a study of wind-resistant safety design of wind turbines tower system

The Seventh Asia-Pacific Conference on 

Wind Engineering, November 8-12, 2009, 

Taipei, Taiwan 

A STUDY OF WIND-RESISTANT SAFETY DESIGN OF WIND 

TURBINES TOWER SYSTEM 

Ching-Wen Chien 1 , Jing-Jong Jang 2 

1 Ph.D. Candidate, Department of Harbor and River Engineering, National Taiwan Ocean 

University/ E&C Engineering Corporation Keelung, Taiwan, Taiwan, 

d93520010@ntou.edu.tw 

2 Professor, Department of Harbor and River Engineering, National Taiwan Ocean 

University Keelung, Taiwan, Taiwan, jangjj@ntou.edu.tw 

ABSTRACT 

During the last decade, many wind turbine towers damage were caused by typhoons strike repeatedly. As the 

cost impact of buckling failure, this study starts to investigate the characteristics of wind turbulence and buckling 

resistance procedure is presented. The theories of the dynamic wind loading in along-wind and across-wind 

with/without the torsional force are applied to analysis the total wind force. Furthermore, the finite element 

analysis is used to obtain the across-wind force of the response. Some basic features of the analysis and the 

design of the prototype of a steel 660 KW wind turbine tower in Taiwan are presented. These results of this 

research highlight the following: First, the 660 KW wind turbines tower should be designed with natural period 

less than one second, and the gust effect factor, G, equal to 2.38. Second, at low mean wind velocities, due to the 

vortex shedding resonance with the fundamental mode of vibration, the across-wind response is higher than the 

along-wind response. Third, the turbulence intensity is observed to 25~44% in Typhoon Jangmi, and the alongwind 

response exceeds the total wind response. Fourth, the total wind force without considering torsional force 

will be underestimated 4.2%, therefore when the maximum deflection is closed to 1 %, the torsional force should 

not be ignored for the safety design. 

KEYWORDS: TORSIONAL WIND FORCE, WIND TURBINE TOWER, TOTAL WIND FORCE, 

TURBULENCE INTENSITY 

Introduction 

There are many different configurations of wind turbine generator system (WTGS) have 

been built in large amounts around the world. As the tubular tower occupies area smaller than 

the lattice steel tower, then the distance of the blade between the shaft is increased to reduce 

the aerodynamic effect (Chien and Jang, 2008a). Moreover, the tubular towers are designed as 

truncated cones with their diameter increasing towards the base in order to increase their 

strength and at the same time to save material (Stathopoulos and Baniotopoulos 2007). 

Consequently, the tubular tower has a large ratio of height to least horizontal dimension that 

makes it a particularly more slender and wind-sensitive than any other structures. On the one 

hand, from structural characteristics of the tubular tower, it is a statically-determinate 

structure, due to lack of the cumulated experience for the design in a proper way with respect 

to serviceability, strength and safety criteria. Therefore, such structures are easier failure than 

common structures. As in these areas with typhoon application of IEC-61400-1 codes or 

National standard wind codes and the fact that information on external conditions is limited 

may result in either too high cost or too high risk (Clausen and Pagalilawan, 2006). In 2003,

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan 

the supporting of self-standing system were collapsed or partially damaged by Typhoon 

Maemi in Okinawa, Japan. The authors have investigated some failures from geometric 

properties of wind turbine tower due to typhoon strike (Jang, and Chien, 2009), developed the 

procedure of Wind-Resistant Design (WRD) for the support structures subjected to windinduced 

excitation from vortex shedding and gust buffeting (Chien and Jang, 2008b). 

