Frame Drift Question

[SteelPE]

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?

 

[JAE]

I believe the 0.7 factor is found in the IBC (not in ASCE 7) and is applicable to Components and Cladding deflections, not MWFRS deflections.



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[P1ENG]

JAE is absolutely correct. Table 1604.3 of the IBC is where the C&C factor can be found. 2009 and older IBC uses 0.7. 2012 and newer IBC uses 0.42 which correlates to the same 0.7 factor with the new ASD load combination wind factor of 0.6 (i.e. 0.7*0.6 = 0.42). Do not apply the 0.42 to ASD factored loads or you are double dipping. MWFRS does not get the benefit of the deflection reduction factor.

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

 

[SteelPE]

I know about the C&C limits found in the IBC however, we have historically used 10 year wind to calculate frame drifts for single story buildings. So if the drift is figured using Vult would and you are looking for a 10 year drift on the frame then would you use 0.42?

 

[JAE]

If Vult and if 10 year then it appears that 0.42 is the appropriate factor.

We don't ever use 10 year on MWFRS drifts typically.

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[bearjew]

We don't ever use 10 year on MWFRS drifts typically.

What do you typically use instead? IMHO, using true 50-year wind for serviceability seems like overkill.

 

[JAE]

50 yr.

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[bearjew]

50 yr.

I meant to say 50-year seems like overkill, typed 5-year instead lol. But interesting. That's above and beyond.

 

[P1ENG]

SteelPE, read the commentary of section L4 in AISC 360. Some very good information there. It seems it is up to the designer. Here are some key points:
[ul]
[li]Common drift limits are H/100 to H/600[/li]
[li]Some designers use 50 or 100 year mean recurrence interval; others use 10- or 20-year mean recurrence intervals[/li]
[li]Use of factored wind loads (nominal wind load multiplied by the iwnd load factor) is generally considered to be very conservative when checking serviceability.[/li]
[/ul]

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[P1ENG]

Similarly, read the commentary of Section C26.5.1: Serviceability Wind Speeds in ASCE7-10 (page 518).

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[JAE]

We've always used the 50 year for drift (i.e. 90 mph service wind) and this was, to me, an unwritten standard for many years.

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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[bearjew]

JAE, thats fair. I just have had trouble getting drift to check on multi-story moment frames with brittle cladding in the past when using 50-year interval. If you have a braced frame or shear walls I doubt its an issue.

 

[P1ENG]

The easiest thing to do in your situation is leave it up to the customer via a paper trail. Since it is a single story building, it is easy enough to size the frames to meet 50-year deflections. Then size them for 10-year. Provide both options to the client and let them choose letting them know what the possibilities are on the 10-year deflections (cracked/damaged cladding). They are more than likely going to choose the cheaper frame, but then you have record of their choice so they don't back to you when things crack.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant