I’m new to this forum this is my first post. I’ve always had an interest in model trains never really got a chance to do anything with it but my stepfather and I are building a dual track layout around a large room. One of the tracks has to cross a 12 and a half foot Gap so I made this bridge
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Welcome on board, OSIHA.
An interesting bridge indeed. I shall be watching the progress.
David
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- traingular shaperd bridge beams need to rest on something attached to walls, not just sit on the roadbed
- need to add ties between roadbed and bridge beams, multiple ties are needed because of the length of the span
- roadbed may need to be thicker or resting on 2 I-beams
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I appreciate your input if you zoom in you can see that I have cables on one side of the bridge after it’s all painted I’m going to put the cables on the other side. The road bed will be a bit thicker and have more support as I put trim pieces to cover up what I’ve done to keep my cables in place. Those triangle trusses are glued and nailed a lot lol. I built one and I cut it wrong so I used it as a test to see what it would take to break it I weigh 260 265 and I had to put all of my weight on it just to get it to give a little bit that thing’s not going anywhere. They are screwed to the side of the road bed in multiple locations. I figure beginning of next week I’ll have it finished I’ll set up another picture when it’s done
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looks like the bridge is supported by the roadbed, not the other way around
I guess it kind of is LOL it’s a very unique system but it’s a 12 and 1/2 ft span and I couldn’t have anything touching the ground. And my step dad didn’t want angle pieces coming down underneath to the wall. So if I attached the angles to the wall I couldn’t get the train onto the bridge. As it sits with just half of it done I put some weight on it and it didn’t go anywhere so we’ll see what happens
Osiha, Welcome to the forum. I’ll be watching to see how it turns out.
Al
I am presuming that the ‘triangle’ arch is bonded to the roadbed at the intersection points. That might tend to keep the roadbed member in tension across the gap, which is what you’d want.
The problem is, as noted, that you want the arch ‘springings’ to rest on abutments. If the arch is effectively ‘tied’ (as this one seems to be), the load on the abutments will be substantially vertical, BUT it needs to be carried there (with a post or bracket of some kind).
What you have is the whole weight of the triangle tarrangement being carried by the little cantilevered pieces of roadbed at the end – and I see them visibly deflecting; this at the very least will cause problems when you try to lay track there or run trains without derailing.
What I’d suggest is that you use tension rods or even wires up to the ceiling to carry the vertical load at the four ‘corners’ of the triangle in plan. If these are threaded at top or bottom, you can easily adjust line and surface of the track and approaches as laid, which is probably the best way to do it. If you don’t mind temporarily opening the ceiling, you could put pieces of wood between joists to give anchor points immediately above the corners for solid framing, if joists are not conveniently located.
I’d also note (although this is something of a kludge, I was a card-carrying kludgemeister in college) that ONE wire from the center of the apex of the triangle could be used to carry most of the effective load of the triangle plus roadbed. There will be a little support loading ‘up’ or ‘down’ on the cantilevers as trains run on and off the bridge, but that would be well within their carrying capacity. You might want to use static weights on the inside of the triangle or under the roadbed to balance if you do that.
First, welcome to the forum, OSIHA!
Second, that is a really neat idea. That’s quite a long distance to cover, and it looks like you’ve done a good job.
Please keep us up-to-date with more photos as you and your step-father continue to build. If you haven’t noticed on this forum, we love to see photos of everybody’s layouts.
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Yes, nice job indeed! That curve at the end looks really sharp, on the photo anyway 
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Simon
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I absolutely appreciate your input. My son’s an architect and he actually designed this for me and I don’t know if you zoomed in but there are cables that hold the roadbed up from the triangle and I understand that it’s a self-supporting object but it’s also held up by brackets on the walls and I keep putting a level on every day and it’s not warping at all. It looks like it’s warping on the ends a little because it goes from two layers to one layer of plywood. I actually have to put cables on the second side after I finish painting it and then I put trim on that is 2 in tall that will help support it also not to mention they’ll be support under the road bed hidden by a quarter inch piece of plywood that will slide in and out in case I need to maintenance anything. But trust me I will definitely try this and put weight on it and see how it does before I run my train on it. It’s almost 8 1/2 ft in the air I don’t think my engine would be in good shape if it fell off LOL again thank you so much for your input. I’m still new at this so I take everybody’s advice. I have however been building and remodeling homes for 52 years and I used to build trusses for for roofs. So I’m hoping that with my son’s design and my ideas that it’ll be okay. I will definitely keep posting and whether it fails or is successful I’ll let everybody know.
