should stair be seismically isolated from building structure? 1

[cmrdata]

I'm reviewing the drawings of an existing 5 story building, located on the west coast, with typical egress steel stairs.
I don't see any special connection detail for the attachment of the stair stringer to the supporting floor beams at each floor landing.
Intuitively, one would think that the stair should be isolated from the building structure itself, as far as lateral restraint is concerned, so it does not change the overall stiffness of the building lateral load resisting systems. Worse yet, it might attract unintended seismic force to the stringer if it was not designed for such force.
I saw many buildings without such "sliding" detail, and I don't recall that the Building Code addresses this issue.
I appreciate any thoughts you might have.
Thank you.

 

[tstructural]

Is the stair framing more rigid than the building lateral system?

I'm not aware of anything in the building code that addresses this but will be checking your post to see what answers you get - Great Question.

 

[MrHershey]

Not stairs, but I've seen it discussed with regards to ramps in parking structures. There has been evidence of detrimental behavior during earthquakes before with ramps in moment frame buildings. Know there was at least one failure in Northridge partially attributed to the ramps acting as part of the lateral system. But those are large ramps for vehicles, no stairs. And believe concern is limited to moment frame buildings, not shear wall buildings which are more similar in stiffness than the braces that the ramps would behave as.

Would think it'd only be an issue if you're a moment frame building and really only if stairs are cast-in-place so they're more like ramps with stair treads on top. And even then would think it would only be real impactful for smaller buildings with long runs of stairs (so squater aspect ratio) and probably would need to be straight runs rather than stairs with midlandings between floors. For your normal short runs of steel stairs the stringers usually aren't that big, would think they'd buckle out of plane pretty easily and then you're just back to your previous lateral system.

Of course if you're truly concerned about it could always try modeling it and see what happens. It's an interesting question.

 

[wannabeSE]

Often times there will be a slotted connection at the slab on grade. I think some engineers may have assumed the steel stairs with the intermediate landings supported by hangers are flexible enough to accommodate interstory drift. I have seen special detailing at each level on newer buildings with special steel moment frames (big drift). The code requirements are in ASCE 7-10 section 13.3.2.

 

[Agent666]

I've always done it because I practice in a high seismic area.

Even trying to accommodate a few mm of interstorey drift can create significant forces in the stair flights, enough to cause yielding of the reinforcement and permanent deformation of the stair flight.

We had a few instances of stairs in the Christchurch earthquakes in New Zealand where the stairs were built in or had insufficient clearances to allow for the required interstorey drift in sliding on a support ledge which lead to collapses.

Basically in the first cycle the stair hit the end of the support and shortened axially as it failed to accommodate the drift, then in the reverse cycle it subsequently fell off the support ledge due to it being too short.

In the case of one building the stairs fell off their supporting ledge and cause a progressive collapse of the stairs flights below over a large portion of the height of the structure. See Link. It was very lucky in this instance that no one was present in the stairwells when the earthquake hit.

Moral of the storey, always provide for the interstorey drift... you are kidding yourself if you think your stairs will accommodate it.

 

[racookpe1978]

And of course, right during and after an earthquake, the power is likely out, the elevators either stuck or out of service, and so the stairs are the only exit from upper floors. And access up for rescue or continued occupant use later in the day. That squeeze-short, then expand-and-fall is troubling. I had thought of that problem before.

 

[cmrdata]

Thank you all for your input and your suggestions.
I gather that there has been no "standard operation" on this subject. For braced frames or shear walls, the presence of the steel pan stair might not distort the building lateral stiffness too much (for more flexible moment frames or concrete ramp, that’s a different story). And I do agree that the stair should remain functional after a major EQ hits and providing some kind of slide connection to accommodate inter-story drift certainly helps.

I’m simply surprised that it was not done, in the high seismic area, more often than it should have been.