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Northwich swing bridges. What stops them floating up in flood?


MoominPapa
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Watching the footage of the floods in Northwich over the last few days, the river level has been close to the bottom of the bridge decks, but the decks have been firmly in place, and traffic using the bridge. 

 

Now my understanding is that these are floating caisson bridges. Ie the pivot area has a large cylindrical tank which extends below water level and floats, supporting much if the weight of the bridge. The ring of roller bearings which are visible as you pass under the bridge only support a small proportion of the weight, and acts to locate the deck as it swings.

 

Surely, then, when the river goes into flood the caisson would experience increased flotation force, and the whole contraption would rise up? I can see that the caisson might be in well with the water level set differently to the river level, but during the floods the whole area in inundated, and the well must surely be filled with water. Maybe some poor unfortunate from CRT has to be dispatched when floods are imminent to open valves and flood the caisson, lest the bridge rise up into the air?

 

I'd love to know the details, does anyone have any information?

 

MP.

 

 

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I have no idea, but perhaps the caissons are normally flooded and  only pumped out when the bridge is swung to take a lot of the weight off the bearings. Perhaps there are extra supports to take the weight off the bearings in the closed position. Similar to the wedges that are hydraulically driven in on many smaller powered swing bridges that take the weight entirely off the rollers and bring the deck an inch or two up to the height of the approach roads. If the rollers have even partial weight on them all the time they risk getting flats put on them and corresponding divots in the bearing surfaces from traffic pounding.

Jen

Edited by Jen-in-Wellies
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The caissons do not take the full weight of the bridge, with the additional weight over that taken by the caissons bearing on a circular trackway. Thus the bridge should not move because of a variation in water level. The bridge is described in a 1900 paper presented to the Institution of Civil Engineers by the Weaver's engineer, S A Saner.

1900 Swing Bridges…Northwich, Saner, ICE.pdf

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22 hours ago, MoominPapa said:

 

Surely, then, when the river goes into flood the caisson would experience increased flotation force, and the whole contraption would rise up? I can see that the caisson might be in well with the water level set differently to the river level, but during the floods the whole area in inundated, and the well must surely be filled with water. Maybe some poor unfortunate from CRT has to be dispatched when floods are imminent to open valves and flood the caisson, lest the bridge rise up into the air?

 

I'd love to know the details, does anyone have any information?

 

MP.

 

 

And stop calling me Shirley.

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18 hours ago, MoominPapa said:

That explains it. Thanks Pluto. The paper is a real period piece: the amount of words expended on the bills for the new-fangled electricity is astounding!

 

MP.

 

Yes, the bridge is not normally 'floating' but the caisson reduces the weight on the bearings and abutments, making it easier to design a mechanism for turning the deck when needed (interesting that Parliament was sufficiently experienced as engineers that they could specify the required opening time before any design had been submitted! Perhaps shipping interests were still more influential than road transport) As far as I can see, both from the paper and recent photos, the bridge deck does not itself have much buoyancy and so flooding will reduce the weight of the bridge on its supports but will only become a problem as and when the extra uplift exceeds the residual weight which, according to p79, is around 70 tons. I could not see any estimate of the deck buoyancy.

 

Also note that on p81 the author specifies a current in volts - even the eminent engineers of that time managed to get their electrical units wrong - how often does that happen on the illustrious pages of CWDF?

 

However, it was interesting to see that even at this fate there was quite a bit of attention to miti9gating the impact of human error, something that Human Factors thinking would take half a century to catch up in some engineering disciplines! (Or even longer if Grenfell Tower is anything to go by)

 

Thanks for posting the reference.

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On 26/01/2021 at 11:00, Mike Todd said:

Yes, the bridge is not normally 'floating' but the caisson reduces the weight on the bearings and abutments, making it easier to design a mechanism for turning the deck when needed (interesting that Parliament was sufficiently experienced as engineers that they could specify the required opening time before any design had been submitted! Perhaps shipping interests were still more influential than road transport) As far as I can see, both from the paper and recent photos, the bridge deck does not itself have much buoyancy and so flooding will reduce the weight of the bridge on its supports but will only become a problem as and when the extra uplift exceeds the residual weight which, according to p79, is around 70 tons. I could not see any estimate of the deck buoyancy.

 

Also note that on p81 the author specifies a current in volts - even the eminent engineers of that time managed to get their electrical units wrong - how often does that happen on the illustrious pages of CWDF?

 

However, it was interesting to see that even at this fate there was quite a bit of attention to miti9gating the impact of human error, something that Human Factors thinking would take half a century to catch up in some engineering disciplines! (Or even longer if Grenfell Tower is anything to go by)

 

Thanks for posting the reference.

The main reason for the buoyancy was to create a bridge which would withstand the subsidence then taking place around Northwich, as the photo from the Waterways Archive shows. In some ways it would make the operational mechanism more difficult as movement of the bridge could affect the mesh of any gears. The second photo shows the Hayhurst floatation tanks out for repair in 1977.

D5621 salt subsidence.jpg

Northwich 611.jpg

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2 hours ago, Pluto said:

The main reason for the buoyancy was to create a bridge which would withstand the subsidence then taking place around Northwich, as the photo from the Waterways Archive shows. In some ways it would make the operational mechanism more difficult as movement of the bridge could affect the mesh of any gears. The second photo shows the Hayhurst floatation tanks out for repair in 1977.

 

 

yes, the 'easier' I was referring to was that the load that would be exerted on the gear mechanism was reduced - and I think that is what the paper says. on p74.

 

The subsidence issue was also important a they needed to be able to keep the bridge deck level with the roads, not wanting to upset the wealthy property owners on the adjacent streets, especially if there was a steep change in height to get onto the bridge. as well as headroom for the influential ship operators.

 

Incidentally, on p75 the buoyancy issue was of concern if the water level was too low not too high!

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