The Bizarre Bases of Antenna Towers

0:01 In 1974, a new world record was set for the tallest structure on Earth.

0:07 Soaring to 646 meters or 2,120 feet,

0:11 the Warsaw Radio Mast was built to broadcast

0:15 radio programs to Polish-speaking audiences across Europe.

0:18 If the atmospheric conditions were just right,

0:22 those signals could be picked up from nearly anywhere in the world.

0:25 But like all big infrastructure projects, building it was only half the battle.

0:30 Maintaining a structure that tall—and that slender—was incredibly expensive.

0:35 Over time, the guy wires that held the tower upright began to wear out.

0:40 By 1991, many of them were frayed and overdue for replacement,

0:44 a job that wasn’t just costly, but also fairly complex.

0:49 To replace a guy wire,

0:50 two temporary guys needed to be attached to the mast first.

0:54 Then the old guy could be removed and swapped out for a new one.

0:59 But on August 8, 1991, the sequence got mixed up.

1:03 Reports vary, but it seems that one of the main

1:06 cables was disconnected before the temporary ones were fully installed.

1:10 A gust of wind twisted the tower,

1:13 pulling the temporary cables away, and the unsupported mast collapsed.

1:17 Incredibly, no one was injured in the failure,

1:21 but it was a catastrophic loss nonetheless.

1:24 Usually, the tallest structures in the world lose

1:27 their position because something else is built taller.

1:30 In this case, a tower in North Dakota regained the lead by default.

1:35 It’s actually not an unusual story.

1:38 This particular type of structure, called a guyed mast,

1:42 has some seemingly bizarre structural characteristics that make it possible,

1:46 including the sometimes unusual bases that seem to defy logic.

1:50 But they come with risks, too.

1:53 At least nine guyed masts taller than 600 meters have collapsed,

1:58 mostly in the US, and hundreds of similar

2:01 shorter structures around the world as well.

2:04 They’re pretty interesting structures: cool to look at, incredibly tall,

2:07 just rare enough that seeing one is kind of special.

2:11 So this video is an ode to guyed masts, and of course,

2:15 I built a little demo in the garage to help explain how they work.

2:20 I’m Grady, and this is Practical Engineering.

2:33 Radio communication is a remarkable technology

2:36 that enables a huge variety of wireless devices,

2:40 from garage door openers to cell phones.

2:44 If humans could perceive the full spectrum of electromagnetic radiation,

2:47 even just the human-made stuff, we would be completely overwhelmed by the volume

2:53 and variety of information moving through the airwaves.

2:57 Many of the frequencies used for communication,

3:00 especially those broadcast by radio and television stations,

3:03 require a clear line of sight;

3:06 the path between the transmitter and receiver has to be relatively unobstructed,

3:10 at least by objects that are opaque to radio waves, like the earth.

3:15 That’s why many antennas are mounted at the tops of hills,

3:19 mountains, or (lacking those) gigantic towers.

3:21 The higher they are, the further their signals can extend.

3:26 Antenna towers are some of the tallest human-made structures in the world,

3:30 with many topping out above 600 meters (roughly 2,000 feet).

3:35 At that height, the distance to the horizon

3:38 is more than 50 miles (or 80 kilometers).

3:41 To achieve that has required some very clever structural engineering.

3:44 Let me show you what I mean.

3:47 This is my model antenna tower.

3:49 Pretty basic; just a steel welding rod stuck in a plate.

3:53 "Whoa!" This isn’t going to match the structural behavior of an actual mast,

3:59 but it’s close enough for a garage demo.

4:02 The main load on a tower like this, besides its own weight, is wind.

4:06 So let’s apply some wind and see what happens.

4:18 "That's big!" The tower’s still standing- it didn’t collapse.

4:23 But structural engineering isn’t all about strength.

4:26 A structure can “not fall down” but still fail.

4:30 We also have to address the concept of serviceability:

4:33 does the structure actually do what it’s meant to?

4:36 And in this case, hopefully it’s clear that the answer is no.

4:40 Many antennas are designed to be directional.

4:42 It takes a lot of power to radiate signals,

4:45 so you don’t want to waste it sending them where they’re not needed.

