Building the Forth and Tay Bridges Today: What Designs Might Be Used?

[Dr_Paul]

Here's a question that has occurred to me when thinking of the Forth and Tay Bridges: were today a railway bridge to be built across a river estuary of similar geographical and geological features to the Firths of Forth and Tay, what sort of designs might be used: would they be similar in some ways to the Forth and Tay Bridges, or would they be of a quite different design?

 

[Mcr Warrior]

The original Tay Bridge was of course, somewhat different in design, before it infamously fell down in a storm in 1879.

That's why its replacement, and the Forth Rail Bridge were so markedly over-engineered, and have stood the test of time.

Modern day replacements would no doubt be of a radically different design.

 

[edwin_m]

Suspension bridges like the 1960s Forth Road Bridge aren't very suitable for rail, as they are quite flexible and the uneven weight distribution doesn't work too well, although there are some out there. However a cable-stayed bridge similar to the recent Queensferry Crossing could be made to work. Due to its much lower height a replacement Tay bridge would probably be a viaduct with similar span lengths to the existing.

 

[Bald Rick]

The Medway Viaduct is your answer.

 

[Mcr Warrior]

The Medway Viaduct is your answer.

They are effectively three parallel viaducts (one for rail, two for the M2 motorway), aren't they?

As a separate approach, are there any examples of modern day bridges (with long spans) and road / rail decking, perhaps at separate levels?

 

[DerekC]

Have a look at the Oresund Crossing between Copenhagen (Denmark) and Malmo (Sweden). It's partly a tunnel and partly a bridge. The bridge has multiple side spans with a 490m cable stayed main span. Two deck - road traffic on top, trains underneath. My guess is that Tay would look something like this, with Forth being like three of the main spans put together.

 

[GRALISTAIR]

Have a look at the Oresund Crossing between Copenhagen (Denmark) and Malmo (Sweden). It's partly a tunnel and partly a bridge. The bridge has multiple side spans with a 490m cable stayed main span. Two deck - road traffic on top, trains underneath. My guess is that Tay would look something like this, with Forth being like three of the main spans put together.

Yes and of course all designed to carry 25 kV OHLE

 

[ABB125]

Another Danish crossing of interest is the one between Nyborg and Korsør. For half the crossing, the road and railway run or parallel bridges; then the railway drops into a tunnel, but the road stays on a bridge. Does anyone know why this is?

 

[Gloster]

Another Danish crossing of interest is the one between Nyborg and Korsør. For half the crossing, the road and railway run or parallel bridges; then the railway drops into a tunnel, but the road stays on a bridge. Does anyone know why this is?

Primarily, to provide sufficient height for shipping, including - unless I am mixing it up with the Oresund [first o should have either a slash through it or two dots over it, depending on whether you are using Danish or Swedish spelling] Bridge - pre-fabricated oil rigs from Finland.

 

[ABB125]

Primarily, to provide sufficient height for shipping, including - unless I am mixing it up with the Oresund [first o should have either a slash through it or two dots over it, depending on whether you are using Danish or Swedish spelling] Bridge - pre-fabricated oil rigs from Finland.

That may well be the case, but surely the road would also need to go in a tunnel (like the Öresund bridge)?

 

[Gloster]

That may well be the case, but surely the road would also need to go in a tunnel (like the Öresund bridge)?

The road bridge is humped-backed, with the centre of the span
being about 70 metres over water level.

 

[John Webb]

.....As a separate approach, are there any examples of modern day bridges (with long spans) and road / rail decking, perhaps at separate levels?

Not certain if it counts or not, but the rebuild of the Britannia Tubular Bridge across the Menai Strait after the fire included a road deck above the railway.

 

[Mcr Warrior]

Not certain if it counts or not, but the rebuild of the Britannia Tubular Bridge across the Menai Strait after the fire included a road deck above the railway.

I had considered that, but it did seem very much to be retrofitting a new road deck to the original Victorian era rail structure.

It does seem that, prior to HS1, there hasn't been all that much building of new railway bridges in the UK anytime recently, other than, perhaps, across motorway alignments, but certainly not across mile wide river estuaries.

