Field Testing of the Signal Head Vibration Absorber ...docs.trb.org/prp/14-5056.pdfconducted extensive studies of a number of different types of vibration absorbers attached to

Christenson, Cashany, Hua and Zuo 1

Field Testing of the Signal Head Vibration Absorber Mitigation Device to Reduce Fatigue in Wind–Excited Traffic Signal Support Structures Richard Christenson* Associate Professor Department of Civil & Environmental Engineering University of Connecticut 261 Glenbrook Road, Unit 2037 Storrs, CT 06269 Phone: 860-486-2270 Fax: 860-486-2298 E-Mail: [email protected] Majid Cashany Graduate Research Assistant Department of Civil & Environmental Engineering University of Connecticut 261 Glenbrook Road, Unit 2037 Storrs, CT, 06269 Phone: 860-486-4058 Fax: 860-486-2298 E-Mail: [email protected] Jieying Hua Graduate Research Assistant Department of Civil & Environmental Engineering Mail Stop 1023 Texas Tech University Lubbock, TX, 79409 Phone: 806-742-3476 Fax: 806-742-3446 E-Mail: [email protected] Delong Zuo Associate Professor Department of Civil & Environmental Engineering Mail Stop 1023 Texas Tech University Lubbock, TX, 79409 Phone: 806-742-3476 Fax: 806-742-3446 E-Mail: [email protected]

TRB 2014 Annual Meeting Original paper submittal - not revised by author


*Corresponding Author Submission Date: August 1, 2013. Total number of words, including figures: 5,930 words Number of words in text: 4,430 words Equivalent number of words from figures: 1,500 words



ABSTRACT The signal head vibration absorber (SHVA) is an effective vibration mitigation device proposed to reduce the in-plane wind-induced vibration and corresponding fatigue of traffic signal support structures. The SHVA was successfully tested in the Advanced Hazards Mitigation Laboratory at the University of Connecticut subjected to both free and force vibration testing utilizing pluck tests and a linear shaker. A new realization of the SHVA has been developed that provides for no modifications of standard signal heads or mounting hardware and is readily installable in the field for both retrofit and new applications. This paper provides research verification of the performance of the SHVA to reduce wind-induced vibration in traffic signal support structures. Successful field testing of a traffic signal support structure with a 60 foot (18.29 m) long mast arm is conducted at the Texas Tech University’s National Wind Institute (NWI) Wind Engineering Research Field Laboratory. The field test results, consistent with prior laboratory testing, show that the SHVA is able to reduce in-plane motion of the mast arm tip by 90%. INTRODUCTION Traffic signals are used extensively around the United States to control conflicting flows of traffic at road intersections, crosswalks and other locations. To ensure clear visibility these traffic signals are typically supported over the travel lanes by cantilever or bridge support structures. Cantilevered signal support structures have a single vertical pole and a horizontal mast arm, while bridge-type structures consist of a horizontal member supported at both ends with vertical poles (1). Cantilevered signal support structures are more commonly used due to less cost and reduced potential hazard the poles result in next to the roadway.

Cantilevered traffic signal support structures, however, are particularly susceptible to wind induced vibration (2). Kaczinski et al. (3) noted that mast arm displacements have been reported in excess of 48 inches under steady state winds with speeds in the range of 10 to 35 mph. Various types of wind loading, including galloping, vortex shedding, natural wind gusts, and truck-induced gusts, can result in traffic signal support vibration: Wind-induced galloping was suggested to often be the cause of excessive in-plane (vertical) vibrations in traffic signal support structures (4). Galloping is due to aerodynamic forces generated on non-circular cross sections, such as traffic signal structures with attachments (signs, traffic signals) to the mast arm (5). In galloping conditions, the amplitude of the excessive vibration increases as the wind speed increases. Another phenomenon that has been identified for traffic signal support structure vibration at low wind speeds is vortex shedding (6, 7 and 8). In this case, the alternating vortices that shed in to the wake of the structure can lock-in with a natural frequency of the structure and create large-amplitude vibration. At higher wind speeds the vortex shedding excites the mast arm less. Natural wind gusts typically result in motion of the mast arm in both the in-plane (vertical) and the out-of-plane (horizontal) directions. Similar to natural wind gusts, truck-induced gusts arise from upward wind gusts occurring as trucks pass under the mast arm and can result in in-plane motion of the mast arm.

