I want to add a Pratt Truss bridge to my HO layout under construction. Problem is, the place I've chosen has a slight grade (less than 1%). I know, I should have considered this in the original design, but didn't. Were these bridges ever built on a grade? I've seen some pictures where there seems to be a grade, but I can't be certain. Thanks in advance for the info.
Hello, and welcome. Yes, one could say that most bridges are erected as part of a grade. That could be something very close to no grade at all up to the maximum permitted by design and policy for the railroad. I have walked across bridges that seemed to be level and on others where the approach on one end was clearly on a grade, which the bridge deck matched.
-Crandell
Welcome aboard!
For a grade of less than 1%, the engineering staff would probably go with a catalog bridge from the steel fabricator, specifying a slightly higher capacity than they would for a bridge on dead level track. Since railroad bridges were usually grossly overbuilt anyway...
OTOH, on a steeper grade, the bridge would be modified to keep the normally vertical members vertical, and the bridge panels would become rhomboid rather than rectangular.
Looking at a poster illustrating a bridge on a curve, it appears that the fixed shoes are on the low end and the flexible shoes are on the high end. It's a multi-span, so both variants are right next to each other on top of a masonry pier.
Chuck (Modeling Central Japan in September, 1964 - including lots of bridges on grades)
Mine is on a 1+% grade. Not that I would have changed anything if it was 2.5% because I'm just not that much of a rivet counter. Anywho, it looks fine and no one has ever said "hey, you're bridge is leaning!"
San Dimas Southern slideshow
A very nice example, too!
I have two bridges, both on grades. This trestle is near 2%.
Selector, I trust your trestle bents are completely vertical.
Mark
spfan01
Almost every bridge I have (13 at last count) is on a grade, between 1 and 2% either ascending or descending. If your using a Pratt, a grade won't disturb it. If you're thinking of a trestle or a steel viaduct, just make sure that your bents are vertical. Other than that, go for it.
Here's my Deer Creek Viaduct on a 2%:
The Bullards Bar bridge on a 2% descending:
Malakoff Diggings bridge on a 1% ascending:
Tom
Tom View my layout photos! http://s299.photobucket.com/albums/mm310/TWhite-014/Rio%20Grande%20Yuba%20River%20Sub One can NEVER have too many Articulateds!
markpierce Selector, I trust your trestle bents are completely vertical. Mark
As near as my eyeball could make 'em so, Mark. I would have used a scale plumb bob, but I thought it was a bit much.