insulated slab interior load limits

[solarmass]

I'm considering making a rock/cement stairway instead of typical wooden stairs in order to add interior mass to our house, but am concerned that the slab won't support it, since we put rigid foam and radiant heat tubing under it. The staircase would rise 10 feet at it's highest point. Any advice? Thanks, Solarmass

 

[boo1]

If the slab is not designed for it, then dont do it. The foam will compress and the slab crack. Typically slabs are reinforced to 8-12 inches with #4 rebar at 12" oc both ways for masonary loads.

 

[jheidt2543]

Check the specs on the insulation, some of the rigid insulation boards have pretty high compressive strengths. You may be able to distribute the stair load over a large enough area to make it work.

 

[BruceSol]

What is the slab thickness and foam type/thickness?

 

[solarmass]

The slab was supposed to be 6" thick, but after the blue board insulation and radiant floor heat tubing, and I'm afraid it is more like 3" of actual concrete and wire mesh. I'm inclined to take Boo1's advice. Per recommendation from jheidt2543, it looks like blue board is 25 psi, and at 150 lbs/sq ft for rock/cement, that would be roughly 100 psi for the max 10 foot high staircase. If there is still some hope to do this, let me know! Thanks to all of you for your input. Sincerely, solarmass

 

[boo1]

Solarmass, the stone masonary stairs will act like a point loading on the stair. Typically the 3000 psi concrete mixes is residential are less the 2000 psi. The radiant tubing adds stress concentrations to the slab. As i stated before slab cracking and movement is likely. The risk is not worth the gain.