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BBAA VI International Colloquium on: Bluff Bodies Aerodynamics & Applications

Milano, Italy, July, 20-24 2008

FIELD OBSERVATIONS OF WIND-INDUCED MAST-ARM LIGHTING POLE VIBRATION

Delong Zuo∗ and Chris Letchford†

∗Department of Civil and Environmental Engineering Texas Tech University, M.S. 1023, Lubbock, TX 79409, USA

e-mail: delong.zuo@ttu.edu

† School of Engineering University of Tasmania, Private Bag 65, Hobart Tasmania 7001

e-mails: Chris.Letchford@utas.edu.au

Keywords: Lighting Pole, Vortex-Induced Vibration, Buffeting, Galloping

Abstract: Under excitation from wind, highway lighting poles of various types have exhibited problematic vibrations. Previous study of these vibrations and the consequent fatigue and failure of the structures was primarily based on analytical formulation and laboratory tests, and the effort has been focused on some specific perceived types of oscillations. The prototype aerodynamic behavior of lighting pole structures still need to be further investigated. This paper presents an interpretation of the lighting pole vibrations observed through a long-term full-scale measurement project. Based on this interpretation, the excitation mechanisms that induced the vibrations were identified.

1 INTRODUCTION Highway lighting poles as slender structures with low-level of damping have been ob-

served to exhibit wind-induced vibrations. If not properly mitigated, such vibrations can lead to fatigue of structural members [1] and in some extreme cases, failure of these structures[2].

Current design of lighting poles are guided by standards based on the concepts of equiva-lent static force and gust effect factor [3, 4]. Limitations still exist in these specifications. For example, in the fourth edition of the “AASHTO Standard Specifications for Structural Sup-ports for Highway Signs, Luminaires and Traffic Signals” [5] in the United States, the wind loading specified are primarily based on results from wind tunnel [6] and water tank tests [7], which may not completely reveal the prototype excitation mechanisms. Also, the so-called “typical lighting poles” commonly found along highways, which are usually 9.14 m to 16.76 m (30 ft to 55 ft) in height, are often overlooked. For example, theses shorter poles are not recommended to be designed for fatigue since they are considered “smaller structures”. In light of the reported lighting pole failures [2], It appears that such recommendations need to be reexamined. Recently, a closed-form method has been developed for buffeting analysis of slender vertical structures such as poles and monotubular towers[8, 9]. On this basis, proce-dures for fatigue analysis of these structures due to buffeting were also proposed[10, 11].

Delong Zuo and Chris Letchford

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These methods and procedures have been combined with laboratory experiments to investi-gate the failure of a large number of lighting poles as well as to study the vibration mitigation of such structures [2]. In the same study, it was also suggested that lighting poles can be sus-ceptible to galloping under certain weather conditions. This suggested that, as structurally simple as lighting poles appear to be, their interaction with wind can be complex.

This paper presents the findings from a full-scale investigation, which followed the failure of a number of lighting poles on a bridge in the state of Texas in the United States. These findings can be used as a context for further advancement of the analytical and numerical me-thods for the analysis of wind-induced lighting pole vibrations, as well as for the development of effective and efficient mitigation strategies.

2 FULL-SCALE MEASUREMENT CONFIGURATION The subject of the study is an aluminum lighting pole with a cantilever arm. It consists of a

10.67 m high linearly tapering pole, which is 25 cm and 15 cm in outer diameter at the base and at the top, respectively. The wall of the pole is 0.5 cm thick. The cantilever arm has a 3.66 m spread and a 1.52 m rise. The main arm is a bent aluminum tube of 0.3 cm in wall thickness. The horizontal portion of the arm has a circular cross-section of 6 cm in outer diameter; the inclined portion tapers from an elliptical cross-section (11.1 cm in outer major axis and 6 cm in outer minor axis) at the arm-pole connection to a circular cross-section of 6 cm inches in outer diameter where it turns horizontal. The arm is reinforced by an inclined strut of 3.8 cm in diameter and a vertical strut with an elliptical cross-section (3.8 cm in major axis and 1.7 cm in minor axis). The structure is anchored on a steel base bolted to a concrete foundation.

The vibration of the structure was monitored by three accelerometers: one on the luminaire, one near the top and one at approximately 2/3 height of the pole. The representative wind was measured by an ultrasonic anemometer in the vicinity of the pole at a height of 3.96 m above ground. Fig. 1 shows schematically the lighting pole and the location of the accelerometers (Ac #1 to #3), as well as a coordinate system for characterization of the vibration and the wind. The Z direction, which is not shown, follows the right-hand rule and points downward.

Fig. 1 Configuration of full-scale measurement system

(0 )°

(90 )°

(180 )°

(270 )°

(360 )°

Luminaire

θWind

7.11 m

3.58 m

0.06 m

2.36 m

3.66 m 0.81 m

Delong Zuo and Chris Letchford

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3 INTERPRETATION OF OBSERVATIONS The data from the full-scale measurement system were used in conjunction with a finite-element model (FEM) to identify the natural modes and frequencies of the lighting pole. Ta-ble 1 shows the natural frequencies of the first several modes of the structure. The plane formed by the X and Z axes are called the cable plane. Vibration in this plane is designated “in-plane” and that perpendicular to this plane (the Y-Z plane) is designated “out-of-plane”. For brevity, the mode shapes estimated using the FEM model are not shown herein.

Mode 1st In-Plane

1st Out-of-Plane

2nd In-Plane

2nd Out-of-Plane

3rd In-Plane

3rd Out-of-Plane

Frequency 1.13 Hz 0.91 Hz 3.60 Hz 2.70 Hz 9.34 Hz 8.95 Hz Table 1: Natural frequencies of the lighting pole

Two types of vibration were observed for the lighting pole. Fig. 2 (a) and (b) show the 30-second mean out-of-plane modal amplitude of the upper pole tip (“pole-U Y”) against the cor-responding mean wind speed and the reduced velocity, respectively. The modal displacements were generated based on the acceleration measurements using a numerical integration and band-pass filtering scheme. The diameter used for calculation of the reduced velocities is that of the pole at the same height of the anemometer. Two groups of data can be identified in these graphs. Group one consist exclusively low-amplitude second-mode vibrations and are associated with relatively low wind speed and a restricted range of low reduced velocity in the vicinity of 5. These characteristics associated with group one are typical of vibrations induced by vortex shedding. Group two consist both first-mode and second-mode vibrations, whose amplitude increased with wind speed and reduced velocity.

Fig. 2 Modal amplitudes vs. (a) wind speed and (b) reduced velocity for out-of-plane upper pole tip vibrations

0 90 180 270 3600

0.5

1

1.5

2

Mea

n po

le-U

dis

p Y

am

plitu

de (c

m)

Wind direction (°)

Mode 2

0 90 180 270 3600

2

4

6

8

Mea

n po

le-U

dis

p Y

am

plitu

de (c

m)

Wind direction (°)

Mode 1Mode 2

Fig. 3 Modal amplitudes vs. wind direction for vibrations in (a) group one and (b) group two

(a) (b)

Group one

Group two Group

one

Group two

Delong Zuo and Chris Letchford

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Fig. 3 (a) and (b) show the amplitude versus the associated wind directions for the two groups of vibration. It is evident that significant vibrations in group one occurred only when wind direction is close to 90° or 270° , which is parallel to the lighting pole plane and normal to the vibration direction. The vibrations in group two were not dependent on wind direction.

Although not shown in this abstract form of the paper, two similar groups can also be ob-served for the vibrations in the plane of the lighting pole, except that group one of in-plane vibrations only occurred when wind direction is close to 0 / 360° ° or 180° , which is normal to the cable plane and again normal to the vibration direction. Full-scale data also show that these vibrations in group one were close to either purely in-plane or purely out-of-plane. This is believed to be due to the fact that the frequencies in the in-plane and out-of-plane directions are considerably different and that vortex-induced vibration occurs primarily in the across-wind direction. The frequency of the vortex-shedding off the pole therefore can only lock in to either an in-plane or an out-of-plane frequency of the structure only when wind approaches from an orthogonal direction. Although the amplitudes of the vortex-induced vibrations were small, since the frequencies were quite high, they have resulted in failure of the connections between the cantilever arm and the pole (, which is clearly not due to first mode vibration). This type of vibration, therefore, has to be considered in the design of lighting poles.

The data also suggest that the vibrations in group two are unsteady both in amplitude and in frequency. This is evidence that these vibrations were due to turbulence in the wind, i.e., buffeting. These vibrations therefore can be analyzed using the methods proposed in [8, 9].

ACKNOWLEDGEMENTS The authors would like to thank Texas Department of Transportation for its support.

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and Structures. 5 (5), 463–478. 2002 [2] L. Caracoglia and N. P. Jones. Numerical and experimental study of vibration mitigation

for highway light poles. Engineering Structures. 29 (5), 821-831. 2007 [3] A. G. Davenport. Applications of Statistical Concepts to the Wind Loading of Structures.

Proceedings of the Institute of Civil Engineers. 19 449-472. 1961 [4] A. G. Davenport. Gust loading factors. Journal of Structural Division, ASCE. 93 11-34.

1967 [5] AASHTO. Standard Specifications for Structural Supports for Highway Signs, Luminaires

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