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Wind, terrain and structural damping characteristics under tropical cyclone conditions
Engineering Structures 21 (1999) 1006–1014 www.elsevier.com/locate/engstruct
Wind, terrain and structural damping characteristics under tropical cyclone conditions J. Shanmugasundaram *, P. Harikrishna, S. Gomathinayagam, N. Lakshmanan Structural Engineering Research Centre, CSIR Campus, TTTI, Chennai 600 113, India Received 12 September 1997; received in revised form 13 March 1998; accepted 16 March 1998
Abstract A large number of buildings and structures, including some well-engineered structures, have been reported to be damaged during tropical cyclones. This stresses the need to study the various characteristics of tropical cyclone winds. A full-scale field experiment on a 52 m tall steel lattice tower has been undertaken to study the wind, terrain and structural characteristics under normal and tropical cyclone wind conditions. Data collected from the instrumented tower during tropical cyclones in June and December 1996 were analyzed and compared with the characteristics obtained during normal wind conditions. The measured wind and terrain characteristics have been compared with the results reported in the literature. The measured structural characteristics, such as the fundamental frequency and damping ratio of the structure, have been compared with the analytical values and those reported in the literature. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Field measurements; Terrain characteristics; Tropical cyclone winds
1. Introduction The annual average number of tropical cyclones occurring all over the world is around 92. The Indian ocean is one of the six major tropical cyclone prone regions of the world. Every year, four to five tropical cyclones occur in the Indian ocean, 80% of which cross the east coast of India. The maximum wind speed during such cyclones can be as high as 260 kmph (163 mph). The characteristics of tropical cyclone winds are quite different from those of well behaved normal winds. At present, research on the effects of tropical cyclone winds on buildings and other tower-like structures, and mitigation of their effects is far less than required. This is due to the complexities involved in assessing (i) the intensity and the severity of wind speeds in tropical cyclones, (ii) the strength of materials and types of materials used, (iii) the quality and method of construction adopted and (iv) the dynamic properties of the structure and the wind. The safe and economical design of buildings and other structures against wind loading requires knowledge on many inter-linked wind and structural
characteristics, such as wind speed and direction, variation of wind speed with height, terrain roughness, turbulence intensity, and the frequency and damping of the structure. As already mentioned, since there are many uncertainties involved in these areas, studies based on theoretical models alone will not be sufficient to understand the realistic nature of tropical cyclone wind forces and to predict the behaviour of structures subjected to them. The available full-scale field experimental data obtained during cyclones is scarce. Realising the importance of the problem, the Structural Engineering Research Centre, Chennai, India, situated about 3 km away from the Bay of Bengal coast, has erected a 52 m tall narrow-base steel lattice tower and instrumented it with anemometers, accelerometers and strain gauges to study the above mentioned problems. Normal and extreme wind data from these sensors were collected during different seasons of the year. In this paper wind speeds and response data obtained during tropical cyclones in June and December 1996 are analyzed and discussed. The structural characteristics evaluated from measurements are also discussed.
* Corresponding author. 0141-0296/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 1 - 0 2 9 6 ( 9 8 ) 0 0 0 5 3 - 4
J. Shanmugasundaram et al. / Engineering Structures 21 (1999) 1006–1014
2. Test structure A 52 m tall narrow-base steel lattice tower on which the measurements were made is surrounded by a few scattered buildings of varying heights (two to five stories of height varying from 6 to 15 m) within a distance of 1–3 km, on the northern, southern and western side of the tower. The sea is 3 km away on the south eastern side. From the visual observation, the terrain under study may be described as between category 2 and 3, in terms of the IS:875 (Part 3)—1987 [1] classification. The tower is a four legged lattice steel structure made of rolled steel galvanized angle sections and connected with bolts. The total height of the tower above the ground level is 52.1 m. The tower is square in plan with a base width of 2.84 m and a top width of 1.8 m. The configuration of the tower is shown in Fig. 1.
3. Instrumentation In order to compute the wind, terrain and structural characteristics, the tower was instrumented with anemometers at four levels, 10 m, 16.4 m, 29.6 m and 51.6 m above ground level, with accelerometers in E & W and N & S directions at 10 m and 51.6 m levels. The anemometers were fixed at the free end of a 2 m long boom assembly, which was bolted to the tower leg. UVW (Gill) propeller type anemometers were used at all levels, except at 29.6 m, where a sonic anemometer was used. The accelerometers used were of piezo electric type, manufactured by Vibrometer Corporation, USA (Model C510M101) with capacity to measure the acceleration in the range of ⫾ 0.25 g. All the sensors are connected to a computer through shielded signal cables. Many sets of wind data were collected during normal winds. Each record had a length of 15 min and a sampling rate of 20 Hz. The wind, terrain and structural characteristics were evaluated for normal winds from this data [2,3].
4. Collection of tropical cyclone wind data Two severe tropical cyclones formed over the Bay of Bengal and crossed the Indian coast during June 1996 and December 1996. The tracks are shown in Fig. 2. Both the cyclones formed around longitude 86°E and latitude 11°N. The June 1996 cyclone was originally heading close to Chennai and then moved off in a northerly direction, finally crossing the coast near Vijayanagaram (longitude 83.25°E and latitude 17.5°N). However the December cyclone, after changing its track many times as shown in Fig. 2, crossed the coast near Chennai (longitude 80.25°E and latitude 12.6°N). The wind speed components U, V, W in three orthog-
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onal directions at four levels, acceleration in two perpendicular directions at two levels, were collected during both cyclones. During June 1996, wind data was recorded between 11.00 a.m. on 13/6/1996 and 3.00 p.m. on 14/6/1996. The maximum and 15 min mean wind speeds recorded at the 52.1 m level were 24.08 m/s and 11.89 m/s, respectively. During the period of measurement, the wind direction changed from 310° to 250° with respect to North. During December 1996, wind data was recorded from 4.00 p.m. to 8.45 p.m. on 6/12/1996. The maximum and 15 min mean wind speeds recorded at the 52.1 m level were 30.31 m/s and 16.76 m/s, respectively. The wind direction during the measurement changed from 300° (North west) during 4.00 to 6.00 p.m., to 0° (North) during 6.00 to 6.30 p.m., and just after land fall to 76° (North east) from 6.30 to 8.45 p.m. Fig. 3 shows the mean, 3 s gust and maximum wind speed during each 15 min period for the two cyclones.
5. Wind and terrain characteristics 5.1. Power law coefficient The characteristics of tropical cyclone winds are quite different from normal winds. Every cyclone has its own wind profile and turbulence characteristics. Fig. 4 shows the variation of power law coefficient, ␣, with respect to mean wind speed at 10 m height during normal and tropical cyclone wind conditions. It can be seen that the power law coefficient for tropical cyclone winds is more than normal winds. The empirical equation (␣ ⫽ 1/(ln ¯ )), suggested by Ishizaki [4] to evaluate power law U coefficient for extreme wind, is also plotted in Fig. 4. The observed values are much lower than those predicted by Ishizaki [4] for the recorded wind speeds, and are around a constant value of [1/(ln 50)]. The power law coefficient referenced to the wind velocity at 10 m height as per the equation of Ishizaki is 0.26 for a wind speed of 50 m/s, which is taken as the basic wind speed in cyclone prone regions. Davenport [5] recommended a power law coefficient of 0.28 for normal winds, for terrain category 3. The values of ␣ obtained for this particular site are close to open terrain condition (terrain category 2) during normal winds, but the value is close to semi-urban terrain condition (terrain category 3) during tropical cyclone winds. Similar findings have been recorded by others [6] and are embodied in the Australian Wind Code [7]. 5.2. Turbulence intensity The turbulence intensity depends upon the surface roughness, but the surface roughness is not the only fac-
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Fig. 1.
Tower configuration with instrumentation.
tor to produce turbulence during tropical cyclones. Turbulence is an inherent property of tropical cyclone winds due to in-built convective fluid accelerations and the coriolis effect due to earth’s rotation and gravitational forces. Generally on any type of terrain, for normal winds, the value of turbulence intensity decreases with height. This trend was observed at the test site during normal winds, but the profile of the turbulence intensity was steeper during tropical cyclone wind conditions. Fig. 5 shows the recorded turbulence intensity as a function of height for normal and tropical cyclone winds for the test site along with Australian Code [7] values for different terrain categories. It is to be noted that IS-875 (Part
3)—1987 [1] does not explicitly give the value of turbulence intensity for different heights and for different terrain categories. However, the Indian and Australian Codes have used the same value of terrain roughness length for different terrain categories. It is seen from Fig. 5 that the values of turbulence intensities for this test site are very close to the values given for terrain category 3 during normal winds, but the value tends to increase during tropical cyclone winds. It can be seen that there is significantly higher turbulence intensities all along the height under tropical cyclone winds than under normal winds. The measured turbulence intensities under tropical cyclone winds tend to
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Fig. 2.
Fig. 3.
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Tracks of June and December 1996 cyclones.
Mean and maximum wind speed during cyclones.
shift from category 3 to category 4 terrains of the Australian code. For the design of structures and structural components which are wind sensitive, this implies that higher fluctuating wind loads prevail over greater heights in tropical cyclones than in normal winds. For the atmospheric boundary layer, this is an indication of increased shear layer thickness in the boundary layer wind. The values of turbulence intensity at the 10 m level during normal and extreme wind conditions are shown
¯ ), as an empirical in Fig. 6. Ishizaki [4] suggested k/(ln U expression to evaluate turbulence intensity, where k ⫽ 0.4, 0.6 or 0.8. He suggested a value of 0.6 for the design, to resist extreme wind, which is shown in Fig. 6. However, the measured turbulence intensity for these two cyclones was always higher than the values suggested by Ishizaki. Based on these experimental results, ¯ ) to evaluate it is suggested the expression of 0.8/(ln U turbulence intensity for tropical cyclone winds. This is
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Fig. 4.
Variation of power law coefficient with mean wind speed.
Fig. 5.
Variation of turbulence intensity along the height.
Fig. 6. Comparison of turbulence intensity values with literature values.
Fig. 7.
Comparison of turbulence length scale values.
shown as an upper bound curve in Fig. 6. The measured turbulence intensities in the lateral direction were observed to be about 75% of the measured turbulence intensities in the along wind direction. The measured turbulence intensities in the vertical direction were observed to have a wide scatter in the range of 35–60% of the measured turbulence intensities in the along wind direction.
at different levels using Taylor’s hypothesis [8]. Fig. 7 shows the measured values of turbulence length scales during normal and tropical cyclone winds, and comparison with the empirical expression given by ESDU [9]. It is seen from this figure that the turbulence length scales in the direction of wind are found to be of the same order during normal and extreme winds, and comparable with the suggested values of ESDU.
