Need help with factors affecting suspension bridge failures

[tiny-tim]

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_Narrows_Bridge_(1940 ), quoting "Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks" (1991, American Journal of Physics 59 (2): 118–124)

 

[studenthelp10]

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 textbook). '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 don't have the same problem.

 

[tiny-tim]

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]

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]

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]

re jambaugh: i found this article on dampers http://science.howstuffworks.com/engineering/structural/smart-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.

 

[studenthelp10]

i just wanted to know one more thing, i have to write a synopsis on physics involved in our spaghetti suspension bridge design and I am ok with all of them except for how youngs modulus of elasticity is applied to bridges.

my answer is : youngs modulus of elasticity applies to bridges because it gives us an idea of what materials to use to build the bridge. Youngs modulus tells us how rigid or flexible an object is, this is because it is based on stress/strain meaning how much stress (compression/tension) needs to be applied before the object changes shape "strains". For a bridge we want a bridge that can take a lot of stress without straining and losing its shape. If more stress is applied to an object and it doesn't change in shape much, this means it is very rigid and strong- this is the type of material needed for a suspension bridge.

 

[studenthelp10]

spaghetti is not very rigid or strong (youngs modulus), so we will need to group them up- . in numbers spaghetti is much stronger under stress, because the stress is distributed over the many spaghetti strands instead of 1 for example. This will mean that the spaghetti bridge will strain less under more stress (compression/tension). By adding more numbers of spaghetti to the bridge it should be more able to take the 4kg load we are putting on it.

is this good ? its my last question

Thanks everyone for youre help and for the group discussions i know know a lot more about bridge failures and why the tacoma fell down :) :D

 

[Travis_King]

The main thing that the new design had that the old one didn't was a trussed side. The original bridge had plating on the sides which created a body for the wind to act on, and forced creation of the vortices. The new bridges in it's place, if you look at them from the side, are trussed, allowing the wind to pass through; reducing the effective area for the wind to act on.

 

[tiny-tim]

… For a bridge we want a bridge that can take a lot of stress without straining and losing its shape. If more stress is applied to an object and it doesn't change in shape much, this means it is very rigid and strong- this is the type of material needed for a suspension bridge.

you're only considering the stiffness of a material, ie how much it bends

something can bend a lot without breaking, or break after very little bending …

so you also need to consider the hardness and the toughness of the material

look all three words up in wikipedia and on this forum ​