Exterior Wall Metal Stud Design Wind Loads?

[StructuralEd]

Doe the IBC allow reducing wind loads on metal studs for deflection checks?
A vendor claims that the code allows it but it seems counter intuitive to reduce a code derived loading and then apply the reduced load to a specified deflection.

 

[spats]

Yes, the IBC allows it. 2009 IBC, Table 1604.3, footnote f. says "The wind load is permitted to be taken as 0.7 times the "component and cladding" loads for the purpose of determining deflection limits herein."

 

[msquared48]

Probably to account for increased I due to composite action from attached materials.

Mike McCann, PE, SE (WA)

 

[StructuralEd]

Thanks for the replies.
I'd like to find out the reasoning behind the load reduction.

 

[XR250]

I thought it was due to 10 year wind versus 100 year wind for deflection calcs

 

[MrHershey]

Not quite 10 vs 100, but right idea. Don't have 2009 commentary on me, but from the 2012 commentary:

There are two aspects to this adjustment. The first is the conversion from a wind speed with a 50-year mean recurrence interval (MRI) to a 10-year MRI event at an allowable stress design level. The mapped wind speeds of the code have generally been based on an MRI of 50 years. Serviceability checks, such as deflection, have typically been based on a lower MRI (for example, 10 years). The ASCE 7 commentary to the 2005 edition provided factors to covert (sic) to wind speeds with MRIs other than 50 years in Table C6-7. For wind speeds between 85 mph and 100 mph, the factor for 10 years in Table C6-7 was 0.84. The conversion applies to the wind speed, V. Since design wind pressure is a function of V², the conversion factor must be squared before applying it to the design wind pressure. The factor 0.84 squared is 0.7056 which is rounded off to 0.7, which was the factor given in previous editions of the code.

Then goes on to indicate the second aspect they reference is the switch from allowable to ultimate level for wind. Reduction factor in 2012 IBC is actually 0.7x0.6=0.42.

Interesting to note that that factor increase as wind speed increases. For my area the base wind speed is 105 mph, which has a factor of 0.74 to convert from 50 year to 10 year. It's not codified this way, but following their logic I could argue for an even bigger reduction of 0.74x0.74=0.55 (versus 0.7 for lower speeds) on my components and cladding loads for deflection checks.

 

[StructuralEd]

10 years is not a conservative recurrence time to base a building design on.
It seems that the trend around here as of late is more severe weather more often.

Thanks again for all the information.

 

[MrHershey]

10 years is for serviceability, not strength. You still have to design for the full 50 year load from a strength standpoint. And the reduction only applies to components and cladding wind loads, not main wind force resisting loads. Idea is it doesn't make a ton of sense to look at fifty year return period when the consequence of exceeding these deflection limits is typically just light damage to finishes and other relatively (in comparison to the building structure) easily replaceable elements.