5.3. Turbulence length scale
5.4. Exponential decay coefficient
The turbulence length scales, which indicate the gust size of wind in the direction of the wind, were evaluated
The exponential decay coefficient, Cz, is considered to be independent of surface roughness in practice, but
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generally this value is higher for rough surface conditions than for smooth surface. The Indian code on wind loading recommends a value of 12 for Cz. This value is assumed to be constant irrespective of the type of terrain and wind speed. From the experimental results, the Cz values were evaluated from the cross spectral values using an exponential fit. It is found that the average value of Cz is 7.5 for mean wind speed of 13.5 m/s, 6.31 for mean wind speed of 10.22 m/s and 4.1 for mean wind speed of 7.36 m/s at 51.6 m level. Fig. 8 shows the Cz values for all the measured data during normal and tropical cyclone wind conditions. From this figure it is found that the value of Cz increases with increase in mean wind speed [10,11]. The best-fit value for Cz can be taken as [3.4*ln(V10) ⫹ 0.3] for structures lying in the test site type of terrain. 5.5. Wind speed spectra Fig. 9(a) and (b) show the plot of non-dimensional power spectral densities for the June and December 1996 cyclones. The spectral plot for the December 1996 cyclone from measured data at 29.6 m height matches very well with the Davenport spectrum. The spectrum of the June 1996 cyclone contains significant energy in the high frequency range. Assuming that the total variance can be written as
2T ⫽ 2N ⫹ 2H
(1)
where 2T ⫽ total variance during June cyclone, 2N ⫽ variance for normal conditions, and 2H ⫽ additional high frequency turbulence; the value of H works out to ¯ ), which matches very well with the square root (0.13 U of area under the spectral plot in the high frequency
Fig. 8.
Variation of Cz values with mean wind speed.
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region. A number of factors, including geographical features, meteorological parameters, and sudden change in the direction of the tropical cyclone track observed, could have contributed to the observed high frequencies. In earlier investigations where attempts had been made to measure the tropical cyclone wind characteristics, the instrument response (sensitivity and sampling speed of the measuring instruments)/response time (time constants in the case of anemometers and sampling rates) were limiting factors [12]. In the present investigation the ultrasonic three dimensional anemometer is capable of measuring digital data at 21 Hz, and this possibly explains the tail end high frequency region obtained in the spectral plot. Since Fig. 9(a) shows a flat section just below the Nyquist frequency ( ⫽ 10 Hz), it is possible that there is significant energy beyond 10 Hz also, and a certain level of ‘aliasing’ is not ruled out. Possibly, in future experiments it may be necessary to find ways and means of improving the sampling rate even beyond 21 Hz, as was done in this experiment. The power spectral densities given by Davenport, data measured by Tamura et al. [12] during Typhoon Mireille, and June 1996 cyclone data presented by the authors, are compared in Fig. 9(c). The deviation from Davenport’s spectrum for both the cyclones are apparent. While increase in spectral energy was observed for ¯ ) in the range of 1.0 and 10.0 in the case values (nLx/U study presented by Tamura et al. [12], the spectral density in the case under discussion, namely June 1996 cyclone, is higher between 10.0 and 100.0. These observations make it very clear that more attention is needed to obtain tropical cyclone wind data based on field measurements so that analytical expressions can be developed. It is also interesting to note that the energy ¯ ) between level is nearly constant for values of (nLx/U 10.0 and 100.0, with the parameter (n S(n)/variance) being constant at 0.025. This has significant implication in structural design. The nearly constant trend seen in the tail end of the spectrum (possibly including aliasing effects) might decay beyond 10 Hz, but the measured data was limited to 10 Hz. This is governed by the sampling rate of 20 Hz. Many of the codes of practice normally recommend dynamic analysis for structures having natural frequency less than 1.0 Hz. This roughly matches ¯ ) values of around 10. As the energy in the with (nLx/U ¯ ) is between 10 and high frequency range where (nLx/U 100 remains nearly constant during June 1996 cyclone, even structures which were hitherto considered as rigid become dynamically sensitive. In fact it has been hard to explain some of the failures of microwave towers [13,14] and transmission towers during tropical cyclones as the wind speeds observed were far lower than those that may lead to the collapse of these towers. In addition, local transverse panel resonance is a distinct possibility leading to progressive collapse of the structure. Hence, for suitable design procedures, the spectrum of turbu-
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Fig. 9. Power spectra of wind speed. (a) June 1996 cyclone (including possible aliasing). (b) December 1996 cyclone. (c) Comparison with literature.
lence needs modification. This can only be done based on field studies. Even low rise industrial structures whose natural frequency often lie in the range of 3–6 Hz have been observed to be damaged under tropical cyclone winds [15,16]. This could be one of the reasons why sheeting on industrial roofs fail in fatigue [17,18] due to high stress range with lower number of cycles at local resonant vibrations. This, however, needs further investigation.
6. Structural characteristics 6.1. Fundamental frequency From the auto-spectral values of the measured accelerations of the structure, the dominant peak has been identified, which is the natural frequency of the structure. The acceleration spectrum for a typical run is shown in Fig. 10. The average value of the measured fundamental fre-
J. Shanmugasundaram et al. / Engineering Structures 21 (1999) 1006–1014
Fig. 10.
