1st International Conference on Natural Hazards & Infrastructure28-30 June, 2016, Chania, Greece

Assessing the performance of vehicular traffic signal assemblies during hurricane force winds

I. Zisis, P. Irwin, B. Berlanga, A.G. ChowdhuryDepartment of Civil and Environmental Engineering, Florida International University, Miami, USA

B. HajraInternational Hurricane Research Centre, Florida International University, Miami, USA

ABSTRACT

Traffic signal assemblies are an essential infrastructure component that helps manage traffic in busy intersections. A significant fraction of existing signals in North America use span-wire supporting systems. However, hurricanes involving wind speeds exceeding 100 mph in the 2004-2005 season caused significant damage to various civil engineering structures, including traffic signal assemblies. In order to prevent wind induced structural damage to these traffic signals, Florida Department of Transportation (FDOT) has sponsored a research program at the 12-fan Wall of Wind (WOW) facility at Florida International University (FIU), USA. The WOW is capable of generating wind speeds in excess of 157 mph that is equivalent to a category 5 hurricane in the Saffir-Simpson scale. Two different configurations were chosen to carry out the first phase of tests at the WOW. The first configuration consisted of a 5-section and a 3-section traffic signal, while the second configuration consisted of two 3-section traffic signals and a 4-section traffic signal. Both these configurations were tested for wind speeds of 10, 20, 40, 70, 100 and 130 mph, or until failure occurred. The inclination and acceleration of the signals, as well as the tensions in the catenary and messenger wires of the span wire system were measured during the tests. Results show that the first configuration consisting of the 5-section and 3-section traffic signals experienced higher wire tensions than the second configuration, and therefore the first configuration was found to be more vulnerable of the two. In both the configurations, the tensions in the catenary wire decreased with an increase in wind load, while the messenger wire experienced an increase in tension with an increased wind load. Furthermore, at wind speeds above 70 mph, an aerodynamic instability resembling galloping was observed for both these configurations.

Keywords: Hurricane, Wall of Wind, Traffic signal, Wire tension, Wind load

INTRODUCTION

Traffic signals are an essential component of managing traffic in cities. Depending on the traffic conditions and flow the signals can come in various shapes, such as: 3-section or 5-section lights. A significant fraction of existing signals in North America use span-wire supporting systems. In these systems the traffic signals are attached to a hanger which is supported by a wire at the top end (catenary wire) and a wire at the bottom end (messenger wire). The two wires are then connected to vertical members (end support posts) on either side of the street. Limited studies on wind induced structural damage of traffic signals has been reported in literature. For instance, McDonald et al. (1995) found that oscillation of traffic signals caused by wind was a major cause of structural damage to the traffic signal. Similarly, Kaczinski et al. (1996) found that cantilevered structures like traffic signals are prone to structural damage caused by wind induced galloping phenomenon. Hamilton et al. (2000) developed a damping device that could reduce vibration of traffic

* Corresponding author email: izisis@fiu.edu, phone: 001-305 348 4869

signals to prevent wind induced structural damage. However, most of these studies were carried out at non atmospheric boundary layer (ABL) wind speeds below 100 mph, while a category 5 hurricane can produce wind speeds up to 150 mph. In fact, during the 2003-2004 hurricanes in Florida, various civil engineering structures including traffic signals were damaged (Cook et al., 2012). Furthermore, existing standards and specifications such as the ASCE 7 and AASHTO, 2013 does not provide adequate design guidelines for the safe design of such structures. It is well known that a faulty traffic signal can cause considerable inconvenience to motorists. For instance, Sivarao et al., 2010 state, “highest number of road accidents occurs at these junctions due to careless and recklessness of the drivers especially when the traffic lights go faulty due to various reasons”. Most importantly, adequate functioning of the traffic signal is necessary for proper traffic management and evacuation purposes during and after major extreme events. Therefore, this explains the need to assess the wind induced loads on traffic signals so that they may be designed to withstand the wind forces and remain functional at all times.

