Structural Behavior Testing of Giant Lighting Towers

Case Studies
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In November 2003, a 140-ft, high-mast lighting tower collapsed in Sioux City, Iowa. Experts from Iowa State University, Purdue University, Lehigh University and University of Minnesota teamed with the Iowa Department of Transportation (IDOT) in a statewide investigation into the behavior and design of these tall, yet relatively flexible structures. State-of-the-art Campbell Scientific data acquisition systems were used to help measure the key factors affecting the safety of the towers.

Physical inspection of all of these lighting towers determined that some needed reinforcement or replacement. The team then set about determining the in-service stresses produced by wind loads as well as determining the overall behavior characteristics of several of the structures.

IDOT’s Office of Bridges and Structures began a study of the towers in 2004, led by Robert J. Connor, PhD, and Ian C. Hodgson, P.E., S.E. The field instrumentation was intended to quantify the stresses induced in critical components of the towers, to measure the wind phenomena that produced fatigue damage and to identify and measure the key dynamic properties of the towers themselves. All of this was accomplished using field instrumentation, testing and long-term monitoring of a selection of towers.

Many towers were studied throughout two phases of testing, and data from a variety of sensors was collected. The investigators attached anemometers, strain gages and accelerometers to the towers. On most of the towers the researchers performed a dynamic test called a “pluck” test where stresses were applied and then suddenly released to let the tower vibrate. The resulting natural frequencies and damping characteristics of these high-mast towers could be determined. A Campbell Scientific CR9000 datalogger collected the data. The CR9000 is a high-speed, multichannel, 16-bit data acquisition system, configured with digital and analog filters to assure noise-free signals.

Two towers were initially monitored for a period of just over one year to measure the naturally occurring wind stresses the towers endured. One tower was not modified in any way and was called the "as-built tower." This tower had no damage but was similar in structure to the one that collapsed in November 2003. A CR9000 was installed adjacent to this pole and collected data from over 20 sensors. Another tower was termed the “retrofit tower,” because it was modified with a steel splice jacket at its base. The sensors at this pole were monitored by a CR5000 datalogger, another high-speed, multichannel, 16-bit logger. Both of these towers were studied for 12 months for their response to natural wind loads. It was important that the instrumentation be dependable enough to perform through various Iowa seasons and to be managed remotely.

Remote communication with the data loggers was established using a satellite Internet connection on the as-built tower. Data collection was performed automatically via a server located at Iowa State University. The satellite link also was used to upload new programs as needed. A wireless link was installed between the as-built tower and the retrofit tower several miles away.

To assure all sensors were functioning properly, a CR9000 was set up in a field vehicle and driven to the various sites to be connected directly to a laptop. The resulting data can now be studied by the researchers and IDOT. As part of a follow-up study, another pole is currently being monitored for a period of 12 months to further enhance the knowledge base related to the in-service response of these critical components of the transportation system.

In all of these study phases, Campbell Scientific dataloggers proved to be essential to acquiring the information researchers needed concerning these high-mast lighting towers.

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