Acceleration spectrum.
quency is 1.17 Hz. The theoretically evaluated natural frequencies of the lattice tower using three dimensional modelling of the structure were 1.18, 5.03, 6.0 and 9.04 Hz. The fundamental natural frequency of 1.18 Hz agrees very well with the measured value of 1.17 Hz. The measured auto spectrum had significant contribution at higher modes as well, under cyclone winds even upto 10 Hz, clearly indicating presence of significant energy in cyclone wind spectrum between 1 Hz and 10 Hz. White band noise or aliasing effect in the wind spectrum cannot lead to the structural response shown in Fig. 10. This further strengthens the presence of significant energy in turbulent cyclone wind even beyond 1 Hz. The figure also shows a local mode vibration at 9.3 Hz, which is due to flapping of the anemometer support assembly, where the accelerometer was mounted. 6.2. Damping ratio The damping ratio of the structure in the fundamental mode was evaluated at different wind speeds. In the present study, since the acceleration response spectrum was found to be narrow banded at the fundamental frequency of the structure, the half power method was used to evaluate the damping ratio. The average value of the damping ratio was found to be 1.60% during normal winds. This value increased to 1.72% during the June 1996 cyclone and to 2.55% during the December 1996 cyclone. The increase in mean wind speed increases the overall damping ratio due to the increase in aerodynamic damping [19] and modal damping. Fig. 11 shows that recorded values of damping ratio with mean wind speed, had a wide scatter, possibly due to directional fluctuations.
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Fig. 11. Variation of damping ratio with mean wind speed.
7. Conclusions The measured wind, terrain and structural characteristics during tropical cyclone winds, presented in this paper have significance for the design of structures in tropical cyclone prone regions. The results of this paper are summarised as follows: 1. The measured power law coefficients are found to be higher than normal winds during tropical cyclone wind conditions, leading to a pseudo rougher terrain at the given site. 2. The power law coefficient during normal and tropical cyclone winds decreases with increase in mean wind speed. 3. A decrease in turbulence intensity with increase in height was also observed. However the turbulence intensity is found to be higher for tropical cyclone winds than normal winds, particularly at lower levels, resulting in a steeper slope of the turbulence intensity profile. This means that the risk of fatigue failure is increased in tropical cyclone winds. 4. The empirical expression suggested by Ishizaki to evaluate turbulence intensity should be modified to ¯ ). 0.8/(ln U 5. The turbulence length scales in the direction of wind is found to be of the same order during normal and tropical cyclone wind conditions and is comparable with the values suggested in ESDU. 6. The value of Cz was found to increase with increase in wind speed at low wind speeds as against the constant value given by the IS Code, which is much higher than the measured values. 7. A study of the power spectral density of tropical cyclone winds indicates that gust energy is available at
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high frequencies beyond 1 Hz. The damage to many of the well designed lattice towers during cyclones may be explained by the above observation. It is essential to develop a modified spectrum of tropical cyclone winds for purposes of design based on field experiments. 8. The measured fundamental frequency values are very close to the theoretically evaluated value. Under tropical cyclone winds, participation of higher modes in the acceleration response of the structure is observed, while only fundamental mode was dominant under normal wind conditions. 9. The damping ratio was found to increase with increase in wind speed.
[5]
[6]
[7]
[8]
[9]
[10] [11]
Acknowledgements The authors acknowledge, with gratitude, the encouragement and permission granted by the Director, Structural Engineering Research Centre, Chennai, for the publication of this paper. The authors would also like to thank their colleagues, Mr M. Arumugam, Mr G. Balasubramani, and Mr K. Sankaranarayanan for all their help in collection and analyses of data.
[12]
[13]
[14]
[15]
References [1] IS:875 (Part 3)—1989. Indian standard code of practice for design loads (other than earthquake) for buildings and structures, Part 3, Wind loads. New Delhi: Bureau of Indian Standards, 1989. [2] Shanmugasundaram J, Harikrishna P, Gomathinayagam S, Arumugam M., Lakshmanan N. Field measurement of wind and response data on a 52 m tall lattice tower. Research Report. Chennai: SERC, September 1995. [3] Harikrishna P, Gomathinayagam S, Shanmugasundaram J, Lakshmanan N. Wind and strain response measurements on a 52 m tall narrow base lattice tower. Research Report. Chennai: SERC, September 1996. [4] Ishizaki H. Wind profiles, turbulence intensities and gust factors
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for design in typhoon-prone regions. Journal of Wind Engineering and Industrial Aerodynamics 1983;13:55–66. Davenport AG. In: Holand I, editor. Wind structure and wind climate, safety of structures under dynamic loading, vol. 1. Trondheim, Norway: Tapir, 1978. Holmes JD, Melbourne WH, Walker GR. A commentary on the Australian standard for wind loads. Victoria, Australia: Australian Wind Engineering Society, 1990. AS:1170.2—1989. Australian standard for minimum design loads on structures, Part 2: Wind loads. North Sydney: Standards Australia, 1992. Flay RGJ, Stevenson DC. Integral length scales in strong winds below 20 m. Journal of Wind Engineering and Industrial Aerodynamics 1988;28:21–30. E.S.D.U. Characteristics of atmospheric turbulence near the ground. Engineering Science Data Unit, Nos. 74030 and 74031, London: ESDU, 1974. Simiu E, Scanlan RH. Wind effects on structures. New York: John Wiley & Sons, 1986. Solari G. Turbulence modelling for gust loading. Journal of Structural Engineering, ASCE 1987;113(7):1550–69. Yukio T, Kenji S, Kazuki H. Wind response of a tower (Typhoon observation at the Nagasaki Huis Ten Bosch Domtoren). Journal of Wind Engineering and Industrial Aerodynamics 1993;50:309–19. SERC. Investigation of the failure of a 91 m microwave tower. Consultancy Report: 327/90. Madras: Structural Engineering Research Centre, 1990. SERC. Failure investigation of Microwave Tower. Consultancy Report: CNP No. 023541. Madras: Structural Engineering Research Centre, 1997. John RD. Case studies on cyclone damages to industrial structures. Course on Wind Disaster Mitigation of Structures. Madras: Structural Engineering Research Centre, 1990. Shanmugasundaram J, Gomathinayagam S, Arunachalam S, Harikrishna P. Damage survey of buildings and strucutures due to November 1996 cyclone near Kakinada, Andhra Pradesh. Research Report. Madras: Structural Engineering Research Centre, 1997. Xu YL, Mehta KC, Reardon GF. Fatigue of metal roof cladding subject to wind loading. Part I: Fatigue-related characteristics of roof pressures. Proceedings of Ninth International Conference of Wind Engineering, New Delhi (India), 1995:1067–78. Baskaran, Bas A, Dutt OM. Evaluation of roof fasteners under dynamic wind loading. Proceedings of Ninth International Conference of Wind Engineering, New Delhi (India), 1995:1207–18. Holmes JD. Along-wind response of lattice towers—II: aerodynamic damping and deflections. Engineering Structures 1996;18(7):483–8.
Wind, terrain and structural damping characteristics under tropical cyclone conditions J. Shanmugasundaram *, P. Harikrishna, S. Gomathinayagam, N. Lakshmanan Structural Engineering Research Centre, CSIR Campus, TTTI, Chennai 600 113, India Received 12 September 1997; received in revised form 13 March 1998; accepted 16 March 1998
Abstract A large number of buildings and structures, including some well-engineered structures, have been reported to be damaged during tropical cyclones. This stresses the need to study the various characteristics of tropical cyclone winds. A full-scale field experiment on a 52 m tall steel lattice tower has been undertaken to study the wind, terrain and structural characteristics under normal and tropical cyclone wind conditions. Data collected from the instrumented tower during tropical cyclones in June and December 1996 were analyzed and compared with the characteristics obtained during normal wind conditions. The measured wind and terrain characteristics have been compared with the results reported in the literature. The measured structural characteristics, such as the fundamental frequency and damping ratio of the structure, have been compared with the analytical values and those reported in the literature. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Field measurements; Terrain characteristics; Tropical cyclone winds
1. Introduction The annual average number of tropical cyclones occurring all over the world is around 92. The Indian ocean is one of the six major tropical cyclone prone regions of the world. Every year, four to five tropical cyclones occur in the Indian ocean, 80% of which cross the east coast of India. The maximum wind speed during such cyclones can be as high as 260 kmph (163 mph). The characteristics of tropical cyclone winds are quite different from those of well behaved normal winds. At present, research on the effects of tropical cyclone winds on buildings and other tower-like structures, and mitigation of their effects is far less than required. This is due to the complexities involved in assessing (i) the intensity and the severity of wind speeds in tropical cyclones, (ii) the strength of materials and types of materials used, (iii) the quality and method of construction adopted and (iv) the dynamic properties of the structure and the wind. The safe and economical design of buildings and other structures against wind loading requires knowledge on many inter-linked wind and structural
characteristics, such as wind speed and direction, variation of wind speed with height, terrain roughness, turbulence intensity, and the frequency and damping of the structure. As already mentioned, since there are many uncertainties involved in these areas, studies based on theoretical models alone will not be sufficient to understand the realistic nature of tropical cyclone wind forces and to predict the behaviour of structures subjected to them. The available full-scale field experimental data obtained during cyclones is scarce. Realising the importance of the problem, the Structural Engineering Research Centre, Chennai, India, situated about 3 km away from the Bay of Bengal coast, has erected a 52 m tall narrow-base steel lattice tower and instrumented it with anemometers, accelerometers and strain gauges to study the above mentioned problems. Normal and extreme wind data from these sensors were collected during different seasons of the year. In this paper wind speeds and response data obtained during tropical cyclones in June and December 1996 are analyzed and discussed. The structural characteristics evaluated from measurements are also discussed.
* Corresponding author. 0141-0296/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 1 - 0 2 9 6 ( 9 8 ) 0 0 0 5 3 - 4
J. Shanmugasundaram et al. / Engineering Structures 21 (1999) 1006–1014
2. Test structure A 52 m tall narrow-base steel lattice tower on which the measurements were made is surrounded by a few scattered buildings of varying heights (two to five stories of height varying from 6 to 15 m) within a distance of 1–3 km, on the northern, southern and western side of the tower. The sea is 3 km away on the south eastern side. From the visual observation, the terrain under study may be described as between category 2 and 3, in terms of the IS:875 (Part 3)—1987 [1] classification. The tower is a four legged lattice steel structure made of rolled steel galvanized angle sections and connected with bolts. The total height of the tower above the ground level is 52.1 m. The tower is square in plan with a base width of 2.84 m and a top width of 1.8 m. The configuration of the tower is shown in Fig. 1.
3. Instrumentation In order to compute the wind, terrain and structural characteristics, the tower was instrumented with anemometers at four levels, 10 m, 16.4 m, 29.6 m and 51.6 m above ground level, with accelerometers in E & W and N & S directions at 10 m and 51.6 m levels. The anemometers were fixed at the free end of a 2 m long boom assembly, which was bolted to the tower leg. UVW (Gill) propeller type anemometers were used at all levels, except at 29.6 m, where a sonic anemometer was used. The accelerometers used were of piezo electric type, manufactured by Vibrometer Corporation, USA (Model C510M101) with capacity to measure the acceleration in the range of ⫾ 0.25 g. All the sensors are connected to a computer through shielded signal cables. Many sets of wind data were collected during normal winds. Each record had a length of 15 min and a sampling rate of 20 Hz. The wind, terrain and structural characteristics were evaluated for normal winds from this data [2,3].
4. Collection of tropical cyclone wind data Two severe tropical cyclones formed over the Bay of Bengal and crossed the Indian coast during June 1996 and December 1996. The tracks are shown in Fig. 2. Both the cyclones formed around longitude 86°E and latitude 11°N. The June 1996 cyclone was originally heading close to Chennai and then moved off in a northerly direction, finally crossing the coast near Vijayanagaram (longitude 83.25°E and latitude 17.5°N). However the December cyclone, after changing its track many times as shown in Fig. 2, crossed the coast near Chennai (longitude 80.25°E and latitude 12.6°N). The wind speed components U, V, W in three orthog-
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onal directions at four levels, acceleration in two perpendicular directions at two levels, were collected during both cyclones. During June 1996, wind data was recorded between 11.00 a.m. on 13/6/1996 and 3.00 p.m. on 14/6/1996. The maximum and 15 min mean wind speeds recorded at the 52.1 m level were 24.08 m/s and 11.89 m/s, respectively. During the period of measurement, the wind direction changed from 310° to 250° with respect to North. During December 1996, wind data was recorded from 4.00 p.m. to 8.45 p.m. on 6/12/1996. The maximum and 15 min mean wind speeds recorded at the 52.1 m level were 30.31 m/s and 16.76 m/s, respectively. The wind direction during the measurement changed from 300° (North west) during 4.00 to 6.00 p.m., to 0° (North) during 6.00 to 6.30 p.m., and just after land fall to 76° (North east) from 6.30 to 8.45 p.m. Fig. 3 shows the mean, 3 s gust and maximum wind speed during each 15 min period for the two cyclones.
5. Wind and terrain characteristics 5.1. Power law coefficient The characteristics of tropical cyclone winds are quite different from normal winds. Every cyclone has its own wind profile and turbulence characteristics. Fig. 4 shows the variation of power law coefficient, ␣, with respect to mean wind speed at 10 m height during normal and tropical cyclone wind conditions. It can be seen that the power law coefficient for tropical cyclone winds is more than normal winds. The empirical equation (␣ ⫽ 1/(ln ¯ )), suggested by Ishizaki [4] to evaluate power law U coefficient for extreme wind, is also plotted in Fig. 4. The observed values are much lower than those predicted by Ishizaki [4] for the recorded wind speeds, and are around a constant value of [1/(ln 50)]. The power law coefficient referenced to the wind velocity at 10 m height as per the equation of Ishizaki is 0.26 for a wind speed of 50 m/s, which is taken as the basic wind speed in cyclone prone regions. Davenport [5] recommended a power law coefficient of 0.28 for normal winds, for terrain category 3. The values of ␣ obtained for this particular site are close to open terrain condition (terrain category 2) during normal winds, but the value is close to semi-urban terrain condition (terrain category 3) during tropical cyclone winds. Similar findings have been recorded by others [6] and are embodied in the Australian Wind Code [7]. 5.2. Turbulence intensity The turbulence intensity depends upon the surface roughness, but the surface roughness is not the only fac-
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Fig. 1.
Tower configuration with instrumentation.
tor to produce turbulence during tropical cyclones. Turbulence is an inherent property of tropical cyclone winds due to in-built convective fluid accelerations and the coriolis effect due to earth’s rotation and gravitational forces. Generally on any type of terrain, for normal winds, the value of turbulence intensity decreases with height. This trend was observed at the test site during normal winds, but the profile of the turbulence intensity was steeper during tropical cyclone wind conditions. Fig. 5 shows the recorded turbulence intensity as a function of height for normal and tropical cyclone winds for the test site along with Australian Code [7] values for different terrain categories. It is to be noted that IS-875 (Part
3)—1987 [1] does not explicitly give the value of turbulence intensity for different heights and for different terrain categories. However, the Indian and Australian Codes have used the same value of terrain roughness length for different terrain categories. It is seen from Fig. 5 that the values of turbulence intensities for this test site are very close to the values given for terrain category 3 during normal winds, but the value tends to increase during tropical cyclone winds. It can be seen that there is significantly higher turbulence intensities all along the height under tropical cyclone winds than under normal winds. The measured turbulence intensities under tropical cyclone winds tend to
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Fig. 2.
Fig. 3.
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Tracks of June and December 1996 cyclones.