The large amplitude and cyclical response from the various wind-induced vibration results in repeated live load stress variations, in particular at the mast arm-to-pole and pole-to-base plate connections, which can significantly reduce the fatigue life of signal support structures. Reducing the effective stress range, the difference between the maximum and minimum stress in a cycle, by reducing the amplitude of the vibration can significantly increase the fatigue life of that structure.



There are two approaches to reduce vibrations: reduce the exciting load on the structure or modify the dynamic properties of the structure. Reducing the excitation on the structure has primarily focused on modifying the aerodynamic properties of attachments to the mast arm or the mast arm itself. This approach, while effective, may limit performance to one type of wind excitation. For example the proposed airfoil approach, whereby a sign blank is mounted horizontally near the tip of the mast arm to serve as an aerodynamic damper, can potentially provide an effective energy dissipating mechanism for galloping but may not provide any benefit for truck-induced gusts (5). As a second approach, changing the mass, stiffness or damping of the structure can be considered. Traditionally, signal support vibration is addressed by increasing the strength and stiffness of the structure. This results in larger poles and mast arms as well as overbuilt connection details. An alternative to the brute force approach is the application of structural control to effectively modify the dynamic characteristics of the structure.

Structural control introduces mechanical devices to redistribute and/or dissipate energy in the structure. In the past decade researchers have examined various applications of structural control, often referred to as vibration mitigation devices, to reduce vibrations and extend the service life of traffic signal support structures. An effective vibration mitigation device can decrease the amplitude and number of cycles, extending the service life of these structures. According to the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals 5th edition (5), traffic signal support structures should be designed for fatigue, considering galloping, vortex shedding, natural wind gust and truck induced gust loading. When a support structure exhibits vibration in the field, a vibration mitigation device can be considered. The AASHTO specifications specify that in lieu of designing for galloping and vortex shedding forces, an effective vibration mitigation device may be used to reduce vertical deflections. The new AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals 6th edition (9) have similar specifications for utilizing vibration mitigation devices, however, do not identify vortex shedding as a concern for fatigue.

A number of mitigation devices have been proposed for traffic signal support structures with varying degrees of complexity and performance (10, 11, 12 and 4). Hamilton et al. (12) conducted extensive studies of a number of different types of vibration absorbers attached to a 50 ft (15.24 m) mast arm. Free vibration tests were conducted and acceleration measurements at the tip of the mast arm were used to determine the critical damping ratio of the traffic signal structure. The uncontrolled critical damping ratio of the mast arm considered in their research was experimentally measured to be 0.15%. The Alcoa dumbbell, Shot put, Hapco, Flat bar and Strand dampers increased the critical damping ratio of the structure to 0.26%, 0.29%, 0.31%, 1.1% and 1.6% respectively. Impact dampers showed more promise, increasing the critical damping ratio to 6.12% for the spring/mass liquid impact damper. This damper was relatively complex and had a high manufacturing cost. Hamilton et al. (12) also considered a dual strut configuration that required a strut to be placed at an angle between the mast arm and the pole. The strut was found to increase the critical damping ratio to 6.00%. However, the strut configuration required an extended portion of the pole above the mast arm connection and further refinement was needed for practical use as strut performance was dependent on the angle of inclination between the strut and the mast arm (13).

A similar test setup was used as part of a two phase study by Cook et al. (14). In the initial phase of the project, laboratory testing was performed on a 37 ft (11.28 m) mast arm structure. The critical damping ratio of the uncontrolled mast arm was measured to be 0.27%



from free vibration tests. Similar to the previously mentioned study, the mast arm was instrumented with accelerometers. In this research a mass oscillator was used to excite the structure to simulate wind induced vibration. The study considered various types of dampers including Horizontal and U-tube liquid dampers, Horizontal spring/mass impact damper, Stockbridge damper and Batten damper with resulting critical damping ratios of the structure increased to 0.38%, 0.40%, 0.78%, 0.42%, and 1.82%, respectively. A Friction damper tested was shown to provide a 6.5% critical damping ratio, however, it was considered unattractive. A traditional tuned mass damper (a.k.a. damped vibration absorber) was found to be quite effective increasing the critical damping ratio to 8.71%. The weight of the traditional tuned mass damper that was added to the end of the mast arm was 12.5 lbs (5.67 kg). The traditional tuned mass damper considered by Cook et al. (11) was discounted because it had to be specifically designed for each specific traffic signal support structure to achieve the desired level of performance.