The study is aimed at analyzing the Typhoon Jammie speed from the Central Weather 

Bureau to compare the design basis of wind turbine towers. The authors deal with the 

classical problems of the dynamic alongwind response and the equivalent static forces, 

referred to as the gust factors, aimed at determining the maximum response for engineering 

application to wind turbine towers design. The authors have also obtained the regression 

formulas of natural period with/without turbine based on the theory of Generalized 

Coordinate System Method and FEM analysis (Chien and Jang, 2005). As the natural period 

is a good verification in the discrimination of 1sec. of natural periods to rigidity and flexible 

structure or not for a wind turbine tower than the ratio of Height/Diameter method (Chien and 

Jang, 2008b). On the other hand, the structure is usually assumed to undergo aerodynamic 

actions partly distributed along the axis of the shaft and partly concentrated in the geometrical 

center of the masses, therefore torsion load is always ignored (Solaria, 1999). As a matter of 

fact, due to the turbulence or typhoon strike, the torsion of the shaft in the acrosswind planes 

should not be ignored. However, it’s still not found the application of torsion theory at present, 

only there are torsion theories in Japan's code (AIJ 1996). In the ASCE7 code adopts the 

eccentricity e for rigid structures to consider the torsional loads (ASCE7 2005). As for more 

accurate prediction of damage due to typhoon strike, it is important to compare the alongwind 

force to total wind force with/without the torsional force. A benchmark model is based 

on the VESTA’s 660 Kw (VESTA, 1998) prototype with Finite Element analysis performed 

by applying appropriately linearly elastic laws for the analysis of across-wind response. 

Furthermore, the design of the steel tower for gravity and wind loadings is according to the 

relevant Taiwan code the results of the analyses to perform. The study includes three parts: 

(1); along-wind response analysis; (2); across-wind response with/without torsional force 

analysis; (3) develops the procedure of WRD for a wind turbine tower. 

Turbulence Intensity and Typhoon Speed 

The turbulence intensity, also often referred to as turbulence level, is defined as 

I 

u 

u ' ( t) 

= (1) 

U 

where 

u ' ( t ) is the root-mean-square of the turbulent velocity fluctuations and U is the mean 

velocity. If the turbulent energy, k, is known 

u ' ( t) 

can be computed as 

1 2 

= + + = (2) 

3 3 

' 2 2 2 

u ( t) ( u( x, t) u( y, t) u( z, t) ) k 

U can be computed from the three mean velocity components U t ( x ) , U t ( y ) and U t ( z ) as 

U U x U y U z 

2 2 2 

= t ( ) + t ( ) + t ( ) 

(3) 

2


Turbulence is the factor which describes short term wind variation/fluctuations. WTGS 

designed according to the special safety class as defined in Table 1. As defined in the IEC 

61400-1 wind turbine design/safety standard, the largest wind speed to be considered is called 

“V e50 ” which is the maximum gust over a 50-year return period for a 3-second averaging time. 

A designates the category for higher turbulence characteristics, B designates the category for 

lower turbulence characteristics, I 15 is the characteristic value of the turbulence intensity at 15 

m/s, a is the slope parameter (IEC-61400-1, 1998). Generally, a WTGS is supposed to be 

designed to stop rotation for their blades , for example, changing pitch of blades in order to 

reduce wind loads on a tower and blades in case of high wind more than 25 m/s. The 

maximum allowable extreme wind speeds are listed in Table 2. Fig. 1(a) shows the path of 

Typhoon Jangmi strike in Taiwan. Fig. 1(b) shows the maximum mean wind speed, the 

maximum instantaneous wind speed were observed 32m/s, 46.1m/s respectively, in the 

Wuchi station by the Central Weather Bureau. The turbulence intensity is equal to 44% by Eq. 

(1) as shown in Table 3. Typically the turbulence intensity is between 5% and 20 %. 

Therefore, due to this circumstance the tower is prone to buckling failure. On the other hand, 

the wind speed of average was 56m/s on the last ten minutes, the maximum instantaneous 

wind speed during the Typhoon Jangmi was investigated exceed 70 m/s by the Taiwan Power 

Company (TPC). Fig. 1(c) shows the broken tower of wind turbines by Typhoon Jangmi 

strike in Taiwan. Those exceeded IEC’s 50 year design wind speed U 50 for class I wind 

turbine. Therefore it cannot be expected that designs made according to the IEC 61400-1 will 

ensure sufficient structural reliability of turbines in regions of the world, where typhoons 

occur. 

