Hey, Elsa... Do you wanna wreck a stop light!
Forget ice and snow! Wind can demolish the structure that holds up a stoplight.
It starts wiggling and wiggling until the whole thing falls down. Like this one in
Chicago. That could fall on a car! Or worse, smack someone on the head!
So what are we gonna do about it? Well, I talk to a couple of traffic signal
structure experts to find out what we can do. And to goof around a little bit, I built this
toy since messing around with a real stop light is kind of expensive and a little dangerous.
We're probably gonna wreck this thing.
(show open)
Imagine being Mark Taylor and getting this video.
(Taylor:) "One of my maintenance techs was out there filmed that and says, 'we've got an issue.'"
(Taylor:) "I looked at it and said, 'Wow, this is a problem!'"
This 60-foot-long pole bobbing up and down over a busy 60-mile-an-hour highway.
(Taylor:) "That was way scary."
The wind introducing a lot of safety problems.
(Lough:) "We get in that situation where our techs can't go up and work on it
to secure those devices because those arms are bouncing all over the place.
When those things are really bouncing, I mean, their signal head parts flying off."
(Taylor:) "What actually kicking the bottom of the traffic signal head from hitting a semi (truck)?"
And most catastrophically, the whole arm could fall off.
(Lough:) "So then we have to consider shutting down the road, you know,
just to just in case it comes down or it throws a head off."
And the problem wasn't going away. There's a lot of wind in this county in Utah,
which due to some wacky spelling, you probably can't pronounce.
Go ahead and guess.
Okay, the answer?
(Taylor:) "TOO-WILL-A"
(Taylor:) "We had to do something quick."
Carl Macchietto makes a living, helping cities and states save their stoplights.
(Macchietto:) "I've been, you know,
studying this for such a long period of time. I find it fascinating."
So, how does wind push a stoplight?
(Macchietto:) "It's when wind blows against the traffic signal structure and hits the frontal
area of the traffic signal itself. But yet the movement is perpendicular to the wind load."
To see it myself, by step outside. Since I'm not tall enough to reach a stoplight,
I pretended the world was turned 90°.
So when the wind hits the surface of these signs that are mounted to the pole, well,
the energy doesn't just go away, it has to go somewhere.
(Macchietto:) "The wind loads hitting the traffic
signals and signs that cause it to vibrate. It's those attachments."
So it goes to where it's mounted to the pole. That starts rocking the pole back and forth.
Stand against it. You can feel it.
(Macchietto:) "So, wind blows this way, movement goes this way."
It might be little tiny movements. You can barely see them, but those little
tiny movements, well they all flex where the pole meets the sidewalk.
And so as it rocks back and forth again and again and again and again— well, you're wearing
out that joint. A big problem in America's most blustery places, but not just there.
America can be a windy country. That's why we call all this middle part here, "tornado alley."
But it's not just their problem. Wind can blow down a stoplight in any of the
50 states. Even the calm ones. The wind doesn't always give a big steady push.
Sometimes even a fairly gentle wind makes a stoplight structure, start bouncing.
And as road fan "52 Pickup" spotted here in Bloomington, Minnesota, once it starts balancing
it takes forever to stop bouncing. It's called galloping. And boy, can it kill a stoplight.
(Macchietto:) "People say, well, the poles were designed to meet
over a hundred mile an hour. Why did they fail? Well, that's because it went into resonance."
A stoplight's metal is like a big version of this paper clip. And imagine this sorry thing
is a stoplight. I can sit here and bend it and bend it, bend it, bend it, bend it, and bend it.
Eventually, the metal fatigues, and it just snaps. Either from banding the metal directly,
or from loosening up one of the bolts from where the two poles me.
(Macchietto:) "And now you only have three anchor bolts, which means the pole itself is starting
to see not symmetrical loading of stress. And therefore can cause the pole to fail."
And then there's another problem. Simple physics tells us that the longer the pole,
the more force there is to rip everything apart.
And that's a problem on America's big arterial streets, like this major east-west expressway,
south of Las Vegas. The horizontal pole needs to be long enough for a left-turn
arrow. But that means the stoplight now has a gigantic lever arm for the wind to push.
You start counting three, 6, 9, 12...
(chipmunk)
...70. This has got to be at least 70 feet long here.
(Macchietto:) "Some of the state of Florida arms are 90 feet or longer."
To show you just how dangerous these levers can be, I built one myself. And a buddy helped me
set it up. And it turns out, I'm not exactly Mark Rober. Mine doesn't stand up on its own.
But thankfully, a thoughtful stranger left this Walmart cart behind for us to use.
The part of the wind today will be played by King Kong. Why him? I don't know.
(Macchietto:) "I mean, some of these emotions can
fail a traffic arm in just a matter of a day or two."
And Mark Taylor definitely did not want to see his stop light in Tooele fall and hurt somebody.
(Taylor:) "And it was obvious by the video that we had to do something quick."
So when the wind picks on a stoplight,
what options do engineers have to keep drivers and pedestrians safe?
There are basically two: One is to use a fatter paper clip, one that takes a
lot more bending before it finally snaps. Or option two, don't bend the paper clip as much.
And just down the road, the state of Nevada bought a very fat paper
clip. A massive 200-foot-long gantry to hold up stoplight signal heads that they
built back in 2007. A hurricane couldn't blow this thing down.
Brute strength always works. But it's also always not cheap.
This stoplight was rolled into a $25 million dollar widening
project. So I have no idea how much this costs, but it's probably fair
to say there's a reason you don't see stoplights built like this very often.
On a normal traffic signal pole that would hold up a stoplight, you can kind of give it a hug.
(Though most people don't.)
But to give you some idea of how massive this one is?
Try to give it a hug...
It's not even close.
Of course, that's an extreme example but all stoplights, even the regular ones,
are getting a little bit beefier. Because back in the middle of the 20th century,
design standards are kind of flimsy.
Look at how narrow this pole is here. Well, this one's really easy to hug.
It gets away with it because that horizontal mast arm is so short. There's not a lot of leverage.
It wasn't until 2001 that the first fatigue standards for
stoplights really started to show up. Which is a lot beefier it works a lot
better to fight the wind. In fact, if you look right over there. A new stoplight,
which certainly looks stronger than the old one. Ready to fight the wind. At least in theory.
This one's harder to hug.
But these stricter standards weren't helping Mark Taylor. This stoplight
is no older than a first gen iPhone. So, even though it was built to the latest,
strongest standards, it was still galloping anyway.
(Taylor:) "He was saying that we were using the Category II steel anyway."
So like most engineers, he has to go with option two,
reduce the galloping. You know, make the wind wiggle his stoplight less.
(Macchietto:) It's very high cyclic stresses that occur when you're bending it that high.
If you don't bend the paper clip, nearly as much or nearly as often,
it's gonna take a lot longer for it to break. So, to make that happen,
we first have to figure out how does the wind cause galloping.
Think about the wing of an airplane, the distance the air travels on the top,
is slightly longer than the bottom. That creates a sucking force,
a vacuum, that pulls the wing and the airplane up in the air.
And on some stop lights, the same thing can happen, a special way that the wind happens
to hit this special stoplight— hitting the signs and the stop lights and the distance that the air
has to travel one way versus the other. It creates that same sort of upward force that
vacuum that pulls it up, gravity pulls it back down, and you start getting that galloping going.
But it's even weirder than that. This animation from Wikipedia shows vortex
shedding. The weird forces the wind causes once it goes around an object.
Complicated fluid dynamics that go far beyond my limited mental capability.
Okay, blow the wind!
So, Texas Tech University found that if the wind was at a 45° angle,
that's when it would typically start galloping the most.
Wind is really unpredictable for engineers to work around.
