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.
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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