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?
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/