need help with factors affecting suspension bridge failures


by studenthelp10
Tags: bridges, tacoma
studenthelp10
studenthelp10 is offline
#19
Apr12-12, 02:05 AM
P: 26
So if im putting this altogether: is it like this?

The tacoma narrows bridge was very light and because of this it had less tension in its cables compared to other bridges. This is because the weight of the bridge pulls down on the cables and gives it tension, but since the tacoma is light -there is less weight pulling down and therefore less tension. The effect of this is that it will take a smaller gust of wind to vibrate the tacoma bridge.

When the wind blew against the bridge, some went under above the roadway of the bridge and some went underneath. If the wind is unequal at the top or bottom it causes the bridge to "twist' (move up and down) . The 'twist ' creates more tension (force pulling something apart) to pull the cement apart. Since cement is better in compression than in tension, and more tension was added it broke apart and failed.

Am i making progress? :)
tiny-tim
tiny-tim is offline
#20
Apr12-12, 05:11 PM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
P: 26,167
Quote Quote by studenthelp10 View Post
The tacoma narrows bridge was very light and because of this it had less tension in its cables compared to other bridges. This is because the weight of the bridge pulls down on the cables and gives it tension, but since the tacoma is light -there is less weight pulling down and therefore less tension. The effect of this is that it will take a smaller gust of wind to vibrate the tacoma bridge.
i don't think the tension in the cables matters, i think the problem is the tension in the roadway

the rest looks ok
When the wind blew against the bridge, some went under above the roadway of the bridge and some went underneath. If the wind is unequal at the top or bottom it causes the bridge to "twist' (move up and down) . The 'twist ' creates more tension (force pulling something apart) to pull the cement apart. Since cement is better in compression than in tension, and more tension was added it broke apart and failed.
studenthelp10
studenthelp10 is offline
#21
Apr12-12, 05:43 PM
P: 26
so is the second paragraph all i have to say - is there any way i can improve it ? by using information from other sources i found.

Im not sure if these were all the physics principles involved in the collapse?
studenthelp10
studenthelp10 is offline
#22
Apr12-12, 06:03 PM
P: 26
Travis Kings post from earlier Noted: "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"

So the chain of events was the wind -> rocked the cables -> tension to no tension at all then back (zero) as said above = twist of bridge (b/c weight of bridge is light- b/c little tension in cables as bridge was light)-> bridge twists up and down-> the wind added to the twisty motion by splitting up when it hit the bridge some on top and some underneath making it twist more dangerously-> twisting adds compression and tension force to cement-> cement can handle compression but is very bad at tension so cement brike apart-> roadway collapses into river underneath the bridge. ?

would that be a better explanation to put into my assignment?
i need to find a way to link the chain of events together i.e. this ccaused this->.... which caused this which made the bridge fail
scutterbob
scutterbob is offline
#23
Apr12-12, 08:04 PM
P: 17
I will also add in, learn what concrete and cement are and that they are not interchangeable. Vortex shedding, resonance and insufficient stiffness to the lateral loads were issues. Newer suspension bridges decks are actually designed to produce a net downward force which stabilizes the system. Reading someone else's conclusion is not really you understanding what happened.
studenthelp10
studenthelp10 is offline
#24
Apr12-12, 11:31 PM
P: 26
thanks scutter bob i took into count what you said and i researched vortex shedding http://www.mecaenterprises.com/vortex_shedding.htm shows a diagram.

What I think is happening is that the wind force blowing from left to right on the diagram and travelling above and below the circle. Then curving back inward and spiralling. Im not sure but i think that the wind is pushing from boths sides i.e. direction right and from the spirals going the opposite way (looking like compression horizontally)- opposite forces pushing object inward. I think this then squeezes the circle together and when the sides are squeezed in the top and bottom of the circle expand (looks like tension) forces pulling object outward in opposite direction to stretch . Maybe this is similar to what happens to a bridge :)?. This was all my own thinking - is it right? just need to confirm :D

the note above wasnt a conclusion - it was a step i didnt understand because they worded it badly- but travis Kings explanation makes it a bit more clearer- im slowly getting there :)
tiny-tim
tiny-tim is offline
#25
Apr13-12, 02:29 AM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
P: 26,167
Quote Quote by studenthelp10 View Post
So the chain of events was the wind -> rocked the cables -> tension to no tension at all then back (zero) as said above = twist of bridge (b/c weight of bridge is light- b/c little tension in cables as bridge was light)-> bridge twists up and down-> the wind added to the twisty motion by splitting up when it hit the bridge some on top and some underneath making it twist more dangerously-> twisting adds compression and tension force to cement-> cement can handle compression but is very bad at tension so cement brike apart-> roadway collapses into river underneath the bridge. ?

