Tae Bo Workout Sent Skyscraper Shaking

MiniMike writes "According to CNN: 'Seventeen people performing a vigorous Tae Bo workout caused tremors that forced the evacuation of a South Korean skyscraper earlier this month, the building's owners say. Scientists recreated the event in the 12th floor gym, according to a report in the Korea Times.' I don't know which is scarier, that they made such a flimsy skyscraper, or the sight of 17 scientists doing a Tae Bo workout. Hopefully they're better at it than the scientists I've seen in the gym."

Comments

Precedented...

[Jah-Wren Ryel]

Korea's got a dramatic history with such things. 500 people died in that one.

Re:Precedented...

[bunratty]

The US does too.

It's that bridge all over again

[oldhack]

The doom of resonant frequency.

Re:It's that bridge all over again

[TheRaven64]

Which bridge? The Tacoma Narrows Bridge, that shook itself apart, is one famous example, but it's not the only one. The Millennium Bridge in London had a similar problem. If people walked across it at a normal walking pace, their footsteps were at the bridge's resonant frequency. Worse, when a bridge starts to resonate like that, people naturally start walking in sync with the vibrations, making it worse. It cost £5m to fix - over 25% of the original construction cost.

Re:It's that bridge all over again

[Ungrounded Lightning]

yea, its really more like the millennium bridge.

Yes.

The Tacoma Narrows bridge problem was caused by the cross wind if I recall correctly.

And by a design of some windbreak structures on the sides of the bridge that caused the twisting motion of the bridge, once the resonance was being pumped up, to modulate the crosswind airflow over the bridge in a way that pumptd the resonance further.

It was essentially a vibratory wind turbine. It failed when the wind down the narrows was finally high enough to pump energy into the resonance faster than the bridge's internal friction at high excursions damped it.

Re:It's that bridge all over again

[dotbot]

Interestingly, for the Millennium Bridge, it wasn't actually the frequency of people's footsteps that caused the problem. (This can be a problem when people step in time and some other London bridges have signs that troops should break time.) The main issue with the Millennium Bridge was that the resonance was lateral, i.e. side to side, which was not well known about, and there was positive feedback: small lateral movements, within normal limits, became amplified because the way people naturally corrected their balance induced the resonant frequency. I haven't read enough about this topic to know whether there was any positive feedback, i.e. whether the movement of the building was affecting their Tae Bo workout (or other people in the building) which in turn amplified the tremors.

Re:Whoops

[MozeeToby]

I was going to post the same Wikipedia article, but then I read it and learned that the failure wasn't cause by resonance (resonance frequency of the bridge was ~1Hz, but the oscillations occurred at ~.2Hz). The Discovery channel taught me wrong but hey, at least I learned something new today.

Scientists are bad at the gym?

[MetalliQaZ]

Scientists are bad at the gym? Is that a stereotype? Never heard that before. I know some marathon-running, rock-climbing scientists that would probably take issue with that.

Re:Scientists are bad at the gym?

[c6gunner]

Scientists are bad at the gym? Is that a stereotype? Never heard that before. I know some marathon-running, rock-climbing scientists that would probably take issue with that.

Sure, and I know some slashdot readers who actually RTFA, but we're talking general trends here!

Not even remotely related

[SuperBanana]

The article concerns a workout which happened to hit the resonant frequency of the structure, which was properly designed. The failure you linked to had to do with criminal negligence by the owners ignoring repeated engineering and construction firms who refused to accept design changes driven by greed (and were fired for it), completely repurposed building usage leading to gross overloading of the structure's strength, and then completely incompetent handling of obvious signs of structural failure (failure to immediately evacuate the building.) For fuck's sakes, they removed columns for escalators, then cut into the remaining columns, then ADDED A STORY WITH POURED CONCRETE FLOORS *and* an air conditioning unit the building wasn't designed for. And then when the building started to fail, they just blocked off areas to hide it from customers because they didn't want to lose sales revenue.

