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Discussion Starter · #1 ·
My dad, the retired metal shop teacher, has "volunteered" to make a steel bridge for us.

I don't want anything too fancy, just a basic steel truss structure - don't want to take too much advantage -

Does anyone know where I could find any basic plans?

How wide should the inside dimension be for two trains to pass side by side?
 

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The width is determined by what you run. 1/20.3 needs more space than 1/29 or 1/32.

Same for tunnels, plan ahead for what you might run in the future.

My aristo plow blade is my widest piece of running gear, and the LGB pustfix car is the tallest. These determined my tunnel width and height.

I ended up using 10 inches for rail head to top of tunnel opening, and I used a 12 inch aluminum culvert for the tunnel. Bachmann Annie and LGB beartrap clear nicely.

I have not had any problems to date with visitors running their equipment, but the K27 might be interesting on the first pass.

OTOH, if running all 1/32, you can get by with much smaller widths and heights.
 

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I made mine as a Pratt(I think is the name) truss. Just type it in on an image search and something should come up. I did mine to hold two tracks so I can have
two loops. More pix on my web site. It's made of 1/2" square tubing that I cut. Made a wood jig to hold them in place and hired a buddy to weld it up.
 

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Discussion Starter · #4 ·
Thanks, Jerry, your idea is exactly what I have in mind.

Oh, and Dan, while I only have 1:29 right now I want to plan for 1:20!
 

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How do you figure out how tall the bridge needs to be so that it is structurally sound? Is 1' high correct for a 14' bridge?
 

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I have no clue Bill, guess I did not consider that it mattered, just made it tall enough for those darned 1:20 guys to run their stuff thru it, if any show up with a big 'un. Not sure that the track needed the supports(red lines in the plans) or not, but thought the cross braces being a foot apart was too big of a distance. Bridge is really sturdy, had thought I might need a center support, but it seems to be fine so far. Doubtless it is over engineered , as much of our stuff seems to be!
 

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Discussion Starter · #7 ·
Bill:
I don't have one yet 8^), and my statics days are waaaay behind me, but at our scale, I think the materials are strong enough that it isn't critical. You can go for pleasing proportions. More important is the support from underneath.
 

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The bridge in the drawing that Jerry provided is called a subdivided Warren truss. Here is a spotters guide to bridges: spotters guide

I'm building a subdivided Warren truss myself. I'm not going off of any plans. I found an old railroad bridge I liked and one weekend I crawled all over it will a tape measure. For my bridge I'm allowing 8" side-to-side clearance. So if you used, for example, 1/2" square tubing for the side columns, the bridge would be a total of 9" wide. The vertical clearance on mine is 11". I take into consideration the hieight of the bottom beams, ties and rail height. So 11" is from top of the rail to the bottom of the top chord.

If you want some inspiration for some bridges these guys are a forum sponsor at MLS: bridges
 
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the inside width for a two track bridge should be 12" by my experience.
that would leave you with about 1 1/2" between two LGB trains and about 3/4" between trains and structure.
(by the way, 6" from middle of track to middle of track is the distance, you get when making sidings with R1 curves and switches)
 

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Bill, There formulas to figure the strength of the bridge, but from my experience I would say that a bridge with the dimensions shown built of 1/4 X .032 wall steel tubing would be strong enough to walk on!
 

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When building bridges at the scales we use, in general, the minimum material sizes are ususally adequate for most brideges except a long clear span. When I used to design bridges 30 years ago, for a girder type highway bridge if the bridge was more or less than 5 to 6% depth to span ratio we took another look at it but that was highway bridges and crane ralils not railroad bridges. As far as trusses go it depends on the type of truss that you want to use as to the depth to span ratio. I would recomend that you find a bridge that you like and make your model bridge of similar propotions.

As far as things to watch the width and height clearences as already noted are going to drive your bridge size and should be generally in line with the bridge prototype that you choose for the design. As far as the construction the only issue I would note would be to be carfull with the joints/connections. As in real life bridges if you have in problems this is where it will likely be the issue. That being said, again, in general, due to the minimum sizes that we use when modeling it will not be a problem but you just need to watch what you are doing.

Bill W
 
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