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How strong is this spiral staircase?

The short answer is, I don't know, but I have learned what they can do. In 1984 when I built my first spiral stair for my home I also had some concernes about the strength. If you look real close at the picture below you'll notice some steel washers I made to prevent any chafing between the spacer pipes and the treads. Before building this stair I'd assumed the treads were going to be moving up and down a little with use. As it turned out there wasn't any movement as soon as the handrail was installed. The washers were a waste of time, and I haven't used them since.
Home stair
The best I can figure is that the handrail and balusters act as a helical truss supporting all the way around the staircase. I've had a half a dozen people on this stair several times and still don't have any problems with it. Click on this small picture to see nearly a thousand pounds of use.
Spiral stair picture

With each tread being attached to the treads on either side of it, and also restrained by the handrail and balusters, I'm thinking there's just too much to displace for any single treads movement.

Once upon a time my basement was extreamly damp and it caused the treads to go through some rather drastic changes. I had a couple of the treads split on their small ends right into the center pole hole, but this has had no effect on the stairs integrity.

Although it isn't shown in this picture, I have a rather odd mounting under the edge of the loft, the center pole is actually holding a beam there. After all these years I can turn the top couple of spacers, but I don't have any movement in the treads when going up or down. At first glance people think the treads are cantelevered off the center pole, which they do look like, but they're really held in place by the balusters and handrail truss attached to their ends. The one movement I have seen is when the stair is over about twelve feet high, or six feet in diameter, there's a slight side to side displacement. The center pole isn't restrained side to side except at its ends so there is a little movement. The fix for this has been to fill the center pole with concrete which makes it extreamly solid in that axis.

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