Years ago I read in a publication, whose name escapes me, that no one is allowed to walk a dog or march in cadence on the Golden Gate Bridge thereby causing harmonic vibrations that would escalate and eventually bring down the bridge. Am I dreaming this or did I really read it somewhere. My family has looked at me since as someone who may have a scrambled brain. Help!
Don’t know about that (at least never heard of it, and I’m a bay area native). What I can tell you is that after having the bridge open to the public on its anniversary, the cables were stretched many inches from supporting such a dense load.
The principle is correct. Marching in cadence can set up harmonics that will structurally weaken a bridge. (I think the hotel bridge-type walkway that collapsed a few years ago went down during a party during which a number of people were dancing on it–although there were some structural problems with the construction, as well.)
Translating that to a “do not walk your dog on the Golden Gate” law sounds like a stretch. I have heard (and never actually investigated whether it was true) that it is the general practice of the military to break cadence when marching over some varieties of bridges.
The Urban Folklore web site ( http://www.urbanlegends.com ) says this about bridges and people walking in step:
The T stands for undoubtedly true, the U for Unanswered or Unknown (and may be unanswerable), the F for false, and Fb or Tb for False Believed (Pretty sure it’s false, but it isn’t proven conclusively) or True Believed.
Going by the above list of entries, it seems that soldiers do commonly break cadence when crossing bridges, but it doesn’t seem to be actually needed. It seems no one has actually seen a bridge fail from the harmonic vibrations of walking in step.
There was also some discussion on the AFU newsgroup some time back about that pier that collapsed. IIRC, the general consensus was that while people were dancing, the weight of the people just standing was over the weight limit, so there was no conclusive evidence that the dancing (even assuming the majority of the people were in step) had as much to do with the problem as just having a “live” load versus a static load. But I’m definitely going by memory here, so a visit to deja news or the AFU archive (on the above referenced web page) is in order if you want to dig up those discussions.
Ugly
>> no one is allowed to walk a dog
That is because dogs have four legs and they resonate diagonally therefore inducing resonance that will destroy the bridge. OTOH you can take a camel for a walk and, even thought it weighs more, the gait has a different set of harmonics that do not induce such resonance.
You don’t really mean to say that the Golden Gate Bridge could actually be destroyed by having a dog walk over it, do you? That was sarcasm there, right?
waterj2, I sure hope that was sarcasm.
Can you a imagine, a stray dog starts across the bridge. Immediately all traffic is close off at both ends of the bridge. An Apache helicopter on alert is immediately dispatched. It rises up to the deck level with it’s guns ready to take out the dog to save the bridge. On the San Fransisco side a demonstration has already started to save the dog. A warning shot is fired over the dog’s head. The shot hits a pidgeon, the dog stops to check out the wounded bird. Everyone breaths a sigh of relief. The bridge is saved again!
Jim
I just wanted to mention, that this is something I would have never expected to see from tomndebb 
For what it’s worth, it is possible for a bridge to be destroyed by resonance effects (witness the Tacoma Narrows bridge), but it has to be at just the right frequency, and unless it’s a tiny little footbridge, a dog’s footsteps probably wouldn’t be enough even with resonance.
Unless, of course, it were Clifford… I definately wouldn’t take him across any bridges; I’d just let him wade across;)
If there is a rule against walking a dog on the bridge, I would suspect it’s because they’re afraid a dog will get loose and cause an accident. this GGB site explains what can and can’t cross the bridge (bikes are ok, but not skateboards). No mention of dogs is made either way.
If it was designed by a computer programmer, it might be destroyed if the programmer didn’t catch the “limping dog weighing exactly 20.5 pounds on a day when the wind is blowing 16 mph from the southwest” bug. 
Nah. That happens all the time. I’m always willing to pass on a plausible story to keep a thread going–I just try to be sure to indicate when I can’t support it. (I can post between compiles and test runs, but I can’t do serious research on company time.)
(Note that RJKUgly’s citation from AFU on the same point is just about as strong an assertion as my memory.) < eg >
Just a few days ago there was a TV program on PBS about bridges and a segment was dedicated to the Tacoma Narrows bridge. The conclusion was that it was woefully underbuilt in every sense. They showed footage of the bridge moving way too much even in the slightest breeze. They showed some graphic computer models that showed how the vertical sides of the bridge caught the wind and induced pressures that tended to amplify the oscillations. The wind itself was constant, it was the effects which were in sync with the oscillations of the bridge. The lessons learn from that bridge served to design reinforcements for other suspension bridges, including the Golden Gate. Footage was also shown of the Golden gate bridge moving way too much. When the wind is above a certain threshhold, it is closed to the public.
The whole program was extremely interesting. But the resonance factor has been totally exagerated. No bridge has ever suffered any damage, nor even come close, by exterior forces because they happened to have the same natural oscillation period. The chances of the frequency coinciding are infinitessimal and the effects would immediately be noticed.
The whole thing about breaking step is just UL. I have heard it myself more than once. The mass of the people is just not even enough to begin swaying a massive bridge.
My mother related the story of her high school class going to the Royal Gorge Bridge in Colorado (circa 1958). The teacher told them all to march in step as they crossed it, but it began the bouncy, trembling shake, and they were told to NOT march in step any longer.
The Tacoma Narrows bridge collapse has to be the most famous suspension bridge when talking about resonance and frequency.
I have a small clip I pulled off the web several years ago here.
Of course, a lot of the bridges soldiers cross have to be assumed to be in less than perfect shape, so the soldiers are told to break step just to be safe.
Cecil touched on soldiers’ breaking step and the Tacoma Narrows Bridge in one of his columns: Can opera singers shatter glass with their high notes?