Why Towers Fall: An Engineer’s Perspective
Prior to my career as a Consulting Engineer at EDT, I worked for over a decade in the telecommunications industry where I performed structural engineering work and inspections services. In this role, I traveled all over the country and worked on thousands of cellular and broadcast towers which gave me a better perspective on this special industry that’s so ubiquitous to all our lives.
One thing that makes these structures so unique is that, unlike most structures we build, they are not usually occupied or even visited. So, much like a tree and its relative volume levels, if a tower falls and no one is around to see it, how do we figure out what went wrong? The answer to that question relies on a thorough understanding of the engineering behind tower structures and a historical knowledge of the issues that have caused tower collapse in the past.
Firstly, while a properly designed, constructed, and maintained tower is resilient to the elements, natural disasters still happen. Major wind events, such as hurricanes and tornadoes, can knock a tower over, or disrupt cell service at the minimum. Ice storms and freezing rain can build up on the tower, causing a significant increase in weight and wind area.
However, many tower failures occur as a result of improper design or construction. For example, I investigated a tower that collapsed during a major hurricane, in which a seven-foot-long portion of one of the tower foundations uprooted from the ground. At first glance, this is just another example of high winds, right? Well, there were two additional facts to consider: One, that seven-foot-long foundation was supposed to be thirty feet long, and two, mud was caked and embedded within the bottom of the concrete foundation. It turns out that there was a partial cave-in of the hole during concrete placement when the tower was built, creating voids within the foundation which provided a weak point that broke during the hurricane.
Improper maintenance is also a frequent cause of tower failures. Since towers are not regularly visited or utilized the same way other buildings or equipment are, a regular and thorough maintenance schedule and checklist are mandated as part of the governing standard for the industry. Because of its high strength and ductility, nearly all tower structures and their components are constructed of steel. If not properly protected, and especially if exposed to moisture, such as in the great outdoors, steel will go through an electro-chemical process in which the iron is converted to iron oxide. This process, known as corrosion, or rusting, will deteriorate the steel until enough material is lost so that the tower can no longer support the load its holding.
Lastly, I may have been misleading before. Many tower failures, especially the ones that make the news, occur while someone is present on-site. Improper construction practices, on the part of contractors or individual crew members, have led to many of the more tragic tower failures. To lift new antennas or equipment up the tower, it is common to connect lifting equipment, or rigging, directly to the tower and use that to pull the load upward. Improper rigging, such as using a carabiner instead of a properly rated shackle, or failing to inspect a damaged sling prior to use, can break mid-lift causing the load and rigging to fall, striking and/or breaking whatever it hits on the way down. Similarly, removing and replacing structural members on the tower, as part of an upgrade, requires a structural engineer to be engaged with the planned construction to determine if the tower has enough excess capacity or requires some form of temporary bracing.
When it comes to tower failures, sometimes the cause seems obvious, but requires further investigation, such as a hurricane blowing over a tower with an improperly constructed foundation. Other times, the cause is less obvious, and more time is spent investigating the wreckage, such as determining where and how the rigging broke. Figuring out the root cause takes a combination of engineering comprehension on how towers are designed and how the materials behave, as well as a practical understanding of how they are built and the shortcuts taken. Determining the root cause is important, not only for determining fault, but for advancing the industry. While the jury may still be out on whether a tree makes a sound when it falls, a fallen tower always has something to say.
About the Author
Luke Meadows, P.E. is a Consulting Engineer at Engineering Design & Testing Corp. (EDT) with a background in structural engineering and failure analysis. Licensed as a Civil Engineer in multiple states including Alabama, Florida, Georgia, Indiana, Louisiana, Mississippi, North Carolina, and Tennessee, Luke specializes in evaluating the performance and integrity of steel and concrete structures, with extensive experience in wind loading, environmental effects, and welded connections.