How prone are the two Severn Bridges to swaying?

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RJDG14
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How prone are the two Severn Bridges to swaying?

Post by RJDG14 »

I've travelled across both the original Severn Bridge and the newer Second Severn Crossing/Prince of Wales Bridge in vehicles before, but have never walked across the original one on foot (you can't walk across the newer one), and while I've never been able to feel any motion on either in a vehicle, wondered if it's possible to on the suspended portions of each if you were on foot? I'd imagine that the original bridge would be more prone to motion since its suspended portion is much longer (most of the newer bridge is actually a viaduct - only about 400m in the middle is suspended) and it was built to older design specs, though I believe has undergone strengthening work several times over the years, which may or may not have an impact on how prone it is to swaying. I know that the original bridge was repainted from a dull grey to white around the time that the new bridge opened, and don't know if there was any other work done on it then, but there was certainly work being carried out about a month ago as it was closed on the two Saturdays that I travelled between England and Wales.

I've seen video clips of the Clifton Suspension Bridge swaying in high winds, but know that it is about a century older than the original Severn Bridge.
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Re: How prone are the two Severn Bridges to swaying?

Post by Isleworth1961 »

I've never felt it sway on a moderately windy day, but it probably sways a bit in very high winds, in which case it's likely to be closed anyway. The recent closures have been due to cable inspections. The bridge was designed to cope with high winds in that the suspended roadway has a sort of streamlined aerofoil shape so wind flows smoothly over and under it.
Last edited by Isleworth1961 on Sun Jul 31, 2022 15:14, edited 1 time in total.
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Re: How prone are the two Severn Bridges to swaying?

Post by DB617 »

I don't think the 'new' bridge sways at all. The distance between the two columns, by inspection, does not look sufficient, nor the length of the stay cables. Suspension bridges I would imagine to be prone to sway due to the longer cables and larger lever arm between anchoring points, but from experience alone it's hard to know because nobody is allowed out onto it when the winds are significant. We must have some bridge engineers amongst us who can shed more light; my structural engineering education has been thready and a bit 'meh'.
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Re: How prone are the two Severn Bridges to swaying?

Post by IAN »

This clip demonstrates the technology used to stop the bridge from swaying.

https://www.youtube.com/watch?v=ruO-JJT ... nceChannel
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Re: How prone are the two Severn Bridges to swaying?

Post by RJDG14 »

Isleworth1961 wrote: Sun Jul 31, 2022 15:11 I've never felt it sway on a moderately windy day, but it probably sways a bit in very high winds, in which case it's likely to be closed anyway. The recent closures have been due to cable inspections. The bridge was designed to cope with high winds in that the suspended roadway has a sort of streamlined aerofoil shape so wind flows smoothly over and under it.
I know that the original bridge is quite prone to being closed in high winds, whereas the newer one very rarely shuts and when it does, the weather is usually severe like what we had back in February.

Given that it opened 2 years prior to the original Severn Bridge and is also longer, is the Forth Road Bridge any more prone to swaying? It looks to have been built by the same contractors as the Severn Bridge but seems to have an uglier, less aerodynamic design.

I don't think Britain has ever had a bridge as badly designed as this:



I know that the original Severn Bridge is definitely not prone to sideways swaying like the original Tacoma Narrows Bridge was, nor do I think the Clifton Suspension Bridge is, but is it at all prone to up/down motion, similar to the Clifton Suspension Bridge?

The worst modern day bridge accident in the UK would probably be the collapse of the Cleddau Bridge during construction, but those issues were subsequently ironed out and the bridge eventually opened. While it is a viaduct and not a suspension bridge, I did notice when walking across it about a month ago that I could feel some slight vibration when on it. I suspect that this may be the case on the original Severn Bridge and Forth Road Bridge.
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Re: How prone are the two Severn Bridges to swaying?

Post by solocle »

RJDG14 wrote: Sun Jul 31, 2022 14:55 I've travelled across both the original Severn Bridge and the newer Second Severn Crossing/Prince of Wales Bridge in vehicles before, but have never walked across the original one on foot (you can't walk across the newer one), and while I've never been able to feel any motion on either in a vehicle, wondered if it's possible to on the suspended portions of each if you were on foot? I'd imagine that the original bridge would be more prone to motion since its suspended portion is much longer (most of the newer bridge is actually a viaduct - only about 400m in the middle is suspended) and it was built to older design specs, though I believe has undergone strengthening work several times over the years, which may or may not have an impact on how prone it is to swaying. I know that the original bridge was repainted from a dull grey to white around the time that the new bridge opened, and don't know if there was any other work done on it then, but there was certainly work being carried out about a month ago as it was closed on the two Saturdays that I travelled between England and Wales.

I've seen video clips of the Clifton Suspension Bridge swaying in high winds, but know that it is about a century older than the original Severn Bridge.
You can definitely feel it when an HGV passes on the motorway. I haven't noticed motion on the wind, but I've only been across it a couple of times.
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Re: How prone are the two Severn Bridges to swaying?

Post by RJDG14 »

solocle wrote: Sun Jul 31, 2022 15:33
RJDG14 wrote: Sun Jul 31, 2022 14:55 I've travelled across both the original Severn Bridge and the newer Second Severn Crossing/Prince of Wales Bridge in vehicles before, but have never walked across the original one on foot (you can't walk across the newer one), and while I've never been able to feel any motion on either in a vehicle, wondered if it's possible to on the suspended portions of each if you were on foot? I'd imagine that the original bridge would be more prone to motion since its suspended portion is much longer (most of the newer bridge is actually a viaduct - only about 400m in the middle is suspended) and it was built to older design specs, though I believe has undergone strengthening work several times over the years, which may or may not have an impact on how prone it is to swaying. I know that the original bridge was repainted from a dull grey to white around the time that the new bridge opened, and don't know if there was any other work done on it then, but there was certainly work being carried out about a month ago as it was closed on the two Saturdays that I travelled between England and Wales.

I've seen video clips of the Clifton Suspension Bridge swaying in high winds, but know that it is about a century older than the original Severn Bridge.
You can definitely feel it when an HGV passes on the motorway. I haven't noticed motion on the wind, but I've only been across it a couple of times.
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I have noticed in the past just how little barrier there is beside the footpath to stop somebody from falling into the Severn - it's little more than a 1m or so railing. I believe the original Severn Bridge has had a reputation for being a bit of a suicide spot, especially since many other bridges have installed measures to prevent suicide attempts. I think one of the Manic Street Preachers members (Richey Edwards) is believed to be among those who jumped off of it since he went missing and his vehicle was found parked at Aust services.

The new bridge doesn't have a footpath but its side barriers look about 2-3 metres, and look designed in a way that prevents cross winds. I think these are the main issue with the original bridge that often leads to closures rather than swaying, due to the lack of barriers that mitigate the effect of heavy winds crossing the carriageway.
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Re: How prone are the two Severn Bridges to swaying?

Post by KeithW »

RJDG14 wrote: Sun Jul 31, 2022 15:28
I don't think Britain has ever had a bridge as badly designed as this:

I know that the original Severn Bridge is definitely not prone to sideways swaying like the original Tacoma Narrows Bridge was, nor do I think the Clifton Suspension Bridge is, but is it at all prone to up/down motion, similar to the Clifton Suspension Bridge?

The worst modern day bridge accident in the UK would probably be the collapse of the Cleddau Bridge during construction, but those issues were subsequently ironed out and the bridge eventually opened. While it is a viaduct and not a suspension bridge, I did notice when walking across it about a month ago that I could feel some slight vibration when on it. I suspect that this may be the case on the original Severn Bridge and Forth Road Bridge.
The Tacoma Narrows Bridge was an early design and the problem of a bridge that moved because of vortex shedding at the same frequency as the bridge swaying was not something that had been encountered before. There was no suspension bridge anywhere near that length in the UK at the time, the Forth Rail Bridge was deliberately overdesigned as a result of the Tay Bridge disaster. Learning through disasters is nothing new - see Severn Railway Bridge, Dee Bridge, Broughton Suspension Bridge, Bull Bridge and Yarmouth Bridge.

As an engineering student the reputation of Leading Edge design meaning Bleeding Edge was drummed into us, see also DH Comet crashes.
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Re: How prone are the two Severn Bridges to swaying?

Post by RJDG14 »

KeithW wrote: Sun Jul 31, 2022 16:05
RJDG14 wrote: Sun Jul 31, 2022 15:28
I don't think Britain has ever had a bridge as badly designed as this:

I know that the original Severn Bridge is definitely not prone to sideways swaying like the original Tacoma Narrows Bridge was, nor do I think the Clifton Suspension Bridge is, but is it at all prone to up/down motion, similar to the Clifton Suspension Bridge?

The worst modern day bridge accident in the UK would probably be the collapse of the Cleddau Bridge during construction, but those issues were subsequently ironed out and the bridge eventually opened. While it is a viaduct and not a suspension bridge, I did notice when walking across it about a month ago that I could feel some slight vibration when on it. I suspect that this may be the case on the original Severn Bridge and Forth Road Bridge.
The Tacoma Narrows Bridge was an early design and the problem of a bridge that moved because of vortex shedding at the same frequency as the bridge swaying was not something that had been encountered before. There was no suspension bridge anywhere near that length in the UK at the time, the Forth Rail Bridge was deliberately overdesigned as a result of the Tay Bridge disaster. Learning through disasters is nothing new - see Severn Railway Bridge, Dee Bridge, Broughton Suspension Bridge, Bull Bridge and Yarmouth Bridge.

As an engineering student the reputation of Leading Edge design meaning Bleeding Edge was drummed into us, see also DH Comet crashes.
The Forth Rail Bridge looks pretty well designed for a railway bridge from the 19th Century. The original Tay Bridge looks pretty flimsy by modern standards with little to no high wind support. The original Tacoma Narrows Bridge looked fairly well designed for its day in terms of its suspension and towers but from the pictures and opening booklet that I can find, the main deck looked very basic with virtually no regard made for airflow underneath.
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Re: How prone are the two Severn Bridges to swaying?

Post by Chris5156 »

RJDG14 wrote: Sun Jul 31, 2022 15:28
Isleworth1961 wrote: Sun Jul 31, 2022 15:11 I've never felt it sway on a moderately windy day, but it probably sways a bit in very high winds, in which case it's likely to be closed anyway. The recent closures have been due to cable inspections. The bridge was designed to cope with high winds in that the suspended roadway has a sort of streamlined aerofoil shape so wind flows smoothly over and under it.
I know that the original bridge is quite prone to being closed in high winds, whereas the newer one very rarely shuts and when it does, the weather is usually severe like what we had back in February.
One thing that hasn’t yet been mentioned is that they are two different types of bridge. The Severn Bridge is a suspension bridge, but the Second Severn Crossing is cable stayed, and cable stayed bridges by their nature are less prone to movement. The Second Severn Crossing was also specifically designed to remain open in high winds to reduce the frequent closures of the M4 that were required in bad weather on the old bridge.
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Re: How prone are the two Severn Bridges to swaying?

Post by Bryn666 »

Any modern girder bridge regardless of span type will bounce up and down when traffic passes over it - this is a critical element of the design, if it didn't then the structure would risk developing fatigue cracks and it'll collapse eventually. This is why they have complex bearing arrangements underneath and that's why the Thelwall Viaduct spent the best part of the 2000s under repairs when several had developed fatigue cracks.

All these design requirements are why I steer well clear of structural calculations; get one element wrong and you're going to kill or maim a lot of people, cf Morandi Bridge.
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Re: How prone are the two Severn Bridges to swaying?

Post by Chris Bertram »

solocle wrote: Sun Jul 31, 2022 15:33
RJDG14 wrote: Sun Jul 31, 2022 14:55 I've travelled across both the original Severn Bridge and the newer Second Severn Crossing/Prince of Wales Bridge in vehicles before, but have never walked across the original one on foot (you can't walk across the newer one), and while I've never been able to feel any motion on either in a vehicle, wondered if it's possible to on the suspended portions of each if you were on foot? I'd imagine that the original bridge would be more prone to motion since its suspended portion is much longer (most of the newer bridge is actually a viaduct - only about 400m in the middle is suspended) and it was built to older design specs, though I believe has undergone strengthening work several times over the years, which may or may not have an impact on how prone it is to swaying. I know that the original bridge was repainted from a dull grey to white around the time that the new bridge opened, and don't know if there was any other work done on it then, but there was certainly work being carried out about a month ago as it was closed on the two Saturdays that I travelled between England and Wales.

I've seen video clips of the Clifton Suspension Bridge swaying in high winds, but know that it is about a century older than the original Severn Bridge.
You can definitely feel it when an HGV passes on the motorway. I haven't noticed motion on the wind, but I've only been across it a couple of times.
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Re: How prone are the two Severn Bridges to swaying?

Post by darkcape »

Bryn666 wrote: Sun Jul 31, 2022 21:57 Any modern girder bridge regardless of span type will bounce up and down when traffic passes over it - this is a critical element of the design, if it didn't then the structure would risk developing fatigue cracks and it'll collapse eventually. This is why they have complex bearing arrangements underneath and that's why the Thelwall Viaduct spent the best part of the 2000s under repairs when several had developed fatigue cracks.

All these design requirements are why I steer well clear of structural calculations; get one element wrong and you're going to kill or maim a lot of people, cf Morandi Bridge.
When I walked across the M48 bridge the bounce due to passing traffic, combined with the water flowing beneath, was enough for me to start feeling a little ill! The Forth Road Bridge also had a significant bounce when walking across, which you feel far more than any sway.

One bridge I built was only 130m long,supported on two piers & steel beams, but even that had a noticeable bounce when HGVs crossed.
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Re: How prone are the two Severn Bridges to swaying?

Post by solocle »

Chris Bertram wrote: Sun Jul 31, 2022 22:24 Nice picture of the bike, but what's the photo supposed to be telling us?
How close the path is to the motorway, the lack of meaningful barriers, and the general geometry of the bridge. Which does play into wind exposure, too, although I haven't experienced the bridge in meaningful winds.
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Re: How prone are the two Severn Bridges to swaying?

Post by fras »

The Severn suspension bridge is shaped to minimise swaying. I remember reading about the shape of the bridge sections many years ago shortly after it opened. As I remember, the consulting engineers were Freeman Fox and Partners.
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Re: How prone are the two Severn Bridges to swaying?

Post by AAndy »

I used to drive it everyday back in the 80's , before the 2nd crossing and of course with toll booths in both directions. It often took an hour to get though the tolls and 55 minutes of that you would be on the bridge eastbound, and in windy conditions I can assure you all it dipped and swayed and bounced to a reasonable concerning amount.
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Re: How prone are the two Severn Bridges to swaying?

Post by FosseWay »

solocle wrote: Mon Aug 01, 2022 08:42
Chris Bertram wrote: Sun Jul 31, 2022 22:24 Nice picture of the bike, but what's the photo supposed to be telling us?
How close the path is to the motorway, the lack of meaningful barriers, and the general geometry of the bridge. Which does play into wind exposure, too, although I haven't experienced the bridge in meaningful winds.
I've cycled over the bridge with an annoyingly strong headwind (though nowhere near enough for them to close it). I don't remember any swaying, but it does vibrate when an HGV goes past. The same applies to my local suspension bridge.

I don't think the cycle path on the Severn Bridge is unreasonably close to the motorway or that the barriers are not "meaningful", tbh. How far from the motor traffic can you reasonably expect the walkway/cycleway to be? As it is, the ped/cycle path on the Severn Bridge is wider than on Älvsborgsbron as shown above (though the latter has cycle paths on both sides). Moreover, the cycle path on the Severn Bridge is at the same level as the roadway. On Älvsborgsbron you're below the level of the passing traffic, meaning you get spray from vehicles right in the face. Both bridges are of similar age, btw.

The wire barrier is designed to deflect most vehicles, which is why 2+1 roads in Sweden with a similar wire barrier down the middle have a higher speed limit than those that don't. It's not going to stop an HGV barrelling directly into it at 100 km/h, but there's not much that will. Bottom line is that you're much less likely to suffer any kind of collision with any kind of vehicle (other than another cyclist) on the bridge than you are on a bog-standard S2.
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Re: How prone are the two Severn Bridges to swaying?

Post by Chris Bertram »

solocle wrote: Mon Aug 01, 2022 08:42
Chris Bertram wrote: Sun Jul 31, 2022 22:24 Nice picture of the bike, but what's the photo supposed to be telling us?
How close the path is to the motorway, the lack of meaningful barriers, and the general geometry of the bridge. Which does play into wind exposure, too, although I haven't experienced the bridge in meaningful winds.
Pretty sure we've seen that before, from someone who'd walked over it.
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Re: How prone are the two Severn Bridges to swaying?

Post by solocle »

FosseWay wrote: Mon Aug 01, 2022 09:39
solocle wrote: Mon Aug 01, 2022 08:42
Chris Bertram wrote: Sun Jul 31, 2022 22:24 Nice picture of the bike, but what's the photo supposed to be telling us?
How close the path is to the motorway, the lack of meaningful barriers, and the general geometry of the bridge. Which does play into wind exposure, too, although I haven't experienced the bridge in meaningful winds.
I've cycled over the bridge with an annoyingly strong headwind (though nowhere near enough for them to close it). I don't remember any swaying, but it does vibrate when an HGV goes past. The same applies to my local suspension bridge.

I don't think the cycle path on the Severn Bridge is unreasonably close to the motorway or that the barriers are not "meaningful", tbh. How far from the motor traffic can you reasonably expect the walkway/cycleway to be? As it is, the ped/cycle path on the Severn Bridge is wider than on Älvsborgsbron as shown above (though the latter has cycle paths on both sides). Moreover, the cycle path on the Severn Bridge is at the same level as the roadway. On Älvsborgsbron you're below the level of the passing traffic, meaning you get spray from vehicles right in the face. Both bridges are of similar age, btw.

The wire barrier is designed to deflect most vehicles, which is why 2+1 roads in Sweden with a similar wire barrier down the middle have a higher speed limit than those that don't. It's not going to stop an HGV barrelling directly into it at 100 km/h, but there's not much that will. Bottom line is that you're much less likely to suffer any kind of collision with any kind of vehicle (other than another cyclist) on the bridge than you are on a bog-standard S2.
The Severn bridge also has paths on both sides, although my understanding is that the north side is generally preferred (the exit is by the services and it's more obvious).

Also note the cycle path is below the level of the motorway in the middle of the span:
Image

The barriers exist, sure, but in comparison to the nearby Avonmouth Bridge, they are insubstantial.
Image
On the Severn Bridge, it really does feel like there's barely anything between you and the motorway - because, other than a knee high wire barrier, there isn't. Of course, the M48 is hardly a busy motorway these days, so I actually like it - the exposure gives quite a buzz. But it probably wouldn't be so pleasant if it was the M25.
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Re: How prone are the two Severn Bridges to swaying?

Post by FosseWay »

I tend to avoid cycling over Älvsborgsbron because it *is* a busy 6-lane highway, you get a lot of turbulence from passing traffic as well as natural wind and it's 50 m high with the surrounding road network at sea level. I can choose to either take the ferry or cycle over the next bridge upstream, which is lower (so less wind and less height gain needed) and has a lower speed limit for vehicles. The downside to that is that if you cross the lower bridge outside rush hour there is a high probability that it will open for a ship and you have to wait. But there isn't a huge amount anyone can realistically do about Älvsborgsbron - as I said before, there's a limit to how far apart the ped/cycle path can be from the traffic and still be part of the same bridge. The ped/cycle path on both bridges, and others like the Humber Bridge, is a sensible width allowing cyclists and pedestrians to avoid each other, and a cycle path of this kind is surely preferable to no cycle path at all (hence the hoo-ha when the Humber Bridge cycle path was closed a while back).
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