An innovative vibration absorber for traffic signal support structures was proposed by Christenson and Hoque (15, 16, 17 and 18). This device, called the signal head vibration absorber, was tested on a 35 foot long mast arm at the University of Connecticut. The traffic signal support structure initially had a natural frequency of 1.27 Hz and a critical damping equal to 0.2%. Free vibration tests were conducted for both the rigidly connected signal head and the SHVA system. The mast arm tip acceleration responses were collected. The comparison of the two sets of free vibration responses showed that the SHVA significantly attenuated the vibration of the mast arm tip. The SHVA reduced the acceleration of the mast arm from 0.5 g to 0.06 g (approximately 3.5 inches (8.89 cm) to 0.4 inches (1.02 cm)) in 2.75 seconds, as opposed to 300 seconds for the rigidly connected signal head. The analysis also showed that the critical damping in the 35 foot (10.67 m) traffic signal support structure had increased to 10.1%. This corresponds to over a 90% reduction in the steady state response of the structure.

This paper conducts field testing of a low-cost vibration mitigation device for typical cantilevered traffic signal support structures called the signal head vibration absorber (SHVA). The approach incorporates the concept of a damped vibration absorber to reduce in-plane vibration. Out of plane vibration is not directly mitigated in the current configuration of the proposed device. In the proposed method, the signal heads themselves are modified so that they are no longer rigidly connected to the mast arm but are allowed to translate vertically relative to the mast arm when the mast arm vibrates. The proposed method uses the signal head mass to provide a supplemental mass to absorb the energy due to wind excitation. The signal head vibration absorber (SHVA) provides a relatively large supplemental mass to reduce vibration while effectively adding no additional mass to the overall system. The SHVA provides increased performance and improved robustness when applied to different traffic signal support structures. In the following sections, the details and benefits of the SHVA are described. Field tests were carried out to investigate the SHVA effectiveness. The field tests and the corresponding results are described in the subsequent sections. PROPOSED SIGNAL HEAD VIBRATION ABSORBER (SHVA)

The SHVA device is based on the basic principle of the tuned mass damper (TMD). The uniqueness of the SHVA lies in the fact that no additional mass is required for this system (17, 18). The signal head itself acts as the supplemental mass. As the mass of the signal head moves vertically, energy is dissipated and vibration is greatly reduced. A new realization of the SHVA, as shown in Figure 2, has been developed that provides for no modifications of standard signal



heads or mounting hardware and is readily installable in the field for both retrofit and new applications.

In the current SHVA configuration two end brackets are attached to a standard signal head. The signal head can consist of 3, 4 or 5 lights oriented linearly. The end brackets have high durability Lexon bearings that allow the signal head unit to translate vertically on two guide rails supported with a standard rigid two-way mast arm mount. Inside the guide rail a tension spring is attached, connected at the mid-point of the guide rail on the top and to a retention rod on the bottom attached directly to the bottom guide plate. This spring, shown in Figure 2 is specifically tuned to reduce vibrations for a wide range of mast arm lengths (18). An eddy current damper, also shown in Figure 2, is mounted on the bottom side of the bottom end bracket to provide the damping needed to insure performance over a wide range of mast arm lengths. Counter weights can be added to the back side of the end brackets during installation, as needed, to insure the signal head translates vertically with minimal friction in the bearings. FIELD TEST DESCRIPTION Field testing is conducted at the Texas Tech University’s National Wind Institute (NWI) Wind Engineering Research Field Laboratory. The traffic signal support structure with a 60 foot (18.29 m) mast arm is used for this study. Previous testing on this structure has considered both horizontal and vertical orientation of the signal heads. Since the current realization of the SHVA allows for only vertical orientation, the vertical signal head configuration will be presented in this study. Future research will evaluate the performance of a SHVA with signal heads mounted horizontally.

The particular 60 foot (18.29 m) mast arm, shown in Figure 3, has been the subject of previous research on wind excitation of these cantilevered traffic structures (8, 19). The traffic pole is a mono-tubular pole and a cantilevered arm with a circular cross-section. The 19.59 ft (5.97 m) pole is tapered with a 21.65 in (0.55 m) diameter at the base and 18.5 in (0.47 m) diameter at the top. The 60 ft (18.29 m) mast arm has an 18.5 in (0.47 m) diameter at the mast arm to pole connection and a 10.31 in (0.262 m) diameter at the free end of the mast arm. The signs and signal head configuration can be seen in Figure 3.

The specific mounting of the SHVA on this mast arm is shown in Figure 4. The SHVA is mounted on the traffic signal located near the tip of the mast arm. This is a 5-head traffic signal SHVA configuration. The counter weights can be seen in Figure 4(b) are used to insure the signal head was balanced for this specific configuration.

A tri-axial accelerometer and anemometer are used to monitor the vibrations of the mast arm and the wind, respectively. The accelerometer is installed at the free end of the mast arm to measure the accelerations in-plane (vertical), out-of-plane (horizontal) and along the length of the mast arm. The acceleration, and corresponding displacement, along the length of the mast arm is negligible and is not reported in this paper. An ultrasonic anemometer is installed on the top of the pole to measure wind speed. The sensors are sampled continuously by a data acquisition system with a sampling rate of 40 Hz.

The dynamics of the structure are dominated by the first (fundamental) mode vibration in the in-plane and out-of-plane directions. The first mode natural frequency is determined, by peak picking the auto-spectral density function of the acceleration response. This fundamental frequency is determined to be 1.00 Hz in-plane and 0.93 Hz out-of-plane (for horizontal configuration of the signal heads). Orienting the signal heads vertically does not change the in-



plane first mode natural frequency, but does decrease the first mode out-of-plane natural frequency to 0.92 Hz.

The tip displacement of the mast arm as a function of the wind speed has been shown to provide insightful performance measure for traffic signal support structures. Since displacements are more directly related to fatigue and serve as a better indicator of performance than acceleration, the response quantity of interest. The measured acceleration time histories are passed through a 5th order high-pass Butterworth filter with a cut-off frequency of 0.5 Hz to remove a DC offset in the measurement. The signal is then numerically integrated, twice, to calculate the displacement time histories. The displacement time histories are next filtered by a 5th order Butterworth filter to estimate the vibrations in the first two in-plane and out-of-plane modes of the structures. The pass band of these band-pass filters were set as 0.5 Hz frequency bands centered at the natural frequencies of the modes of interest. For each modal displacement record is partitioned into 1 minute segments and transformed using the Hilbert transform to estimate the modal amplitude and frequency of the vibration (20). FIELD TEST RESULTS Data for the traffic signal support structure uncontrolled, without the SHVA, was collected between December 2012 and January 2013. In late January 2013, the SHVA was installed on the 5 head signal. Data for the SHVA controlled structure was collected between February 2013 and May 2013. To evaluate the performance of the SHVA, the one minute modal displacement is examined as a function of wind speed for both cases – with and without the SHVA. The results are shown in Figure 5.

The field tests of the horizontal signal head configuration, without a mitigation device, clearly indicates that vortex induced vibration occurs in the in-plane direction around 10 mph (5 m/sec) and 16 mph (10 m/sec). The response due to vortex shedding is observed as large as 4.1 in (10.41 cm) and 5.7 in (14.48 cm) at 10 mph and 16 mph, respectively. The SHVA is able to reduce the peak one minute modal displacement to 0.37 in (0.94 cm) and 0.61 in (1.55 cm) around the wind speeds of 10 mph and 16 mph, respectively. This corresponds to a reduction in in-plane one minute modal displacement of 91% and 89%, and effectively eliminates vibration in the mast arm. Over the wind speed measured, from 3 mph to 45 mph, the largest one minute modal displacement of the mast is reduced from 5.7 in, at 16 mph, for the uncontrolled case to 1.1 in at over 30 mph for the SHVA mast arm. The SHVA performance is observed to be robust with respect to the wind speed, providing displacements of less than 1 in. It should be noted that 99.96% of the calculated displacements for the SHVA are less than 1 in (2.54 cm). The performance of the SHVA is independent of the particular source of the wind excitation, since the mitigation device modifies the dynamic characteristics of the traffic signal support structure and is independent of the loading.

The SHVA does not have any significant effect on the out-of-plane motion. This is as expected since the SHVA is intended to only address the vertical motion of the mast arm. It is noted that the in-plane vibration is reduced to approximately the same amplitude as the out-of-plane motion. This result indicates perhaps that the vertical response is limited to that due to gusting conditions.

