Wednesday, February 16, 2011

The scale of things...

Although Doyle, International and Scribner may sound like the names of three different types of bridge designs they are really the names for three different standard lumber measurement systems. If you want to learn more details about them Google - Number of board feet in a log - and read on.

If you have more time on your hands than you know what to do with (maybe one of the reasons you are reading this blog) or want to learn about Burr Arch Truss, Stephen Long's design, Howe or Pratt Truss to name a few. Google - Town Lattice Truss An Appropriate Bridge Technology for Developing Countries - a 2010 MIT thesis by Todd Radford.
Other sources:
US Dept of Transportation 2005 report - Covered Bridge Manual, good terminology reference.
Milton S. Graton's book The Last of The Covered Bridge Builders, a great story and pictures.

If a picture is worth a thousand words then it probably takes 1,000 words to explain it, right?
Well I will resist the urge and let the pictures do the talkin'. If you have questions I welcome them.

I have found in this process (Google - timber framed covered bridges - and click on the site that lists Darlington High School if you want to see the pictures and story on the first 3 bridges) that model and scale building to be the keys to understanding and problem solving before it gets to the real thing. Like Bruce, the student from Black Hawk High School working on this bridge said, "Make a mistake in cutting one of the real pieces and you have to cut down a whole tree to replace it!" He's right you just don't go to the local lumber yard and get a rough sawn 2" x 12" - 20 foot long board.

I have found 1/4" foam core board to be ideal material for model making. You can rip pieces on the band saw for 1/8 scale boards (works out nice as 1/4 represents 2" well). It cuts with a utility knife and can be screwed or hot glued together. You saw the results in the previous post 1/8 scale model picture.

The next step is to scale things up a bit to get more practice and the feel of working with real wood. Drawing and reading the scale plans, laying out the pieces, cutting and assembling them is great hands on work that teaches the important lessons and drives them home because most likely things will get screwed up. Nothing is as easy as it looks. You learn best the things you teach yourself. It is your brain that tells your hands what to do but there is no substitute for actual hands on experience. Finding the balance in the education of individuals is the tricky thing. Because I had some leftover rough sawn 2 x 6 material the model in the pictures above is full scale in board thickness ( 2" ) and 1/6 scale in board width and length. Great learning experience laying out and assembling the larger heavier material. The plan is to have Bruce dis-assemble this 1/6 scale model to "reverse engineer"  the process. Time permitting we may them transport the pieces to South Wayne and reassemble them at the bridge site to "proof out" our plans for the real deal.

Puzzler - Look at the pictures above and make a 3 view Orthographic sketch of some part of the bridge you see.

Tech Vocab - Orthographic, Isometric, View Projection

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