 

[ABB125]

The road bridge is humped-backed, with the centre of the span
being about 70 metres over water level.

Ah right, thanks. It's not so easy to see things like that on Google maps!

 

[Bald Rick]

They are effectively three parallel viaducts (one for rail, two for the M2 motorway), aren't they?

They are, although the motorway viaducts aren’t relevant to the question.

When it opened, I understand that the Medway Viaduct has the longest single span on a high speed railway anywhere in the world. The approach spans are built to a standard design, and the main span supports are of a (then) special design.

 

[edwin_m]

They are, although the motorway viaducts aren’t relevant to the question.

When it opened, I understand that the Medway Viaduct has the longest single span on a high speed railway anywhere in the world. The approach spans are built to a standard design, and the main span supports are of a (then) special design.

I think the rail bridge was constructed so the piers aligned with the road bridge - not sure if it was to reduce visual impact, because that was just the best place to put the piers, or for some navigation reason.

 

[ABB125]

They are, although the motorway viaducts aren’t relevant to the question.

When it opened, I understand that the Medway Viaduct has the longest single span on a high speed railway anywhere in the world. The approach spans are built to a standard design, and the main span supports are of a (then) special design.

I'm sure I read somewhere that the Medway viaducts are related to the (nowhere near as long) bridges on (I think) the A435 heads of the valleys road, but I've never seen any from the ground so can't really comment.
Either way, the Medway viaducts look impressive!

Google Image Result for https://upload.wikimedia.org/wikipedia/commons/d/de/MedwayViaducts2013.jpg?utm_source=en.wikipedia.org&utm_campaign=index&utm_content=original

 

[Bald Rick]

I'm sure I read somewhere that the Medway viaducts are related to the (nowhere near as long) bridges on (I think) the A435 heads of the valleys road, but I've never seen any from the ground so can't really comment.
Either way, the Medway viaducts look impressive!

Google Image Result for https://upload.wikimedia.org/wikipedia/commons/d/de/MedwayViaducts2013.jpg?utm_source=en.wikipedia.org&utm_campaign=index&utm_content=original

They are impressive, particularly the rail one, and especially when you are stood on it

 

[busesrusuk]

They are effectively three parallel viaducts (one for rail, two for the M2 motorway), aren't they?

As a separate approach, are there any examples of modern day bridges (with long spans) and road / rail decking, perhaps at separate levels?

The Tsing Ma suspension bridge in Hong Kong carries the six lane Lantau Highway on top whilst underneath is the Airport Express and Tung Chung MTR lines (two tracks - the two lines share the infrastructure). At one point it was the longest double decker road/rail suspension bridge; not sure if that is still the case.

 

[DelW]

Not certain if it counts or not, but the rebuild of the Britannia Tubular Bridge across the Menai Strait after the fire included a road deck above the railway.

I had considered that, but it did seem very much to be retrofitting a new road deck to the original Victorian era rail structure.

Not just a new road deck, since the railway deck and all the supporting steelwork was new as well. Only the piers and some of the approaches survived from the original bridge.

Incidentally, regarding railway suspension bridges, I believe that the tall towers, with their openings near the top, were to allow for suspension chains to be added if the wrought iron tubes proved to be inadequate on their own.

 

[Mcr Warrior]

Not just a new road deck, since the railway deck and all the supporting steelwork was new as well. Only the piers and some of the approaches survived from the original bridge.

Thanks for clarifying.

 

[Dr_Paul]

The Tsing Ma suspension bridge in Hong Kong carries the six lane Lantau Highway on top whilst underneath is the Airport Express and Tung Chung MTR lines (two tracks - the two lines share the infrastructure). At one point it was the longest double decker road/rail suspension bridge; not sure if that is still the case.

This came as a surprise: a suspension bridge carrying a railway line. I didn't think that such a bridge would have the strength to do that, especially over such a long span. I notice that the route uses EMUs and that only one train is allowed on the bridge at a time, but altogether it's an indication of what modern engineering can achieve.

Thanks for all the information so far. I had a feeling that a modern Tay Bridge-style structure would be like the Medway Viaduct (only a fair bit longer). Presumably we won't be seeing anything like the Forth Bridge constructed in the future: a shame, but that's progress.