A common concern for the SHVA, where the signal head is allowed to translate relative to the mast arm, is the potential for excessive amplitude of vibration of the signal head – even with reduced mast arm displacement. Figure 6 shows results of the one minute modal



displacement of the SHVA installed on the 60 ft mast arm. It is clear in this figure that the displacement of the signal head is similar in magnitude to the displacement of the mast arm tip, and thus no concern is warranted.

CONCLUSIONS The signal head vibration absorber (SHVA) is proposed as an effective vibration mitigation device to reduce the in-plane wind-induced vibration of traffic signal support structures. This paper provides research verification of the performance of the SHVA. A new realization of the SHVA is first presented that provides for no modifications of standard signal heads or mounting hardware and is readily installable in the field for both retrofit and new applications. The field testing at the Texas Tech University’s National Wind Institute (NWI) Wind Engineering Research Field Laboratory is described. Three months of monitoring the SHVA is presented and compared to prior monitoring of the traffic signal support structure with no SHVA. The SHVA is shown to reduce the vortex induced vibration of the mast arm by approximately 90%. Further, the one minute modal mast arm displacement at the mast arm tip is effectively reduced to less than 1 in over the range of wind speeds, from 3 mph to 45 mph measured. The SHVA performance is independent of the type of wind excitation and is demonstrated in these field tests to reduce vertical vibration over the full range of wind speeds collected.

ACKNOWLEDGEMENT The authors gratefully acknowledge the Texas Department of Transportation for funding this field testing under the guidance of Wade Odell, Research Engineer, Safety & Operations, Structures & Hydraulics, Texas Department of Transportation, Research & Technology Implementation Office. This report does not constitute a standard, specification, or regulation. The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data herein. The contents do not necessarily reflect the views of the Texas Department of Transportation. The authors also acknowledge the National Cooperative Highway Research Program's Innovations Deserving Exploratory Analysis (NCHRP-IDEA) program for funding the original work through NCHRP-IDEA Project 141 and the support of the Connecticut Department of Transportation (ConnDOT). REFERENCES (1) Davidson, J.S., F.H. Fouad, R.S. Abdalla., B. Calvert, E. Barton, and R.A. Warr.

Computer-Based Sign, Luminaires, and Traffic Signal Support Design Tools for State and County Engineers. Report No. FHWA/CA/OR-, April 30, 2004.

(2) Dexter, R. J., and M.J. Ricker. Fatigue-Resistant Design of Cantilevered Signal, Sign, and Light Supports. NCHRP Report 469, Transportation Research Board, National Research Council, Washington, D.C., 2002.

(3) Kaczinski, M. R., R. J. Dexter, and J. P. Van Dien. Fatigue-Resistant Design of Cantilevered Signal, Sign and Light Supports. NCHRP Report 412, Transportation Research Board, National Research Council, Washington, D.C., 1998.



(4) Pulipaka, N., P.P. Sarkar, and J.R. McDonald. On Galloping Vibration of Traffic Signal Structures. Journal of Wind Engineering and Industrial Aerodynamics, Vol. 77-78, 1998, pp. 327-336.

(5) AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals. Fifth Edition, 2009, Chapter 13.

(6) Cruzado, H. J. Risk Assessment Model for Wind-Induced Fatigue Failure of Cantilever Traffic Signal Structures. Lubbock: Texas Tech University, 2007.

(7) Zuo, D., and C. W. Letchford. Field Observations of Traffic Signal Structure Vibration. Conference on Wind Engineering Proceedings, 22-26 June 2009, San Juan, Puerto Rico, pp. 1-6.

(8) Zuo, D., and C. W. Letchford. Wind-Induced Vibration of a Traffic-Signal-Support Structure with Cantilevered Tapered Circular Mast Arm. Engineering Structures, Vol. 32, 2010, pp. 3171-3179.

(9) South, M. S. Fatigue Analysis of Overhead Sign and Signal Structures. Physical Research Report No. 115, 1994.

(10) McManus, P. S., H. R. Hamilton, and J. A. Puckett. Damping in Cantilevered Traffic Signal Structures under Forced Vibration. ASCE Journal of Structural Engineering, Vol. 129, No. 3, March 2003, pp. 373-382.