Buckling Resistance 

As the thickness is less than the radius of the shaft, the wind turbine tower should be 

considered as a thin wall structure. However, the formula of moment in inertia is derived 

3 

as I = π R t ; R is the average of radius; t is the thickness of the shaft. Usually, the 

manufactory and owner increased the radius R than the thickness of the shaft to obtain 

stronger the stiffness of the structure under the same total amount of steel weight. Due to this 

circumstance the tower is prone to local buckling. Concerning the fatigue limit state, the 

width to their thickness ratio should be satisfied the following of buckling resistance 

requirements. First, geometric design should be based on the diameter-thickness (D/t) ratio 

less than 125 to avoid the local buckling of wind turbine tower. Second, if the ratio of D/t is 

large than 125, moreover by Eq. (4) to control the buckling failure as follow (Schilling, 1965) 

50 

25 

0 5 10 15 20 25 

Max. Ave. Wind Speed 

40 

Max. Gust Wind Speed 

20 

Wind Speed(m/s) 

30 

15 

10 

20 

5 

10 

0 5 10 15 20 25 

Time (HR.) 

0 

Fig. 1: (a) Typhoon Jangmi (b) Wind Speeds in Wuchi Station (c) Broken WTGS 

3


Table 1: Basic Parameters for WTGS Classes 

WTGS Class I II III IV 

U ref (m/s) 50 42.5 37.5 30 

U ave (m/s) 10 8.5 7.5 6 

A I 15 (-) 

0.18 0.18 0.18 0.18 

a (-) 2 2 2 2 

B I 15 (-) 

0.16 0.16 0.16 0.16 

a (-) 3 3 3 3 

Table 2: Wind Speed According to IEC/GL 

Max. 10 Max. 3 sec. Gust Stop wind speed 

min. mean mean max. acc. 

V47 – 660 kW 50 m/s 70 m/s 10 m/s 2 25 m/s 

Table 3: Wind Turbulence Intensity 

Stations Max. 10 

min. mean 

Max. 3 sec. 

mean 

Gust 

max. acc. 

turbulence 

intensity 

Wuchi station 32 m/s 46 m/s 14 m/s 2 44% 

TPC station 56 m/s 70 m/s 14 m/s 2 25% 

D 

< 0.1259⋅ E s , (4) 

t F y 

D is diameter of shaft, E s is modulus of elasticity, and F y is yield stress. Third, the ratio of 

lateral deflection-height should be controlled less than 1% to avoid the buckling failure. 

Analysis of Along-Wind Load 

Davenport derives a factor, which the R.M.S. (Root Mean Square) component would be 

exceeded with a probability of 50%. After further refinement Davenport recommended the 

following expression for peak factor 

0.577 

g = 2× ln(3600 n) 

+ 

, (5) 

2× 

ln(3600 n) 

where n is the natural frequency. Thus, maximum instantaneous wind velocity is then 

Umax = U + g ⋅ σ ( U ) , (6) 

The along-wind response is usually obtained by the gust effect factor method. This approach 

is based on the maximum response that can be represented by the corresponding mean 

response multiply a gust effect factor. The F 

d 

total drag force at any point for a wind turbine 

tower is given by Eq. (7), and the mean displacement of mean along-wind velocity at the 

average height point is obtained by Eq. (8) 

1 

H 2 

Fd = ⋅Cd ⋅ ρa 

⋅ ∫ V( z ) ⋅ B( z ) ⋅φ( z )dz 

0 

2 

, (7) 

Fd 

X ave = ; K 1 =m 1 (2πn 1 ) 2 , 

K 

(8) 

1 

4


On the one hand, the total along-wind displacement 

X 

a(max) 

( H ) can be divided into two parts: 

First, the average displacement x (H ) is caused by average wind speed. Second, the turbulent 

displacement x 

a(max) 

( H )is caused by the turbulence wind speed. Hence, the total along-wind 

displacement of a structure subjected to along-wind force is given by Eq. (9), the gust effect 

factor G(z) is refered to Eqs. (8), (9) to obtain the Eq. (10): 

where 

X ( H ) = x( H ) + [( g σ ) + ( g σ ) ] = x( H ) + x 

a(max) 

( H ), (9) 

2 2 1/ 

2 

a(max) B Bx R Rx 

x a (max) 

g = 2× ln( 3600n ) + 

R 

G( z) = 1+ x( z) 

, (10) 

0. 

