[Note that this article is a transcript of the video embedded above.]

In 1974, a new world record was set for the tallest structure on Earth. Soaring to 646 meters or 2,120 feet, the Warsaw Radio Mast was built to broadcast radio programs to Polish-speaking audiences across Europe. If the atmospheric conditions were just right, those signals could be picked up from nearly anywhere in the world. But like all big infrastructure projects, building it was only half the battle. Maintaining a structure that tall—and that slender—was incredibly expensive. Over time, the guy wires that held the tower upright began to wear out. By 1991, many of them were frayed and overdue for replacement, a job that wasn’t just costly, but also fairly complex.

To replace a guy wire, two temporary guys needed to be attached to the mast first. Then the old guy could be removed and swapped out for a new one. But on August 8, 1991, the sequence got mixed up. Reports vary, but it seems that one of the main cables was disconnected before the temporary ones were fully installed. A gust of wind twisted the tower, pulling the temporary cables away, and the unsupported mast collapsed. Incredibly, no one was injured in the failure, but it was a catastrophic loss nonetheless. Usually, the tallest structures in the world lose their position because something else is built taller. In this case, a tower in North Dakota regained the lead by default.

It’s actually not an unusual story. This particular type of structure, called a guyed mast, has some seemingly bizarre structural characteristics that make it possible, including the sometimes unusual bases that seem to defy logic. But they come with risks, too. At least nine guyed masts taller than 600 meters have collapsed, mostly in the US, and hundreds of similar shorter structures around the world as well. They’re pretty interesting structures: cool to look at, incredibly tall, just rare enough that seeing one is kind of special. So this video is an ode to guyed masts, and of course, I built a little demo in the garage to help explain how they work. I’m Grady, and this is Practical Engineering.

Radio communication is a remarkable technology that enables a huge variety of wireless devices, from garage door openers to cell phones. If humans could perceive the full spectrum of electromagnetic radiation, even just the human-made stuff, we would be completely overwhelmed by the volume and variety of information moving through the airwaves. Many of the frequencies used for communication, especially those broadcast by radio and television stations, require a clear line of sight; the path between the transmitter and receiver has to be relatively unobstructed, at least by objects that are opaque to radio waves, like the earth. That’s why many antennas are mounted at the tops of hills, mountains, or (lacking those) gigantic towers. The higher they are, the further their signals can extend.

Antenna towers are some of the tallest human-made structures in the world, with many topping out above 600 meters (roughly 2,000 feet). At that height, the distance to the horizon is more than 50 miles (or 80 kilometers). To achieve that has required some very clever structural engineering. Let me show you what I mean.

This is my model antenna tower. Pretty basic; just a steel welding rod stuck in a plate. This isn’t going to match the structural behavior of an actual mast, but it’s close enough for a garage demo. The main load on a tower like this, besides its own weight, is wind. So let’s apply some wind and see what happens. [Beat]. The tower’s still standing - it didn’t collapse. But structural engineering isn’t all about strength. A structure can “not fall down” but still fail. We also have to address the concept of serviceability: does the structure actually do what it’s meant to? And in this case, hopefully it’s clear that the answer is no. Many antennas are designed to be directional. It takes a lot of power to radiate signals, so you don’t want to waste it sending them where they’re not needed. This varies a lot depending on the end use. Radio and TV broadcasts are less sensitive to movement than microwave communications, but in general, we can’t have antenna towers wobbling around like floppy wet noodles in the sky.

You can imagine that to adequately stiffen this tower, it would have to be a lot wider at the base. And that’s just what we do with so-called self-supporting towers. They’re designed to be freestanding and stable against the wind entirely on their own. Self-supporting towers don't take up much space, so they are ideal in urban areas where land comes at a premium. But, they are expensive to build because of all the extra material required for stiffness and stability against lateral wind loads. In fact, their cost goes up roughly proportional to the height squared. For guyed masts, it's roughly height to the power of 1.5. You need more land for a guyed tower since the guys extend so far out, so there is more cost there, but above a certain height (that depends on those land costs), it becomes the most economical option. And for really tall towers, it’s really the only technically feasible one. They are just so structurally efficient, it's almost unbelievable. To give you an example, at 324 meters tall (or 1,060 feet) the Eiffel Tower weighs around 7000 tons. A guyed tower of the same height would weigh roughly five percent of that.

So let me add some guys to my tower and we’ll see how it works. Of course, you can’t add just one. Wind can come from any direction, and don’t forget one of the most important adages of civil engineering: you can’t push a rope. So it takes at least three guys to get some tension in every direction. Some towers use four lanes, but most stick with three. This seems like a more stable situation, but now we’ve got a new problem. Watch what happens when I apply a lateral load. It's still just not that stiff, and actually, the tower buckles. And here’s why:

The guys can’t pull horizontally on the tower to resist lateral loads directly. They have to be anchored to the ground, which means they meet the tower at an angle. Any tension in the cable is going to necessarily put the tower in compression as well. And what happens with skinny compression members? They buckle.

Steel can take a lot of compression. Theoretically, this rod is strong enough to hold my entire weight without a material failure. If it were short, it’d be more than capable of bearing a full Grady, but when it’s tall and skinny like this, it can barely hold its own weight. When the tower takes a lateral load, the guy wires transfer that into compressive force. And unless the structure is stiff enough, it buckles. If I move the guys out so they’re at a shallower angle, you can see it takes a lot more wind load to buckle the structure. Less cable tension is needed for an equivalent horizontal force. And this is one of the many structural tradeoffs with guyed towers. You have to balance the land cost of extending anchors outward against the cost of a stiffer tower that can withstand steeply angled guys.