(11) Cook, R. A., D. Bloomquist, M. A. Kalajian, and V. A. Cannon. Mechanical Damping Systems for Traffic Signal Mast Arms. Report No. WPI 0510775, Engineering and Industrial Experiment Station, University of Florida, Florida, 1998.

(12) Hamilton, H. R, G. S. Riggs, and J. A. Puckett. Increased Damping in Cantilevered Traffic Signal Structures. Journal of Structural Engineering, Vol. 126, Issue 4, April 2000, pp. 530-537.

(13) McManus, P. S. Evaluation of Damping in Cantilevered Traffic Signal Structures under Forced Vibrations. Master Dissertation, Department of Civil and Architectural Engineering, University of Wyoming, Laramie, Wyoming, 2000.

(14) Cook, R. A., D. Bloomquist, D. Richard, and M. A. Kalajian. Damping of Cantilevered Traffic Signal Structures. Journal of Structural Engineering, Vol. 127, Issue 12, December 2001, pp. 1476-1483.

(15) Hoque, S., and R. E. Christenson. Reducing Fatigue in Wind–Excited Traffic Signal Support Structures Using an Innovative Vibration Absorber. In Transportation Research Record: Journal of the Transportation Research Board, No. 2251, Transportation Research Board of the National Academies, Washington, D.C., 2011, pp. 16-23.

(16) Hoque, S., E. Warner, C. Henriquez, C. Huynh, R. E. Christenson, J. Caicedo, and G. J. Yun. Vibration Mitigation of Traffic Signal Support Structures–Strut versus Signal Head Vibration Absorber. Presented at 91st Annual Meeting of the Transportation Research Board, Washington, D.C., 2012.

(17) Christenson, R. Smart Vibration Absorber for Traffic Signal Supports. WO Patent 2,011,085,184 2011.

(18) Christenson, R. Reducing Fatigue in Wind-Excited Traffic Signal Support Structures using Smart Damping Technologies.NCHRP-IDEA Program Project Final Report 2011.

(19) Hua, J., and D. Zuo. Experimental Investigation of Wind-Induced Vibration of Slender Tapered Cylinders. The 12th Americas Conference on Wind Engineering, Seattle, Washington, USA, 2013.



(20) Bendat, J. S., and A. G. Piersol. Random data. John Wiley & Sons, Inc., Hoboken, New Jersey, 2010.



LIST OF FIGURES FIGURE 1 Signal Head Vibration Absorber (SHVA): (a) drawing; (b) mounted on a mast arm in the laboratory at the University of Connecticut. FIGURE 2 Components of the signal head vibration absorber (SHVA): tension spring (left); and eddy current damper (right). FIGURE 3 Field Test of 60 foot mast arm with Vertically Mounted Signal Heads (shown without the SHVA installed on the 5-head signal). FIGURE 4 Signal Head Vibration Absorber Installation at Texas Tech Wind Field Station. FIGURE 5 One Minute Modal Displacement of Mast Arm Tip versus Mean Wind Speed: (a) uncontrolled (no SHVA); and (b) SHVA installed. FIGURE 6 One Minute Modal Displacement of the SHVA installed on the 60 ft Mast Arm.



(a) (b)

FIGURE 1 Signal Head Vibration Absorber (SHVA): (a) drawing; (b) mounted on a mast arm in the laboratory at the University of Connecticut.

springs

dampers

guide rails

signal head translates vertically

standard bracket

end bracket



FIGURE 2 Components of the signal head vibration absorber (SHVA): tension spring (left); and eddy current damper (right).



FIGURE 3 Field Test of 60 foot mast arm with Vertically Mounted Signal Heads (shown without the SHVA installed on the 5-head signal).



(a) (b)

FIGURE 4 Signal Head Vibration Absorber Installation at Texas Tech Wind Field Station.



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FIGURE 5 One Minute Modal Displacement of Mast Arm Tip versus Mean Wind Speed: (a) uncontrolled (no SHVA); and (b) SHVA installed.



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FIGURE 6 One Minute Modal Displacement of the SHVA installed on the 60 ft Mast Arm.


Documents

Field Testing of the Signal Head Vibration Absorber ...docs.trb.org/prp/14-5056.pdfconducted extensive studies of a number of different types of vibration absorbers attached to