At the first phase of this study the focus was on two different configurations, one of which consists of a 5-section and a 3-section traffic signal, while the other consists of two 3-section and one 4-section traffic signal. The main objective of these tests and the subject of this paper is to identify the more vulnerable of the two configurations, so that it may be used as a ‘benchmark’ for future studies. In summary, the wind induced forces and moments on the catenary and messenger wires, besides the inclination and acceleration of the signals were measured for wind speeds varying from 0 to 130 mph. The following section presents the experimental set up and the different configurations tested in the WOW followed by the experimental results and their critical discussion.

WALL OF WIND EXPERIMENTAL SET UP AND MODEL CONFIGURATIONS

The Wall of Wind (WOW) experimental facility at Florida International University (FIU), USA is a large scale research laboratory that has been involved in various aspects of wind engineering, such as assessment of wind-induced loads on low-rise and tall buildings and building components, wind loads on infrastructure systems, wind-driven rain effects on buildings etc. (Mooneghi et al., 2014, Kargarmoakhar et al., 2015). The intake side of WOW consists of 12 fans of 700 horsepower each (see Figure 1a) and the test section includes the flow management system and the 16 feet turntable (see Figure 1b). The flow management system can be configured to simulate different types of atmospheric boundary layer profiles (ABL). In the present study, the spires and roughness elements produced an open terrain (exposure C, ASCE 7-10), with both velocity and length scales of 1:1.

Figure 1. 12-Fan Wall of Wind experimental facility: (a) intake side, (b) open jet test-section side

The traffic signals to be tested were installed on a special test rig with a span of 21 feet 11 inches. There were two different hollow structural sections (HSS) that were utilized, HSS 10” x 6”x 3/8” and HSS 6” x 6” x 3/8”. The total length of the test rig is 24 feet and the width is 7 feet and 6 inches. The two support columns are 15 feet 6 inches tall supported on top of HSS 10” x 6” x 3” sections. A photo of the test rig on top of the WOW turntable is shown in Figure 2.

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Figure 2. Picture of test rig frame on top of the turntable

A sag length of 4 feet on the catenary wire (top most wire) was used to simulate full scale conditions. Figure 3 shows the location of the load cells to measure the forces and moments and their respective directions. Load cells 1 and 4 are on either side of the catenary wire, while load cells 2 and 5 are on either side of the messenger wire. In addition, accelerometers (Acc.2, Acc.3, Acc.5) and inclinometers (Incl.3, Incl.4) were used to measure the acceleration and inclination respectively of the traffic signals (see Figure 3).

Figure 3. Location of the load cells, accelerometers and inclinometers on the test rig and traffic signals

The first signal combination to be tested was one 3-section vertical and one 5-section (i.e. configuration1). Both signals were placed with a 5 feet center to center separation at approximately mid span of the test rig. There was a distance of approximately 8 feet 6 inches from the outside edge of the 3-section signal to the front face of the steel column as shown in the sketch of the first signal combination setup in Figure 4a.

The second signal combination to be tested was two 3-section and one 4-section (i.e. configuration 2). Signals were placed with a 4 feet center to center separation at mid-span of the test rig. There was a distance of approximately 6 feet 11 ½ inches from the outside edge of the two outer signals to the front face of the steel column as shown in sketch of second signal combination setup Figure 4b. The bottom of all signals for both combinations tested was at approximately 4 feet 6 inches above the concrete floor.

Figure 4. Schematic of the 3+5 signal configuration (a) and 4+3+3 signal configuration (b) tested at WOW

The wind perpendicular to the face of the traffic signal represents 0o. The wind speeds were gradually increased from 10 mph to 20 mph, 40 mph, 70 mph, 100 mph and 130 mph, or until failure occurred. The term ‘failure’ is defined as a condition when a part of the traffic signal fails. This may include: signal light not functioning due to electrical malfunctioning caused by excessive wind loads and vibration, or lack of alignment caused by excess inclination of the traffic signal, preventing the motorists from clearly viewing the signals. Failure also included, if any of the wires (catenary or messenger) ruptured during the test, although damage to the ‘back plates’ was not considered a ‘failure’. The measurements were started only after about 3 minutes during which time a stable flow in the WOW was achieved.

