Building the Octagon Pt.13. Beams, rim joists and water levels.

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I think discussion on the CN Observatories forum is leading towards a design consensus. The upper 2x6 rim joists will be doubled to 4 x 6 and screwed together along their length. [90x145] The posts will be bracketed each side with 2x4s to better support these doubled joists. Each completed joint will be plated over inside and out to stop the main support posts from pulling apart under load. The nailing plates main purpose is to control tension loads between timbers. Not to provide resistance to compression, twisting or shear.

I am trying to arrive at an attractive combination of 4"x4" and 2 x 2"x4"s timber brackets. Hopefully one which won't look like an afterthought once attached in a "clump." I am torn between mitering the bottoms of the brackets or making them curved on the bandsaw. The squared off bottom of the 4x4 bracket looks decidedly "amateur" and unfinished to my eyes.

I can see the need for some 45° braces but don't want to start cutting joints. These might actually weaken the structural components locally if it involved removing material to make mortises and tenons.

I bought some clear 1/2" hose to make a water level. This required that I raise my "reference" post slightly to allow the other post's ground clamps to be above minimum height. I was using rather a long length of hose to start with and getting some erratic levels. So I changed to the shorter "half" instead and noticed a single, large bubble. It took some shifting but eventually the bubble rose to the surface of one pipe and vanished. I could then work my way around all the posts resetting their heights via the footing's screws jacks to match the "reference."

An opaque garden hose, with clear end pipes, which is sometimes recommended, would be very difficult to check for bubbles. Garden hose is also rather prone to flattening at bends unless considerably pressurized. Which is why I chose to use thick wall, clear plastic hose instead.

All large bubbles must be eradicated for the level to work properly. The diameter and length of the tube has no bearing on accuracy but the weight increases very rapidly with larger diameter hose. [Pi x r^2 x L] It is best to arrange a length where the middle can rest comfortably on the ground or scaffolding to take up most of the weight. Particularly where the open ends are raised considerably above ground level. A plug in the open end[s] will stop the water shooting out if one end is lowered accidentally. But MUST be removed again for the level to work. Both ends must be maintained at a level close to each other and the sometimes wild "see saw" in levels allowed to subside before the level is "read."

The image shows my method of attaching each end of the hose to the upper rim joists. This is my "reference" end with an arrow showing the meniscus. The level of water in the tube has actually dropped a little since I left it in place an hour before the photograph was taken. This may be just the hundreds of tiny bubbles 'evaporating' over time. The level is extremely sensitive to water being added to top up to a marked line. I simply stuffed a small funnel in the end of the hose while both were elevated and added a little water at a time until the hose was just, full enough. The water [hose] level is a very old device and extremely accurate. Some users like to add some food colouring to the filling water to make the levels more obvious. I had little or no problem seeing the meniscus from across the 3m octagon in bright sunshine. 

My expert advisors on the CN forum keep changing the rules on my octagon building. They want to overbuild the thing compared with the original spec. I'm trying to find an alternative way around this if only to save effort lifting massive timbers while working alone. I suppose I could take a week's rest and then start building again when the design is fully hammered out. I am joking of course and no criticism is intended. I am very grateful that anyone who would try to help me put together a strong and safe building based on their own, long experience.

Click on any image for an enlargement.

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