Seismic relative displacement ASCE-7 Section 13.3.2

[Gnique]

I have looked into this topic quite a bit before I came here to bother everyone with a question. As I understand Section 13.3.2, the effects of relative displacement must be accounted for. What I can't figure out is how do I go about determining the stress induced by deflection in a very complex assembly like an egress stair system. So, the building moves some amount and forces the stairs to move and thereby induces stress into the stair assembly. What is the induced stress? Don't I have to know Moment of Inertia to derive stress from deflection? Thank you for your help. Nicholas

 

[Mjkkb2]

I believe this paragraph has to do with serviceability issues of non structural components during earthquakes. It is not really addressing stresses on the components, just the expected movement of it due to the structure moving. Any potential stresses caused by the movement are in my opinion beyond the scope of this section of the code.

 

[MrHershey]

I've typically seen this applied mostly to glazing or other elements at the building perimeter that could fall on people below if dislodged. The idea is that the connections and perhaps even elements themselves need to be able to accommodate the story drift without dislodging. Don't necessarily need to withstand the inelastic deformation fully intact (I'm not expecting my windows to remain pristine in a large scale earthquake), but shouldn't become a fall hazard.

The way the code is written, you could in theory claim this should also apply to other architectural elements (pre-engineered stairs, interior stud walls, etc.), though I don't think I've ever actually seen it considered for most elements beyond just designing for their own seismic mass. Though I've worked in a few seismically active regions, I haven't worked in California. Perhaps they're more strict.

 

[wannabeSE]

I have analyzed stairs using a forced-displacements to calculate the demand caused by inner story drift. These demands can be huge and these forces need to transferred into the building. Typically, it is easier to design connection that slip to accommodate the drift. It is critical that the stairs remain operational after an earthquake so the occuopants can evacuate. The design must accomodate drift parrallel to the stair run and perpendicular. If the stairs have a straight run between floors with fixed connection at each end, they can act like a braced frame and change the buildings response to seismic.

I am guessing that drift is not considered in design for many stairs in California. It depends on the jurisdiction whether or not plan check even looks at the stair structural design. Some definitely do, and I think it is becoming more common. Egress stairs are oftentimes a deferred submittal and left to the contractor to hire an engineer for the design.

 

[SocklessJ]

I'm dealing with a similar issue right now. I was trying to figure out how to design the stairs in an industrial steel structure.

I think typically stairs are left out of the structural model, since they're not considered structural steel. People also don't like their stairs picking up lateral load. But in my mind, unless the stair has a sliding joint detail, it WILL pick up lateral load in reality. And we should design for that load, so that the stairs remain funcitonal after a seismic event. This seems to be the intent of the ASCE 7 Chapter 13 requirements for relative displacements. I'm not sure how you satisfy that if you analyze your stair seperately, unless you apply a forced displacement (which I doubt many are doing).

I could detail a sliding joint, but this would raise some eyebrows in an industrial structure. My solution is to use the method now described in Section 13.5.10 of ASCE 7-16. I included the stairs in the structural model, and did my code check based on my overstrength load combinations. The reactions I give to the connection designer will also be based on overstrength loads. Sure, some of my stringers are picking up lateral load, but that just forces me to stiffen up my system.

 

[WARose]

I've been up against similar problems when it came to stair support myself. One trick I typically used to handle it was make the stair support framing very flexible. I.e. something like kickers for the frame [LFRS] and slotted connections where the first stringer(s) attaches to the slab.

Just make it more/as flexible than the building itself and it should go along for the ride.

 

[Mjkkb2]

It is a standard practice where I am to delegate the stair design to the contractor's engineer. I gotta be frank - I never thought about this on my previous projects. However, thinking about the potential failure mechanism, the stair must either:
1. go along for the ride (move with the structure)or,
2. take on some of the lateral forces.
IF the latter occurs then it would either start yielding; develop cracks etc... which in turn would relieve it from the seismic forces or if it was strong enough, transfer the forces applied to the foundation or other lateral system members. I guess there is a possibility that it could fail but seems to me that if there are load paths (lateral systems) for the seismic load to make its way back to the soil, then there's no need to check all components (not part of the lateral system) just because they have some stiffness and may attract lateral load.
What are the panel's thoughts on this reasoning?

 

[JoshPlumSE]

To me, the interface between steel stair systems and the structure is an issue that isn't identified enough in design. I don't know if it's the delegation to subcontractors or what. But, if the stairs are not isolated from the structure in some way (like with slotted connections like WARose suggested), then you run the real risk of the stairs absorbing a lot of load from the main structure that they weren't designed to resist. And, the last thing we want is to get significant damage / failure in the main means of egress from the structure.