Can a bridge fall apart?

[downinOZ]

Recently a new TV program has hit my part of the world which has shown all sorts of experiments most of us would love to have tried (re: making a chicken gun or blowing holes in a pressurized plane cabin to see if passengers really can get sucked out of the hole!?!).  Mythbusters ran one segment where they tried to disprove the myth that armies around the world break step when going over a bridge as marching could cause the bridge to reverbrate and collapse.  Their proof that bridges do move was the Tacoma Narrows Bridge (a suspension bridge).  Most of us who have some experience in decking bridges would probably agree that it is very possible for bridges to...have a fair bit of play.  My most memorable experience would be decking an MGB where two sections lined up, picked up the decking, and walked/jogged into place.  Talking to a retired RSM from the Australian Combat Engineers, he had the same opinion - there is truth to the reasons behind breaking step.

Does anyone have further experiences that could prove/disprove this practise?

The Mythbusters decided to say the myth was false...twice.  The first experiment was clumsy and inaccurate, and the second bridge was, nothing more than a stress test on wooden simple bridges.  I did write and suggest they contact the College of Knowledge and ask to borrow their MGB model, but I think they've had enough of that line of experimentation.

Chimo

 

[George Wallace]

I am not sure whether or not a large number of Troops, marching In Step, would actually be enough to cause any bridge to collapse, but the possibility is there for certain types of bridges.  I remember doing the Nijmegen Marches and on the Bridge in Nijmegen there are large signs posted instructing "Marching Troops Not to March In Step".  As we crossed, a moment when a large number of troops may have stepped as one happened, and the whole bridge did a quick sway to the left.  It was quite a scare.  Anyone who has been to Nijmegen and seen the bridge, knows how large (4 to  6 lanes of traffic) and long that suspension bridge is.  Could enough strain be put on that bridge, in this fashion, to cause serious damage or collapse is only something an Civil Engineer could determine.  It, however, can cause some concern to those crossing.  ;D

 

[AwSnap]

:soldier: i think they were smart to break step, but then again u see cars, vans and what not crossing bridges. say 1000lbs a piece. so 20 cars in a row... 20,000 lbs+ the people inside 200lbs a piece 24,000lbs PLUS the 18 whlrs 10,000lbs (give or take) a piece and there are 10 of them thats 126,000(extra 2000 for the people inside the 18whlrs) all of them on ONE bridge. now that is just an example, usually you can see 100 or even MORE cars trucks vans and 18s........................TO ADD, bridges are made to withstand horrible weather. so with 800,000lbs of steel on a bridge on a windy day when its rocking back and forth.


can you say "march away!"?

 

[Inch]

:soldier: i think they were smart to break step, but then again u see cars, vans and what not crossing bridges. say 1000lbs a piece. so 20 cars in a row... 20,000 lbs+ the people inside 200lbs a piece 24,000lbs PLUS the 18 whlrs 10,000lbs (give or take) a piece and there are 10 of them thats 126,000(extra 2000 for the people inside the 18whlrs) all of them on ONE bridge. now that is just an example, usually you can see 100 or even MORE cars trucks vans and 18s........................TO ADD, bridges are made to withstand horrible weather. so with 800,000lbs of steel on a bridge on a windy day when its rocking back and forth.


can you say "march away!"?

Have a read on "resonance" as well as the Tacoma Narrows Bridge. It had nothing to do with the weight that's on the bridge and everything to do with the rhythmic stepping of the troops. If that rhythmic stepping matches the natural resonance frequency of the bridge, the waves will get bigger and bigger until the bridge collapses. Wind can also cause resonance in the bridge as was witnessed with the collapse of the Tacoma Narrows bridge.

 

[Kat Stevens]

The theory goes that the rhythmic marching will create what is known as a harmonic resonance.  that resonance would work in much the same way as ultrasound is used to break up bone spurs, kidney stones etc.  I have read of reports that, back in the 30s, the ground would vibrate as German troops goose-stepped by.  A lot would also depend on the type of bridge, IE supported bridge versus suspension.  I personally thought the Mythbusters were more concerned with making a cool looking marching machine than actually proving the myth... By the way, Tacoma Narrows was cool ;D

CHIMO,  Kat

 

[downinOZ]

You make a good point about supported and unsupported bridges.  The longer a span is unsupported, the longer the wave has to form or grow.  The Bailey and ACROW bridges I've worked on never seemed to move very much at all, but the longer double-story MGB without a pier or link re-inforcement moves wildly. 

 

[alan_li_13]

Guys, Awsnap is 14, they don't learn bout ressonance until 3/4 years later, lol  ;D
But i guess it is a very interesting thing to learn even though you might not understand. First time i heard about its effects, i went home and waited till no one's in the house, and started marking time on my dinner table. The consequences were not what i wanted though.

 

[Kat Stevens]

The Mythbusters decided to say the myth was false...twice.  The first experiment was clumsy and inaccurate, and the second bridge was, nothing more than a stress test on wooden simple bridges.  I did write and suggest they contact the College of Knowledge and ask to borrow their MGB model, but I think they've had enough of that line of experimentation.

Chimo

It would not be accurate to take a scale model and expect to replicate the stresses of 1000s of tons of steel.  The bridge the Mythbusters build was not a fair representation either.  As an example of a resonance wave,  We built 110' of 3/1 Bailey once upon a time.  I was on the far bank with my hand the last panel.  The boys at the other end were pounding out the pins on the jacking posts on the home side.  A vibration went up my arm and set my crazy bone tingling. thought nothing of it at the time, but a prolonged rhythmic pounding, I believe, MAY set up the kind of wave we're looking at..., Just MHO, as usual...

