Does anyone know about suspension bridges???

[Spinney]

Cycling over the Severn Bridge a couple of days ago (lovely bridge!).
I've seen these things on the suspension cables before - anyone know what they are for?
They only appear on the cables once the cables are longer than the lamp posts, and are in pairs (i.e. two bits of wire mounted near each other, both with those 'blobs' on the ends).
When the suspension cables get even longer (i.e. 2 x lamp post height), another single 'thing' appears higher up.

My only guess is something to help stop the cables vibrating in the wind? But why pairs, and why the weird shaped blobs on the ends?

 

[HorTs]

http://www.newscientist.com/blog/lastword/2007/06/cable-clingers.html

http://en.wikipedia.org/wiki/Stockbridge_damper

 

[AndyRM]

Alien eggs.

Run. While you still can.

 

[neil_merseyside]

Yup as suspected they're vortex shedders - don't want this happening

http://upload.wikimedia.org/wikiped...coma_Narrows_Bridge_destruction.ogg.480p.webm

 

[Keith Oates]

I've not seen those before so I'm glad you asked the question Spinney. As they say you learn something new every day. !!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[Spinney]

Thanks @HorTs !

I thought of googling, but it's a bit difficult when you don't know what the things are called!

One of the replies to the New Scientist question stated that the pairs on the Severn Bridge cables were set at 90 degrees to each other to damp vibrations from any wind direction - they did look as if they were at right angles.

But on the longer cables, where there was another 'cable clinger' higher up, there was only one, not two of them. Maybe those are just adjusted for the prevailing winds?

 

[Archie_tect]

Building foundations in high risk earthquake zones also have mass dampers for the same reason... modern skyscrapers also have them on the roof to counteract wind oscillation.

 

[swansonj]

We have them in our business too. We don't do such specific tuning as the Severn Bridge ones, we just slap one or more standard models at the end of the span, but the distance out from the clamp is related to the likely position of nodes and antinodes. Here is a younger me and and even younger and considerably prettier colleague (who quite rightly went on to be my boss for a while) almost sitting on some:

 

[Smurfy]

We have them in our business too. We don't do such specific tuning as the Severn Bridge ones, we just slap one or more standard models at the end of the span, but the distance out from the clamp is related to the likely position of nodes and antinodes. Here is a younger me and and even younger and considerably prettier colleague (who quite rightly went on to be my boss for a while) almost sitting on some:
View attachment 83017

OK, now the OP has been answered, could I take the liberty of asking Swanson to explain how those multi part insulators work that he's sitting on?

 

[andytheflyer]

Fascinating @HorTs, thanks for that link. So they are also in use on the wobbly Millennium Bridge in London.

Don't think so - they added weights to damp the vibrations, but it's not a suspension bridge, so the vibration was different. IIRC.

 

[Cuchilo]

If it was invented in 1928 there has to be a dogbone damper blues song somewhere !

 

[slowmotion]

Don't think so - they added weights to damp the vibrations, but it's not a suspension bridge, so the vibration was different. IIRC.

Two types of damper were used according to Wiki...

After extensive analysis by the engineers,[9] the problem was fixed by the retrofitting of 37 fluid-viscous dampers (energy dissipating) to control horizontal movement and 52 tuned mass dampers (inertial) to control vertical movement. This took from May 2001 to January 2002 and cost £5M. After a period of testing, the bridge was successfully re-opened on 22 February 2002. The bridge has not been subject to significant vibration since. In spite of the successful fix of the problem, the affectionate "wobbly bridge" epithet remains in common usage among Londoners.[10][11]

 

[vernon]

Yup as suspected they're vortex shedders - don't want this happening

http://upload.wikimedia.org/wikiped...coma_Narrows_Bridge_destruction.ogg.480p.webm

The Tacoma bridge collapse was a totally different form of failure. The major factor in the collapse of the bridge was the large amplitude low frequency oscillations caused the slab sided decks catching the wind and not the relatively high frequency low amplitude oscillations of the cables.

 

[Cuchilo]

The Tacoma bridge collapse was a totally different form of failure. The major factor in the collapse of the bridge was the large amplitude low frequency oscillations caused the slab sided decks catching the wind and not the relatively high frequency low amplitude oscillations of the cables.

I concur .

 

[neil_merseyside]

I concur .

Ta for clarification, still impressive to see it buckle and fail!