RESULTS AND DISCUSSION

The results and discussion are divided into three parts. First representative results from the wind induced forces measured from load cells 1 and 5 are discussed, followed by the root mean square (rms) of accelerations and, inclinations for the different types of signals tested.

The comparisons on wind-induced tensions on catenary and messenger wires for both configuration tested are shown in Figure 5. More specifically, Figure 5a shows the catenary tension on load cell 1 that was placed on the left side of the test rig (see Figure 3 for exact location). Configuration 1 generated an increase in the tension from 550 lbs to 587 lbs, from wind speeds of 0 to 32 mph. However, beyond 32 mph up to 60 mph, the tensions reduced from 587 lbs to 570 lbs. Similar trends were also observed in Configuration 2, although the reduction in the tensions were even more pronounced, since the tensions reduced from 548 lbs at 30 mph to 418 lbs at 96 mph.

The reversed phenomenon was observed in the case of the messenger wire; as wind velocity increased the tension in the messenger cable increased as well. Figure 5b shows an increase in messenger tension on load cell 5 which was on the left side of the test rig and below load cell 1 (see Figure 3 for exact location). In both configurations the tensions increased from approx. 550 lbs at 32 mph to approx. 1310 lbs at 96 mph.

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Figure 5. Forces measured by load cells: a) Catenary tension (load cell 1); b) messenger tension (load cell 5)

The above results can be justified as follows; data indicates that lift force generally contribute to reducing the tension in the catenary cable as wind speed increases. Drag force initially contributed to an increase in tension in the catenary after which there is a decline in tension as wind speed increases. Span wise force also

(a) (b)

initially contributes to an increase in tension in the catenary cable followed by a decline in tension as wind speed increases.

Data collected from the accelerometers installed in the 3-section signals revealed that there were higher rms accelerations on the bottom corners of the signal as opposed to the top center (see Figure 6). In point of fact, from the onset of the test at very low speeds the bottom right corner accelerometer was recording higher rms accelerations than the other two. All three accelerometers indicated rms accelerations to be increasing as wind speed increased. This behavior was similarly observed in 3-section traffic signals for both configurations.

For the case of the 4-section signal, data collected from the accelerometers revealed that the bottom right and left corners appeared to have similar rms accelerations throughout the test. The top center of the signal had similar rms accelerations initially but decreased slightly later in the test. The difference in rms accelerations between the three locations was not as dispersed as seen in the 3-sections signals. The behavior of the 4-section signal for Configuration 2 is shown in Figure 7. Very similar behavior to that seen in the 4-section signal was observed in the 5-section signal, as shown in Figure 8. The bottom right and left corners have similar rms accelerations whereas the top center of the signal had similar rms accelerations early on but decreased slightly later in the test..

Figure 6. RMS acceleration of the 3-section traffic signal for Configuration 2

Figure 7. RMS acceleration of the 4-section traffic signal for Configuration 2

Figure 8. RMS acceleration of the 5-section traffic signal for Configuration 1

Of great interest to the performance of the signal assembly was the data recorded by the inclinometers. As previously mentioned, the inclination of the signal assembly is one of the main criteria for its functionality. If the signal exceeds a certain incline then the drivers may not be able to clearly identify their indication which makes them not operational. A representative case from the ‘inclinometer 3’ placed at the bottom of the 5-section traffic signal (see Figure 3) for Configuration 1 is presented in Figure 9. As expected, results show that with increasing wind speed the inclinations of the traffic signal increased. For instance, the inclination measured along an axis perpendicular to the wind was about 35 degrees at 63 mph. Results also indicate that at wind speeds above 45 mph, the inclination is about -2 degrees (signal veers to the right), when measured along an axis parallel to the wind direction. In fact, at wind speeds above 70 mph, an aerodynamic instability similar to ‘galloping’ was also noticed in some cases. Overall, observations in the data revealed that much higher inclinations occurred relative to the axis perpendicular to the wind than relative to the axis parallel to the wind for all traffic signals utilized in both tests.

