Investigation of High-Mast Lighting Tower Large-Amplitude Vibrations

Video evidence and field-measured time-history data have demonstrated high-mast lighting towers (HMLTs) are susceptible to large-amplitude vibrations. The videos depict HMLTs in first-mode harmonic resonance with tip deflections over five feet. Similar events have been observed throughout the US, in states such as Alaska, Kansas, Nebraska, South Dakota, Utah, Wisconsin, and Wyoming. Further, HMLT owners have reported collapse and severe fatigue damage during follow-up inspections. As most HMLTS are located directly adjacent to travel lanes, failure poses a high safety concern to the motoring public. For example, some of the video evidence, captured by passing motorists, depicts HMLTs vibrating in the median between travel lanes.

Large-amplitude vibrations can cause high stress ranges at fatigue sensitive details, leading to premature failure. Limited field-measured time-history data suggests a single event has the potential to expend the entire fatigue life of a pole. Additional data is required to identify the loading conditions that result in these large-amplitude vibrations. The objective of the proposed research is to conduct long-term remote monitoring to quantify the loading conditions resulting in large first-mode vibrations, thereby enabling the development of improved design procedures and effective mitigation strategies.

The objective of the research is to establish the loading condition(s) and determine potential mitigation strategies for HMLT large-amplitude vibrations. Study results should include design recommendations to prevent large-amplitude vibration of new structures and/or propose mitigation strategies for existing inventory.

HMLTs are located directly adjacent to the motoring public; as such, any failure could have catastrophic results. The large-amplitude loading condition(s) have the potential to create large fatigue cracks in a short time period. Further, typical removal and replacement of a single HMLT is approximately $80,000. Therefore, identifying the cause of the vibrations will reduce the number of premature failures, while implementing design and/or mitigation procedures will maximize the value of department of transportation (DOT) infrastructure investment today and in the future.

The previous and on-going data collection has demonstrated the large-amplitude vibrations are rare and difficult to capture. However, while rare, the large-amplitude event has a critical impact on the performance of the structure, and potentially catastrophic consequences. Additional monitoring locations are required to capture a sufficient amount of data in a reasonable timeframe. Ultimately, the proposed study will increase the likelihood for the loading condition(s) to be captured and quantified, thereby enabling potential strategies to be developed for future design and mitigation. As a result, the study has the potential to directly impact the LRFD Specifications for Structural Supports for High Signs, Luminaries, and Traffic Signals.

In March 2011, a video was recorded in Watertown, South Dakota of a HMLT experiencing large-amplitude first-mode harmonic resonance. The initial observation occurred near the completion of an NCHRP study focused on developing the fatigue design loading for HMLTs (NCHRP Report 718). The NCHRP study monitored 11 HMLTs at eight locations over 24 months. Following the video evidence, the NCHRP 718 researchers examined the triggered time-history data from the study for any large-amplitude events. Two possible instances of large-amplitude first-mode harmonic resonance were identified during the review; however, neither case had calculated tip displacements on the order of magnitude of the displacements estimated from the Watertown, SD video.

Following the events from Watertown, SD and NCHRP 718, large-amplitude HMLT vibrations have been observed in additional states, specifically Alaska, Kansas, Nebraska, Utah, Wisconsin, and Wyoming. In addition, many HMLT owners have reported premature fatigue cracking and/or collapse, with some of the failures occurring within several years of initial installation. As a result, in 2017 the Wyoming Department of Transportation funded a study attempting to capture the large-amplitude vibrations. The objective of the research was to use field-collected data to understand the loading condition(s) resulting in the rare loading phenomenon. The ongoing study is monitoring four HMLTs across the state of Wyoming. To date, the research has captured three events at two different monitoring locations. The largest recorded event had a peak stress-range of 78 ksi, corresponding to a calculated tip displacement range of 216 inches. Additional data is required to fully characterize the loading condition to make necessary improvements.

The following are a list of major tasks required to successfully complete the research:

Task 1: Detailed literature synthesis

Task 2: Determine selection criteria for field testing and long-term monitoring and identify the monitoring sites

Task 3: Identify analytical methodology for predicting loading conditions and mitigation strategies

Task 4: Conduct field and analytical studies

Task 5: Develop proposed design recommendations and mitigation strategies

Task 6: Submit revised design recommendations and mitigation strategies (draft final report)

Task 7: Prepare and submit final report

Research results would be provided to state DOT designers and leveraged to improve new HMLTs. Similarly, maintenance personnel would employ proven mitigation techniques to improve in-service performance of the existing inventory. Training seminars and webinars would be developed and used to rapidly educate appropriate personnel to be equipped to effectively implement the research results, namely the mitigation strategies to current structures. Further, a long-term implementation strategy includes updates to the LRFD Specification and a mitigation guidelines document.

The study has the potential to directly impact the LRFD Specifications for Structural Supports for High Signs, Luminaries, and Traffic Signals.