577 

2× 

ln( 3600n ) 

is the resonant peak factor, gB 

≅ 3.5 . σ 

BX is 

the R.M.S. value of the non-resonant displacement; σ 

RX is the R.M.S. value of the resonant 

displacement. 

Analysis of Across-Aind Load and Torsional Wind Load 

Due to vortex shedding, when the mean design wind speed (U H ) is greater than the critical 

across-wind speed (U cr ) that always falls in the resonance area. U H (m/sec) can be calculated 

by local wind code. When the wind velocity is located in vortex resonance area, design the 

wind turbine tower should be considered in the across-wind loads. Vortex resonance area is 

between 0.50 and 1.30U H as defined here. Lock-in effect is subjected to the vortex shedding 

frequency and structure natural frequency nearly; within certain range of wind velocities. 

With the round cylinder as an example when there is turbulence, it’s S t =0.2 and the U cr value 

can be obtained by Eq. (11): 

5D 

Ucr 

= , (11) 

T 

T j is the natural period of structure (j=1, 2….n). However, vortex shedding although can 

"lock-in" and continue as the velocity increases or decreases slightly, if the velocity changes 

by more than 30%, the vortex shedding will stop, and across-wind load is no need to consider 

at outside this range. The across-wind force is obtained by Eq. (12) (Tsai, et al., 2006). 

2 Z 

Wrz = Ucr ⋅ ⋅Cr 

⋅ A , (12) 

h 

W rz is the across-wind force at height z; C r is the wind force of vortex resonance; A is the 

project area of the structure. The tip displacement of the wind turbine tower in across-wind 

force is obtained by Eq. (13) (Wang, 1995) 

2 

Ucr 

⋅ D ⋅φ( Z) 

X 

c 

= 

, (13) 

2 

8000⋅ξ 

⋅m⋅ω 

where ξ is the damping ratio of structure (=0.02); φ ( Z ) is a function of viration mode, the 

critical wind speeds for torsional wind force is obtained by Eq. (14) (Tsai, et al., 2006) form 

the torsion theories in Japan's code (AIJ 1996). 

j 

' Z 1 

Tz 

1.8 ( ) 

T Z T 

1 

TR 

M = ⋅ q h ⋅ C ⋅ A ⋅ B ⋅ g R 

H 

⋅ ⋅ + β 

, (14) 

5


q( h ) is a velocity pressure, C is the parameter of torsional wind force, 

' 

T 

A 

Z 

is the projected area at 

R is the coefficient of 

height Z, g 

T 

is the coefficient of peak factor of torsional wind force, 

TR 

resonance factor of torsional wind force, β is the damping ratio, B is the projected breadth. 

The total dynamic wind load effect could be calculated as follow: 

X = X + X , (15) 

2 2 

t(max) 

a max c 

X t is total dynamic wind load; X a(max) and X c are effect of along-wind load and across-wind 

load, respectively. 

Case Study and Procedure of Wind Resistat Design 

A benchmark of steel tubular tower is based on a VEST’s V47-660kw to set up where is 

located in the Wu-chi as shown in Figs. 2(a)~(c). Design of tower is made by steel structure 

with the piling foundation. As simplification calculation, the stepped cylindrical section is 

transformed into a uniform thickness. Therefore, the analysis is adopted the parameters of 2/3 

height for the wind turbine tower, the diameter is equal to 3(2.45) m, the thickness of shaft is 

15 (10) mm; the height is about 50m. The procedure of WRD as follow: First Step, the 

natural period of the benchmark is control in first mode by eignvalue analysis as shown in 

Table 4. As the benchmark is a symmetrical structure, their vibration periods are close to each 

other, the first two fundamental modes of the model are dominated by lateral translations. 

Second Step, the design wind speed is used by the Typhoon Jangmi (U 10 (C) =56m/s), the gust 

factor, along-wind displacement (X a ) is calculated by Eqs. (8)~(10) as shown in Table 5. 

Third Step, the analysis of across-wind resonance calculated by Eq. (11), one shall consider 

the across-wind force in the resonance area as shown in Table 4, and the results of acrosswind 

force with/without torsional force is calculated by Eqs. (12), (14) as shown in Table 6. 

