Straw Bale Construction/Techniques/Pushing the Limit
Pushing the Limit
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Arches and vaults
Designing the catenary arch
A catenary shape helps you build the sturdiest vault construction.
The gravity approach
You will need:
- a heavy rope at least as long as the circumference of your arch (thin metal wire rope would be ideal)
- an even, rectangular wall at least as high and wide as the arch you want to build. Hint: you can make one out of plywood, pallets or anything you want.
- screws and a screwdriver (or nails and a hammer)
- a wide roll of paper
- a spirit level
- spray paint
- If you haven't got the required wall, build one from lightweight materials. Just make sure it's perfectly vertical and level.
- Tape the paper to the wall. Hints: Vertical is easier. Have them overlap a few centimeters. Number the segments.
- Mark a level line on the paper a bit higher than the arch you want to build. Double check the line is level.
- Mark a second line on the paper exactly as far down as the height of the arch you want to build.
- Mark two points on the top line as far apart as the width of the arch you want to build.
- Fix the rope to one nail, as close to the paper as possible. Lower it from the other nail until it reaches the base line.
- Spray paint the rope. Hint: You may also want to quickly spray paint the overlap between paper rolls.
The mathematical approach
You will need: a computer, a pencil, some exact measuring skills, a roll of paper, a long measuring device, adhesive tape, a flat surface the size of your arc.
- Calculate the coordinates of your catenary. You can either use a catenary designer like https://github.com/ddjokic/pyCatenary-NoElast/blob/master/pycatenary.py or http://www.cgl.uwaterloo.ca/~smann/catenary/ , or use the cosh(x) function yourself.
- Tape the paper together to form a big paper surface that could cover the entire catenary's surface.
- Start out by setting out the triangle defined by the top and the two base points of the catenary.
- Precisely mark the coordinates of one point every 50 cm approximately. Keep in mind that the precision of this drawing is key to the stability of your vault, so take your time.
Building the formwork
You will need:
- planks. They don't need to be thick, but they do need to be high. Recycled pallet planks are a perfect fit.
- a saw. A circular saw will work best, but a hand saw will do.
- nails and a hammer or (preferably) screws and an electric screwdriver
- your catenary arch design
- straw bales (if available)
Let's start building the arches:
- Put the paper arch design on a flat and dry surface (the floor?).
- Put the planks on the inside of the arch design. You will have to saw the planks a bit for this, especially at the top.
- Put a second layer of planks on top of the first one. Screw them to the first layer. The idea is to have them overlap with the first layer to get some basic rigidity.
- Mark every 25cm on the outside of the arch. This is where the horizontal battens will link the three arches.
- Reinforce the arch by screwing two planks in an X shape to the inside of the arch. These should also make carrying the final formwork easier.
- Build three arches by repeating the above steps three times. The arches should be almost identical in shape.
- Measure the length of your bales.
- Space your three arches with a distance of one bale's length between each arch.
- Hint: In some cases it might be easy to: lay the first arch on a flat and dry surface, put bales on top as spacers (keep the length side vertical), lay the second arch on top of that, lay another row of bales, lay the third arch.
- Triple check the arches are all 100% level and perfectly aligned.
- Screw the battens to the three arches, removing any spacer bales as you proceed.
The following link can help: http://minke-strawbaledome.blogspot.com
In France, at least three "paligloo" straw/pallet domes have been built: http://paligloo.free.fr .