Seismic RC Stair Design

[Trenno]

I'm currently designing several multi-storey RC buildings within a high seismic region (PGA = 0.16 m/s^2).

In all cases the lateral stability system consists of isolated and grouped RC cantilever shear walls.

I want to confirm my train of thought is correct - if all egress stairs for the building are contained within a shear wall core arrangement, the forces acting upon the stairs will be minimal due to the fact they would move with the stair core. Therefore special detailing of movement joints to isolate the stairs from the base structure wouldn't be required?

I understand that in sway/moment framed buildings the detailing of RC stairs is much more critical, in that they can compress/buckle or even slip off the end of the corbel.

Canterbury Earthquakes Design Guidance Report
Reference Paper
NZ Earthquake Stair Checklist

 

[JAE]

I think you are correct as long as the story-to-story drift amount within the concrete shear walls are considered.
They will be fairly stiff, of course, but there will be deformation in them and the resulting drift.

This would just fall under deformation compatibility checks.

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[KootK]

if all egress stairs for the building are contained within a shear wall core arrangement, the forces acting upon the stairs will be minimal due to the fact they would move with the stair core.

I think that there may be a logical error here. I'd expect the forces developed in the stair to be the same whether they were in the core or thirty feet off to the side of the core. They just respond to the inter story drift as JAE suggested. The stairs are not shielded in any way because they are located within the core. That said, the forces will tend to be lower in a shear wall building than in a moment frame building. That is simply because you get more drift in your average moment frame building however.

I've yet to see anybody detail movement joints in RC stairs located in RC shafts, even in very high seismic regions. My gut tells me that, under a design seismic event, you might get some stair damage at the floors nearest the shear wall plastic hinging but that that damage wouldn't render the stairs unfit for their intended purpose.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.