Mina Dawood, assistant professor of civil and environmental engineering, has published a report outlining factors that contribute to cracking of high mast illumination poles during fabrication. The report also provides strategies to prevent these cracks and potential failures from occurring.
High mast illumination poles, which range from 100 to 175 feet tall, are typically used at major highway interchanges in urban areas. There have been scattered reports from across the United States of these poles failing, presumably due to wind-induced fatigue, although no such failures have been reported in Texas. Most of these failures start with cracks near the area where the pole is welded to a base plate and connected to a concrete foundation. These cracks first form during fabrication, when poles are galvanized, or coated with zinc, to protect them from corrosion.
“The way these poles are designed, they should have an essentially infinite life,” said Dawood. “The question is if there’s a preexisting crack at the base plate connection, how does that affect the fatigue life of the pole.”
The Texas Department of Transportation (TxDoT) gave Dawood a one-year $50,000 grant to study the problem through computer simulations and analysis of existing data.
To perform this work, Dawood took into account many different factors that could impact pole life, including: eight different pole designs; pole fabrication processes; roughly 30 wind speed regimes and various wind effects; the surrounding terrain, such as buildings or open plains, and its impact on wind; the presence of a ground sleeve that reinforces the pole at its base; and wind and terrain conditions specific to the state’s five major urban areas of Austin, Dallas/Ft. Worth, El Paso, Houston and San Antonio.
Dawood’s report identified pole fabrication practices that were most and least likely to induce cracking during fabrication, as well as pole configurations that were most susceptible to wind-induced fatigue in given areas and under given wind regimes.
The report also identified what is likely the single most determining factor of the ‘safe’ serviceable life of a pole with a pre-existing crack: the presence of a ground sleeve. “The TxDoT ground sleeve detail is incredibly effective at maintaining the safe serviceable life of poles even if there is a crack,” Dawood said. As a result, TxDoT is considering making the use of ground sleeves on high mast illumination poles mandatory, he added.
While this particular research project concluded in August of 2011, Dawood said he hopes to work with TxDoT in the future to expand upon his findings.
“We’d like to add some field work and experimental work that is specifically relevant to the state of Texas and to these connection details,” he said.