# need help with factors affecting suspension bridge failures

by studenthelp10
Tags: bridges, tacoma
 P: 26 ive read articles and it says that the tacoma suspension bridge failed because of its length and it being too thin so when the wind blew on it it fell. an question i have to answer from school is investigate bridge failures and explain why they failed using physics principles so exactly how does the blowing of wind on a bridge affect it? does it put more stress on it - so it strains more and then breaks . How does this relate to compression and tension forces? could you please explain simply
 PF Patron HW Helper Sci Advisor Thanks P: 25,521 hi studenthelp10! welcome to pf! first look at a video of it, particularly the sideways views (eg http://www.google.co.uk/url?q=http:/...HHUjLEKOQ5A5Lg ) what do you think is happening in the bridge?
 P: 26 thanks for the reply, i saw the video u posted and it shows the bridge swinging violently from left right? then collapsing, while some of the concrete is held up by the cables. then the bridge getting closed. Because of this I think the bridge must be made of pretty weak material to be pushed by the wind that way.. but i need some solid facts i did some more research and found this site, but dont understand terminology: https://docs.google.com/viewer?a=v&q...dHP6dCy9fDxewg
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## need help with factors affecting suspension bridge failures

looking at the video, what do you think are the tension and compression forces in the bridge?
 P: 1 It's all about harmonics. A steady wind with a velocity that closely matches the natural harmonic length of particular span(determined, largely, by its length and tension) sets up a sympathetic vibration in that span, in much the same way a bottlle produces sound when you blow across its top. This suggests it may be a good idea, among other things, to build bridge spans of unequal length and setting dampers between the spans. All physical objects, including people, possess thier own particular set of harmonics. A loud enough sound tuned to one's primary harmonic could blow one's body apart! The Tacoma bridge event has been shown to all first year physics and engineering students as an example of the power of harmonic amplification of sound waves.
 P: 26 i know from research the tension force is from the cables being pulled apart from the load on the bridge.. and this compresses the towers- but im unsure how wind comes tp play in this? - a guess would be that the wind is twisting the bridge creating more tension = stress and then strain because the bridges cables cannot handle the stress and cannot stretch in length- so it fails and the cables break? . then when the bridge loses its cables its platform falls? im not to sure but i thinkk this is what happenned???? :) i hope I get this answer its been bugging me all week and i got to finish my assignment soon
 P: 26 thanks for the reply estein :) could you please explain abit more simply? please :) like does the vibration increase tension or something? according to definition of sypathetic vibration it is : sympathetic vibration (plural sympathetic vibrations) the vibration of a body, at its natural frequency, in response to that of a neighbouring one having that frequency; resonance i think this means that every object has a limit to vibrating? when u increase the vibrations it breaks apart? solid->liquid?
 P: 26 i hope so to :S
 P: 26 sry guys i new to all of this type of physics and im struggling to answer this question, so i need simple explanations please :) I really appreciate your replys though :D
 P: 26 I think ive solved it!!!!! yay! :D - i was researching and found some good info i think it says that when the tacoma narrows was designed engineers forgot to take into count other factors like the wind. These added more tension and compression to the cables. when the wind blew the cables tension force was put under greater compression (pushed together) then going back to tension (pulled apart) causing the platform/cement of the the bridge to swing side to side. the movement then broke up the cement and because the material is more rigid/ not very flexible ; it broke apart and the bridge collapsed. :) i will probably use this in my assignment, (correct me if im wrong) . funny thing i solved my own problem :D
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hi studenthelp10!

(just got up )
 Quote by studenthelp10 … i was researching and found some good info …
stop trawling the internet, and use your own eyes and brain!
 i think it says that when the tacoma narrows was designed engineers forgot to take into count other factors like the wind. These added more tension and compression to the cables. when the wind blew the cables tension force was put under greater compression (pushed together) then going back to tension (pulled apart) causing the platform/cement of the the bridge to swing side to side. the movement then broke up the cement and because the material is more rigid/ not very flexible ; it broke apart and the bridge collapsed
can cables go into compression?

watch the video
do any of the cables break?

what does break?

what are the internal and external forces on the thing that breaks?
 P: 26 I need actual facts/ research then I can use my brain to figure out how things go together :S ? and understand it better none of the cables break but the roadway does. Im not sure what the internal and external forces are sorry? would the external be wind and internal be compression/tension in cement? the article says :As the diagram above shows, the cables were anchored at each end, and supported in the middle by several raised towers. This allowed for the tension in the cables due to the weight of the cars and road to be conveyed into the ground. The reason the Tacoma Narrows Bridge did not last is that engineers never considered aerodynamics and wind forces, which added both a compression and tension force to the bridge. Every time the wind blew at strong gusts the tension force of the cables would be overcome by compression, then back to tension causing galloping oscillations of the deck or road. It didn’t help that the engineers built the bridge so light either. Without the weight of the bridge to dapper the oscillations they could be very intense. The material holding the deck during these vigorous movements finally tensed to a point of collapse and the bridge went down. is this wrong?
 P: 26 i may have mis understood this bit "Every time the wind blew at strong gusts the tension force of the cables would be overcome by compression, then back to tension causing galloping oscillations of the deck or road" after reading it through again i think its probably the road that is getting more compression by the wind causing more tension in the cables then when the wind stops it goes back to normal then starts again.... causing the twisting of the road. probably because the bridge caught more air under neath it (like a parachute) when the wind blew under it. I think this created those compression forces
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 Quote by studenthelp10 after reading it through again i think its probably the road that is getting more compression by the wind causing more tension in the cables then when the wind stops it goes back to normal then starts again.... causing the twisting of the road. probably because the bridge caught more air under neath it (like a parachute) when the wind blew under it. I think this created those compression forces
yes, i think you have to concentrate on the tension and compression of the road

form the video, the chains were perfectly capable of supporting the weight of the road, it was the road itself that was at fault (and of course the chains can swing in any direction, so they're contributing almost no structural stability to the shape of the road)

i don't think a road much minds being in compression, surely it's only tension that's a problem?

as you say, the video shows clearly that the wind is causing the road to twist … perhaps you should consider why it isn't twisting equally all the way along?
 P: 705 If you want to research something look up torsional harmonic motion. Consider what happens when air hits a surface, and also what happens when it travels over and under it. Also, was it that the bridge was light that caused a problem? The bridge they erected in it's place after the incident was very nearly a copy except for a glaring detail...trusses...which even potentially made it "lighter" Lightweight isn't a problem if something is properly designed. Note: When they say the tension in the cables was overcome by compression, what they mean is that the tension in the cables (usually taut) went to, basically, zero as the bridge sections they were attached to shifted. They word it poorly and, in my opinion, improperly.
 P: 343 The original excitation that started the torsional vibration of the Tacoma Narrows bridge was vortex shedding as wind vortices were shed, first off the top, then the bottom of the bridge deck. This drove the torsional motion that happened to coincide with a torsional natural frequency of the deck and the amplitude grew to the point of catastrophic failure.