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
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
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.
|Sponsoring Committee:||AKB10, Innovative Highway Structures and Appurtenances
|Research Period:||12 - 24 months|
|RNS Developer:||Sam Fallaha, Florida DOT|
|Source Info:||The following list includes organizations interested in the research results and could help support implementation:|
• AASHTO Committee T-12, Sam Fallaha
• TRB Sub-Committee AFF10(1), Carl Macchietto, P.E
|Index Terms:||Electric transmission towers, Amplitude (Physics), Vibration, Remote sensing, Loads, Structural health monitoring, |
Bridges and other structures