spiral stair support

[mcmook]

Any advice re supporting a spiral stair. The stair will be quite large (Class C diameter approx 1900mm) and likely to be a steel construction.

I was planning to just span 3 joists with 18mm ply and 1 1/4" floor boards on top in a square 900mm (which would make up the height i need to be flush with the final floor) Joists are approx 7" x 2" and at

450mm centres.

But belt and braces... should I be thinking about putting in a concrete footing between 2 bridled joists. Since most (all?) the weight is on the central colomn.

[dom]

Timber domestic floor designs are usually based around a 150Kg per sq metre static loading. I'd imagine a steel spiral staircase would be more like 1000Kg.

Haven't you/your structural engineer had to specify the supporting structure (with detailed calculations) to Building Control?

[John Rumm]

Is it starting on the ground floor? Is that solid or suspended? The base of the staircase will be where the load in concentrated.

[mcmook]

The stair will start on the ground floor. Its a timber suspended floor - though its not really suspended more 7" x 2" joists laid on bricks every metre or so directly on the solum.

[John Rumm]

It sounds like there that you will need to focus your efforts then, make sure the floor at that point is strong enough to carry the stair. It might be that you need to strengthen the floor, or alternatively find a way to transfer the load to the sub floor/brickwork. Do you know how much the complete stair will weigh?