Re:Whoops

[Daetrin]

First, you and MozeeToby are both correct. However it's also true that Galloping Gertie was used for decades as an object lesson for the problems resonance can cause in large engineering works. Even though the attribution of the fault was (somewhat) incorrect, we still learned something from it that was entirely correct when applied appropriately, something that apparently _wasn't_ applied in this case.

Second, and maybe i'm wrong here, but as best as i can understand from the complicated physics the problem with Galloping Gertie wasn't a simple case of "elementary forced resonance." However it was still a case of resonance, just really complicated resonance. From the wikipedia page on Aeroelasticity: "These interactions may become smaller until a condition of equilibrium is reached, or may diverge catastrophically if resonance occurs." So to my mind the real lesson from Galloping Gertie was that you can't just account for elementary resonance, you have to account for all possible sources of resonance. In this case however they didn't even take care of the elementary kind.

Bring him in.

[Datamonstar]

Have they questioned this guy yet?

RTFA

[SuperBanana]

Did you read the article? The resonance could only be felt on one very narrow range of floors. The whole building wasn't shaking, which is what tuned-mass and active dampers are designed for. They're for countering earthquakes, wind-induced vibrations, etc.

Re:RTFA

[Jah-Wren Ryel]

So, I'm no expert here, but is it REALLY possible to design a building which does not resonate at any frequency?

You need a way to dump the energy into some other form, there are multiple ways of doing that, I linked to one in an earlier post. You don't have to dump all of the energy, just enough to prevent it from becoming structurally dangerous within certain margins. I think it is entirely reasonable to expect a building to avoid shaking 19 floors because about 10 people are jumping around. It isn't like they are putting a whole lot of energy into the structure to begin with.

Re:Whoops

[HornWumpus]

Too bad you learned something wrong.

Galloping Gertie was brought down by aeroelastic flutter, aka forced resonance. Aeroelastic flutter is more specific, but not a correction.

Galloping Gertie was visibly resonant in its second harmonic, in torsion. Structures have more then one resonance frequency. All those frequencies have harmonics.

Re:yes

[MiniMike]

OP here. The end of my submission, where I include that IAAS (scientist), was cut off, so I would like to state that I wasn't trying to convey a "where (sic) better than they are" attitude. I could have stated my intent with the last surviving sentence more clearly. While many scientists I see in the gym are in great shape, some of them are in quite poor shape and uncoordinated. The ones at that end of the spectrum are usually in the beginner group exercise classes. It is that group I was trying to reference in the post. The last sentence should have read 'some of the scientists'. Hope this clears it up.

Re:resonance

[ehrichweiss]

I wonder what the Mythbusters will say in regards to their "Marching In-step over a Bridge" episode. I was one of several to speak up and AFAIK they never addressed resonance. I said the same thing about their Tesla Earthquake Machine episode except I pointed out that these earthquake machines are used in demolition to this very day; I got the same lack of response.

Re:resonance

[DeathElk]

They're obviously having too much fun blowing shit up to worry about responding to your points.

Re:resonance

[ShakaUVM]

A bunch of Koreans doing Tae Bo while listening to Snap and collapsing a skyscaper around themselves would definitely get my vote for most amusing tragedy of the decade.

Re:resonance

[Anonymous Coward]

I wonder what the Mythbusters will say in regards to their "Marching In-step over a Bridge" episode. I was one of several to speak up and AFAIK they never addressed resonance. I said the same thing about their Tesla Earthquake Machine episode except I pointed out that these earthquake machines are used in demolition to this very day; I got the same lack of response.

The problem seem to be that you are under the impression that Mythbusters is something other than an entertainment TV show.

Re:resonance

[Shark]

The problem seem to be that you are under the impression that Mythbusters is something other than an entertainment TV show.

Apparently that's enough credentials for the US military... And I'm not trying to prove you wrong here. I'm baffled by it.