Building the Octagon Pt.14 A new and superior floor design!


My expert advisors on the CN forum have been teasing me with regular design updates to my original octagonal building. They wanted to overbuild the thing compared with the original spec. I'm trying to find an alternative way around this if only to save myself having to lift long and 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 absolutely no criticism is intended. I am very grateful that anyone who would try to help me put together a strong and safe building. I am, after all, enjoying the voluntary efforts of a personal design team built on their own, long experience in the field. Bouncing ideas off one another has resulted in  a better design from my own point of view.

To which I can now cheerfully add a new obs. floor design and support structure from my ever-willing advisers. I get to keep my present octagon structure and the additional 2"x8" support beams are still manageable in size and length. The image shows the underfloor beam structure. To which I am indebted to Midnight Dan a member of Cloudy Nights forums.

By using doubled beams, well fixed to one another, I don't have to lift great weights of 4" thick timber in one go. These beams can also be lifted onto solid timber brackets already bolted firmly in place on the support posts. So no more silliness, trying to fix a heavy piece of timber, while simultaneously holding it up at arm's length!

I knew my shed was rather close to the octagon but the new, extended beam design would interfere with the pitched roof. Easily fixed by curtailing the veranda flooring and supports on a line at the outside of the nearest posts to the shed. The veranda will also stop at the outer post line with the balustrade closing off the extended, front and back "corners." A 2"x8" beam will be supported on timber brackets to carry the overhanging veranda on that side.

The image shows how an extended 8" deep beam would interfere with the shed roof. The thin plank laid on top is just to provide a line of sight for maximum clearance at the top of any projecting beam. Anything lower, than this, soon collides with the roof. Better, I think, to stop the veranda at a beam attached solidly to the posts and running parallel with the shed.

I can start by fixing the 2x8s which butt against the upright posts with 22.5° bevels. This will greatly increase the stiffness of the octagon ready for the next step. Longer 2x8s need to be fixed beside the first beams but passing right beside the support posts to carry the cantilevered veranda. Which needs to be no more than 2' wide. [60cm]

This means I shall have to remove at least some of the existing, upper, 2x6 rim joists. Probably to replace them with 2x8s  to butt against the new beams with 22.5° bevels. First, the posts will need to be fixed firmly together with cross members to avoid the octagon becoming unstable again.

The long 2x8s will need to reach their raised position as easily as possible. So great care is needed not to obstruct a straight, clear lift. Temporarily tying the tops of the upright posts together with timber might best achieve this. I could use the existing 2x6s which are already cut to length and beveled for this. Or, I could keep the 2x6 rim joists in their present positions but shorten them with new bevels. The 2x8 cross beams carry all the weight into the support posts via solid timber brackets. So the 2x6 rim joists are adequate to their task if they are retained.

I could lower the crossbeams to bring the observatory floor to its original height. My own plan was to add the floor joists between the rim joists. Not to add a whole new layer of joists on top. Which would obviously raise the finished floor height by the height of the floor joists. Presumably 15cm or 6" with normal 16" joist spacing.

I need to make a decision about this depending on downstairs ceiling height and the loss of wall height upstairs in the observatory. None of this is truly critical to function but it is worth serious consideration before I start adding the heavy beams. My initial decisions on obs. floor height were a desire for a clear view down to the horizon over the shed's ridge. I have no need of any more in the way of extra height of the obs. floor.

The downstairs floor is now about 8" over average ground level. The underside of the present obs. floor rim joists is 2.32 meters above the ground floor. I think I can afford to lose some height to bring the ceiling height downstairs to 2.1m or about 6'7".

Which leaves me with the decidedly, non-trivial option of adding all of the cross beams without having to remove the present, upper rim joists. I can lift the beams onto their brackets so that they just clear the undersides of the rim joists. The octagon of 2x8 rim joists can then be fixed in place. Once the beams and heavier rim joists are all safely in place the present 2x6 rim joists can be removed. So that the parallel floor joists can go on top.

Just having a solid platform to observe from would be a huge improvement on standing at the bottom of the present, green "well." With the only views of the Moon and planets visible from upstairs windows at bedtime.

Click on any image for an enlargement.