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Humen Bridge
3

Humen Bridge

RE: Humen Bridge

Looks very suspiciously like Nov 7 1940 Tacoma Narrows
https://www.youtube.com/watch?v=j-zczJXSxnw
80 more years of experience and tons of software later and we're still making the same mistakes?

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

RE: Humen Bridge

Of course this reminds a lot of people of the Tacoma Narrows bridge.
What I found humorous were the comments below that video on YouTube:

gotta be more careful with the future designs man. Building bridges is one of the hardest things in my opinion because of the law of physics.

The bridge has never experince something like that since it open 23 years ago, even with those extreme typhoon in 2017 and 2018.

This bridge was designed by UK 23 years ago, they must check the issue!

There is something spirit ram is going on in China....💀💀💀💀

Scary stuff. Seems like the construction held though, so as they say - all's well that ends well.

wrong calc in natural resonance frequency. should have put air vents to let the wind through.

Need do a big job at once before it collapsed if wind excess more than 100 mph.
Just replace a big job or add bottom ties to make it less wave to avoid metals stress.

They should consider banning music being played on bridges ;)

It was later found that theyr built 2 water counter shaking things on each end of the bridge . They now taken it down so it does not shaking anymore

Those big dummies

CIA in action? Or HK TW thugs?

The bridge got infected by an alien virus and became alive

This footage actually reflects extremely sound engineering. The bouncing of the bridge is purposely tolerated by the structure to account for aeroelastic flutter. Only inferior people wouldn't know this.

Dam man this virus is lasting longer than made in China bridge lmao

better than most italian bridge




RE: Humen Bridge

We visited the Royal Gorge suspension bridge in years past. It was reasonably windy, not bad, though. Looking down the bridge it looked like it was moving maybe a foot or so.
I was talking to an older lady at the church up there. She said when they built it, it would move enough that you couldn't see cars at the far end, and that at some point, they added additional cables to restrain that.

RE: Humen Bridge

In the loosest sense, yes it is like Tacoma - wind+suspension bridge gives unexpected deflections. But Tacoma was torsional instability caused by flutter, this doesn't look torsional. Apparently the problem started after a new 4 ft high wall was added recently.

I imagine that is now being un-added.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Humen Bridge

Quote (GregLocock (Automotive) 14 May 20 22:13)

... this doesn't look torsional.

That's an interesting observation. I wonder if the sustained wind was diagonal to the deck somehow spawning vortex rolls down the length of the bridge? The geography of the greater area (22.793056, 113.608611) is flat river delta but the east end of the bridge is anchored in a rise of hills, which could create some interesting scenarios as well.


From Google Earth with 1.3x vertical exaggeration.


From above video at 19 sec. Note debris blowing in the wind.






RE: Humen Bridge

Further to my previous post, the weather in Hong Kong on May 5, from timeanddate.com

RE: Humen Bridge

But that bridge is a long way from Hong Kong, maybe 130 km.

RE: Humen Bridge

Sorry, I wasnt standing on the bridge at the time! ;)

RE: Humen Bridge

Gallopin' Gertie Redux

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: Humen Bridge

In case anyone is interested in the meteorology that caused this event, I found some better info. If anyone can improve on this with charts from the region, please do.

Generally, the wind was from the south, rising to 36 km/hr in the afternoon. The airport is located 27 km from the bridge.


Map from flightradar24.com.


From Google Earth and Wikipedia


Weather data from rp5.am.

RE: Humen Bridge

It appears that temporary safety barriers set out along the windward edge were indeed the problem. It is rather incredible, though not unforseeable, that such a small item could have such a drastic effect.

A screen grab of the barrier being dismantled.


From Youtube.

RE: Humen Bridge

Looking closely the original barrier has spaces under and around it which was then blocked off by the solid barriers. Clearly enough to set up a different frequency of vortex.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Humen Bridge

And when you consider that the bridge is designed to withstand hurricane force winds, the winds that afternoon were but a breeze. I'm thinking there just wasn't enough leverage to pick up the leading edge w.r.t. the trailing edge. Or another way of analyzing it, the center of lift was too far aft, i.e. over the center of the bridge as opposed being closer to the leading edge.

RE: Humen Bridge

It happens in real aircraft as well.

Some of them even have emergency checklist for when its starts happening.

The saab2000 has had more than a few occurrences of it. A mate that was flying one when it happen was not happy in the slightest afterwards. Not a change of underpants but it was the first time he had ever been airsick in his life. Pretty much everyone that was sober barfed their guts up. Unfortunately they were half way across the North Sea when it started so they had 40 mins of it before they could land.

Its called aeroelastic stability

RE: Humen Bridge

Very few, if any, engineers or scientists completely understand this type of dynamic behavior. We try to avoid it by either tweaking or brute force, but sometimes the physics is elusive.

