Frame Drift Question
Frame Drift Question
(OP)
I have an extremely small single story structure that is utilizing moment frames for a LFRS. The code is IBC 2015. I have calculated the wind loads and estimated the drift for the moment frames. I now have to figure out if these drifts are in compliance with engineering standards. Using ASCE7-05 I would calculate my drift ratio's usig a value of .7*(frame drift under full wind). Now that we are using ultimate wind loads I suppose I should be using 0.6*0.7*(frame drift under full wind) or .42. Is this correct?






RE: Frame Drift Question
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RE: Frame Drift Question
The 0.7 factor takes your mean occurrence period from 50 years (Risk Category II) to 10 years. The code is more lenient on damage to components and cladding, but you certainly don't want your MWFRS experiencing that damage.
Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant
RE: Frame Drift Question
RE: Frame Drift Question
We don't ever use 10 year on MWFRS drifts typically.
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RE: Frame Drift Question
What do you typically use instead? IMHO, using true 50-year wind for serviceability seems like overkill.
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RE: Frame Drift Question
I meant to say 50-year seems like overkill, typed 5-year instead lol. But interesting. That's above and beyond.
RE: Frame Drift Question
Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant
RE: Frame Drift Question
Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant
RE: Frame Drift Question
The commentary in ASCE 7-10, for Chapter 26 (C26) discusses this.
Part of it all depends on the nature of the building (occupants, use, cladding, etc.) and is left to we engineer's discretion with very little to go on.
We've just leaned back on the 50 year unless we have struggles with that (usually don't) and then if we do we might look at something less onerous.
AISC had their nifty table that they published some years ago that dealt with drift limits related to exterior cladding (AISC Journal, 1st Qtr, 1993)
They also put this information in their Design Guide 3.
Here's a short snippet of their commentary on return period:
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RE: Frame Drift Question
RE: Frame Drift Question
Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant