Girder Bridge

[jtlan]

Happy November, Train Tech.

I feel like I've occasionally seen bridges posted here, but for most bridges some sort of elevation change or slope is needed to integrate it into the layout. I wanted to share a simple design I came up with that addresses this issue.

The basic idea is that the parts of the bridge that hold on to the edge of the gap don't extend underneath the track, so the "floor" of the bridge is at the same level as the rest of the layout:

I tried to keep the bridge easy to build, using mostly basic bricks, plates, and Technic beams. The trickiest parts to find are probably the studless beams.

While writing this post I realized I had made a mistake, and the bridge shown in the first photo actually spans a gap of 34 studs, instead of some multiple of 16; I've since fixed it in the CAD files. The idea is that the bridge should be as long as common baseplate lengths to integrate into layouts easily. For example, here's a 48-stud long version:

Since there are repeating 16-stud sections in the bridge, it's easy to extend it. I haven't tested to figure out what the longest stable length is, but the bridge should be stronger in scenarios where the table extends down and gives the underhanging part of the bridge something to push against (like in the second image above).

What do you think? I've attached the CAD files if you want to play around with them.

Brickshelf (pending moderation).

girderBridge_32.lxf

girderBridge_48.lxf

[marook]

Looks good.

Nice easy structure for getting over an open area. :-)

[legoman666]

Looks good, I made a similar truss bridge that spanned 6' with a single support in the middle. My track wasn't table level like yours though, it was 1 brick off the table. It only sagged a little, yours pushing against the table edge might alleviate that to some extent. I think most of the flex comes from the Lego bending though.

[LEGO Train 12 Volts]

Very nice project!

The archietcture looks strong!

[Dutchiedoughnut]

Looks nice - although I question the engineering a little bit. In a bridge of the bridge will be in tension (it'll get pulled apart) and the top is in compression. It'll be very helpful if the girder extends to the bridge head, or if there is a diagonal connecting to the bridge head. You will notice all girder bridges have this.

[jtlan]

Looks nice - although I question the engineering a little bit. In a bridge of the bridge will be in tension (it'll get pulled apart) and the top is in compression. It'll be very helpful if the girder extends to the bridge head, or if there is a diagonal connecting to the bridge head. You will notice all girder bridges have this.

Right. It's part of why I suggested that the bridge would be stronger if placed against tables with a tall side, because then the forces would be redirected into the table which would resist the bending moment in the bridge:

I still need to test with longer spans, but I think the design could be improved from a strength perspective. The goal was to highlight the technique of making the floor of the bridge level with the track, which makes the bridge easier to integrate into layouts.

[Man with a hat]

Nice work and well thought through.

Did you try to fix the technick bricks at an angle to create triangles? That should stiffen things as well.

[WholeWheatLego]

Nice work and well thought through.

Did you try to fix the technick bricks at an angle to create triangles? That should stiffen things as well.

This is a cool idea, keeping the track level, but as Man w/ Hat said, a truss (repeating triangles) would be a lot stiffer than the rectangles.

Something along these lines, perhaps?

edit: typos, man... they're everywhere...

Edited November 5, 2015 by WholeWheatLego

[BricksMcgee]

Nice concept! Resting against the table edge should really help, especially on smaller spans.

Was thinking of using something similar for next year's displays, as moving a display from venue to venue usually means dealing with unknown/variable table heights. Was trying to think of a way of using the minifigure baseplates as "cladding" for my bridge, but haven't got beyond a few sketches yet. Will post any progress.

[RacerGTR Productions]

Thats nice, really simple but it looks technical. Thats goooooood

[jtlan]

This is a cool idea, keeping the track level, but as Man w/ Hat said, a truss (repeating triangles) would be a lot stiffer than the rectangles.

Something along these lines, perhaps?

For the length of this span, I'm not sure it makes a huge difference, but you could use 6L half-beams or the bent liftarm to get triangles. The main point of weakness I was concerned about is that the shear force for the entire bridge passes through only 4 16L beams, so it's important to be able to divert some of those forces into the side of the table.