eMeS
Member
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?!
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?!
Why?!?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.
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 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.
I hadn't heard anything about the Green Party's proposal - trying to understand what problem it might solve?
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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...
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/