connection between concrete wall and staircase

[n3jc]

Im dealing with an outer staircase which leads to basement. There is a concrete wall that is enabling this. What is a normal/usual connection between the wall and staircase?

The wall has to be build first I guess since it enables a room to even build staircase?

But after the concrete wall is standing is there any connection between staircase and wall? (dowels/rebar from the wall to the staircase) or nothing (i think some cracking will apear).
Maybe some thin (1 cm) thermal insulation as dilatation between?

Thoughts?

 

[SteynvW]

1) Build the wall first
2) Design the staircase to span in its length.
3) If the stair fixing conditions are relatively rigid
you can probably keep the staircase apart, otherwise
you should dowel into the wall.

My 2cents!

 

[TehMightyEngineer]

My $0.02: Do it out of precast (but I'm biased) and then it can slide against the wall all it wants. For something like this we would pour the walls and that lower slab first, and then lay the precast stairs on a bed of crushed stone. I'd use the lower slab as a thrust block and let the stairs float between the two walls unconnected. I would expect the soil lateral load on the walls would keep any gaps naturally closed but still allow for thermal expansion and/or shrinkage.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[XR250]

Teh,

Are you pre-cast stairs sloped on the bottom? If so, how to you get a sloped bed of stone to fit?

 

[n3jc]

staircase span is pretty big (long) so I should probably use dowels (design them based on shear force?)

I have another question - what about support conditions in stairacase model? Should I use pinned supports or fixed at both ends of staircase (staircase will be doweled in foundation at the bottom and in the concrete wall at the top with rebar).

 

[TehMightyEngineer]

Are you pre-cast stairs sloped on the bottom? If so, how to you get a sloped bed of stone to fit?

They are sloped on the bottom. I typically design them to span the full length (from end to end) as we assume the contractor can't always get the crushed stone to be perfectly support the stairway. Often we'll dowel into a stem wall at the top and leave the bottom floating freely (pin-roller supports if you want to ideal it that way n3jc).

Some examples:

Here you can see we supported the ends on stem walls. If I recall correctly this one was free-floating with dowels connecting the stairs into the curbs to tie everything together. We used the weight of the stair to hold everything in place.

They got the rough profile of the crushed stone and then packed in what they could at any low spots under the stairway. You can see they also had to grout a bit at the top to make up a gap.

Our historical performance for our precast stairs has been quite successful, not very often we hear about any issues. I imagine a local precaster for the OP might be able to get similar quality.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[XR250]

That would seem to work in that particular situation. It looks like in the OP's case, he has a wall on each side which would not allow post placement packing.

 

[TehMightyEngineer]

Agreed, this is why I typically assume full-length span and any additional support by the crushed stone is a bonus. Overall the reinforcement difference is negligible, especially for precast where I have the lifting and handling loads as well.

I remembered we also had a wall similar to the OP's recently (in a stairwell) and for that one the architect specified a matching sloped surface cast into the concrete walls so the stairway could span the short direction from wall to wall. I'll have to dig around to see if I have any pictures from that job.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com