RE: Humen Bridge

Truly a humbling moment. cat2

RE: Humen Bridge

If you look deep enough into old films of the Tacoma Narrows bridge, it would oscillate in pure bending modes at lower wind speeds. The ultimate failure occurred during exceptionally high storm winds, and did involve torsional modes as well.

RE: Humen Bridge

I just travelled with a canoe on top of my car yesterday. Right when I started I noticed a loud flutter coming from the flat strap I used to strap it on the rack. After a little thought at the rest stop about killing the excitation I rolled the fluttering section into a circular profile with duck tape and that stopped it really effectively. I'm not sure if I reduced the the excitation or the torsional response by doing this but it worked.

RE: Humen Bridge

Is that why when I see loads strapped to a flat bed trailer there always seems to be a half twist in the strap between the top of the load and the trailer?

RE: Humen Bridge

@davidbeach - My dad was a trucker way back when and he told me that's exactly why there's the 1/2 twist in those straps.

RE: Humen Bridge

davidbeach - I suppose it is - It's similar to the rotating spiral you see on fixed diameter metal chimneys - the idea is that there isn't a fixed diameter and hence not a fixed frequency.

Having a quick look it seems that yes it reduces flutter, especially when in a free air situation, but can fall foul of the DOT, if they interpret "twisted straps" a bit too far.

One half twist is OK, more than that and you reduce the tension strength.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Humen Bridge

I wonder if the vortex on the towers have any contribution to the problem. It might not be all in the deck.

RE: Humen Bridge

The wind was 36 kph, which is nothing in the grand scheme, and the temporary barriers were an obvious point of issue. Having said that, it would be interesting to learn how the towers behaved as part of the oscillating system.

RE: Humen Bridge

I once strapped a ladder to the top of my pickup and the vortex shedding off of every rung made a tremendous noise at 100kph.

That's impressive if it turns out the row of barriers could have that much effect.

www.sparweb.ca

RE: Humen Bridge

SparWeb,
I wonder if the rungs were at some critical spacing such that there was a reinforcing effect as air traveled down the ladder. I could easily see that being catastrophic at the right (i.e. wrong) speed.

(But what do I know; I bailed on the fluid mechanics option after taking one look at the Navier-Stokes equation... )

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: Humen Bridge

Once we had a substitute in either calc 3 or diffeq, it was the department chair. He was a great story teller, I heard he was a magician outside of his work at the community college. I think he had set up a system that could resonate or something like e^(bt)*sin t or something, and then he was like 'look at us here, all guys today. What do guys do together? They tell stories, and I've got one for ya. Well I was in the back yard, by the pool and I had just turned a fan on cause it was hot, and the fan was blowing JUST SO and these little waves started to form in the pool from the breeze, getting bigger and bigger, bit by bit. Then I went inside to answer a phone call and I made some iced tea and I came out and the waves were HUGE, the water was splashing out of the pool it was crazy! So what did I do? I turned the fan off and it all settled down.'

That was it. He left us to scratch our heads later and figure out why it was a fib (no damping in the system).

RE: Humen Bridge

Vibration wise the Humen Bridge could be in pitching mode with one end dips while the other end rises. It could come from one of the six rigid body motions or the lowest natural frequencies (vertical, lateral, logitudinal, rocking, pitching and yawing. I say this because section between the two towers has a monolithic steel box girder structure supported by flexible vertical hangers. The two end approach spans are structurally seaparate as they are supported by short RC beams on columns (no vertcal hangers with the main cables). In a way the centre section is a simply supported beam on closely spaced spring supports. Had the system been relatively rigid the first mode should be the same shape as the deflection under its self weight. However if the structural system is relatively flexible the first natural frequencies would be the deck's free body movements.

The Engineer has since discovered couple of badly corroded hangers and one of them had totally severed. It is possible the spring supports were locally weakened when the vibration was observed.

To some including myself the vibration was set off by vortex shedding from the across wind effect. For slender structure the across wind effect is always more critical than the inline wind effect. I personally got involved with two steel chimenys that vibrated under modest wind. One actually had 40% of the base cleanly severed by fatque. I would say every steel chimney around 100m or about will have across wind vibration problem requiring mitigating measures like welding helical strakes at the upper one-third.

Tacoma Bridge was initially torsional along the longitudinal axis.

An excellent example of vortex shedding induced vibration is enclosed below.
https://www.youtube.com/watch?v=8OExYy28moc

RE: Humen Bridge

That's a wild video Saikee119. I used to get light posts at the bus stop oscillating by pumping them in time. In a couple parking lots i've seen light posts connected to eachother by plastic line, I assume to damp vibration.

RE: Humen Bridge

moon161,
Yep a vibrating pole can be damped down by using cable stays.

The other method is to use dampers. Some tall buildings use such technique to upset the vibration pattern making it difficult to sustain. We have in London the Millennium footbridge initially setting out excitations by the foot traffic. It was subsequently cured by dampers.

