How prone are the two Severn Bridges to swaying?

[RJDG14]

I had the opportunity of walking across the original bridge today and can say that in calm conditions, there is virtually noticeable sway. I'd say that what little motion there is would probably be comparable to a car ferry when moored in a dock.

What I did definitely feel was vibrations from moving vehicles. These were possibly more noticeable than what they are on the Cleddau Bridge. I think the deck as it is immediately below the footpath is quite thin, possibly only a foot or so. I could feel these vibrations across the whole structure (Aust Viaduct, Severn Bridge, Beachley Viaduct and Wye Bridge) , not just on the suspension bridge portion.

[KeithW]

I had the opportunity of walking across the original bridge today and can say that in calm conditions, there is virtually noticeable sway. I'd say that what little motion there is would probably be comparable to a car ferry when moored in a dock.

What I did definitely feel was vibrations from moving vehicles. These were possibly more noticeable than what they are on the Cleddau Bridge. I think the deck as it is immediately below the footpath is quite thin, possibly only a foot or so. I could feel these vibrations across the whole structure (Aust Viaduct, Severn Bridge, Beachley Viaduct and Wye Bridge) , not just on the suspension bridge portion.

Both bridges move up and down as the vehicles are a moving load, the suspension cables are carrying the load and elongate slightly when a heavy load is applied by a moving HGV. The decks were designed as aerofoils to provide stability. Any elastic structure will deform under load and that includes the bridge deck. In fact on the deck and the towers of the Severn Bridge were strengthened in the 1980's to handle the increasing load from HGV's in particular.

When the bridge was designed the max weight of an HGV was 24 tons , by 1996 it had increased to 38 tons and I believe its now 44 tons. The Cleddau bridge is a rigid box girder so less vibration is normal especially as internal diaphragms act as straighteners, it was the inadequate thickness of these that caused the collapse when it was being built.

[RJDG14]

I had the opportunity of walking across the original bridge today and can say that in calm conditions, there is virtually noticeable sway. I'd say that what little motion there is would probably be comparable to a car ferry when moored in a dock.

What I did definitely feel was vibrations from moving vehicles. These were possibly more noticeable than what they are on the Cleddau Bridge. I think the deck as it is immediately below the footpath is quite thin, possibly only a foot or so. I could feel these vibrations across the whole structure (Aust Viaduct, Severn Bridge, Beachley Viaduct and Wye Bridge) , not just on the suspension bridge portion.

Both bridges move up and down as the vehicles are a moving load, the suspension cables are carrying the load and elongate slightly when a heavy load is applied by a moving HGV. The decks were designed as aerofoils to provide stability. Any elastic structure will deform under load and that includes the bridge deck. In fact on the deck and the towers of the Severn Bridge were strengthened in the 1980's to handle the increasing load from HGV's in particular.

When the bridge was designed the max weight of an HGV was 24 tons , by 1996 it had increased to 38 tons and I believe its now 44 tons. The Cleddau bridge is a rigid box girder so less vibration is normal especially as internal diaphragms act as straighteners, it was the inadequate thickness of these that caused the collapse when it was being built.

Did the bridge move up/down any more prior to its strengthening, then? I believe the bridge is currently undergoing another strenthening scheme, hence why it was closed in June when I got some photos of it at Beachley, and had its outer lanes both closed today.

I can sort of see a small amount of up/down motion in video clips I've seen but didn't see this while on the bridge, even with a bit of careful looking while stationary, nor could I really see it in today's weather conditions at the Aust viewpoint. I did feel a very small amount of probable motion when walking along it (though couldn't see it), similar to being on an extremely calm ferry crossing to Ireland.

I also got some very nice pictures on a 2007 DSLR that I bought secondhand yesterday:

[DB617]

AFAIK the current VolkerLaser works are cable inspection and maintenance, rather than strengthening.

[RJDG14]

AFAIK the current VolkerLaser works are cable inspection and maintenance, rather than strengthening.

Yep, that looks to be the case.

Does anyone here know what the four bars are under the deck of the original bridge? They appear to be moveable, and their positions have shifted over the years.

I was on the bridge in very calm conditions and couldn't really feel any movement aside from vehicle vibrations, but if you were walking along the bridge in 50mph winds would you be able to feel any subtle motion? Some suspension bridges have a noticeable amount of up/down motion in high winds, although I know the Severn Bridge was unusual at the time it was built in that it has triangular suspension wires which I believe reduce the amount of vertical movement.

There is definitely a small amount of visible movement on the Forth Road Bridge from a timelapse I found. It uses regular vertical cables:



The motion of the Severn Bridge definitely seems subtler than this.

[Isleworth1961]

Does anyone here know what the four bars are under the deck of the original bridge? They appear to be moveable, and their positions have shifted over the years.

That's pretty obvious - maintenance access platforms. Even the Clifton Suspension Bridge has one.