Mean and maximum wind speed during cyclones.
shift from category 3 to category 4 terrains of the Australian code. For the design of structures and structural components which are wind sensitive, this implies that higher fluctuating wind loads prevail over greater heights in tropical cyclones than in normal winds. For the atmospheric boundary layer, this is an indication of increased shear layer thickness in the boundary layer wind. The values of turbulence intensity at the 10 m level during normal and extreme wind conditions are shown
¯ ), as an empirical in Fig. 6. Ishizaki [4] suggested k/(ln U expression to evaluate turbulence intensity, where k ⫽ 0.4, 0.6 or 0.8. He suggested a value of 0.6 for the design, to resist extreme wind, which is shown in Fig. 6. However, the measured turbulence intensity for these two cyclones was always higher than the values suggested by Ishizaki. Based on these experimental results, ¯ ) to evaluate it is suggested the expression of 0.8/(ln U turbulence intensity for tropical cyclone winds. This is
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Fig. 4.
Variation of power law coefficient with mean wind speed.
Fig. 5.
Variation of turbulence intensity along the height.
Fig. 6. Comparison of turbulence intensity values with literature values.
Fig. 7.
Comparison of turbulence length scale values.
shown as an upper bound curve in Fig. 6. The measured turbulence intensities in the lateral direction were observed to be about 75% of the measured turbulence intensities in the along wind direction. The measured turbulence intensities in the vertical direction were observed to have a wide scatter in the range of 35–60% of the measured turbulence intensities in the along wind direction.
at different levels using Taylor’s hypothesis [8]. Fig. 7 shows the measured values of turbulence length scales during normal and tropical cyclone winds, and comparison with the empirical expression given by ESDU [9]. It is seen from this figure that the turbulence length scales in the direction of wind are found to be of the same order during normal and extreme winds, and comparable with the suggested values of ESDU.
5.3. Turbulence length scale
5.4. Exponential decay coefficient
The turbulence length scales, which indicate the gust size of wind in the direction of the wind, were evaluated
The exponential decay coefficient, Cz, is considered to be independent of surface roughness in practice, but
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generally this value is higher for rough surface conditions than for smooth surface. The Indian code on wind loading recommends a value of 12 for Cz. This value is assumed to be constant irrespective of the type of terrain and wind speed. From the experimental results, the Cz values were evaluated from the cross spectral values using an exponential fit. It is found that the average value of Cz is 7.5 for mean wind speed of 13.5 m/s, 6.31 for mean wind speed of 10.22 m/s and 4.1 for mean wind speed of 7.36 m/s at 51.6 m level. Fig. 8 shows the Cz values for all the measured data during normal and tropical cyclone wind conditions. From this figure it is found that the value of Cz increases with increase in mean wind speed [10,11]. The best-fit value for Cz can be taken as [3.4*ln(V10) ⫹ 0.3] for structures lying in the test site type of terrain. 5.5. Wind speed spectra Fig. 9(a) and (b) show the plot of non-dimensional power spectral densities for the June and December 1996 cyclones. The spectral plot for the December 1996 cyclone from measured data at 29.6 m height matches very well with the Davenport spectrum. The spectrum of the June 1996 cyclone contains significant energy in the high frequency range. Assuming that the total variance can be written as
2T ⫽ 2N ⫹ 2H
(1)
where 2T ⫽ total variance during June cyclone, 2N ⫽ variance for normal conditions, and 2H ⫽ additional high frequency turbulence; the value of H works out to ¯ ), which matches very well with the square root (0.13 U of area under the spectral plot in the high frequency
Fig. 8.
Variation of Cz values with mean wind speed.
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region. A number of factors, including geographical features, meteorological parameters, and sudden change in the direction of the tropical cyclone track observed, could have contributed to the observed high frequencies. In earlier investigations where attempts had been made to measure the tropical cyclone wind characteristics, the instrument response (sensitivity and sampling speed of the measuring instruments)/response time (time constants in the case of anemometers and sampling rates) were limiting factors [12]. In the present investigation the ultrasonic three dimensional anemometer is capable of measuring digital data at 21 Hz, and this possibly explains the tail end high frequency region obtained in the spectral plot. Since Fig. 9(a) shows a flat section just below the Nyquist frequency ( ⫽ 10 Hz), it is possible that there is significant energy beyond 10 Hz also, and a certain level of ‘aliasing’ is not ruled out. Possibly, in future experiments it may be necessary to find ways and means of improving the sampling rate even beyond 21 Hz, as was done in this experiment. The power spectral densities given by Davenport, data measured by Tamura et al. [12] during Typhoon Mireille, and June 1996 cyclone data presented by the authors, are compared in Fig. 9(c). The deviation from Davenport’s spectrum for both the cyclones are apparent. While increase in spectral energy was observed for ¯ ) in the range of 1.0 and 10.0 in the case values (nLx/U study presented by Tamura et al. [12], the spectral density in the case under discussion, namely June 1996 cyclone, is higher between 10.0 and 100.0. These observations make it very clear that more attention is needed to obtain tropical cyclone wind data based on field measurements so that analytical expressions can be developed. It is also interesting to note that the energy ¯ ) between level is nearly constant for values of (nLx/U 10.0 and 100.0, with the parameter (n S(n)/variance) being constant at 0.025. This has significant implication in structural design. The nearly constant trend seen in the tail end of the spectrum (possibly including aliasing effects) might decay beyond 10 Hz, but the measured data was limited to 10 Hz. This is governed by the sampling rate of 20 Hz. Many of the codes of practice normally recommend dynamic analysis for structures having natural frequency less than 1.0 Hz. This roughly matches ¯ ) values of around 10. As the energy in the with (nLx/U ¯ ) is between 10 and high frequency range where (nLx/U 100 remains nearly constant during June 1996 cyclone, even structures which were hitherto considered as rigid become dynamically sensitive. In fact it has been hard to explain some of the failures of microwave towers [13,14] and transmission towers during tropical cyclones as the wind speeds observed were far lower than those that may lead to the collapse of these towers. In addition, local transverse panel resonance is a distinct possibility leading to progressive collapse of the structure. Hence, for suitable design procedures, the spectrum of turbu-
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Fig. 9. Power spectra of wind speed. (a) June 1996 cyclone (including possible aliasing). (b) December 1996 cyclone. (c) Comparison with literature.