But you can see we’re not quite out of the woods here. Some shorter guyed towers can get away with one level of supports, but mine is still pretty flimsy in the middle. Lateral forces can still deflect it quite a bit, and it’s still prone to buckling under compressive loads, like, for example, the weight of an antenna mounted to the top. And now this is kind of like a bridge on its side.

We’ve got supports on both ends and loads trying to bend the structure in the center. So we can do what the bridge engineers do: either stiffen the structure or add more intermediate supports. It’s a little more complicated than that though, since every guy adds additional compressive load on the tower, in addition to providing lateral support to reduce the unbraced height. You’re kind of adding to both sides of the equation. Luckily, the lower you go on the tower, the shallower the angle of the cable. Just as a little demonstration of this, let’s compare the loads my little tower can support as we add more guys. With just one level, it’s right around 50 grams. This can barely support its own weight, let alone any extra on top. With a second level halfway up, it’s quite a bit stiffer. I could get 100 grams on top with no failure. Adding two more levels, now this thing feels rock solid. I’m not sure if it comes across on camera, but the change in stiffness is dramatic. It passes the wind test with flying colors. It couldn’t quite hold a kilogram, but Brady could sit on it just fine, even if it made him a bit uneasy (since his hard hat is still damaged from the last demo).

One of the other tradeoffs with this is the pre-tension of the cables. These guys sag along their length; they’re not perfectly straight. Under high wind, they tighten up and add stiffness. But in calm conditions, that slack can cause the tower to wobble. The obvious solution is to pre-tension the guys to take the sag out, but again, that pretension puts extra compression on the tower, requiring stronger members or more guys. So this is a balancing act as well.

And then there’s the base. You have essentially two choices here. We’re used to seeing large columns with a rigid attachment to the foundation. I did a whole video on base plates diving into this topic deeper if you want to learn more. You can see in my model that, with a fixed connection, my tower holds itself up just fine without loading. Obviously, this rod is solid steel - not a thin latticework of individual members - so the behavior is a little different. But remember that buckling is a function of the end connections of the column. With the bottom fixed, it takes about 140 grams to buckle the rod. When it’s free to rotate at the bottom, it buckles at around half that. The problem in this case is that fixing such a tall tower rigidly to the foundation makes the design a lot more complicated.

If you want rigid restraint, you have to have a way to transfer the loads into the ground. So the foundation has to be designed to resist rotation and pullout forces, and for not a lot of structural benefit. So the other option is to use a spherical bearing or pin support. And if you keep your eye out, you’ll see that a lot of these masts have these sorts of unusual bases where they taper down to a narrow point. In this way, you can just rely on the guys to handle almost all the restraint. The foundation only has to resist the vertical force, and maybe a touch of shear. This allows some movement or settlement of the foundation without inducing stress into the structure. And it just makes the design process easier. Removing the restraint simplifies the structural response and makes the tower more predictable, so you don’t have to be super conservative or spend tons of engineering effort and use sophisticated modeling software in the design. Finally, some towers aren’t used to mount antennas; they are the antennas themselves. For lower frequency transmissions like AM radio, you need a big antenna, so the tower itself is energized. In those cases, the base needs to be electrically insulated from the ground, which is much easier to do at a single point. If you look closely at some towers, you’ll see they’re actually standing on a ceramic disc.

Beyond structural design, these masts come with a lot of other engineering challenges. Of course, there’s the hazard to aircraft. Aviation regulations often require them to be painted in alternating orange and white bands and equipped with warning lights, whose color and flash rate are carefully prescribed, and can even be synchronized with nearby towers to avoid dazzling pilots at night.

Ice is another big one. These towers stretch into colder, wetter layers of air where ice can build up on the mast and guys. That adds weight, but it also adds surface area, sometimes dramatically increasing wind loads. When it melts, it can fall and damage anything below, so often you’ll see protective structures over the radio transmission lines.

Lightning is another threat. For most towers, it’s not a question of IF, but rather HOW OFTEN they’ll be struck. Towers are often equipped with lightning rods or other protection devices and robust grounding systems to keep stray voltage out of the transmission lines and sensitive equipment on the ground. Obviously, those mast radiators I mentioned earlier, where the entire tower services as the antenna, can’t be grounded for lightning protection. So most use some type of spark gap to keep the tower insulated. If lightning strikes, the air in the gap ionizes, allowing the surge to safely reach the ground.

Like all infrastructure, antenna towers need maintenance - painting, changing light bulbs, and servicing antenna equipment. Technicians with specialized training for heights and electrical hazards have to do the work. Some tall towers are even equipped with elevators to provide access, but most require some manual climbing. Although the frequencies used for radio communication are non-ionizing (meaning the waves can’t break apart molecules), that doesn’t mean they aren’t dangerous. Electromagnetic radiation can generate heat; it’s the fundamental principle of a microwave oven. And if the tower itself is energized, a person can become part of the circuit.

With so much of our telecommunication happening through the internet these days, it’s easy to forget the importance of large-scale radio broadcasting and communications. The cells for cellular communications are small, so we’re used to seeing those antennas relatively close to the ground. But you have to look way up to remember how critical the other wireless systems are, especially in emergency situations where radio and television signals can be an essential link to information. So next time you pass one of these towers by, take a closer look, and I hope you’ll appreciate some of the thoughtful engineering that goes into them.