(act break)
(Taylor:) "You know, it's not every storm where that happens. Things
have to be just right. But boy, when that thing gets going, it's scary."
Over the years, engineers have found a few tricks that seem to work. Ways
to mess with the vortex shedding and change up that wing shape.
One, which we talked about in the last video, turn the signal heads sideways. Some states see some
good success with that, but as you can see, wind is complicated. And as this video from McAllen,
Texas shows, they can still gallop anyway. And fall apart.
So, trying something else
engineers would give it a spoiler. (Taylor:) "The old way of doing it,
where you would just you know just mount a sign horizontally like an actual fin."
And then I accidentally see one. Right here, mounted on this traffic pole on Tropicana Avenue,
just a couple of miles away from where we had our Tropicana live stream.
That horizontally-mounted sign that's not really a sign unless it's, you know,
people in airplanes looking down at it.
(Taylor:) "For the most part, that worked out actually pretty good."
A fin right at the end of the lever arm, scooping the air as it goes up and down,
helps slow the galloping down. States like Texas go crazy for these things.
(Taylor:) "That's what a lot of practitioners would do. It wasn't bulletproof by any means."
Like this mast arm holding up road signs in Pennsylvania. It has a nice big sign
blank working as a drag, but the wind seems to have no problem making it gallop anyway.
Another idea: Engineers have tried removing the back plate. The little sun visor around
the stoplight. Which can act like a sail and grab the wind. But Utah is a sunny place,
and at the right time of day, that back plate is really important to see the red light.
Or take the skinny short horizontal fast arm. Trade it for one that's longer and
fatter. Chop off the skinny bit at the end that's extra. Leaving them
with a weird-looking, but fatter, short mast arm.
Or they could try moving the entire stoplight structure closer to the street,
which will let you get away with a shorter horizontal mast arm.
But along a 60-mile-an-hour highway, that's not so great.
(Taylor:) "Yeah, there's a few options you've got,
right? I grabbed one of my local engineering consultants."
And they came up with this contraption. A vibration damper made out of a cable stay.
Crews stretched a cable from the mast arm all the way around to the
base. Then snugged that cable up really tight, so the wind can't move the pole.
(Taylor:) "And we were working with our structures folks, letting them know what
we did after the fact. They came back and said, 'Well, that may work right now...'"
It could be a maintenance nightmare.
(Macchietto:) "They had to tighten the turnbuckle to get the slack back out of it."
(Taylor:) "So, they were aware of research going on by the University of Connecticut."
They thought, you know, it's too bad there's not a way we could put some sort of like
counterweight on the traffic signal structure. So, as it starts galloping up and down that weight
could cancel out the vibration and get it to calm down. Kind of like a shock absorber on a car.
Trouble is there's a physics principle that makes this really difficult to do.
All objects in the universe have a natural frequency that they vibrate to. This includes
stoplights. And when the wind finds that frequency, weird things happen.
It's like swinging on a swing, you sort of get into this pattern. But the wind can show up,
and it can start pushing, pushing really hard. But wind tends to be random, so it pushes you
kind of at the wrong time. It doesn't really help you swing. In fact, it can even slow you down,
get you out of that pattern. But even a modest wind pushing at the right time—
one that doesn't even have to be that hard...
Well, I could start getting you to swing really high, and that is called harmonic resonance.
Which can be really bad, and the most famous example, when the wind found
the harmonic frequency of a bridge near Tacoma, Washington. And kept pushing on
it and pushing on it at just the right frequency... until it all fell apart.
But this physics principle can be really good when an engineer understands it, and works with it.
The wind pushes on tall buildings, just like King Kong climbs them.
Ah, I found a way to make it work.
For super-tall buildings like Taipei 101 in Taiwan, which is about ten times
taller than this thing, it's got such a long lever arm— because of its height,
that a strong breeze could pull the whole building over. So, architects and
engineers cleverly put in a counterweight, tuned exactly to the building's harmonic frequency.