lence needs modification. This can only be done based on field studies. Even low rise industrial structures whose natural frequency often lie in the range of 3–6 Hz have been observed to be damaged under tropical cyclone winds [15,16]. This could be one of the reasons why sheeting on industrial roofs fail in fatigue [17,18] due to high stress range with lower number of cycles at local resonant vibrations. This, however, needs further investigation.
6. Structural characteristics 6.1. Fundamental frequency From the auto-spectral values of the measured accelerations of the structure, the dominant peak has been identified, which is the natural frequency of the structure. The acceleration spectrum for a typical run is shown in Fig. 10. The average value of the measured fundamental fre-
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Fig. 10.
Acceleration spectrum.
quency is 1.17 Hz. The theoretically evaluated natural frequencies of the lattice tower using three dimensional modelling of the structure were 1.18, 5.03, 6.0 and 9.04 Hz. The fundamental natural frequency of 1.18 Hz agrees very well with the measured value of 1.17 Hz. The measured auto spectrum had significant contribution at higher modes as well, under cyclone winds even upto 10 Hz, clearly indicating presence of significant energy in cyclone wind spectrum between 1 Hz and 10 Hz. White band noise or aliasing effect in the wind spectrum cannot lead to the structural response shown in Fig. 10. This further strengthens the presence of significant energy in turbulent cyclone wind even beyond 1 Hz. The figure also shows a local mode vibration at 9.3 Hz, which is due to flapping of the anemometer support assembly, where the accelerometer was mounted. 6.2. Damping ratio The damping ratio of the structure in the fundamental mode was evaluated at different wind speeds. In the present study, since the acceleration response spectrum was found to be narrow banded at the fundamental frequency of the structure, the half power method was used to evaluate the damping ratio. The average value of the damping ratio was found to be 1.60% during normal winds. This value increased to 1.72% during the June 1996 cyclone and to 2.55% during the December 1996 cyclone. The increase in mean wind speed increases the overall damping ratio due to the increase in aerodynamic damping [19] and modal damping. Fig. 11 shows that recorded values of damping ratio with mean wind speed, had a wide scatter, possibly due to directional fluctuations.
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Fig. 11. Variation of damping ratio with mean wind speed.
7. Conclusions The measured wind, terrain and structural characteristics during tropical cyclone winds, presented in this paper have significance for the design of structures in tropical cyclone prone regions. The results of this paper are summarised as follows: 1. The measured power law coefficients are found to be higher than normal winds during tropical cyclone wind conditions, leading to a pseudo rougher terrain at the given site. 2. The power law coefficient during normal and tropical cyclone winds decreases with increase in mean wind speed. 3. A decrease in turbulence intensity with increase in height was also observed. However the turbulence intensity is found to be higher for tropical cyclone winds than normal winds, particularly at lower levels, resulting in a steeper slope of the turbulence intensity profile. This means that the risk of fatigue failure is increased in tropical cyclone winds. 4. The empirical expression suggested by Ishizaki to evaluate turbulence intensity should be modified to ¯ ). 0.8/(ln U 5. The turbulence length scales in the direction of wind is found to be of the same order during normal and tropical cyclone wind conditions and is comparable with the values suggested in ESDU. 6. The value of Cz was found to increase with increase in wind speed at low wind speeds as against the constant value given by the IS Code, which is much higher than the measured values. 7. A study of the power spectral density of tropical cyclone winds indicates that gust energy is available at
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high frequencies beyond 1 Hz. The damage to many of the well designed lattice towers during cyclones may be explained by the above observation. It is essential to develop a modified spectrum of tropical cyclone winds for purposes of design based on field experiments. 8. The measured fundamental frequency values are very close to the theoretically evaluated value. Under tropical cyclone winds, participation of higher modes in the acceleration response of the structure is observed, while only fundamental mode was dominant under normal wind conditions. 9. The damping ratio was found to increase with increase in wind speed.
[5]
[6]
[7]
[8]
[9]
[10] [11]
Acknowledgements The authors acknowledge, with gratitude, the encouragement and permission granted by the Director, Structural Engineering Research Centre, Chennai, for the publication of this paper. The authors would also like to thank their colleagues, Mr M. Arumugam, Mr G. Balasubramani, and Mr K. Sankaranarayanan for all their help in collection and analyses of data.
[12]
[13]
[14]
[15]
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