i need to find a way to link the chain of events together i.e. this ccaused this->.... which caused this which made the bridge fail
the chain of events does not start with the cables, the wind doesn't rock the cables, the cables merely respond to what the roadway is doing

the cables, basically, are springs, each one will adjust to whatever load is pulling it down

when a heavy vehicle goes onto the roadway, the roadway changes shape slightly, and the tension in the nearest cables reacts to that

the wind vortices are behaving like heavy vehicles, except that they can also reduce the load

they're a bit like the air pockets (of reduced pressure) that an aircraft sometimes flies through
Quote Quote by studenthelp10 View Post
What I think is happening is that the wind force blowing from left to right on the diagram and travelling above and below the circle. Then curving back inward and spiralling. Im not sure but i think that the wind is pushing from boths sides i.e. direction right and from the spirals going the opposite way (looking like compression horizontally)- opposite forces pushing object inward. I think this then squeezes the circle together and when the sides are squeezed in the top and bottom of the circle expand (looks like tension) forces pulling object outward in opposite direction to stretch
it isn't left and right, it's up and down alternately, causing the roadway to twist

see this image, showing the vortices rolling off the roadway, alternately above and below, from http://saba.kntu.ac.ir/eecd/Ecourses...s/vortex10.gif



(the word "alternative" should of course be "alternate"! )

the accompanying text explains … (http://saba.kntu.ac.ir/eecd/Ecourses...ter/vortex.htm)
On the side of the bluff body where the vortex is being formed, the fluid velocity is higher and the pressure is lower. As the vortex moves downstream, it grows in strength and size, and eventually detaches or sheds itself. This is followed by a vortex's being formed on the other side of the bluff body . The alternating vortices are spaced at equal distances.

The vortex-shedding phenomenon can be observed as wind is shed from a flagpole (which acts as a bluff body); this is what causes the regular rippling one sees in a flag. Vortices are also shed from bridge piers, pilings, offshore drilling platform supports, and tall buildings. The forces caused by the vortex-shedding phenomenon must be taken into account when designing these structures. In a closed piping system, the vortex effect is dissipated within a few pipe diameters downstream of the bluff body and causes no harm.
studenthelp10
studenthelp10 is offline
#26
Apr13-12, 06:29 PM
P: 26
ooh so its a bit like on the diagram - the flow of wind hits the surface of the object and runs on top and beneath the object. the high velocity fluid is the wind flow and the high velocity fluid is closest to the object and has high pressure, where as the high velocity fluid further away from the object has low pressure. So this then makes the spirals curving inward because the high pressure is on the inside pushing the wind more inward?- sheds. The alternative vortices /spirals are equally spaced and alternate (move one after the other on each side). This then causes the object to twist up and down, following the same motion as the vortices/spirals.- I thinks thats what the diagram is trying to say... but im unsure if its correct.

Two questions? you said "it isn't left and right, it's up and down alternately, causing the roadway to twist" does that mean the wind is blowing from underneath the bridge upward- like vertically? If it is , I dont get it because isnt the wind blowing horizontally and hitting the surface of the bridge to make it go up and down? because of the vortices or spirals alternating?

Secondly, the twisting makes the compression and tension forces right? because i read an article showing a spaghetti getting compressed (pushed together) and when it bends there is tension in the centre, because the centre is getting stretched.
studenthelp10
studenthelp10 is offline
#27
Apr13-12, 06:37 PM
P: 26
http://science.howstuffworks.com/eng...il/bridge2.htm
has a diagram of a plank under tension and compression at the same time- from bending
tiny-tim
tiny-tim is offline
#28
Apr14-12, 03:56 AM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
P: 26,167
Quote Quote by studenthelp10 View Post
The alternative vortices /spirals are equally spaced and alternate (move one after the other on each side). This then causes the object to twist up and down, following the same motion as the vortices/spirals.
that's correct

