View single post by jtrain
 Posted: Sun Dec 29th, 2013 11:19 pm
PMQuoteReplyFull Topic

Joined: Sun May 27th, 2012
Location: Missoula, Montana USA
Posts: 1006
I've made a break through! After thinking about Martin's and Bill's comments, that got be thinking on how to better transfer weight from the valance to the layout deck and under frame. The main problem lies in the supports to hold up the valance. Originally I wanted to use cut plywood brackets in a squared off C shape to use as ribs that would support the backdrop, layout deck and valance all in one while simultaneously transferring weight from the top to the bottom effectively. Plywood alone is not strong enough to do this, especially with as much as 60-70 pounds of additional weight on top (assuming 1 module weighs 35 pounds and the modules are stacked three high in transport).

Here is how my thought trend has been going:

My idea -> brackets -> Bill's idea -> Martin's idea -> suspension -> bridges -> bridge types -> truss bridge -> truss frame -> truss bracket !BINGO!:!:

Finally get to use the light bulb icon!

Below is what I'm thinking of for brackets:

To keep weight down while maximizing stability and strength, Railroads as well as most other forms to transport utilize a truss bridge design for. Beam bridges are good for short spans, suspensions bridges are great for long spans, but truss bridges combine aspects of both.

One thing to consider is that there are three main types of forces acting on any bridge: tension, compression and sheer.

Tension is a force that attempts to bend or stretch a material in the direction of gravity. The ropes that hold up a swing are under tension.

Compression is a force that attempts to break or squish (for lack of a better word) a material. Columns are under compression, holding up a roof.

Sheer is a force that attempts to break a material along a line perpendicular The 'pin' in the link and pin coupler was always under a sheering force.

A rope or cable can take tension better tan most materials while wood products take compression better than most materials. Plywood, due to it's cross grain strength, can take both compression and tension better than dimensional lumber. Metal is best under a sheering force, or under tension.

Since I don't work with metal, that's out. Rope has no use for be either. So I'm stuck with wood. The key is to minimize any tension and turn it to compression. My idea is to make a sort of truss framework out of 1x2 or 1x3 and some 2x2 pieces of lumber which will transfer forces so that the majority of the lumber is under compression. The green represents compression and the red represents tension. All that force, when exerted on screws in each joint, will be under a sheering force. Since screw are metal, they will withstand sheering quite well if given a good hold and some glue.

I then cut out two pieces of thin plywood and sandwich the framework between the two pieces to make a solid bracket that is quite strong.

With that, I can simply add a light frame made out of plywood around the brackets to get my module. This should hold significantly more force than a simple bracket made out of plywood or dimensional lumber alone. Plus, the bracket has nothing sticking out making the shape awkward.

Each bracket is aligned as so, each connected to the others with plywood. To stiffen up the structure, I can add 1x2 supports where necessary along the plywood.

Here is a cross section of the module with the foam, backdrop and the plywood skin (grey perimeter).

And the full assembly, painted and ready for scenery:

So what does everyone think?

Again, Martin's and Bill's ideas are back ups, bu they are good ideas none the less. I will have to build one of these brackets to see how well they hold weight.


James W.

New Blog (permanent this time)
Close Window