Bridge Over Troubled Waters

Concurrent with the experimentation in modeling water was bridge construction. The curved track in the area was to be carried by a deck plate girder bridge. I mocked up a bridge with card stock (manilla folders) because that’s what I had on hand:

cardstock brisge_1

cardstock bridge_2

The idea here was to get an idea of how the bridge would look, the dimensions the styrene would have to be cut to, and where (and how many) piers would be needed. The first step was the piers.

The Northern Pacific seemed to favor cut stone rounded piers for their bridges. A perusal of the Internet didn’t reveal any likely candidates. The nearest analogue was a tapered pier, and that wasn’t close enough. On to scratch-building!

A note: Much of my modeling discussion going forward will be using the metric system. It’s more precise than the English system, and I’m very comfortable with it. Every class I took in engineering school used the metric system, and I worked for years for a company that did much of their business with the Federal government. The Feds have required for some twenty years that docs be submitted in the metric system. Most of my engineering reference manuals are metric. It’s like any other language: don’t try and translate, just accept and you’ll be fine.

From the mock-up I knew that I’d need three piers. Consulting Kalmbach’s Bridges, Trestles, and Tunnels publication (extremely useful; buy it), I figured the pier depth to be about 10 mm. That translates to 3/8″. 3/8″ half-round is a thing, but not a thing outside specialty stores that don’t exist in my area. Contemplating this in the local home improvement store standing in front of 3/8″ round, I hit upon a method to turn round into half-round with a reasonable chance of success. I cut the dowel just slightly oversize for the width of the miter box so it would friction fit, then used a hacksaw (thinner kerf) to cut it in half. It worked. I used white glue to bind the pieces together.

Girder Bridge Piers 1

Girder Bridge Piers 2

The core is balsa. I sanded the half-round with a bench grinder.

Girder Bridge Piers 3

The cores complete.

That solved the basic shape, but what of the cut stone look? I had another look at Creative Models paper textures. I’d used their brick and concrete textures for the coal trestle, and wasn’t entirely satisfied with the quality. They’ve doubled their price since I last ordered, but also appear to have raised their quality. This time around, I found the quality of the printed product to be acceptable.

Girder Bridge Piers 4

The piers are topped with a paper concrete texture, and the stone texture is on the right. I had a plan to conceal the overlap (the same method as patching drywall), but this proved to be unnecessary.

Girder Bridge Piers 5

The piers after wrapping, dusting with black pastel chalk, and sealed with Dullcote. For some reason the paper texture has a faint greenish tinge. I don’t know if this is part of the file, or a printer artifact, but for a water application it works well.

The Bridge Proper

From the mock-up I’d determined the length of the bridge spans. I decided to go with a ballasted deck girder, and drew plans accordingly. I elected to go with 0.030 styrene for girder construction, primarily for appearance reasons.

Girder Bridge Sections 1

I braced the sections with 7/32″ tubing, because that’s what I had on hand that would do the job, and the bracing isn’t visible. The tubing is left over from the conduits for wiring. I used MEK for the cement.

My work area on a typical project:

Typical work session

I’m sure yours is similar.

Spacing of stiffeners on the sides of the girders was problematic. I used the expedient method of checking how many ties were between stiffeners (again, the book), and pulled out a piece of track, Spacing was 10 mm. I drew lines on the girder plates at that spacing with a T-square, and positioned 0.25 mm x 0.75 mm styrene strip on the lines. I cut the strip longer than the sides, and trimmed them after the glue had set with a pair of scissors.

Shoe Are You

Because I knew I’d be building a bridge, I ordered N-scale bridge shoes from Micro Engineering. Prototype bridges use fixed shoes on one end, and sliding shoes on the other to account for expansion and contraction during the year. No such distinction is made in N-scale, and really, the distinction isn’t visible. I would expect that in larger scales, say S and above, there might be merit in distinguishing between the types of shoes.

I’ve seen forums discussing where to mount bridge shoes, but there really shouldn’t be any discussion. Common sense and  a perusal of prototype photos indicate that they are mounted on the end of the girder, where the weight of the structure can be directed into the foundation. The only problem I had was how to mount the shoes on the abutments. A prototype photo in the aforementioned reference shows they are mounted normal (perpendicular) to the span. On the prototype these would be the fixed shoes.

The shoes from ME are white metal castings, and the lower surface needs to be trimmed so they sit level. Be careful! This is N-scale, and it’s fiddly. Easy does it. I attached the shoes to the lower part of the girder with CA.

A look at how the pieces stack up thus far:

Girder Bridge Sections 3

Some adjustments need to be made, but it looks like things are coming along nicely.

 

 

 

 

 

 

 

 

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