Author Topic: Fabricating an arching bridge deck  (Read 1504 times)

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Zox

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Fabricating an arching bridge deck
« on: January 22, 2012, 09:05:12 PM »
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I'm building a highway overpass that has an arch in its surface; that is, the center of the bridge is higher than the ends. I chose to treat this as a problem in fabrication, rather than trying to carve/sand the correct profile out of bulk material. I used my Silhouette cutter pretty extensively for this; without it, I don't think I could have gotten the shallow curves involved to come out consistently.

I started by creating the sides of the bridge from 0.40" and 0.20" styrene, and joined them together with a flat "tray" to form a base structure:



I then cut several curved ribs to fit into the tray, with the same profile as the sides but .040" lower, and .25" shorter at each end to fit inside the tray:



These ribs are a smidge over .125" high at the ends (they would be exactly .125" if they extended the additional .25" to the end of the bridge), and .188" high at the center. I put one rib inside each side wall of the bridge, and 4 spaced across the middle, with full-height blocking at the center to add rigidity and glue surface:



I then laminated two pieces of 0.20" sheet styrene over the top of the ribs and between the sides to form the road surface. The pieces were allowed to "run wild" during the gluing process, and trimmed to match the ends of the bridge once the glue had set:



This was about the point where I knocked my bottle of Weldene styrene cement over, so I'm very lucky that the only damage to the bridge (or anything else!) was a couple of dirty fingerprints. The surface will eventually be covered by a paper road anyway.

So, here's the overpass in its current state, set into position:



There's still work to be done on the underside structure, along with painting and minor details--like the entire rest of the scenery on the module. :) Eventually, it's supposed to be reminiscent of this:



Hey, it's a start. :)

(edit: corrected the name of the glue)
« Last Edit: January 22, 2012, 11:02:22 PM by Zox »
Rob M., a.k.a. Zox
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nscalemike

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Re: Fabricating an arching bridge deck
« Reply #1 on: January 22, 2012, 11:17:45 PM »
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Way to go in taking what could have been a simple bridge to the next level and match the prototype!  Just goes to show the types of things that can be done when you think outside the box.   Looking forward to the completed scene!

Mike

Zox

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Re: Fabricating an arching bridge deck
« Reply #2 on: January 28, 2012, 07:33:35 PM »
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Just an update to show some of the context that's coming together around the overpass:



I also got some preliminary paint on the bridge, so the contrast between the concrete top and steel underpinnings is more obvious, as it is in real life.

I'd hoped to have this largely ready by the Timonium show next weekend, but I think that was...overly optimistic, shall we say. :) (Especially when my basement's hovering around 60 degrees, and neither glue nor paint wants to dry in less than geologic time.)

You can see how the ends of the bridge are "trapped" by the abutments, and will largely be hidden by landscaping. This is true to some extent on the prototype, but here the bridge extends further back over the abutments. That's because the bridge and the abutments have to be removable, so the module will "pair" with my Kankakee River module for storage/transport. (The railroad bridge on the river module will nest into the gap left by removing the overpass.) The Gatorfoam profiles visible at the back of the abutments show the limits of the permanent scenery.
Rob M., a.k.a. Zox
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pnolan48

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Re: Fabricating an arching bridge deck
« Reply #3 on: January 28, 2012, 10:53:46 PM »
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Hey Zox,

How are you getting the .040 to feed into the Silhouette? I'm having a hard time with .030, let alone .040. The .030 eventually grabs, after a few tries; I shudder to think about trying .040. But then I'm loading 15-24" long sheets. Does it work better with shorter sheets?

Zox

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Re: Fabricating an arching bridge deck
« Reply #4 on: January 28, 2012, 11:29:41 PM »
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Pete,

I've just been working with the Evergreen 6x12" sheets, so that may be part of the difference, but I haven't had any problems feeding .040" styrene.

I do use a cutting sheet to engage the outside rollers, while the styrene sits in between them and only engages the middle rollers. Perhaps having the rollers go up in two steps (onto the cutting sheet, then onto the styrene) helps them to clear the styrene?

Also, one thing I do that's non-standard is that I have a bit of an "infeed table" to support the cutting sheet (and attached material) in front of the cutter:



A stack of eight slimline CD cases (or, presumably, 4 standard cases) is just the right height.
Rob M., a.k.a. Zox
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pnolan48

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Re: Fabricating an arching bridge deck
« Reply #5 on: January 29, 2012, 02:53:03 PM »
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Thanks, Zox! It makes sense, of which I am painfully short these days.

I have used your ideas for an infeed table, and also an outfeed table. Slim line cases on the infeed, Epson printer cartridge boxes for Ultrachrome on the outfeed. I just ran out of CD cases (all packed away), and the cartridge boxes were about the same size. It made a world of difference in cutting 33-inch long ship pieces. I gave up on that size ship for a while, as I decided to learn with smaller ships in the 12- to 16-inch range.
« Last Edit: February 04, 2012, 10:15:49 PM by pnolan48 »

lashedup

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Re: Fabricating an arching bridge deck
« Reply #6 on: January 31, 2012, 10:36:35 AM »
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Great idea Rob. Skibbe is trying to build a bridge with an arched span and tried doing it with a wood form instead. It seems to be working, but might be settling a bit. Your method is an additional way to do it that looks to work well. Nice job!

-jamie

Zox

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Re: Fabricating an arching bridge deck
« Reply #7 on: February 01, 2012, 10:33:14 PM »
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Mike and Jamie, thanks for the comments. Pete, glad I was able to help.

Eventually I'll be going back to the Silhouette cutter to make the handrails for the bridge, but for now the fabrication work on and around the bridge is done. The next step is the surrounding scenery, which unfortunately will still be unfinished styrofoam for this weekend's show. :( Then it's on to scratchbuilding more structures than I want to think about, where the Silhouette will come back into play in a big way. :)

At this point, I've got the abutments and wing walls built, primed, and mocked into place:



As with my Kankakee River module, I've got a smorgasbord of materials in play. So far I've used dimensional lumber (the body of the abutments), basswood, styrene, foamcore, Gatorboard, and three different glues to bind them. Paper and spray adhesive are on the to-do list... :)
Rob M., a.k.a. Zox
z o x @ v e r i z o n . n e t
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It is said a Shaolin chef can wok through walls...