CHIMO,  Kat

 

[54/102 CEF]

The mythbusters is a mythical example of mythuse of TV dollars - much better to see Trebuchet enthusiasts fling big rocks at log forts in Scotland

here are some examples - gunners get busy!

WGBH BOston TV show http://www.tfguild.org/projects/fling0.html
--- firing the monster  http://www.tfguild.org/projects/fling/fling11-1.html

http://198.144.2.125/Siege/Trebuchet/Trebuchet.htm

No ammo for Pet this year? Not a problem Colonel - but make friends with the sappers first !

 

[Kat Stevens]

I think Mythbusters is quite well done most of the time, but that chicken cannon experiment is getting pretty tired.  The trebuchet, and a bunch of Archimedes and Leonardo's war machines, are reproduced on Secrets of Ancient Civilizations, and they're awesome!  living out here in the boonies of Alberta, I'm putting my plans together for a mediaeval punkin chunker... the little trick or treaters won't know what hit em this year....mwahahaha!

CHIMO,  Kat

 

[pjocsak]

On a slightly different note,

Here in the UK they recently constructed a footbridge across the Thames called the Millennium bridge. I recall watching a show on it a while back, and it related a story about problems the bridge encountered soon after it opened. Apparently, soon after it opened the bridge began to sway violently. They had done extensive wind tunnel tests on models prior to constructing the bridge so they knew that it wasn't an airflow issue. After some study, they concluded that it was the rhythm of the foot falls made by the pedestrians crossing the bridge that causing the movement. They had to retrofit the structure with hydraulic dampers (or something to that effect, I'm a political scientist, not an engineer so I don't have all the details) which has solved the problem.

Possibly not significant, but at least anecdotal.


P

 

[Kat Stevens]

it's actually quite relevant. People take for granted that the bridges they drive or walk across won't fail. Dampers would absorb res waves, the biggest factor is how the structure is supported, and fastened to those supports.  If you get a chance to see the Tacoma Narrows Bridge video, give a decco, you'll be astounded at how external forces act on these structures....plus, it's just damned cool!

Kat

 

[pjocsak]

Kat,

I have seen a video of the bridge. IIRC they called it "galloping Gerty" or something to that effect. It's absolutely amazing. I remember watching the video and thinking that it was amazing just how much like waves the surface looked. The peaks and valleys were several meters appart. Kind of awe inspiring really, when you think about it. I'm not sure, and you can correct me if I'm wrong, but the bridge in question eventually collapsed not as a result of the traffic going over it, but as a result of the vibration created by the airflow around the bridge. Probably splitting hairs since the result is the same.

P

 

[Kat Stevens]

That's the theory, that high winds passing over the edge of the bridge caused it to vibrate, much like a saxophone reed.  The frequency of the vibration matched the natural resonance of the structure, and the result, as you saw, was catastrophic failure. Same idea if you take a piece of rope and stretch it tight in a wind, see how it vibrates.  Physics is cool ;D

CHIMO,  Kat

 

[Spr.Earl]

That's the theory, that high winds passing over the edge of the bridge caused it to vibrate, much like a saxophone reed.   The frequency of the vibration matched the natural resonance of the structure, and the result, as you saw, was catastrophic failure. Same idea if you take a piece of rope and stretch it tight in a wind, see how it vibrates.   Physics is cool ;D

CHIMO,   Kat

Also it was discovered her deck acted like a Air Foil in high winds.
The bridge was not of box girder construction.

As too the marching thingee,in London they had to put in anti sway bracing on a foot bridge that was built of late as the foot bridge swayed when the locals walked over it.

 

[Standards]

A fellow by the name of Henry Petrofski has written a couple of books that touch on the topic (including the Tacoma Narrows Bridge).  Although he is a Professor of Civil Engineering at Duke University (in the US), they are not textbooks and are actually quite readable.  Check out:
To Engineer is Human: The Role of Failure in Successful Design
Design Paradigms: Case Histories of Error and Judgement in Engineering

Another book that is readable and touches on design failure is Why Buildings Fall Down by Matthys Levy and Mario Salvadori.  Chapter 7 is entitled Galloping Gertie and is all about the Tacoma Bridge failure.  The new edition also includes a chapter on the World Trade Centre.

Doug

 

[Colin2]

I saw the show to and agree that while entertaining, they can be quick and loose with the physics. I believe that the concern about marching troops date quite far back and many of the bridges were quite lightly built (with some fabulous exceptions of course) Hence were more susceptible to the resonance of marching feet.

Modern bridges do have more ability to deal with weight, resonance, weather and temperature effects but as Tacoma Narrows show, the right set of circumstances show that they can and will fail. I was on the Wooden Truss Willow river bridge near Prince George and it vibrated for almost 40 seconds after the logging truck went over. (same bridge was taken out be a lowbed with an excavator on the back, which took out the cross beams of the truss. The Arcow bridge they replaced it with also likes to vibrate after trucks pass over it.

Vehicles tend to set up pressure waves in the bridge in advance of the vehicle and traveling in the same direction, these wave often get broken up by oncoming traffic. This is also a major concern on Ice roads and bridges.

Most of the bridge failures I deal with concern the failure of abutments or gross overloading. Except of course the ancient old wooden logging bridges that have died of old age. The modern modular bridging used by companies such as Ruskin is quite cool. They can do a 100 ton 40m bridge good for 15 years in 3 days. 
By the way my friend built his son a 10' high trebuchet for his 7th birthday. We went down to Jericho beach and flung rocks into the water. Would have been fun to see the Bylaw officer try to ticket us.