I’ve hadd to custom build a bridge. At first I hadd a 1/2" roadbed of foam. I found out it was to flimsy and I was at the maximum supports I could put. I didn’t want to make the bottom thicker as I was already at a 3 1/2" clearence in tall areas. So I cut another peice of foam put it on top and carved a grade into it. The only problem was it was a 5% grade. There are just too many what-ifs with a 5%. I’m thinking of shaving it down so it’s 3-4% grade. But then it could be to flimsy. So if I wrap it in plaster cloth, wil the bridge be stronger?
I would rework those grades for sure. As far as expecting to reinforce foam by wrapping in plaster cloth, it may work somewhat, but feel there are far better options. I assume this bridge is ballasted deck of some sort, to be expecting foam to span the distance. Any sides applied to the bridge, girder, stone etc should supply the support. The span would be better done with a strip of plywood/ pine, then your roadbed.
The existing span can be reinforced by trimming back some foam @ the sides and add wood strips.
I would agree that plaster cloth isn’t a good enough support medium for this application. Even 2 or three layers still can flex and crack.
Joe
Plaster cloth is only as strong as its supporting base, so I´d recommend looking for a more solid solution.
I like the idea of using wood as a fresh start but how would you recommend supporting seeing as the to points (point A an point B for further reference) are both woodland scenics inclines. As well under point a I want to leave space for a ground throw to put on the switch below where the bridge will go.
Cut a notch into the foam incline at each end (points A & B) for the wood bridge to rest on. The top level of the wood bridge should be the same level as the top of the WS foam inclines.
Plaster cloth is what the medical profession uses to make casts - but they’re typically 1/4" - 1/2" thick. Using it in that thickness would negate any possible grade reduction.
If your bridge is going to be visible wood is probably the best choice. If the bridge is going to be hidden, a short length of steel stud (nominal 2x3) laid like a through girder would be the ideal match between thickness and strength. I have a few such in the netherworld. The total thickness from railhead to underside of girder is about 1/4 inch.
Chuck (Modeling Central Japan in September, 1964 - on steel stud benchwork)