Tuesday, June 23, 2009

Thin Walls

I call this "Thin Wall Construction" as an easy name for one-eighth inch thick, eight foot high, structural walls for model railroading. What follows is a description of the problem and the construction of a solution.
Many model railroaders always seem to complaining that they do not have enough room for their layouts. Solutions can include designing a smaller layout, joining a club with a large layout, moving to a larger house, or building a building specifically for your layout.
The need for this extra space may be more mainline, more yards, or wider aisles. The approach I took was to minimize some of the benchwork.

I am building an HO scale layout that I anticipate will be multi-level in its later phases. Originally, I considered building stud walls and cantilevering the benchwork for each level off the wall. Stud walls are usually constructed with 2x4’s with the wide side cross wise. 2x4 dimensions are 1½”x 3½”. The 3½” dimension equals 25.375 scale feet -enough room for another track or a backdrop flat. If we turn the wall sideways so the 1½”is cross wise, that will reduce the space to 10.875 scale feet.

Walter Naumann, our model railroad club's physicist asked “What happens if you use 1/8” plywood? If the plywood is curved it would create structural strength. Adding external ribs below the track and anchoring the edges should gain stability from buckling.” I noted that would reduce the space to 0.90625 scale feet. Since the ribs are below the track and support it they don’t add to the 1/8” thickness at the track level where the scenery will be.
So we agreed one day to try it. It is much like the external ribs on some tanker trucks.

First we provided anchors for the edges of the 1/8” plywood. We marked the floor with the approximate location of the wall. Then we cut ¾” boards to place on the ceiling so we could fix the wall on the top. We secured one stud at the beginning of the wall, and then secured one end of the plywood to the stud. The floor to ceiling distance is 99”. The plywood is 96” tall. In order to secure the wall, we attached some wood blocks to the floor. The blocks were anchored to the floor first and then the wall was curved to place. The wall was secured top and bottom using blocks to maintain the curvature. After the first 4’x8’ panel was up, the second was attached to the first using liquid nails and temporarily attached with short 1” drywall screws through both pieces of plywood and into a 1x2.
Marking the floors

Attaching ceiling boards
Floor Blocks

End Stud

First two panels showing temporary 1x2

Walter lifting his weight on the first external rib

We then proceeded in adding the external ribs. Using an improvised divider to scribe a piece of ¾” pine, we cut a rib that fit nicely into the curve of the plywood. The rib was attached. Just to see how strong it was Walter lifted his weight on the rib and the wall did not move. It looked like it was working, so we continued with more panels.

Initially we used Luan plywood which is readily available at many “big box” stores. We also bought a sheet of Birch plywood to compare. The one panel of Birch ply seems as if it is more structurally stable.

About a week latter we removed the 1x2’s and the screws, which had temporarily held the joints in place while the glue set. We added several additional exterior ribs to stiffen the areas at the inflections of curvature. Once again Walter decided to test the wall. This time he put his full 200 pounds of weight on the wall by climbing the external ribs. Again it held. Clearly more weight than will be applied to the wall by standard NMRA weighted cars.

Walter on the wall

In addition to strength, the wall is stiff. It is effectively a wall that is eight
inches thick with a 1/8 wall at each edge. It is as stiff to hand hammering as a 2x4 wall. For the math inclined, stiffness increases as the cube of the thickness and the effective thickness is eight inches.

Additional Thoughts
The wall is up but not completed. I would like to try some laminated hardboard spline instead of the ¾” pine for some of the external ribs. The addition of ribs at the other levels will further strengthen the wall. The ribs should be spliced together with a doubler under them for maximum strength.

The joints on the panels still need to be filled before the basic “sky blue” backdrop can be painted on. Drywall compound should do well to smooth the crack discontinuity and patch the edges. A thin coat over all may be the quickest, least expensive way to fill the grain in the wood.
The bottoms and tops and edges of the plywood should be anchored continuously on about one inch spacing. The screws can be small because the strength limit is the plywood. This is most important at the extremes of the curves, and in most places that is the only places at the top and bottom we attached. Again, the strength seems to be there without screws at the inflections, but like continuous ribs, screws there will prevent buckling.

Some of the curves we put into the wall were down to a track radius. Less curvature would work because of our more than sufficient strength. The one area made with birch plywood 1/8” thick has a curvature of about 2 inches over 36 inches with a greater curve at the end. It has roughly the same stiffness as the Luan plywood with greater curvature,
The beauty of the curved walls in not only esthetic but the track will curve through these areas to partially take advantage of them. Finally, the walls will provided that necessary view block to locations beyond the train location which is so important in timetable/train order operation and for creating the feeling of distance on the railroad.

Walter says he appreciates that I was willing to try another of his model railroading “crazy” ideas.

No comments:

Post a Comment