Thanks for earlier comments on bridges in general. I’d like advice on bridge abutments. I’ve ordered a Central Valley 150’ HO truss bridge kit for a 21" span that will cross (among other things) a lower track. I know I need 3" clearance above the lower rail. I’m using 5/8" plywood subroadbed and cork roadbed, so that means 3.3" above the lower subroadbed. The bridge is said to take 15/16" height from support shoes to railhead.
I’m looking at Chooch bridge abutments and see #8440 and #8445. The former is said to be useable for the CV bridge. But its 2.75" height to its bridge support shelf gives only 2.44" clearance to the bridge from the lower railhead by my calcs, not enough to go over my 0.3" lower railhead height. So I’d have to shim it up by 0.56" I suppose. Conversely, #8445 is a bit tall, creating 3.44" clearance (which would unnecessarily increase grade) so I suppose I could cut it lower by 0.44" to eliminate some grade, correct?
If I’ve got this right, my choices (besides accepting a higher grade) are to shim the too short #8440, cut off 0.44" from the #8445 bottom, build something of desired height from scratch (wood plus a covering?), or find one from another supplier. What do you suggest? If cutting the too tall one, how best to get a nice 90 degree cut through the resin component. I’ve got as band saw, for instance. Thanks as always for your help.
I didn’t follow all your calculations, but I didn’t see where the 15/16" number got added in. And when you say to the railhead, is that the railhead on the bridge? If the clearance under the bridge is what you’re after, you need the dimension from the bottom of the bridge shoe to the bottom of the bottom girder of the bridge. That value plus the distance from the bottom of the bridge shoe down to the top of the lower railhead is what will determine the clearance under the bridge.
There are multiple ways to handle this. You can cut taller abutments easily on a chop saw or table saw, or use various materials to shim shorter ones. The shims can be hidden by your scenery.
All of my bridge abutments are scratchbuilt from styrene to match prototype photos. I tend to make them a bit shorter than the total clearance I’ll need, then shim to get everything lined up just right.
Here’s a CV truss before being cut into the roadbed (note the hollow styrene abutments):
And here’s the same bridge at left once the track went in. The abutments are temporarily shimmed with scraps of thin wood. I have since roughed in the scenery and permanently mounted the necessary shims to keep the brigde in alignment.
Maxman, I included the info on the shoe to bridge railhead height just as info. I was looking at 3 things: underbridge clearance to lower railhead, actual net rise (for grade) from lower railhead to upper (bridge) railhead with needed underbridge clearance, plus how much the plywood subroadbed rise needs to be to get the plywood’s railhead aligned vertically with the bridge railhead. You’re right in pointing out that the shoe to railhead height may be more then the bridge girder to railhead height whcih will help the clearance. That plus seeing two lengths quoted for the bridge is why I won’t (cookie cutter) cut any plywood until the bridge arrives next week.
Wp8thsub, I like the approach you’ve taken and lean toward copying that. I would get exactly the height I need plus can make custom size wing walls to go with adjacent scenery shape. Can you add some more info, e.g., what thickness styrene you use, how you plan to finish it off to look like concrete, etc. Is there a way to make styrene look more like block or stone than concrete?
A related question. WIth this bridge, one adds the rail (in my case code 83). Should I plan to extend it past the ends of the bridge into the adjacent approaching flextrack ties to aid vertical alignment over the bridge-to-approach roadbed joint, avoiding a rail joint there?
Rob, another detail question…what height do you suggest making the top of the abutments relative to the rail? Do you leave room just for ties (in my case flextrack) atop or suggest leaving room for subroadbed also (in my case cork) and running the cork to the bridge itself?
I used .030" I think. That’s a minimum to really keep the sheet straight, and .040" is probably better. Although it doesn’t show much in the photos, the abutments are already painted with Testors Camouflage Gray. I’ll do final weathering to look like aged concrete with chalk, acrylic washes, and airbrush after the plaster work is done. As for represeting stone, I’d start with a commercial stone abutment or otherwise carve the stone into a cast plaster block.
I extended my rail about 6" onto the adjacent roadbed for this purpose, and handlaid it on wood ties matching my flextrack ties.
Yes, it’s a good kit. It’s not an easy, shake-the-box project, but take your time and you’ll get a good result. Be careful to keep everything straight.
Depends on how you want to handle it. I usually run the abutment right up to the ties. Note that the thickness of my abutments is only that of the styrene, so the roadbed is only cut short enough so the styrene can fit next to it.
Well, here are some photos of what my installation look like right now. I removed the left side deck girder bridge so that you can see the abutment at the end of the bridge. There is also a rule in the photos to give you some idea of the dimensions. I put a piece of code 100 snap track on part of a cork strip to help gage clearance. The top of the abutment where the bridge shoe sits down to the homasote is about 3 inches. From the top of the rail to the bottom of the bridge ended up being about 2-3/4 inch. All of the supports/abutments in the phote are made from wood.
Concerning the length of track overhanging the end of the bridge, I left enough for about 3 ties to be slipped on, along with a rail joiner, at each end. My track is not handlaid, and I wanted to be able to remove the bridge for final painting. The approaching trackbed elevation was adjusted so that the cork roadbed would be at the proper elevation for the track coming off the bridge.
About the length, I measured mine and it is 21 inches end to end of the bottom girder.
I liked the bridge kit. However, I did not like the nylon (plastic?) line material that they supplied for some of the diagonal tension members. I just could not get this to straighten out. I used a suitable diameter brass wire as a substitute.
Wow, great, on target info folks! Thanks for the extra effort to share your expertise. First time encountering these layout details is an interesting challenge and hopefully your kind of help will minimize painful learnings.
Sorry, but I think that you’re over-complicating things. Simply build the bridge, place it temporarily in the spot where it will eventually be installed, blocking it up to give the correct clearance over the lower track. Measure down from the bottom of the bridge shoes to get the height required for the support abutments. While the bridge is still in place, raise the roadbed, with cork and track installed, to the correct height to lin
