on Miner's rule [20] shown in Eq. (4) for G-S-900 and G-D-900 because amplitude of input acceleration was increased step by step

> *i*=1 *n* ( *P*\_\_*i P*) *m*

∙ *ni* (4)

than 500 μ, even the model of tower was reused repeatedly.

*Neq* = ∑

116 Stability Control and Reliable Performance of Wind Turbines

**4.3. Evaluation of fatigue life**

for these cases.

This chapter shows research of stability of supporting structure of onshore wind turbine foundations based on field measurements, finite element (FE) analysis and laboratory experiment. The vibration characteristics of tower were clarified by field measurement. Then, the damage process of reaching failure was examined by FE models. In addition, limit state of foundation was defined by fatigue limit state of concrete. The space-averaged second invariant of strain was proposed as useful index. Consequently, the stress-number of cycle (S-N) diagram derived from laboratory experiment was shown. That suggested that the fatigue life of the specimens vibrated in two directions tends to be shorter than lives under single directional vibration.