So, each time the wind pushes the building this way, that tuned mass damper swings in
the opposite direction, perfectly. And that cancels out the wind force.
And it's all thanks to knowing a building's harmonic resonance.
To save my stoplight, we need to find its harmonic frequency.
Figuring out the harmonic frequency of a skyscraper is really difficult.
But once engineers do, they're done because there's only one skyscraper.
But a city can have thousands of stoplights, each one just a little bit different. How the signal
heads and signs are mounted to the pole, the length of the pole, how the wind hits.
And to make that counterweight that shock absorber work on a traffic signal structure,
engineers would have to calculate the harmonic frequency for every single one.
(Macchietto:) "There's short arms, they're long arms, all having different natural frequencies.
It's not just the pole, it's what's mounted on the pole. So, you change the frequency if
you take one of the signals off. Or if you had a signal, you changed the frequency."
You'd have to put every single stop light in a wind tunnel,
and that's just too expensive. That's never going to happen.
So unfortunately, putting a mass tuned damper on a stoplight is basically impossible.
Until it wasn't.
This is where Carl gets really excited.
(Macchietto:) "What we tried to do is incorporate technologies within the damper to counteract or to
make it more forgiving. So, you don't have to match the natural frequency exactly."
His company takes the spring and counterweight and mounts it inside a thin metal tube.
(Macchietto:) "There's magnets on the way against the aluminum shroud. So,
when the pole wants to move up and down and the spring wants to move up and down your generating
power. You're taking the mechanical energy of the system and converting it to electrical energy.
(Macchietto:) "There's these baffles on the top and the bottom,
and you've got a hermetically sealed tube. So, now you also have pneumatic damping going on.
(Macchietto:) "Because the damper is situated up and down, it takes care
of the galloping up and down. You can have a short arm with a fast natural frequency and
it works. If you have a long arm, it's lower natural frequency. It will work.
So back to Tooele where we first started, does it work?
(Taylor:) "Not only did they put up their device that would work, but they installed
a bunch of the probes on the mast arm and the poles that looked at how much it was moving.
(Macchietto:) "If you watched the one with...."
Carl's side-by-side video on experiment day.
(Macchietto:) "You'll see the yellow is represented..."
Technicians wiring it up with sensors.
(Macchietto:) "...decay of the vibration displacement decay is much faster..."
Then yanking on the pole, the force it to gallop.
(Macchietto:) "...and this is without the vibration damper."
The yellow is showing the counterweight in action.
(Macchietto:) "Can you see it killed it?"
In just over one and a half minutes, the traffic signal arm is calm and happy. But left to mother
nature, it takes 11 minutes. His company has a small collection of video trophies.
(Taylor:) "And it just, I mean, it's very effective.
I just haven't seen any vertical galloping, even on windy days. I see a little bit of
horizontal movement. But talking to the structures folks, they're saying that
horizontal movements just don't matter much. It's the vertical, what causes the damage."
But these mitigators are not cheap. He tells me a set of four, one for each of
an intersection's four poles, runs about $6,000. But if it makes the stoplight's
$30,000 worth of metal last a little bit longer, then it might be money well spent.
(Lough:) "And what we're hoping for is not just to prevent galloping,
but also extend the life of the arms because there's less cyclic vibrations in it."
So, Utah has purchased thousands of these. You'll see them all over the state.
You know, sometimes when I come across a cool engineering toy like Valmont's Mitigator,
the video starts to sound like a commercial for Valmont.
Editorial note: Valmont paid me NOTHING for this video, which is how we're able to have
King Kong in here representing the wind even though that doesn't make much sense.
If you want to help pay for these videos, we can have more chickens and King Kongs and who
knows what else— consider subscribing on Patreon at Patreon.com/RoadGuyRob,
where you pay for me to talk about transportation.
I'll see you next time.
(Captioning made possible by Patreon contributors)