(but i don't understand your explanation preceding it …)
the flow of wind hits the surface of the object and runs on top and beneath the object. the high velocity fluid is the wind flow and the high velocity fluid is closest to the object and has high pressure, where as the high velocity fluid further away from the object has low pressure. So this then makes the spirals curving inward because the high pressure is on the inside pushing the wind more inward?- sheds.
Two questions? you said "it isn't left and right, it's up and down alternately, causing the roadway to twist" does that mean the wind is blowing from underneath the bridge upward- like vertically? If it is , I dont get it because isnt the wind blowing horizontally and hitting the surface of the bridge to make it go up and down? because of the vortices or spirals alternating?
no, the horizontal force isn't a problem

only the vertical forces from the "air-pockets" (vortices) matters

if the bridge designer could stop the vortices from forming (by streamlining, as in vehicles),
the problem would disappear
Secondly, the twisting makes the compression and tension forces right? because i read an article showing a spaghetti getting compressed (pushed together) and when it bends there is tension in the centre, because the centre is getting stretched.
Quote Quote by studenthelp10 View Post
http://science.howstuffworks.com/eng...il/bridge2.htm
has a diagram of a plank under tension and compression at the same time- from bending
yes, if you bend something stiff, it goes into tension on the outside of the bend, and compression on the inside

this does not apply to chains cables ropes etc … they can only be in tension
that's an interesting link ("How Bridges Work" by Robert Lamb and Michael Morrissey, 13 pages) …

on page 11 it deals with resonance, including …
… the wind that day was at just the right speed and hit the bridge at just the right angle to set off the deadly vibration. Continued winds increased the vibrations until the waves grew so large and violent that they broke the bridge apart. The effect is similar to that of a singer shattering a glass with her voice.
… so you see, it wasn't the strength of the wind that mattered, only the exact speed, just as you only need the exact frequency to shatter something
studenthelp10
studenthelp10 is offline
#29
Apr14-12, 08:31 AM
P: 26
The explanation you didnt understand was when i was trying to explain the diagram to you using the technical words in the diagram and fully understand the text underneath you posted earlier about wind shedding. Im trying to understand how the vortices are created after it hits the object - i know it spirals inwards because it travels around but - i need a reason. would the reason be because wind pressure is higher on the inside and lower on the outside so it is pushed in.

basically what i was trying to say was that when the wind hit the object, did it go around the object above and below and because of the high pressure closer to the object it created the spirals/vortices?
jambaugh
jambaugh is offline
#30
Apr14-12, 08:47 AM
Sci Advisor
PF Gold
jambaugh's Avatar
P: 1,767
It may also be helpful to consider the situation in terms of energy. It takes a certain amount of energy to stretch and break a cable or bend a beam. Forces are rate of energy expended per distance moved. The pieces of the bridge are somewhat elastic so that stretching cables and bending beams stores the energy from the imparted forces but when the force is too great they are pushed beyond their elastic limit and break.

Now consider the bridge as designed. The engineers accounted for the force of gravity via the weight of the bridge and the cars it carries. They also accounted for direct force of the wind. What they did not account for was the accumulation of kinetic energy, due to oscillations of the bridge driven by strong winds.

The wind applies a force, the bridge elastically deforms storing that energy, the deformation changes the wind force and the bridge springs back releasing that stored energy which become the kinetic energy of the moving bridge. If the wind stops now the bridge will oscillate with the energy cycling between kinetic energy of motion and stored energy in the springiness of the cables an beams.


Normally that would dissipate over time but before that can happen the wind adds more force and so adds more energy. As explained, whether the bridge will continue to accumulate energy or not depends on resonances. When the wind is blowing just right the oscillatory turbulence will be in time with the oscillatory motion of the bridge so that the wind adds a bit of energy each cycle. The bridge continues to gain energy rocking farther and harder until the material is pushed beyond its elastic limit and beams bend without springing back and wires stretch without springing back. You then have a bridge outside of the design specs and all the engineers carefully balanced forces are out of balance. Wires snap, beams break, and it all comes crashing down.

You may also find it helpful, to research exactly what the engineers do now since this disaster to prevent similar events. Understanding why bridges since then do not fall down is a good way to get at why that one did.
tiny-tim
tiny-tim is offline
#31
Apr14-12, 09:53 AM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
P: 26,167
Quote Quote by studenthelp10 View Post
basically what i was trying to say was that when the wind hit the object, did it go around the object above and below and because of the high pressure closer to the object it created the spirals/vortices?
vortices are caused at certain speeds depending on the shape of the obstruction, see http://en.wikipedia.org/wiki/Kármán_vortex_street and http://en.wikipedia.org/wiki/Vortex_shedding

however, according to wikipedia, the tacoma narrows disaster was not caused by these vortices, but by "aeroelastic flutter", see http://en.wikipedia.org/wiki/Tacoma_...s_Bridge_(1940), quoting "Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks" (1991, American Journal of Physics 59 (2): 118–124)
studenthelp10
studenthelp10 is offline
#32
Apr15-12, 12:16 AM
P: 26
oh yeah jambaugh- i read that they used "dampers" to stop the bridge from moving up and down any further. I think i need to read what dampers actually do- think it says they absorb vibrations. I also read that they build bridges in different sections was a second solution.

also tiny tim- would an explanation of aero elastic flutter be going too in depth my explanation on the failure of the tacoma narrows bridge collapse at my level- high school :). ?

anyway I think i get it now! :) ok so ill try to explain what happenned to make the bridge collapse simply and i hope its right

So the wind force horizontally hit the bridge, some of the wind went above the bridge and some underneath it (ref "Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks") diagram posted above. The wind created vortices/spirals which are equally spaced apart- so the vortices/spirals twisted the bridge in the same motion. On the diagram it shows the vortices pushing the bridge downward on an angle. Because it is downward acts like a parachute, so when the next vortices/spirals came it pushed the bridge up 'lift'- rocking the bridge up and down creating a twisting effect. The twisting/vibration grew more and more as the wind added more force- like a swing being pushed) gaining more and more energy. when the vibrations got closer the the natural frequency the bridge could handle, it began to collapse (resonance- from school text book). 'twisting' creates compression and tension forces on the bridge. The bridge was made of concrete which is good under compression but bad under tension. Because the twisting added more tension- the concrete was bad under tension so it split apart. The increasing vibrations of the bridge from the wind did not help- they made the compression and tension forces grow larger. Since then engineers have used dampers to absorb or reduce vibrations so they do no grow larger so future bridges dont have the same problem.
tiny-tim
tiny-tim is offline
#33
Apr15-12, 02:57 AM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
P: 26,167
yes that looks ok

just one thing about vortices … they are alternately above and below the bridge … and they form in chains ("Kármán streets"), and each chain is in the same direction as the wind (in other words, the vortices move away from the bridge, not along it), though there will be lots of chains at different points along the bridge
jambaugh
jambaugh is offline
#34
Apr15-12, 09:07 AM
Sci Advisor
PF Gold
jambaugh's Avatar
P: 1,767
Quote Quote by studenthelp10 View Post
oh yeah jambaugh- i read that they used "dampers" to stop the bridge from moving up and down any further. I think i need to read what dampers actually do- think it says they absorb vibrations. I also read that they build bridges in different sections was a second solution.
Yes, they absorb the energy of motion. They are essentially the same thing as shock absorbers in your car and for the same reason. In your car, when you hit a bump, the spring suspension would absorb some energy and you would bounce around and get seasick from the oscillations. Shock absorbers act as dampeners which dissipate this energy as heat.

A dampener essentially introduces friction in movement.
studenthelp10
studenthelp10 is offline
#35
Apr15-12, 05:23 PM
P: 26
re: tiny tim: yes that makes sense - the vortices are connected and they move away from the bridge and because they are in chains they are 'connected' so one after the other the vortices hit the bridge and alternate.
studenthelp10
studenthelp10 is offline
#36
Apr15-12, 05:37 PM
P: 26
re jambaugh: i found this article on dampers http://science.howstuffworks.com/eng...structure2.htm

I think i know what the dampers doing? i think that it is resisting the (tension) of the building being pulled outwards because the mr liquid turns solid and as the metal piece in the middle pull out - it has to go against the solid (friction) . This reduces the tension.


Register to reply

Related Discussions
PhD Duration and affecting factors Academic Guidance 5
factors affecting the internal energy Classical Physics 1
Factors Affecting Friction Lab Introductory Physics Homework 1
Factors affecting inductance experiments Introductory Physics Homework 1
Factors affecting performance of Antacids Biology, Chemistry & Other Homework 0