Fourth step, the across-wind force is applied into SAP2000 model to obtain the across-wind 

displacement as shown in Fig. 3. As the maximum deflection is closed to 1 %, the ration is 

just satisfied to the buckling resistance requirements as shown in Table 5. Fifth step, Fig. 4 

compares the base moment of total wind force under U cr =15.6 (m/sec) with the only 

consideration of the alongwind force under Typhoon Jangmi speed (U 10 =56m/s) or Taiwan 

wind code (U 10 =32.5m/s) for design the wind turbine tower. 

Figure 2: (a) Prototype of 660KW (b) Benchmark Model (c) Prototype (1 st mode shape) 

6


Table 4 Analysis of Across-Wind Resonance (U h =U 10 (C)=56m/s) 

Numbers of 

mode 

D 

(m) 

T j 

(sec) 

U cr 

(m/sec) 

U h 

(m/sec) 

Across-wind 

resonance 

1 2.45, 3 1.039, 0.984 12, 15 71 

U h > U cr 

Lock-in 

2 2.45, 3 0.169, 0.223 73 , 67 71 U cr >20m/s 

Table 5 Analysis of Displacement and Gust Effect Factor (U 10 (C)=56m/s, D=3m, t=15mm) 

X ave X a(max) X c X t(max) Total 

Deflection 

G(Z) 

By equation Eq. (8) Eq. (9) Sap2000 Eq. (15) Eq. (15)/H Eq. (10) 

Unit (cm) (cm) (cm) (cm) % 

Benchmark 3.25 4.47 50.4 50.59 1 2.38 

60 

3000 

50.4 

D=3m , t=15mm 

Displacment(mm) 

40 

22.95 

36.38 

Base moment (t-m) 

2000 

Ma=Bending Moment(T-M) 

M a (U 10 =56 m/s) 

M a (U 10 =32.5m/s) 

M a (w/o Torsion force) 

M a (with Torsion force) 

20 

1000 

11.29 

3.11 

0 

0 

0 

0 10 20 30 40 50 

Height (m) 

Figure 3: Sap2000 Analysis of 

Across-Wind Response (U cr =15m/s) 

0 10 20 30 40 50 

Height(m) 

Figure 4: Comparison of Basemoment 

Conclusions 

It’s recommended that the 660 KW wind turbines tower should be designed with natural 

period less than one second, and the gust effect factor, G, equal to 2.38. Presently, the G value 

is greater than 1.77 for rigid structures according to the Taiwan building code 2006. As for 

results, Typhoon Jangmi speed (U 10 =56m/s) in alongwind force is large than the total wind 

force (U cr =15.6m/s, U 10 =11.37m/s in the alongwind and crosswind) of WTGS. However, due 

to the large shaft flexibility, the torsional loading in the acrosswind response should not be 

ignored. It’s underestimated 4.2% in the total wind force without torsional wind force for 

safety design. When the maximum deflection is closed to 1 %, the torsional force should not 

be ignored for the stiffeners (stiffening rings, door tingers, ribs and frame) and the flanges to 

check against local buckling. 

7


Table 6 Analysis of total wind force with/without torsional wind force for wind-power tower 

(U cr =15m/s, U 10 =11m/s, D=3m, t=15mm) 

Height 

Dynamic 

alongwind 

force 


force w/o 

torsion force 

Torsional 

force 


force with 

torsional force 

Total wind force 

w/o torsional 

force 

Total wind 

force with 

torsional force 

(m) (kg) (kg) (kg) (kg) (kg) (kg) 

50.00 325.97 14048.94 618.37 14667.31 14053 14671 

40.00 609.72 22478.30 989.40 23467.70 22487 23476 

30.00 559.31 16858.72 742.05 17600.77 16868 17610 

20.00 495.25 11239.15 494.70 11733.85 11250 11744 

10.00 382.15 5338.60 234.98 5573.58 5352 5587 

1.00 90.72 252.88 13.60 266.49 269 282 

total 2463.12 70216.58 3093.10 73309.69 70278.26 73368.67 

Acknowledgements 

The authors would like to thank the Taiwan Natural Science Council for finance support this 

research (NSC-98-2221-E-019-016). 

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8

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