ANSI NEMA C136.30-2015 Roadway and Area Lighting Equipment - Pole Vibration.pdf

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1、NEMA Standards PublicationNational Electrical Manufacturers AssociationANSI C136.30-2015Roadway and AreaLighting Equipment - Pole VibrationANSI C136.30-2015 American National Standard for Roadway and Area Lighting Equipment Pole Vibration Secretariat: National Electrical Manufacturers Association Ap

2、proved January 13, 2015 American National Standards Institute, Inc. 2015 National Electrical Manufacturers Association NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the document at the

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18、dards by calling or writing the American National Standards Institute, Inc. Published by National Electrical Manufacturers Association 1300 North 17th Street, Suite 900, Rosslyn, Virginia 22209 2015 National Electrical Manufacturers Association. All rights, including translation into other languages

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20、t prior written permission of the publisher. Printed in the United States of America ANSI C136.30-2015 ii 2015 National Electrical Manufacturers Association Foreword At the time this standard was approved, the ANSI C136 committee was composed of the following members: Alabama Power Company American

21、Electric Lighting Caltrans Ceravision City of Kansas City, Missouri City of Los Angeles, Bureau of Street Lighting Cree, Inc. Duke Energy Duke Energy Florida Eatons Cooper Lighting Edison Electric Institute EPRI EYE Lighting International of N.A., Inc. Florida Power a frequency of 1 Hz means there i

22、s one cycle or oscillation per second. Friction Damping: a rubbing action between the objects. The damping is relatively constant, except at some low force value where the relative motion will stop (breakaway force); the damping will then be zero. Fundamental Mode of Vibration: The fundamental mode

23、of vibration for a system is the mode having the lowest natural frequency. ANSI C136.30-2015 2 2015 National Electrical Manufacturers Association Galloping: This phenomenon occurs when a luminaire or device is suspended from a horizontal cantilevered arm attached to a pole in a moderate-to-strong wi

24、nd. As the wind passes the luminaire, it initiates a vertical motion causing the system to oscillate vertically with high amplitudes. When designing a support system for suspended devices, this should be taken into account. Natural Frequency: a frequency of vibration of a system whereby the system o

25、scillates freely. There might be a number of natural frequencies corresponding to various normal modes of vibration. The fundamental frequency (also called a natural frequency) of a periodic oscillation is the inverse of the pitch period length. The pitch period is the length of time to complete a u

26、nit of oscillation. One pitch period thus describes the periodic oscillation completely. Resonance: the tendency of a mechanical system to oscillate at greater amplitude at some frequencies than it does at other frequencies. This phenomenon, in which a relatively small, repeatedly applied force caus

27、es the amplitude of an oscillating system to increase, and in some cases become very large and detrimental to the system. In a mechanical system such as a pole and luminaire assembly (system), there can be a number of frequencies at which resonance can occur, each being a function of the stiffness,

28、geometry, mass distribution, and other factors. Strouhal Number: a dimensionless parameter that characterizes the frequency of vortex shedding of an object. It represents the ratio of the width of a body placed in an air stream to the wavelength of vortices shed from the body. Viscous Damping: dampi

29、ng that is proportional to velocity. Examples of viscous damping devices are shock absorbers and rubber mounting pads. Vibration: mechanical oscillations about an equilibrium point. These oscillations may be periodic or random. Vortex Shedding: the alternating or periodic passing of vortices behind

30、an object in an air stream. This shedding of vortices causes alternating low-pressure zones. The object is pulled toward these low-pressure zones causing oscillating motion. If the air flow is such as to create this shedding at the resonant frequency of the object, the motion will be sustained. Vort

31、ex shedding can occur on poles of any cross-section. 4 Types and Sources of Pole Vibration 4.1 GENERAL All poles can vibrate with certain combinations of pole geometry, site terrain, prevailing winds, and luminaire type, but it is impossible to predict with certainty which poles or when the poles mi

32、ght exhibit signs of vibrations. 4.2 WIND EXCITATION Wind excitation is vibration caused by wind acting on the overall lighting structure. It is most commonly caused by vortex shedding. Pole motion typically will be at right angles to the wind direction. Each individual vortex pulse adds a small inc

33、remental displacement to the pole motion. It may require 100 or more in-phase cycles from a steady state wind for a pole to reach its maximum vibration amplitude. As the vibration intensity increases, the damping factor also increases until it matches the cyclic displacement values caused by the win

