It’s impossible for a steel bridge to be curved… They don’t work. Period. When you want a curve, you use a bunch of connected straight bridges.
And even curved wooden trestles aren’t really curved. The bed the track sits on is still made of a bunch of short straight pieces.
Your photo really shows how they are offset from the center on a single-span curved bridge. Thanks for posting!
It has been much easier to design/build bridges using combinations of straight assemblies. However, it is not impossible to construct such a bridge. With current CAD technology over the past 20 years or so, there have been a number of rail/highway bridges that have ‘curved’ members. For modelers; if you are using ‘off the shelf’ parts - it is still easier to build them out of straight box assembles. And if you are modeling most any era before the late 80’s, straight box assemblies will be used.
Jim
I’m mucking about with building trestle that is curved by cutting straight beds at angles–that is enough fiddling for me thank you very much----especially at 36" in N scale!!!
It’s easy to curve a 1/87 plastic I beam/girder - steel is a little harder to do in 1/1 scale!![:)]
It is possible to both design and fabricate a curved steel bridge. However, the curvature introduces enormous additional stress into the steel - the structure necessarily becomes so beefy that any economy is completely blown away. And we all know how much railroads enjoy throwing unnecessary money at infrastructure [;)] thus, straight bridge segments.
I know the don’t carry the same kind of load factors, but all the new highway overpass and pedestrian bridges (bike trail bridges) that I see in my area have truly curved steel girders, often quite long. Maybe it’s a breakthrough in the technology.
Never have seen a curved RR steel girder bridge, though.
I suspect a double breakthrough:
- Computer-calculated stress models, using truly hairy algorithms far beyond the ability of earlier number-crunching techniques.
- New alloys that are better able to resist torquing and shear stress.
How well they can be adapted to the higher loadings of rail-supporting structures is one question. Whether the much more conservative designers of railroad infrastructure would be willing to risk them is another.
Chuck (Modeling Central Japan in September, 1964 - with curved track on straight bridges)
Speaking here as a practicing structural engineer
It’s true that there’s mind boggling software out there that can calculate stresses with very high precision. I use some of it. It’s also true that experience and history have demonstrated that an engineer who designs a steel structure to the hairs-breadth tolerances of fancy software output is a fool who’s flirting with disaster.
Steel material development is a very mature science. There’s been one significant upgrade in the 20+ years I’ve been working. What doesn’t change is the fact that steel is and always will be mostly made of iron, and thus has certain physical limitations. We can make it stronger but not any less deformable, so we can make a beam that is really light and strong but would still bounce unacceptably precisely because it’s so small and light.
The biggest issue with curved bridges is that the load moves off of the chord between the supports. When this happens, the bridge tries to twist off of the abutments, pillars, etc. I believe that highway bridges can be curved because the load is light enough that the structure can be built to withstand the twisting. Trains are much heavier and would place much more force on the bridge when at the center of a curved span.
I’ve actually tested this on my own layout. Out of necessity, I built a wood truss bridge with a 22" R curve. With a brass model 20T shay on the the bridge it deflects about .040" on it’s outer edge and about .030 on the inner edge. If the bridge were straight, the deflection on both sides would be equal and there would be no tendancy to twist. Fortunately, the carpenters glue I fastened it in place with is strong enough to take the force. The bridge is named “Impossible Bridge” and I tell folks it was designed by a graduate of the Colorado State University engineering program (I’m a Wyoming alumnus - we have a rivalry with CSU).
Here’s a picture of the bridge. By the way, the trestles were made “straight up (I didn’t violate any engineering principles)” with short straight spans between the bents:
The way I bent the bridge was by laminating strip wood around a form. Then I built it just like I would have built a straight bridge.