Originally published by Inside Towers
Author: Jim Fryer
Why do tower fall down?
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 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.
“Even if they’re built correctly,” Griswold said “they fall down. 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 is 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 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.
Self-standing sites have a much wider range of potential failure caused by inadequate bracing, unsecured footings, 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 seen tens of thousands of downed steel structures lying in a crumpled mess waiting for their final inspection and assessment.
Guyed towers, of course, fail more often due to the feature built into their name: the wire and wire connections themselves that tether them into an upright position. Anchor corrosion, loose cables that cause “galloping,” shallow footings in relation to the soil and climate all contribute to their demise, according to Griswold.