4:49 This varies a lot depending on the end use.

4:52 Radio and TV broadcasts are less sensitive

4:55 to movement than microwave communications, but in general,

4:58 we can’t have antenna towers wobbling around like floppy wet noodles in the sky.

5:03 You can imagine that to adequately stiffen this tower,

5:06 it would have to be a lot wider at the base.

5:09 And that’s just what we do with so-called self-supporting towers.

5:12 They’re designed to be freestanding and stable

5:15 against the wind entirely on their own.

5:18 Self-supporting towers don't take up much space,

5:21 so they are ideal in urban areas where land comes at a premium.

5:25 But, they are expensive to build because of all the extra

5:29 material required for stiffness and stability against lateral wind loads.

5:33 In fact, their cost goes up roughly proportional to the height squared.

5:38 For guyed masts, it's roughly height to the power of 1.5.

5:42 You need more land for a guyed tower since the guys extend so far out,

5:47 so there is more cost there,

5:49 but above a certain height (that depends on those land costs),

5:53 it becomes the most economical option.

5:55 And for really tall towers, it’s really the only technically feasible one.

6:00 They are just so structurally efficient, it's almost unbelievable.

6:04 To give you an example,

6:06 at 324 meters tall (or 1,060 feet) the Eiffel Tower weighs around 7000 tons.

6:13 A guyed tower of the same height would weigh roughly five percent of that.

6:20 So let me add some guys to my tower and we’ll see how it works.

6:24 Of course, you can’t add just one.

6:26 Wind can come from any direction,

6:28 and don’t forget one of the most important adages of civil engineering:

6:32 you can’t push a rope.

6:34 So it takes at least three guys to get some tension in every direction.

6:39 Some towers use four lanes, but most stick with three.

6:43 This seems like a more stable situation, but now we’ve got a new problem.

6:47 Watch what happens when I apply a lateral load.

6:50 It's still just not that stiff, and actually, the tower buckles.

6:54 And here’s why: The guys can’t pull horizontally

6:57 on the tower to resist lateral loads directly.

7:00 They have to be anchored to the ground,

7:02 which means they meet the tower at an angle.

7:05 Any tension in the cable is going

7:07 to necessarily put the tower in compression as well.

7:11 And what happens with skinny compression members?

7:14 They buckle.

7:15 Steel can take a lot of compression.

7:17 Theoretically, this rod is strong enough to hold

7:20 my entire weight without a material failure.

7:22 If it were short, it’d be more than capable of bearing a full Grady,

7:26 but when it’s tall and skinny like this, it can barely hold its own weight.

7:31 When the tower takes a lateral load,

7:33 the guy wires transfer that into compressive force.

7:35 And unless the structure is stiff enough, it buckles.

7:40 If I move the guys out so they’re at a shallower angle,

7:43 you can see it takes a lot more wind load to buckle the structure.

7:47 Less cable tension is needed for an equivalent horizontal force.

7:51 And this is one of the many structural tradeoffs with guyed towers.

7:55 You have to balance the land cost of extending anchors outward against

8:00 the cost of a stiffer tower that can withstand steeply angled guys.

8:04 But you can see we’re not quite out of the woods here.

8:08 Some shorter guyed towers can get away with one level of supports,

8:12 but mine is still pretty flimsy in the middle.

8:16 Lateral forces can still deflect it quite a bit,

8:19 and it’s still prone to buckling under compressive loads,

8:22 like, for example, the weight of an antenna mounted to the top.

8:28 And now this is kind of like a bridge on its side.

8:31 We’ve got supports on both ends and loads

8:33 trying to bend the structure in the center.

8:36 So we can do what the bridge engineers do:

8:39 either stiffen the structure or add more intermediate supports.

8:43 It’s a little more complicated than that though,

8:45 since every guy adds additional compressive load on the tower,

8:49 in addition to providing lateral support to reduce the unbraced height.

8:53 You’re kind of adding to both sides of the equation.

8:56 Luckily, the lower you go on the tower, the shallower the angle of the cable.

9:00 Just as a little demonstration of this, let’s compare the loads

9:04 my little tower can support as we add more guys.

9:10 With just one level, it’s right around 50 grams.

