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The Forth Rail Bridge - reimagined

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eMeS

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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?!
 
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waverley47

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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

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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

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

Journeyman

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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

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

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...
 
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HSTEd

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

mrgreen

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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

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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

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

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
 

EastisECML

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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.
 
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