34、d. If the input energy stops, then the pole vibration amplitude will decay over a similar number of cycles. Unfavorable surroundings that contribute to wind excitation can be: a) Flat open area in regions of high wind energy. b) Open terrain with a constant height wall. ANSI C136.30-2015 2015 Nation

35、al Electrical Manufacturers Association 3 c) Flat open terrain with a berm or earthen barrier. d) A building corner or sloped roof generally in flat open terrain. Wind excitation on a vertical pole shaft is primarily in the horizontal direction, resulting in a bending force about the pole base. Wind

36、 acting on a horizontal luminaire support causes a vibrational force in the vertical direction. 4.2.1 Characteristics of Wind-induced Vibration Wind-induced pole vibration is generally limited to wind velocities between 5 miles per hour (mph) and 40 mph. Higher velocities do not cause significant vi

37、bration in poles. As the wind varies in velocity, it will coincide with some discrete resonance modes. When a particular mode frequency is excited, then the resonance condition tends to dominate (lock-in) until there is significant wind velocity change. In the case of tapered structures, the frequen

38、cy of vortex shedding at a particular wind velocity varies over the entire length of the structure. As the wind velocity increases, resonant excitation occurs first at the smaller-diameter portion of the structure and then shifts to portions with larger diameters. Typically for free-standing tapered

39、 supports, the region of maximum excitation is at approximately three-fourths of the structures height and moves downward for higher modes of vibration. 4.2.2 First Mode Fundamental, or first, mode is the lowest mechanical resonant frequency (generally less than 1.0 Hz on large poles). The motion is

40、 sometimes referred to as sway. The maximum motion occurs at the top of the pole. Normally, first mode oscillation has been shown to occur around 1 Hz. First mode wind-induced vibration occurs in varying amounts on most metal lighting poles and is often easily visible. There is only one node (the lo

41、cation of minimum motion): at the ground line location of the pole. High bending stress occurs at the nodes. High acceleration forces occur at points of maximum motion (anti-nodes). First mode oscillation has been shown to cause failures in square straight poles and in round and square straight pole

42、s with low EPA fixture loading. The resonant wind speed can be in the range of 25-40mph. Shaft stress due to resonant first mode oscillation can exceed 15 ksi in some pole types. 4.2.3 Second Mode Second mode is motion at the next higher resonant frequency, generally three to 10 times the fundamenta

43、l frequency. There are two nodes, one at the base and one near the top. Maximum displacement is normally at an elevation midway to two-thirds of the pole height. Vibration at this mode is generally at low amplitude and not visible at a distance. The pole often appears to be shivering when viewed at

44、close range. This mode generally results in low shaft stress of 1-5 ksi, but can lead to intense luminaire acceleration and fatigue to both luminaire and pole. Second mode wind-induced vibration might not be visible. It can be detected by resting ones fingers lightly on the pole and feeling the occa

45、sional shivering-type motion. The maximum displacement in the second mode occurs typically just above the midpoint of the pole. The condition that is most likely to set up second mode vibration is a 10-20 mph wind. 4.2.4 Third Mode Third and higher modes are somewhat rare. A few occurrences of third

46、 mode wind-induced vibration have been observed at frequencies between 1.4 and two times the second mode value. 4.3 FORCED EXCITATION The oscillation of the system is forced if the response is imposed by the excitation. An example of this is traffic-induced vibration on a bridge where lighting struc

47、tures are mounted. The floor of a bridge carrying vehicular traffic moves in the vertical direction at resonant frequencies of the bridge. Motion of the massive bridge being independent of the lighting structure resonances, damping, etc., will impose vertical motion on the lighting structures. Respo

48、nse of the lighting assembly will depend on the geometry and ANSI C136.30-2015 4 2015 National Electrical Manufacturers Association weight distribution of the system. Traffic excitation may be transmitted by other means, such as through the soil or pavement, but the above example is the most common.

49、 The following examples, cases 1, 2 and 3, illustrate the response of some bridge-mounted lighting structure configurations to forced vertical excitation. 4.4 EXAMPLES 4.4.1 Case 1: Assembly with Balanced Weight Distribution An example is a straight pole with a pole-top luminaire centered over the shafts vertical axis. The stress due to in-line motion is much lower than that caused by bending loads. The vertical resonant frequency of the pole will be higher than the excited frequency, thus there will be negligible resonant amplification. NOTEThis and other bridge-mounted s