9:13 This can barely support its own weight, let alone any extra on top.

9:17 With a second level halfway up, it’s quite a bit stiffer.

9:21 I could get 100 grams on top with no failure.

9:26 Adding two more levels, now this thing feels rock solid.

9:30 I’m not sure if it comes across on camera,

9:32 but the change in stiffness is dramatic.

9:35 It passes the wind test with flying colors.

9:41 It couldn’t quite hold a kilogram, but Brady could sit on it just fine,

9:45 even if it made him a bit uneasy (since

9:48 his hard hat is still damaged from the last demo).

9:52 One of the other tradeoffs with this is the pre-tension of the cables.

9:56 These guys sag along their length; they’re not perfectly straight.

9:59 Under high wind, they tighten up and add stiffness.

10:03 But in calm conditions, that slack can cause the tower to wobble.

10:07 The obvious solution is to pre-tension the guys to take the sag out, but again,

10:13 that pretension puts extra compression on the tower,

10:16 requiring stronger members or more guys.

10:18 So this is a balancing act as well.

10:21 And then there’s the base.

10:22 You have essentially two choices here.

10:24 We’re used to seeing large columns with a rigid attachment to the foundation.

10:29 I did a whole video on base plates diving

10:32 into this topic deeper if you want to learn more.

10:34 You can see in my model that, with a fixed connection,

10:37 my tower holds itself up just fine without loading.

10:40 Obviously, this rod is solid steel- not a thin latticework

10:44 of individual members- so the behavior is a little different.

10:48 But remember that buckling is a function of the end connections of the column.

10:53 With the bottom fixed, it takes about 140 grams to buckle the rod.

10:57 When it’s free to rotate at the bottom, it buckles at around half that.

11:01 The problem in this case is that fixing such a tall

11:05 tower rigidly to the foundation makes the design a lot more complicated.

11:10 If you want rigid restraint,

11:11 you have to have a way to transfer the loads into the ground.

11:15 So the foundation has to be designed to resist rotation and pullout forces,

11:20 and for not a lot of structural benefit.

11:22 So the other option is to use a spherical bearing or pin support.

11:27 And if you keep your eye out,

11:29 you’ll see that a lot of these masts have these sorts

11:32 of unusual bases where they taper down to a narrow point.

11:35 In this way, you can just rely on the guys to handle almost all the restraint.

11:40 The foundation only has to resist the vertical force,

11:44 and maybe a touch of shear.

11:46 This allows some movement or settlement

11:48 of the foundation without inducing stress into the structure.

11:51 And it just makes the design process easier.

11:55 Removing the restraint simplifies the structural

11:57 response and makes the tower more predictable,

12:01 so you don’t have to be super conservative or spend tons

12:04 of engineering effort and use sophisticated modeling software in the design.

12:08 Finally, some towers aren’t used to mount antennas;

12:12 they are the antennas themselves.

12:14 For lower frequency transmissions like AM radio,

12:18 you need a big antenna, so the tower itself is energized.

12:22 In those cases, the base needs to be electrically insulated from the ground,

12:26 which is much easier to do at a single point.

12:30 If you look closely at some towers,

12:32 you’ll see they’re actually standing on a ceramic disc.

12:35 Beyond structural design,

12:36 these masts come with a lot of other engineering challenges.

12:40 Of course, there’s the hazard to aircraft.

12:45 Aviation regulations often require them to be painted in alternating

12:49 orange and white bands and equipped with warning lights,

12:53 whose color and flash rate are carefully prescribed,

12:57 and can even be synchronized with nearby

12:59 towers to avoid dazzling pilots at night.

13:02 Ice is another big one.

13:04 These towers stretch into colder,

13:06 wetter layers of air where ice can build up on the mast and guys.

13:11 That adds weight, but it also adds surface area,

13:14 sometimes dramatically increasing wind loads.

13:16 When it melts, it can fall and damage anything below,

13:20 so often you’ll see protective structures over the radio transmission lines.

13:25 Lightning is another threat.

13:26 For most towers, it’s not a question of IF,

13:29 but rather HOW OFTEN they’ll be struck.

