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My wife asked me if these kind of stairs could be done in our new house:

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The house I'm building is made of concrete blocks (which would be left unfinished in the first floor). Now, is there a safe way to affix the treads to the wall? I know cement blocks are tough, but they are also hollow, and we are talking about holding the whole weight of one or more people on the same step.

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Are you sure the floor is going to be concrete block? Concrete block is used for vertical structures, not horizontal. They won't hold the weight laterally. –  The Evil Greebo Oct 6 '13 at 16:25
    
Hum? I'm talking about the walls of the first floor being unfinished concrete blocks, and not the floor itself. –  Luiz Borges Oct 6 '13 at 17:12
    
OH! My mistake! I missed the part where you said you wanted to affix to the wall.. Oops –  The Evil Greebo Oct 7 '13 at 12:31
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There are concrete block anchors that can hold substantial loads, but only if located within a specific area of solid grouted cells. Anchors into hollow cells cannot hold much weight. Assuming you have a mix of hollow and solid cells, you can't reliably anchor each tread where it occurs, but you could anchor a continuous stringer plate to the wall, anchored where it crosses solid cells. The treads or their supports can then be welded or bolted to the stringer plate.

If the wall is completely hollow your only reasonable option is to run a structural steel stringer along the wall that is supported by other means at each end.

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That's what I thought. Still, a plate can be quite thin and do this job (I guess 6mm/1/4" is enough). Then I could anchor this plate in the top slab, the bottom slab, and in the middle where I have a concrete column. –  Luiz Borges Oct 6 '13 at 20:09
    
Yes, relatively thin. My instinct is 6mm is adequate. Detailed analysis may show a problem with slenderness ratio, depending on span and loads, but being restrained by treads help a lot. Attaching narrow flanges along with stiffeners at supports would completely mitigate this, but it's a fair amount of extra work. –  bcworkz Oct 7 '13 at 19:09
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