The Forth Rail Bridge - reimagined

[eMeS]

Just supposing that the century old Forth Rail Bridge didn't exist - how would we design and build one today? Or, would we simply not bother?!

 

[waverley47]

Just supposing that the century old Forth Rail Bridge didn't exist - how would we design and build one today? Or, would we simply not bother?!

This is definitley speculative, but definitley interesting.

Best bet would be a tunnel from the almond valley to Inverkeithing. It's a big too long and too deep for a suspension bridge to work for the dynamic loads that trains bring, and the problem would have been solved before cable stayed bridges became a thing, so a tunnel most likely.

 

[snowball]

The Scottish Green Party is proposing a north-south tunnel under the Forth. A sketch map of the route has been published in either Modern Railways or Rail magazine (possibly both) but I can't immediately find a copy online. Its south end would connect to the existing network somewhere east of Waverley.

 

[swt_passenger]

In this alternate world, doesnt the answer to the question “would you build it” depend on whether the two road bridges already exist?

 

[Journeyman]

The Scottish Green Party is proposing a north-south tunnel under the Forth. A sketch map of the route has been published in either Modern Railways or Rail magazine (possibly both) but I can't immediately find a copy online. Its south end would connect to the existing network somewhere east of Waverley.

Why?!?

 

[eMeS]

I hadn't heard anything about the Green Party's proposal - trying to understand what problem it might solve?

== Doublepost prevention - post automatically merged: 22 Mar 2021 ==

My original query was wondering whether modern materials might shift designs away from something as large and monumental as the Forth Rail Bridge, to perhaps a cable stayed structure*. Perhaps tunnelling is the best way forward if nothing exists.
*I'm no structural engineer...

 

[HSTEd]

The obvious solution would either be an extradosed bridge, or an immersed tube tunnel

 

[SouthernR]

The main benefit is reduced journey times. See - New Civil Engineer https://www.newcivilengineer.com/la...lan-considered-technically-viable-15-01-2021/

 

[mrgreen]

The obvious solution would either be an extradosed bridge, or an immersed tube tunnel

I 'd never heard of an "extradosed bridge" so had to look it up in Wikipedia... it's a box-girder bridge with additional support from cable-stays fanning out from low towers. Wiki says they are best-suited to spans of 100m to 250m which sounds a bit short: the Forth Road Bridge (suspension) main span is 1000m and Queensferry Crossing (cable-stayed)) is 650m.

The Forth Road Bridge is already de-rated to just take light traffic and is decaying so will eventually need demolition or a long, expensive closure for re-cabling. The Queensferry crossing carries only two lanes and a hard-shoulder each direction. So there'll be the need for more road capacity in the future. Maybe a combined road/rail bridge would have been the way forward, if the rail bridge wasn't already in place. Cable-stayed, I'd guess.
When the Queensferry crossing was proposed, there was strong pressure to build a road tunnel instead, but a bridge was chosen as being cheaper and quicker to build. So a combined road/rail crossing would probably also be cheaper built as a bridge, not tunnel. If the crossing was only for rail then the relative economics of bridge or tunnel would be different.

 

[HSTEd]

I 'd never heard of an "extradosed bridge" so had to look it up in Wikipedia... it's a box-girder bridge with additional support from cable-stays fanning out from low towers. Wiki says they are best-suited to spans of 100m to 250m which sounds a bit short: the Forth Road Bridge (suspension) main span is 1000m and Queensferry Crossing (cable-stayed)) is 650m.

The Forth Road Bridge is already de-rated to just take light traffic and is decaying so will eventually need demolition or a long, expensive closure for re-cabling. The Queensferry crossing carries only two lanes and a hard-shoulder each direction. So there'll be the need for more road capacity in the future. Maybe a combined road/rail bridge would have been the way forward, if the rail bridge wasn't already in place. Cable-stayed, I'd guess.
When the Queensferry crossing was proposed, there was strong pressure to build a road tunnel instead, but a bridge was chosen as being cheaper and quicker to build. So a combined road/rail crossing would probably also be cheaper built as a bridge, not tunnel. If the crossing was only for rail then the relative economics of bridge or tunnel would be different.

The longest heavy rail bridge span in existance that I am aware of is ~1150m for a cable stayed road-rail bridge in China.
There is a Hong Kong Metro line on a 1400m suspension span, but other than that, nothing longer

 

[bspahh]

I hadn't heard anything about the Green Party's proposal - trying to understand what problem it might solve?

== Doublepost prevention - post automatically merged: 22 Mar 2021 ==

My original query was wondering whether modern materials might shift designs away from something as large and monumental as the Forth Rail Bridge, to perhaps a cable stayed structure*. Perhaps tunnelling is the best way forward if nothing exists.
*I'm no structural engineer...

The tunnel has been discussed in this thread

https://www.railforums.co.uk/threads/leith-to-kirkcaldy-forth-rail-tunnel-what-do-you-think-of-this-idea-is-it-feasible-and-how-could-the-idea-be-improved.213067/

 

[d9009alycidon]

If the bridge had not been built, it may have been possible to adapt the existing tunnel under the Forth for rail traffic https://www.scottishfield.co.uk/culture/the-forgotten-tunnel-that-crosses-the-river-forth/

 

[Egg Centric]

If the bridge had not been built, it may have been possible to adapt the existing tunnel under the Forth for rail traffic https://www.scottishfield.co.uk/culture/the-forgotten-tunnel-that-crosses-the-river-forth/

Way too far to the west

 

[EastisECML]

New line into a station beneath Edinburgh airport then under the water to reemerge and follow the motorway to Perth with a branch onto the Perth-Dundee line.