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I’m assuming you mean the radius looks too sharp. It’s the same as the other three I put in the room so it should work out. God I hope it does I don’t have to tear that thing apart and redo it LOL
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In real steel bridges, except for girder bridges, bending stress is not applied to the components. Only compression and tension are applied. This is to maximize the strength of the components.
Your bridge has bending in every component, which is worrying the participants of this forum. However, if the components have a large reserve of strength, it may be possible. Please be aware of deformation of components over time, not to mention when passing trains.
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I appreciate the input. I’ve been remodeling building for over 50 years and I had a buddy that taught me how to build trusses you learned from school I don’t know if this will work or not so far so good I hope it does but rather than just telling me how it won’t work and why it won’t work you have actually told me what problems I might have so I will keep an eye on that. Thank you so much
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Unlike vehicles and buildings, steel bridges are generally built not to sag. Below is a video of a 540-foot-span steel bridge in Kyoto, Japan, observing how much it sags when a train passes over it.
There is almost no sagging visible to the naked eye.
The reason the trains are slow is because there is a sharp curve at the north end of the bridge. The bridge is a Pennsylvania truss type, and the steel was imported from Bethlehem Steel and completed in 1928.
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The resistance to sag in rigid trusses is due to keeping the geometrical compression members from bending (which could quickly progress to buckling). The actual structure of your bridge as you first described it can be treated as one elongated panel of a truss, in which the angled ‘arch’ acts in compression, and the roadbed is in tension between the springings (a tied arch). Note that the roadbed is in tension if the tie points where connected to the ‘arch’ are as firmly nailed as you say.
Now, with that whole length of roadbed in tension, it will be self-correcting to level if it ‘bows’ under weight, just like the deck of a suspension bridge. When you add tension stays or hangers between the roadway and the arch, you are putting in periodic vertical ties to keep the roadbed from deflecting, so the roadbed will not as visibly deflect but couple any additional load on it into the beams, where it will be balanced across the rigid joint at the arch crown (and, as in a truss, any bending moment between the triangular members carried by glue, gusset plates, etc. reinforced against rotation).
But this only balances forces between the arch springings – imagine if the bridge were suspended by that single cable from a ball-and-socket joint or clevis right at the upper tip of the triangle. The ends of a truss are the points a real bridge NEEDS support from below. That support is either supplied by abutments or, in the case of rigid-member bridges, “shoes” which can float or rotate while communicating vertical load. (Interestingly, you might do this with an arrangement as simple as four ball bearings resting in shallow cups, which is exactly the arrangement installed on the Hernando de Soto Mississippi bridge, an interesting thing to see!)
The concern is the support in the existing design. To an extent, the portion of the roadbed outside the tied-arch springings is a cantilever beam, which has relatively poor geometrical arrangement of material for such use (consider the arrangement of tension, compression, and shear in an I-beam cantilever vs. a thin horizontal slab). Eaasentially half the weight of the entire bridge structure is acting down on this cantilever, so even a short approach span will be under substantial stress, nearly all of which can be relieved by supporting the bridge mass in tension, so any load is strictly a rotation and have a comparatively short cantilever moment arm.
As with a suspension bridge, you can expect the bridge to deflect somewhat downward when a train runs onto it, and to swing slightly in reaction due to the effect of the curve at the right. All this would be self-correcting when the load runs off, rather than sagging toward failure.
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while we studied bridge construction in school, i learned more when studying aircraft wing design which uses the same principles.
tension and compression can easily be demonstated by gluing a thin sheet (1/32") balsa to one side of a piece of foeam (e.g. 3/4") and put packing tape with the glass fibers along the opposite side of the foam.
the foam easily bends when the tape is compressed but is surprisingly “stiff” when stressed in the opposite direction, compressing the balsa and putting the tape into tension.
looks like the bridge is supported by the roadbed. The bridge should be self supporting, no need for any roadbed,.
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Well, the bridge has been up for a week all the track is on it and obviously I’m running the train across it has not wavered one little bit and it’s only half done. Hopefully once the other cables are on and all the trims on it will never give me an issue. I appreciate everybody’s input on this and concerns. If it does give me problems I’ll post that also.
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