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Hoque, Warner, Henriquez, Huynh, Christenson, Caicedo and Yun 2 ABSTRACT Over the years, a number of cantilevered traffic signal support structures (traffic poles) have collapsed as a result of fatigue failure of the mast arm and vertical pole connection. Due to dynamic wind loading, excessive and sustained vibrations have led to the fatigue cracking and subsequent collapse. A number of different methods have been suggested to reduce the excessive wind induced vibration of traffic poles, including struts and vibration absorbers. The Signal Head Vibration Absorber (SHVA), developed at the University of Connecticut, is a promising type of vibration absorber for traffic signal support structures. This device has experimentally shown to increase the critical damping of the traffic signal structures to 10.1%, eliminating any steady state deflection, and protecting the structure from fatigue damage. In South Korea, where the cantilevered traffic poles are shorter and lighter, struts connected at an angle between the pole and mast arm are used exclusively throughout the country to reduce vibrations. The shorter mast arm length, coupled with the use of a strut, made the structures lighter than average poles in the United States. This paper explores the potential benefits of using a SHVA versus a strut to control the vibration of traffic signal support structures. A 55 foot American pole is evaluated against a 17 foot Korean pole to examine the potential benefits and relative merits of these two methods of reducing excessive wind induced vibration of traffic poles over various traffic pole configurations. INTRODUCTION Cantilevered traffic signal support structures are used all over the world to ensure a clear view of the signal head over traffic lanes. Throughout the United States, the geometrical and structural designs of these structures vary from state to state, whereas, in South Korea, they are constructed uniformly throughout. These structures are widely used because they cost less than the alternative bridge structures and provide increased motorist safety by reducing the chance of collision with a single vertical pole versus two poles. However, these structures are vulnerable to fatigue damage due to slenderness and dynamic loading. The traffic signal support structures are very flexible and lightly damped. This flexibility results in a low natural frequency in the range of 0.7 Hz to 1.4 Hz (1), while the critical damping ratio has been measured to range between 0.15% and 0.5% (2). This low damping results in a dynamic response much higher than that of the static response for the same load and can result in fatigue failure for these cantilevered traffic signal support structures. Fatigue problems of the cantilevered traffic signal support structures have led to significant research over the past two decades. The research mainly focused on finding: (i) the root cause of fatigue problem, and (ii) the methods of mitigation. It has been well established that the traffic signal support structures are primarily susceptible to wind induced vibration. The American Association of State Highway and Transportation Officials (AASHTO) lists four types of wind fatigue design loads in the 2009 Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals: galloping, vortex shedding, natural wind gusts, and truck induced gusts (also known as buffeting). AASHTO(3) also states that the maximum vertical deflection at the free end of the traffic signal mast arm must be limited to 8 inches, due to galloping and truck induced gust (4). It should also be noted that as a cantilever increases in length, so does the likelihood that it will gallop (5). The maximum tip deflection limit is set to ensure that the motorists can clearly see the attachments and do not feel unsafe to drive beneath the structure. However, in most cases, the truck induced fatigue loading can be excluded for fatigue design of cantilevered traffic signal structures. This is because many of these structures TRB 2012 Annual Meeting Paper revised from original submittal.
