minimum height bridge

[NittenDormer]

I have a spiral that crosses over itself after approx 360°. The upward track is basically 2nd radius. Even with a 2% gradient, the track-to-track difference is barely 60mm. It looks like I am going to have to make the climb even steeper, I may even need to break out the magnets and steel undersheets but in the meantime:

1) what is the thinnest material that can be used for a model bridge span?

 

2) what is the real-life under track depth for different bridge types? I deffo don't have room for an arch, or anything with under girders.

 

thanks

[Michael Edge]

I've used steel sheet (I think about .020") for one of the bridge decks on Carlisle. It's well braced, not just flapping in the breeze and it produces an interesting chnage of sound as the trains pass over it. This is the flyover bridge (replaced the original flat crossing in the 1880s) where the WCML crosses the goods lines south of Citadel station and clearance was minimal here, it's the steepest grade on the layout as it is.

[davetheroad]

Rail top to underside of bridge - minimum 15 feet or 60mm in 00 scale.

[SHerr]

It all depends on the span of the bridge. Modern steel bridge decks for a span of around 10-20 meters are about 900mm so about 12mm if your modelling OO. Then there's the ballast to go on top - about 200-300 mm so another 3-4mm then sleepers and track. The outer girder top should be 300mm above the rail level so another 4mm which would need a handrail.

 

Larger spans 20m plus would have a deck of about 1000 to 1200mm so about 16mm then ballast and track, the girders would be deeper - the Wills varigirder is a good representation for these middle span bridges.

 

Older bridges were shallower so probably a better bet if your tight for space/height and these often sat on Long timbers rather than cross sleepers, often sat in steel/iron troughs. This allows a much shallower construction, the timber would be about 250-300mm with just the steel trough plate below - probably about 60mm so total depth under rail is probably only about 300 to 350mm so 4-5mm in OO, but would really only span up to about 10m spans, bigger spans up to 20m would have often had a trough deck (like large corrugated sections) about 300-400mm which often had very little ballast sometimes down to 50-100mm which would mean you could go down to 500 to 600mm below sleepers - 7-8mm but maybe these would have bigger girders like the Wills ones again.

 

If you Google 'trough railway underbridge' there are quite a lot of diagrams/sketches, or 'waybeam railway bridge' for those with long timbers.

 

In terms of headroom as posted above this would normally be about 15 feet but it's not uncommon on more rural or restricted roads to go down to 11-12 feet so you could go down to 44mm below the girder to the road.