13:32 Towers are often equipped with lightning rods

13:35 or other protection devices and robust grounding systems

13:38 to keep stray voltage out of the transmission

13:41 lines and sensitive equipment on the ground.

13:44 Obviously, those mast radiators I mentioned earlier,

13:47 where the entire tower services as the antenna,

13:50 can’t be grounded for lightning protection.

13:53 So most use some type of spark gap to keep the tower insulated.

13:57 If lightning strikes, the air in the gap ionizes,

14:01 allowing the surge to safely reach the ground.

14:04 Like all infrastructure, antenna towers need maintenance- painting,

14:07 changing light bulbs, and servicing antenna equipment.

14:10 Technicians with specialized training for heights

14:13 and electrical hazards have to do the work.

14:16 Some tall towers are even equipped with elevators to provide access,

14:20 but most require some manual climbing.

14:23 Although the frequencies used for radio communication are

14:26 non-ionizing (meaning the waves can’t break apart molecules),

14:30 that doesn’t mean they aren’t dangerous.

14:33 Electromagnetic radiation can generate heat;

14:36 it’s the fundamental principle of a microwave oven.

14:38 And if the tower itself is energized, a person can become part of the circuit.

14:44 With so much of our telecommunication happening through the internet these days,

14:48 it’s easy to forget the importance

14:51 of large-scale radio broadcasting and communications.

14:54 The cells for cellular communications are small,

14:57 so we’re used to seeing those antennas relatively close to the ground.

15:01 But you have to look way up

15:04 to remember how critical the other wireless systems are,

15:08 especially in emergency situations where radio and television

15:11 signals can be an essential link to information.

15:15 So next time you pass one of these towers by, take a closer look,

15:19 and I hope you’ll appreciate some

15:21 of the thoughtful engineering that goes into them.

15:25 Funny behind-the-scenes story about antenna towers:

15:28 I plan these videos out in advance,

15:29 and I actually bought a telephoto lens for my camera about a year

15:33 ago so I could get some of the shots in this video.

15:36 So, I went outside to test it out,

15:38 and the first thing I took a picture of was a bird.

15:41 Of course, then I wanted to learn what kind of bird it was.

15:44 And that basically snowballed into a full-on new hobby of birding.

15:48 I’ve got feeders in the backyard, fancy new binoculars,

15:52 and the Merlin app on my phone.

15:53 I try to get out at least once a week, and so far I’ve seen about 160 species.

15:57 But once you start paying attention and learning more about birds,

16:02 it can be a little disheartening.

16:04 For example, I see Loggerhead Shrikes pretty regularly here in Texas,

16:08 but if you live in the northeastern US, they’ve pretty much disappeared.

16:12 The species has lost about three-quarters

16:14 of its population in North America since 1966, and that’s just one example.

16:19 Another is Little Owls- cute little guys who have

16:23 seen major population declines in some parts of Europe.

16:26 My friends at Planet Wild have been working

16:28 with conservationists in Germany to help re-establish the population there.

16:32 Planet Wild is a community-based organization dedicated

16:36 to protecting our natural world, including wildlife.

16:39 It’s basically crowdfunding for nature.

16:42 Every month, all the members fund a new

16:45 project related to endangered species, oceans, or forests.

16:47 And then they produce a video documenting the project so you can

16:51 see for yourself where your money is going and the impact it’s having.

16:55 I love the idea, which is why I’m a member.

16:57 It makes me feel more connected to the causes

17:00 I care about and part of a community

17:02 who are working together to accomplish something bigger

17:05 than any individual could do on their own.

17:07 If you’ve been looking for a neat way to give back,

17:10 I think Planet Wild is a great way to do it.

17:12 And to prove it, for the first 100 people who sign up,

17:16 I’ll cover your first month.

17:17 Just scan the QR code or click the link in the description

17:20 and use my code PRACTICAL10 to get your first month free.

17:24 No catches- you can cancel anytime.

17:26 If you’re not sure yet,

17:28 go check out their project protecting the Little Owl in Germany.

17:31 You can give whatever amount- big or small- that feels right to you.

17:35 Your money will go towards really cool conservation

17:37 projects that you can watch happen here on YouTube.

17:40 I hope you’ll consider joining.

17:42 Thank you for watching, and let me know what you think.