Allowable Story Drift - ASCE 12.12-1, footnote c

[CEmonkee]

Hi -

I have a question about the allowable story drift in ASCE 7 Table 12.12-1. Footnote c has the following:

There shall be no drift limit for single-story structures with interior walls, partitions, ceilings, and exterior wall systems [highlight #FCE94F]that have been designed
to accommodate the story drifts[/highlight]. The structure separation requirement of Section 12.12.3 is not waived.

Can someone explain what "that have been designed to accommodate the story drifts" means? Specifically for a wood structure - what needs to be incorporated into the design?

Thanks for your help!

 

[r13]

The walls, ceilings are detailed/designed to accommodate the drift without failing the strength and service criteria.

 

[CEmonkee]

Thanks r13

 

[JoshPlumSE]

What it really means (to me at least) is that you will have to demonstrate that these elements (interior walls, partitions and such) are able to accommodate the inelastic drift of the structure. That doesn't sound too difficult in concept. But, I can foresee difficulty with some plan check engineers.

 

[r13]

Won't a second order analysis suffice?

 

[JoshPlumSE]

R13 -

Good question. There are times when this (a P-Delta analysis) will suffice. Like if you have an interior column and you need to demonstrate that it can handle the P-Delta effect (which is still usually based on the elastic deflections). This is especially true when considering the P-Big Delta effect and the column is well connected to the diaphragm. Essentially, just a gravity only / leaning column.

However, even in that case there are sometimes questions about whether connections can survive the level of inelastic deflections that occur. And, whether those "gravity only" columns or connection can survive the P-Delta effect based on inelastic deflections (see some of the Northridge failures of apartments over parking). Not nearly as much of an issue with 1 story structures. But, you get the concept.... Let's make sure something doesn't "pull out of a socket" or something and pancake at the inelastic drift level.

That being said, a similar concept applies to non-structural or semi-structural elements. Ceiling supports (i.e. T-Bar hanging ceilings), permanent partitions and such. Even dry wall, cladding and windows and such. Not such a big deal with one story structures, but it can have an effect for non-ductile elements. Just make sure they can accommodate the drift.

BTW, I'm not expert on this subject.... As my design career has been mostly with industrial structures and equipment supports. Even when I was doing OSHPD work, that was a lot of equipment supports too.

 

[r13]

Got you. Thanks.

 

[glass99]

I can tell you as a facade guy that the story drift is a pretty important design criteria for us. Small story drift means we can do nice simple silicone seals, but large movements like you get in LA require much more complex and ugly membrane connections. P-delta would only tell you about your primary structure. There is a really big variation in story drift in my experience, from H/800 to H/50.