Drift Limitations, 0.025 Ever?

[MTLoeb]

Hey All,
I'm looking for some guidance on table 12.12-1. More than anything I guess I'm looking for a poll on the matter. Most of my engineering has been outside of seismic country, so my code understanding is pretty academic. What would the limit of a single story RC-II SMF building be for drift?
Asking around so far 100% of the answers have been 0.02. Honestly, I'm not convinced, and I have a hunch that it's more of a way to make sure we aren't blamed for giving a drift that cladding folks can't support. There are NEHRP design guides for 3 story steel offices using 0.025, but NEHRP doesn't get angry RFI's. Am I way off base? Is 0.025 for barns only (as I've been told)

 

[JAE]

Being an international website you might want to post what code or standard you are designing under...thanks!

 

[MTLoeb]

Ya, That's a good point.....
ASCE 7-16, or 7-10 for that matter, and I'm specifically asking about seismic drift. By RC-II I mean Risk Category 2.
Thanks JAE

 

[jittles]

Technically if you have a single story RC II SMF, and you're meeting the "...designed to accommodate the story drifts" line, then your drift limit would be... no drift limit, per the exception in that table.

Practically, without final information on those systems needing to be 'designed to accommodate the story drifts', then I would utilize the .020hsx limit to be cover your bases.

 

[MTLoeb]

Thanks Jittles, That's definitely the consensus so far.

 

[Agent666]

I think you've got to be mindful that your potential saving in structure by pushing drifts out to larger values has as you've illuded impacts on other trades. You might make your design of the main structure more economic, but you make some other persons design effort bordering on the impossible. I think it's an iterative process for the best result for the client.

 

[MTLoeb]

Would you say the table implies that 0.02h drift does NOT require detailing then?

 

[E720]

I will just chime in with my experience. It really depends on what lateral system you are using. If you have braced frames or shear walls from what I have read it is HIGHLY unlikely that the building will ever see 2% drift. As an example this can be checked for braced frames by calculating the yield strain in the brace multiplied by Cd/Ie. I have never seen this inelastic drift even exceed 1%. One good method to determine drift would be to run an elastic analysis and then multiply the results by Cd/Ie -- then use the maximum of that drift and 2%.

 

[MTLoeb]

Thanks E720,
Really only a moment frame or wood shear wall could ever see this drift. I wouldn't say I think its a good idea to exceed 0.025, Actually from a drift induced damage stand point i'm surprised it's permitted beyond risk category 1 or lower occupancy industrial. Although drift as a percentage is more important than total drift I'm also surprised to not find any guidance on absolute inelastic limits they way you might find for vertical movement of beams supporting windows....

Anyway, I was able to poll about 40 engineers through the gram (I'm part of a podcast with a small instagram audience) and it was actually 50/50 on who used 0.025h vs 0.02h. I talked to some industry reps as well who said they sometimes see 0.025, but it's less common. I was getting a bit worried that people interpret 0.02 as the opposite of 0.025...such as you don't have to design cladding/walls/etc for the drift value. So, that's my rant, thanks for the info everyone.

 

[sandman21]

BRBF drift can get up there in value. The answer without anything else is .02, it rare to find an architect who will want to go through the effort to accommodate the design as it is not simply windows or cladding. I did work for an A/E firm that design everything under 4 to item #1 in the table.

 

[E720]

Sandman21 - Where have you seen BRBF drifts being high? I am genuinely interested in what your experience has shown. I would think that BRBF's would have very low drift, even lower than SCBF because the connection region of the BRB's can sometimes significantly increase the brace stiffness, you sometimes see BRB's with a KF factor (stiffness modification factor) of up to 2. One thing to consider with BRB's however is that if you specify a 2.5% drift on your plans or spec then the BRB's have to be designed to twice that or 5% drift per AISC 341 section F4.2. I have never got a good answer on why this is though.