Peter Fox said:
In Witham there is a perfectly good and very useful footpath/cycle
crossing across the East Anglian main line.
Quoted message said:the line would be widened to 4 tracks from the present two.
Quoted message said:Here is the Q: If you increase the span by 33% what is the order of cost
increase? (I know the total cost included approaches - let's just
concentrate on the main span.)
Ermm, I'm not sure where 33% comes in? I'm not sure I fully
understand the scenario, but I'll answer some different questions and
see if they encompass what you want:
Typical price for a footbridge across two tracks would be 250k to
design and install. That doesn't include for paying off train
operators, and probably doesn't include possession costs. It assumes
you can get a reasonable number of possessions. Basically, that's the
easiest scenario situation.
If you have four tracks, you'll probably try and do that in a single
span - a central pier is a pain, since you need more possessions to
put the foundations in, and it would need to meet collision loading
requirements, so will be much more than is necessary simply to hold up
the deck. A structure between lines is also a maintenance liability,
and might obstruct sighting distances and whatnot. You really want a
single span if you can.
The span will be twice as long, but the approaches are the same size.
Design cost will be the same (the main span will be almost
off-the-shelf design). The main span is repetitive anyway,
the approaches are more expensive per metre of structure. Probably
50% more expensive to bridge four tracks rather than two.
BUT (big BUT), with railways the cost is strongly dependant upon
getting the structure in place - if you can easily get 24 hr
possessions every weekend, and 6 hr ech weeknight, and there's access
for a nice big road-going crane right up to the site, life's a doddle.
If the only possession you can get is boxing day 2008, and the nearest
road is the other side of a river and a couple of miles of boggy
flood-plain, then it's more entertaining.
A bigger span makes everything more so. You'll probably want your
main span as a single lift - build it elsewhere, bring it to site,
lift it as one piece and plonk it down. I've done that easily, when
you can get a crane right up to the tracks, and with difficulty -
lifting a bridge in over a house in one occasion, and a major signal
gantry (nearly as big as a footbridge) over a warehouse on another.
If your span is bigger, the lift is heavier, the reach is greater, you
might need to tandem lift, and install costs can rocket.
If you're just talking about a 33% bigger span, installation is no
more complex, everything is easy, it might only be 10% more expensive.
However, it's unlikely to be that straightforward.
regards, Ian SMith
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