Figure 9. Inclination of the 5-section traffic signal for Configuration 1 (inclinometer 3)

CONCLUSIONS

Traffic signals underwent significant wind induced damage during hurricanes in the past, causing severe disruption in traffic. To address this issue, experiments were conducted at the Wall of Wind (WOW) experimental facility at Florida International University (FIU), USA. Two different configurations were considered for the study; one of the configurations consisted of a 5-section traffic signal and 3-section traffic signal, while the second configuration consisted of two 3-section traffic signals and one 4-section traffic signal. Wind velocities were gradually increased from 0 to 130 mph, or until failure occurred. The objective was to identify a ‘benchmark configuration’ that can be used in future experimentation. The forces and moments were measured using load cells, while the inclinations and accelerations were measured using inclinometers and accelerometers respectively. Results show that for both configurations, the catenary wire experienced a gradual increase in tension at low wind speeds, but at wind speeds higher than about 30 mph, there was a drop in the tensions. However, the messenger wire experienced an increase in tension with an increase in wind speed for both configurations. This is because, at increasing wind speeds, the traffic signal assembly pushes the messenger wire backwards increasing its tensile forces, while the catenary wire leans forward thereby reducing tensions in the catenary. In general, the first configuration experienced greater tension than the second configuration, and is therefore more vulnerable to wind induced damage. An

aerodynamic instability similar to galloping was also observed, especially at wind speeds exceeding 70 mph which is planned to be investigated further.

ACKNOWLEDGEMENT

The financial contributions of the Florida Department of Transportation (FDOT), Florida, USA is gratefully acknowledged. The opinions, findings and conclusions expressed in this publication are those of the authors and not necessarily those of the Florida Department of Transportation or the U.S. Department of Transportation.

REFERENCES

ASCE 7-10, Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers, ASCE, 2010, Virginia, USA.

AASHTO, American Association of state Highway and Transportation Officials, Standard specifications for structural supports for highway signs, luminaires and traffic signals, 6th edition, Washington, USA, 2013.

Cook, R.A., Masters, F., Rigdon, J.L. 2012. Evaluation of Dual Cable Signal Support Systems with Pivotal Hanger Assemblies. FDOT Contract No. BDK75 977-37, University of Florida, Department of Civil and Coastal Engineering, 2012, Gainesville, FL, USA.

Hamilton, HR, Riggs GS, Puckett JA. Increased damping in cantilevered traffic signal structures. Journal of Structural Engineering. 2000, Apr;126(4):530-7.

Kaczinski MR, Dexter RJ, Van Dien JP. Fatigue-resistant design of cantilevered signal, sign and light supports. Transportation Research Board, NCHRP report 412, National Research Council; 1998.

Kargarmoakhar, R., Chowdhury, A.G., Irwin, P. Reynolds number effects on twin box girder long span bridge aerodynamics, Wind and Structures, Vol. 20, No. 2 ,2015, 327-347.

McDonnald, J.R., Mehta, K. C., W. Oler, and N. Pulipaka. Wind load effects on signs, luminaires and traffic signal structures. Report 1303-1F, Department of Transportation, Texas, 1995, USA.

Mooneghi, M. A., Irwin, P., Chowdhury, A. G. Large-scale testing on wind uplift of roof pavers, Journal of wind engineering and industrial aerodynamics, 128, 2014, 22-36.

Sivarao SK, Esro M, Anand TJ. Electrical & Mechanical Fault Alert Traffic Light System Using Wireless Technology. International Journal of Mechanical and Mechatronics Engineering. 2010; 10(4):19-22.

Zisis, I., Irwin, P., Chowdhury, A., Azizinamini, A., Berlanga, B. Development of a Test Method for Assessing the Performance of Vehicular Traffic Signal Assemblies during Hurricane Force Winds, Report no. BDV29 TWO 977-20, 2016, FDOT, Fl, USA.