[banghead]Anyone know how they may have done this? I thought by shorting each inner curve pannel it would make a curve right?
Usually if you had to have a curve on a deck plate type bridge you would either see multiple shorter spans connected together to form the cord of the curve, or for a shorter bridge they would widen the bridge a bit to allow for the curve. In model form both are easy to replicate. The first is just some short or cut down spans strung together, the second can be done using a double track bridge and just lay your curve through the center. I’ve never seen or heard of an actual “curved” deck plate span. Of course, now that I’ve said that everyone will be posting pictures of them all over the place! [;)]
There are no curved bridges. There are only series of straight bridges strung together to make a curve. In a wooden trestle, the straight pieces are close together, one per pair of bents. For others, they are longer and the sides are adjusted to form the semblence of a curve. If the track is to be curved on top of a straight bridge, the width has to be adjusted to accomodate the curve.
It is not cost effective to make curved parts in the real world. I have seen pics of some nice curved models, but they do not look right to me.
Kalbach has a publication called: Bridges and Trestles that shows a step by step procedure for building a curved track using plate girder bridges and sheet styrene…I built one using the procedure in this publication and it came out nice… [img]http://us.st11.yimg.com/us.st.yimg.com/I/kalmbachcatalog_1906_23216243[/img]
The supports on straight deck bridges are subjected to only compressive stresses, which concrete is extremely suited to withstand. Putting a curve into this structure means the base will then have to moment or torque forces, which concrete cannot withstand by itself without the inclusion of elaborate re-bar strucures. Trains (both model and prototype) do not like flexing roadbeds. The roadbed has to remain stationary. The center of a curve-plate bridge would have a tendency to sag (drop down) from the added weight of the train of the train as it moves along the bridge. In order to resist this, the brdige structure itself would have to heavier, which further complicates the design of the abutments. Typical bridge construction anchors one end of a bridge on its abutment while supporting the other end of the bridge rests on a sliding bearing which allows for adjustment for temperature changes. Incorporating the required torque absorption with this temperature accomodation would further complicate the design, if not make it impossible. Design complication = Higher cost. This makes the curved plate bridge uneconomical.
While, I have not seen them used in railroad contruction, Penndot used true curved girders when it rebuilt the massive King of Prussia interchange of the PA Turnpike, Schuykill Expressway, US 202 and US 422. Granted they are road bridges, and the spans are pretty short. But true curved bridge girders do exsist.
Nick
Nick, you are right, if you are modeling the 21st century. If you are using wood, there are no curved anythings (except scythe handles). The Kalmback book pictured is my main resourse. The wood deck bridge is my current project, but it is taking some modification on width do deal with my 20 inch curved track work. I will not have many pics of this for a month.
The main difuiculty is that its a long span that is about 20 to 22 inch span that ships need to pass under and it is curved so I picked a style thats comonm and might work. Do you have any sugestions what to do here?
Perhaps a series of short spans to form a curve towards the center, then a longer, straight span in the center for ships to pass under, then more short spans to finish out the curve.
Art is correct. True curved girders are recent development. In addition they are also used in deck grider applications, I have yet to see them used in through girder applications.
For your particular situation, I agree with Phillip. A series of short straight girders, forming a rough curve, leading to the shipping channel. Then a longer straight girder over the shipping channel. Depending on the clearances involved you may want to go with some sort of thru girder design.
Nick
Of course, any good freelancer on this forum would say: “It’s your layout; curve the girders if you want! Heck, add a curved truss span in there while you’re at it!”
Or something like that…
Yeah, Nick, that is a sweet curved girder on 202. I remember driving under it every day… Forgot about it…
Brian
Remember, there are lots of bridge designs to choose from. That center span could be of a competely different design. This is a great primer for stirring ideas. I use it pretty often.
that does make sence. So… …by straiting part I need to add that to the curved ends, and with an 18 degree curve now… …(to stay realistic) looks like that part of the park will make due with 35’ cars and maby a B-B electric swicher. That or get rid of the staging yard which defeates the point of the curve. Damn. I think I’ll use two track sections and hand lay the rail spiked evrywhere possible. Thanks.
I added a third girder in the middle so the weight is always centered on 2 girders. I doubt that a prototype existed but it didn’t look right with only 2. This is an 18" curve; Micro Engrg & Central Valley bridge parts.
Not sure if the wood supports are enough for the extra girder weight, but I like it. The idea is the old single track was replaced by a double truss as engines got larger. Lots of weeds and rust on it.