Editor’s note: this article ran last week and, in covering a highly technical subject, glossed over several key points that it’s subject, Bill Griswold wanted to share and clarify on an important issue. To that end, we are re-running the article with revised content. JF
NATE Unite 2019
Bill Griswold, Jr. PE, President of Griswold Tower Software, doesn’t have a quick answer to that. Monopoles, self-standing and guyed towers all face different laws of physics when it comes to structural failure and occasionally it’s Griswold’s job to figure out what put a perfectly erect tower on the ground.
In an exclusive interview with Inside Towers, Wednesday at the NATE Unite 2019 show in Dallas, Griswold explained the myriad of factors that come into play in a tower collapse from wind vortex shedding, welding micro cracks, metallurgy failure, bad grouting and the primary catalysts that create the stress: cars, planes, ice and wind.
Monopoles are more likely to fatigue at the base.” While a non-expert might place the blame solely on wind-loading stressing the foundation, Griswold said it can be due more to vortex shedding on the pole itself in winds as mild as five-to-twenty miles per hour that get the pole’s vibrating frequency up (“all structures have a natural frequency”, he said). High winds, in fact, often obliterate the offending vortex while poor grouting and welding can exacerbate a tower’s failure, Griswold said. The TR14 committee has done considerable work on the vortex shedding (wind induced structural oscillations) that is now addressed in TIA-222-H, Annex M. They continue to develop additional vortex shedding criteria as part of the continuous improvement of the TIA-222 Standard.
Self-Supporting Towers have a much wider range of potential failure causes. These can include inadequate bracing, loose bolts, rust and large objects impacting them at a high rate of speed on the ground or in the air. Griswold is often brought in as the CSI-type investigator hired by insurance companies, carriers or towercos to develop a forensic study of the fallen victim. In his decades on the job, he has been involved in the engineering of over 10,000 towers. The number of failures he has investigated number less than 2 dozen.
There are numerous reasons a Guyed tower might fail. These can include anchor corrosion, impact, “galloping of the guy wires”, shallow footings in relation to the soil all can contribute to their demise.
Another dangerous time for any type of tower is during construction activities, such as modifications and equipment changes and/or additions. There have been several failures in the past few years that occurred when a contractor did not follow proper procedures. Griswold works closely with many tower contractors to prevent these failures.
February 12, 2019