Weight. A curved-girder span would have to built much more robustly for a RR bridge than for a road bridge. So it's usually cheaper to build many short straight spans than a few longer curved spans. You might have to build a curved span if clearances under the bridge require it, but if it's longer than a 100ft or so, it would be cheaper to realign the track to enable a straight span instead, even at the cost of tighter curves leading into that straight span.
Ah, it's just a semantic problem. Most people think of the deck when they think of a bridge, is all. What we actually have is:
a) straight spans, straight decks, straight track --> a straight bridge b) straight spans, curved decks, curved track --> a curved bridge c) straight spans, straight decks, curved track --> a straight bridge (usually single span***), or a curved trestle bridge d) curved spans, curved decks, curved roadway or track --> a curved bridge
I think I've covered all cases.
***MR some years ago published a photo of a wide through girder bridge with a curve over it. Span looked to be about 40ft, inside width about
20ft. An overpass carrying a spur into a factory, IIRC.
I don't see it being that "much less" -- my experience is that railroad agencies actually allow more deflection than highway agencies do. Here in Wisconsin USA, I design road and rail bridges for a living and we need to meet the following live load deflection limits for steel bridges: Union Pacific RR: Span Length / 640 Canadian Pacific RR: Span Length / 800 Wisc. Dept. of Trans. highways: Span length / 1200
If you look at what would be considered the "bare bones" deflection criteria as prescribed by AREMA and AASHTO, the limits are: span length / 640 for railroads [AREMA 126.96.36.199] span length / 800 for highways [AASHTO 10.6.2]
As you can see the deflections allowed on railway bridges are larger than on road bridges in the USA, just the opposite of what you wrote.
You seem knowlegable on this subject... what sort of deflection limits are used on steel railroad and highway bridges Denmark? ____ Mark
Sorry, I wrote based on the situation in Denmark. I didn't imagine that it would be that much different in the USA.
I don't have any values on hand but I will try to find them.
I remember a design case from 1963 between the isles of Falster and Lolland in the southern part of Denmark. The bridge which was named "Frederik 9th. Bridge", consists of two parallel steel plate girder bridges, a four-lane road bridge and a single-track railway bridge. The bridges are in fact curved, consisting of a nomber of straight spans including a bascule span in each of the bridges.
The road bridge was constructed from RRSt. 52-3 (later Fe510C, tensile strength 510 MPa) whereas the railway bridge was constructed from RRSt.
37-3 (later Fe360C, tensile strength 360 MPa) (sorry, these are DIN and ISO steel designations, I don't know the equivalent US designations). The reason for the different materiels used was that the higher strenght of the RRSt. 52-3 couldn't be utilised on the railway bridge as deflection was the limiting factor.
I participated in rebuilding the track switches and interlocking on the bascule span of the railway bridge around 1985.
Despite that set of specs, I'd bet that the railroad bridges are a lot stiffer than highway bridges. I'll also bet that you can much more freely use concrete in highway bridges than railroad bridges. Being a civil engineer, you know the problems with curved bridges and their loading and difficulty in construction as a result.
-- Why do penguins walk so far to get to their nesting grounds?
Sorry, the IIRC refers to the destination or purpose of the bridge, not the bridge itself. It was shown in a pic I encountered when I was clipping stuff out of my MR collection before sending the remainder to the recycler. I may have kept it, but I'm not about to go looking for it
-- I waste enough time on this NG as it is. ;-)
FWIW, and risking diversion of this thread into another endless exchange, MR and RMC have both published pics of switches (turnouts) located partly or wholly on bridges (and I don't mean the Keddie Wye bridge.)