I need a curved bridge 2 tracks wide, with an inner radius of 22" and an outer radius of 24". I also want it to look reasonably prototypical for my city layout. After flipping through my Walthers catalog, I discovered is there really isn’t “curved” bridges available that aren’t wood trestle in design.
I really like the Micro-engineering bridges. But how do I kit bash a straight bridge into a curved one? ack [:0]
Any ideas?
~Thanks
~D
One prototype - see whether you can find any pictures of the Metlac Viaduct on the Mexican Railway. It’s not urban, but you would probably find it an easy project - stone piers (actually steel tubes with a stone sheath, but they look stone), connected by simple girders.
Many curved bridges are simply a series of short straight bridges. Feather River is that way as is the John Allen Bridge. Some one a while back built a curved plate girder bridge by capping the plywood roadbed with plastic in the form of a plate girder. A wooden trestle is the only true curved bridge and that is a series of striaght pieces between bents. There are good articles on this in the various bridge books from MRR mag and others. In the long run, you need to decide what knid of bridge, then learn how those are built and then build one. If it looks good to you it is right unless you are modeling a specific bridge. Have fun. This is a great challenge
Typically railroad bridges that need to support curved track are done using wider straight sections, and multiple segments. Girder style, either deck or through, is most common, probably due to cost and design simplicity.
Some kitbashing and detail work on a few simple Atlas girders would probably work very well for what you want to do.
Lay a sheet of paper flat, spanning an open space between two stacks of books. It will sag in the middle.
Now fold the paper longways twice, making three flat planes, dividing the sheet into three sections, two vertical, the center one flat, U shaped in cross section.
Now it will span the gap with much less sag. The side verticals stiffen the deck, much like beams stiffen the deck of any bridge. When you fully understand why this works, you understand why a load carrying beam cannot be curved when spanning an open space between two supporting piers or columns. The beams have to be straight.
From there, all else follows. You can have straight beams and a curved deck, or straight beams and straight sections of deck, each successive section between piers taking an angle with respect to the previous section, but either way, the beams have to be straight.
That said, you can heat up a styrene Warren truss model in hot water or with a blow dryer and flex it around a curve (with the deck detached), and it may carry the weight of model trains, but it is not prototype, and I don’t recommend doing it to support models you care about.
If there’s an exception to every rule, then two trusses or plate girders curved between piers, could concievably be stabilized by using frequent X bracing in the cross section, to hold the beams immovable with respect to each other, and by anchoring all beam/pier attachment points against both downward and upward thrusts and loads, but again, I do not recommend the practice. Keep the beams straight, and you and your models will be a lot happier.
Curving the trusses will simply not work and would look totally bizzare. If trusses must be used, the design will have to be trapizoidal segments, meaning that on the inside of the curve the truss will shorter, and on the outside longer.
How long is the span???
It would be a fairly straight forward kitba***o remove one panel from the inside, and add it to the outside, giving you the length difference you need. The only question is, is it enough?
The place to start is make a paper or cardboard template of the area you need to span, marking your track centers. Then figure clearances, this will tell you the lengths and widths you require. You might need 2 segments if it becomes too wide.
One Solution for curved track on a bridge is a stone or concrete underpass by using double track tunnel portals as the bridge sides. The underpass is straight but the track on it is curved. I have one such underpass on my HO layout.
One other caveat:
Unless you are running old, shorty equipment, the two inch radius difference may be inadequate to avoid interference if two trains pass each other on the bridge. You would be better off if the outer radius was at least 24-3/4". 25" radius outer curves would be fool proof as far as interference goes, especially if you want to run modern (six-axle) diesels or 85-89 foot cars.
Curved bridges are beginning to be built for highways. I have seen some in the Washington DC area on the interstates. I beleive they tie the girder beams together to form a box beam. They look funny as they seen to have no means of support. Railroads have higher loading problems and continue to use short straight sections to form a curve. If you are buliding a city scene use the micro-engineerring parts you mention but model it after an elevated railway. Prototype curved bridges do not have much clear space under them due the large number of supports needed.
what Big_Boy is suggesting is a series of isosceles trapezoids . see the 3rd example on this page http://id.mind.net/~zona/mmts/geometrySection/commonShapes/trapezoid/trapezoid.html (a picture is worth a thousand words [:)] )
Thank you, that’s exactly what I had in mind for the top view. The sides would be normal design with the number of panels adjusted to fit, and the top bracing would need the ends customized.
Again, this would not be a real railroad’s first choice to build, due to cost. But at least from an engineering point of view, it is more realistic than bending.[;)]