[ChrisM]

Baz, The stairs are generally (see below) 600 mm wide and rise 2.35 metres, with a total of 12 steps including the landing. The key component was the 200 x 10 mm plate that I had rolled to a helix with a radius of 600 mm less than my observatory radius (inner wall). From memory, this was pretty close to a radius of 1100 mm. Of course, the helix has to rise the distance that the stairs do. (So for your obs, if the inside radius of the wall is say 2400, and you want stairs 600 wide, then the helix would have a radius of 1800.)

The sketch that I presented to the rolling shop showed a cylinder of 1090 mm radius, with a band (forming the helix shape) rising from the ground up to the top of the cylinder over a specified portion of the circumference. I've attached the sketch that I gave to the rolling shop.

As it turned out, they rolled it for the wrong hand (ie. did not follow my drawing), and they had to do a fresh one for me. They also had trouble keeping a constant radius, with the result that my stairs reach a minimum width of 545 mm about 3 steps from the top, but they flair out to 800 mm wide at the bottom. At first this seems to be not good, but in reality, the flair is perfect. It gives the stairs a more grand appearance!

The steps are solid hardwood (Vic Ash) 25 x 160 mm. On the stringer, I bolted a 50 x 50 mm angle for each step, and secured the step with a couple of wood screws to each one (from underneath). The outer end of each step rests on a pine batten which is itself screwed to the stud frame.

I welded a flange to the top of the stringer and it's bolted to one of the floor joists (190 x 45 F17). The bottom of the stringer is bolted to the concrete slab with a small bracket. The whole result is very solid.

Some quick pics attached.

Hope that is of some help.

Chris