bridge design decisions: where's Matt?

You might consider a through-truss bridge similar to what I have on my model railroad as shown below. The tracks can be supported by longitudinal beams or girders directly beneath the rails and the solid base that you have to carry the tracks can be removed. This type of construction is realistic and eliminates the need for ballast. The bridge can then then be lifted out for access to the rear area just by disconnecting or removing the tracks. My bridge should have walkways at the sides of the tracks but I like the look of the open structure and have never installed them. It is 36-inches long, made of basswood, and has been on this railroad since 2004.

MELGAR

That... is a good looking bridge Melgar!

So here's what a rough mockup looks like of that style bridge. It isn't built correctly as a truss should be, or to scale, just something to get an idea from.

keep in mind that it is 8' long! So it needs to be about a third higher (and more angled sections). I think the track's steel girders alone should be taller than 2" for the span?

 I need to look for a site on designing one or match up to an existing structure that's 8' scaled up. 8 X 48 = 384' ??

If it is deep it may require a suspension type bridge, if you can use a support in the center a truss will work.

Similar to Melgar's bridges, this one is a kit offered by Plastruct and it's 30" long, but easy to extend. I built one before they offered it in O scale.  Took their HO plans, had them enlarged, and developed the bill of materials.  It is very strong having survived a 6' fall with modest, and easily repairable, damage.

https://plastruct.com/shop/tra...eries-kits/kit-3002/

I have the O scale plans and bill of materials it you'd like to consider this project.  

I couldn't use the Plastruct design as shown since I had curves entering the bridge which would have raised havoc with the big engines, so I inverted them and made them deck truss bridges. I had bought two kits, but because a deck truss doesn't need to be as high or wide as a through truss I was able to save enough material that I build another deck girder bridge and its supporting pylon. With a deck bridge the truss lies directly under the rail gauge. It's very strong and could probably support my weight. I used Keil Line bridge shoes for one end and a sliding support on the other. I drew the plans on CorelDraw and made full-size templates which I used to directly build the trusses. The Plastruct plans were not to scale and were disappointing. Their instructions are generally not so hot.

Instead of punching out the rivets, I used Archer Fine Transfers O'scale rivet decals. It was my first use of this interesting "tool". I bought them from MicroMark, but they're also available directly from the vendor.

The walkways were built on bass wood cross members with balsa wood strips cut with the balsa plank cutter. I used Bowser white-metal cast "hairpin stanchions" threaded with 0.032" brass wire. The track is specially made Ross bridge track with a full-length guard rail.

And yes… that's a real Loch Ness Monster lurking below.

Engineer-Joe posted:

So here's what a rough mockup looks like of that style bridge. It isn't built correctly as a truss should be, or to scale, just something to get an idea from.

keep in mind that it is 8' long! So it needs to be about a third higher (and more angled sections). I think the track's steel girders alone should be taller than 2" for the span?

 I need to look for a site on designing one or match up to an existing structure that's 8' scaled up. 8 X 48 = 384' ??

Some through-truss bridges on the Ohio River have spans of (approximately) 400 to 600 feet. Deck trusses are often used for the approach spans where clearance above river can be smaller than the crossing of the main channel. An eight foot model length should be possible but probably will require splices in the lower chord of the truss. Sounds like an interesting project.

MELGAR

Thanks all.

We have a famous upside down truss in Lockport,NY. I never understood why they put it that way?

It would make it tough to get under to move stuff.

I also like the look of that Plastruct bridge and thought I saw a G scale version too? I might get a couple for the lower G scale if I can make them removable.

Otherwise I would just put a couple of girder bridges underneath the G for better looks.

I really don't want a support right in the middle. I'm getting older and I struggle to duck under there. Usually we send our grandson under to get stuff!

Engineer-Joe posted:

Thanks all.

We have a famous upside down truss in Lockport,NY. I never understood why they put it that way?

It would make it tough to get under to move stuff.

I also like the look of that Plastruct bridge and thought I saw a G scale version too? I might get a couple for the lower G scale if I can make them removable.

Otherwise I would just put a couple of girder bridges underneath the G for better looks.

I really don't want a support right in the middle. I'm getting older and I struggle to duck under there. Usually we send our grandson under to get stuff!

Just looking at the width of the channel, it probably is not wide or deep enough to handle large, deep-draft vessels. So high clearance was not necessary and it was easier to build a deck truss. The height of the bridge above the water may have been dictated by the terrain on either side rather than clearance from water to bridge. I agree that a model bridge without a center support would be much more spectacular. Your sample truss looks like a good start but needs to be deeper.

MELGAR

Interesting Tom. I didn't look at concrete styles for this. I saw an engineering show a few weeks ago that had an amazing concrete span in another country. It was amazing because of how far it spanned, and how thin it was.

 Melgar, I had planned for the bridge to be about a third higher basically. So that prop is around 11 1/2" high and I figured somewhere around 16"??

 It seemed easy enough so I tried to draw one out on a sheet of plywood for a template. I wasn't happy with the three different attempts and I never completed anything.

 I put this bridge area together a few years ago and with our youngest grandson's interest in trains, I wanted to go forward with something for him to enjoy soon. I'm probably getting near the end of these toys capturing his interest. He keeps surprising me so I keep pushing forward.

I’m right here

Looks like that would be easy Tom. Very modern looking. Some of them look like they span the distance too.

Would be so much easier than having to build a double steel truss. I could dress up some plywood or masonite!

Hello Matt, always good to see ya!

now where's SD Matt?

I wish I would have posted this earlier before I began cutting. I had the appropriate sized sides to make the slopes. Now, they are gone!

So I have to rebuild. I know better than to just start cutting. I thought I had a direction.

I have a shipping cardboard tube of steel shapes in my shed for a steel bridge across my water feature that never got built. I could borrow from that supply.

I have to agree with Tom. On a large bridge span, you're going to have sag issues unless you build over a steel roadbed support spine. "Concrete" arch bridges are the simplest way to disguise your steel angle iron, steel tube (a great option as you can run wires through it) or I-beam. Also, fake concrete bridges are easiest to fix when someone hits it with a chair (or their head if it's low). You can also fake girder bridges pretty easily by sticking girder castings onto a supportive roadbed. I don't know if I posted these photos, but the viaduct is supported by fake concrete piers (1x3's and mouldings) supporting HDPE ladder roadbed with girders glued to the sides of the roadbed. To give you a perspective, the span on the right (the first one I built) is four feet long and 16" high at the deck.

Thanks Matt! I look for your advice on bridges.

Joe, looks like a great opportunity for a nice, long bridge.

Three more ideas:

Basswood with balsa trim, painted in Santa Fe colors.  Nearly seven feet long.

Double Lionel bridges, 40 years old.  Extended with bass and balsa wood.  You'd need a 3 inch wide, basswood plank underneath to replace the plastic piers that come with them.  I added four 5m LED lamps to the tops of each.

From Bridgeboss.  Heavily custom modified to fit the height of that upper level.  Jim has a lot of great bridge stuff. 

Jerry

Here's a mock-up of what I might go with. I drew two sets of lines to allow for the sides to go up 1"+ on the sides of the roadbed. However I cut to the wrong line so that decision is already made! Duh.

I went with taller ends to look more massive and I can always cut them down too.

added some details to see what look I like the best?: