Building the Pyramids Pt.2 No rise and fall pier.

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Monday morning: I made a shorter, 90cm tall, pyramid adapter but it offered no real advantage. I had made the middle post 4" shorter to fit the screw jack in there but there was nothing to locate the posts or adapter. It was all too unstable and downright floppy! Most of the time it is just the obs. floor joists and floorboards keeping the pier under control.

So now I've made a simple, crossed 4"x4" [using a halving joint] with an alu. top plate. This will rest on top of the pyramid and provide the necessary 4" spacing for a 12" square, thick plywood, top plate..  It will also locate the tops of the pyramid legs to a clearly defined height.

The posts can then be scribed using a guide of known depth and the posts compound mitered to the line. I really ought to fetch a sheet of plywood from the timber yard to clad the top of the pier. At present there is nothing to stop the pyramidal pier posts from rotating and sliding about relative to each other. I will then be able to judge whether further bracing is necessary.

It is already 77F [25C] again so it will be a long, hot afternoon. It was too, as it crept toward 78.3F in the shade. Before going in for a cooling shower and dinner I decided to fix the feet of the pyramid, pier posts. While adjusting the bracket height on one foot I had noticed another had lifted right out of the brackets. A couple of screws per bracket, four per foot, fixed that. Suddenly the quadpod felt very different!

It hadn't occurred to me that the feet needed to be fixed to achieve geometric stiffness. I had assumed the sheer weight involved would, quite literally, hold everything in place. I was obviously wrong.

I had no time left to investigate further. The tops of the posts are only using friction with the cross-shaped marking guide for location. Fixing the plate might have a similar, beneficial effect on the geometric stiffness of the pyramid. Or I could screw through the legs into the cross to achieve a more precise location. Just to see the effect. I have some long, hex-head roofing screws which might do the job. Nothing else I have is remotely long enough except for studs.[All threads] I'll be back tomorrow.

Tuesday morning: I applied two 150x6mm, 6"x1/4" roofing screws to the top of each post at right angles after drilling. The screws locked up the top end quite nicely and re-tightening the footing brackets has made for a rigid pyramidal structure.

There is a natural resonance when the pier is thumped. This is due to the un-damped free length of the 4x4 posts. Cross bracing near the middle should kill the vibrations. Though too much mass should not be added. Or it could seriously lower the resonant frequency and actually make things worse! Having the braces rise in steps around the pyramid should safely remove the fundamental resonance.

Cladding the upper part of the pyramid in plywood will help to damp any vibrations by shortening the free length as well as adding its own extra stiffness by triangulation. I have to be careful not to hinder the free climb by ladder up to the obs. floor. With the ladder now rising through the pyramid there isn't much headroom on the west side if I completely close off the top half with plywood. I could line the inside of the top of the pier as well as the outside. The box sections produced should really stiffen things up.

I still haven't found a suitable and affordable ladder with treads. So ought to avoid making one impossible to climb by obstructing the climb. The ladder's inclination is an important factor in providing safe headroom. I tried changing the angle of my normal ladder and it did not help head clearance at all!

I could climb to the veranda first but that introduces other problems. Door height to the Obs.would be limited. Then there is the matter of weather. Having to clear snow [or ice] off the veranda just to get into the observatory would be a bore. Which is why I have chosen ground floor access to an internal ladder. Snow can lie here for several months in a bad year.

Click on any image for an enlargement.

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