Normally for a robust structure a static design is all that is needed. For a slender structure, like a long suspension bridge, a dynamic analysis is often required. Structures subject to vibratory or repeated impact loads like machine foundations the design codes are invariably extended to cover the structure performance during dynamic conditions.

RE: Humen Bridge

Saikee,
Thanks for the video.
On a number of occasions, at a local Costco parking lot, I have notice the light poles vibrating. I find it most interesting when they oscillate in the second mode, not the first. The lamp heads are moving a bit but the midpoint has much more amplitude and opposite in phase. This happens at wind speed about 30kph.

www.sparweb.ca

RE: Humen Bridge

SparWeb,

Thanks for letting us know. I have seen a few slender poles vibrating in first mode but your case is the first one in second mode. I suppose if the base and the top sections were strengthened leaving the middle section less stiff relatively then the second mode could be excited under certain wind speeds.

AFAIK if the environmental condition is favourable to sustain the mode shape the excitation doesn't have to be the first mode. Higher modes have more complicated mode shapes and are more difficult to sustain that is all.

If we idealise a cantileveer with one element (two nodes with bottom node fixed) we get one mode shape and one natural frequency. The same cantilever idealised with two elements gives us 1st and 2nd modes plus two natural frequencies. However if we idealise it with 1000 elements we get 1000 theoretical mode shapes and natural frequencies. The fundamental mode and frequency will not change materially in all cases. However the number of natural frequencies can be as many as we model it mathematically.

RE: Humen Bridge

"I suppose if the base and the top sections were strengthened leaving the middle section less stiff relatively then the second mode could be excited under certain wind speeds."

I suggest you do some calculations. Sitffening the tip of a cantliever has very little effect on the low order modes.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Humen Bridge

Stiffening the structure is not always the answer. While excitation method is different, machinery vibration is essentially the same problem Spring - Mass - Damper system. Changing any of the variables can help. I have found setting a sandbag on top of vibrating machinery often damps the vibration to the point where it is no longer a problem. It is a good way to buy time while finding a better solution.

Some steel stacks use a tuned damper pendulum near the top
https://www.google.com/url?sa=i&url=http%3A%2F...

RE: Humen Bridge

"AFAIK if the environmental condition is favourable to sustain the mode shape the excitation doesn't have to be the first mode. Higher modes have more complicated mode shapes and are more difficult to sustain that is all."

Vibration of cylinders subject to vortex shedding can be excited whenever the frequency of the vortices being shed coincides with a natural frequency of the cylinder. This most commonly occurs in the first sway mode, but higher modes can be excited if the conditions are right.

In the case of slender light masts, the first sway mode frequency may be low enough that the wind speed at which excitation is induced is low and you don't see much excitation in practice. Second-mode excitation can be developed at a higher wind speed, and you can see the lights swaying to the left while the middle third of the mast sways to the right. (I have only seen this on very slender masts.) In most practical circumstances, the wind speeds required to generate second- and higher-mode excitation tend to be gusty rather than steady, and the higher mode vibrations damp out quickly - the conditions have to be "just right" to maintain higher-mode excitation.

http://julianh72.blogspot.com

RE: Humen Bridge

jhardy1 (Structural),

Agreed with what you said.

I just want to add the vibration when permitted to continue will eventually lead to the strutural failure by fatique. People seldom investigate if a lamp post or flag pole breaks. They just replace it with another one. The vibration amplitude of a vertical cantilever from a street furniture can be alarmingly large. However each structure has a finite number of stress reversals, usually varies with intensity of the stress, which must be taken into consideration in the maintenace work.

RE: Humen Bridge

Perhaps the added mass from the light at the end of the pole is sufficient to dampen the first mode and result in a second-mode oscillation?

RE: Humen Bridge

@Craig H:
It's not that the first mode is being damped out by the mass of the lights; when you see a light mast swaying in the second mode, the system is being driven by a forcing function with a frequency which is close to the second natural frequency, rather than the first.

Vortices are shed from a cylinder alternately from the lift and right sides, with a frequency which is proportional to the wind speed, forming a "Karman vortex street", generating an oscillating lateral load on the cylinder. When the wind speed is such that the frequency of vortex shedding coincides with a structural frequency, resonance can occur, leading to an amplified response. (And also providing a positive feedback mechanism to lock-in the frequency of vortex shedding with the frequency of vibration.)

A typical light mast will therefore have one wind speed which tends to generate first-mode sway, and a higher wind-speed which tends to generate second-mode sway without generating a simultaneous first-mode response.

http://julianh72.blogspot.com

RE: Humen Bridge

Thanks for that explanation, folks! The frequency of vortex shedding makes far more sense.

RE: Humen Bridge

Another bridge with a similar problem from a similar cause - upgrades! https://twitter.com/markkrueg/status/1269073081231...

Who knew that adding a highly regular repetition to the spacing of close orifices would lead to unpleasant resonance?
We all do now.

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