When checking out-of-plane deflection from wind can importance factor always be equal to 1.0?

[TDIengineer]

For serviceability, should I = 1.0? I am under California building code which references ASCE 7-10. I am not seeing anything specific in the code that says I can do this. When checking allowed seismic drift you divide by I which essential makes it 1.0 for this check.

I have colleagues telling me it is generally accepted, but I am seeing mixed opinions here on a different thread.

I am designing a horizontal wind girt (HSS) for support of the exterior metal studs and the span is significant. I would like to keep my tube size reasonable and don't necessarily want to use Iw=1.15 for checking deflection.

 

[strucguy]

I couldn't find anything in ASCE 7 that specifically let's you do that. But, you bring up a very good point.

 

[XR250]

When I do metal stud design, I try to use 1.0 for deflection when Iw = 1.15, but most of the time the EOR puts the kibosh on it. Don't understand why. I figure that the 1.15 is intended for safety purposes - not serviceability.

 

[TDIengineer]

Excel Engineering said,"When I do metal stud design, I try to use 1.0 for deflection when Iw = 1.15, but most of the time the EOR puts the kibosh on it. Don't understand why. I figure that the 1.15 is intended for safety purposes - not serviceability."

If I was EOR I wouldn't do that as I agree with you 100%. ASCE needs to improve their discussion on serviceability. It is basically glazed over within the Appendix. I have primarily done healthcare design for my career with I=1.5 and Iw=1.15, I always applied the 1.15 for serviceability as well for wind loading, but really it doesn't need to be there. The connections and members should meet the higher loading, but serviceability isn't life safety.

 

[wannabeSE]

TDI,
It is true that the seismic drift is divided by the importance factor. But Risk (Occupancy) Category IV have more stringent drift limit in table 12.12-1. Also, this is used for drift of the structure not the deflection of cladding.

Also note, the 2010 California Building Code (CBC) references ASCE 7-05. The soon to be published 2013 CBC will reference ASCE 7-10. I think the 2013 CBC will take effect 1/1/14.

 

[TDIengineer]

Wannabe said,"It is true that the seismic drift is divided by the importance factor. But Risk (Occupancy) Category IV have more stringent drift limit in table 12.12-1. Also, this is used for drift of the structure not the deflection of cladding.

Also note, the 2010 California Building Code (CBC) references ASCE 7-05. The soon to be published 2013 CBC will reference ASCE 7-10. I think the 2013 CBC will take effect 1/1/14."

Yes, correct about table 12.12-1. I like how the code removed the importance factor from the drift check and instead capped the allowed drift per that table based on occupancy. Much more logical.

And yes, I got ahead of myself with ASCE 7-10 being adopted... not there yet!
On another note... the code should do a better shop in prescribing allowed out-of-plane deflection for veneer. It's clear the ledger should meet L/600, but out of plane requirements are not discussed. I have seen out-of-plane veneer design limits vary from L/300 to L/600. "Other promulgated deflection limits for brick veneer include L/360 by steel stud manufacturers, L/600 according to the Brick Industry Association (BIA), and L/720 based on Canadian Research."

I like to work my out-of-plane deflection to L/360. Maybe I should start a new thread on this topic...

 

[WillisV]

Serviceability is not a life safety issue and in many cases depends on what the owner is willing to accept. The code is largely silent on many serviceability issues for good reason.

 

[XR250]

@TDI Engineer. I agree on the L/360. Most jobs I work on are L/600 or L/720.
Sometimes we get a bonus and the specs do not call for anything. Then the contractor has me design for L/240 - (brittle exterior finishes)
I was inspecting a house recently. It had a really large gable roof. The end wall was platform framed and you could easily shake it 2-3". I pointed that out to the contractor. They never did anything about it, but once it was veneered, it would not budge.
They feel they dodged that bullet

 

[Hokie93]

In my opinion you can use a reduced wind load for deflection and drift calculations. This concept is discussed in ASCE 7-05 commentary section C6.5.5 wherein it states "it may be desired to use wind speeds associated with MRIs other than 50 years". MRI is the mean recurrence interval. The Commentary to Appendix C of ASCE 7-05 recommends using 0.7W for deflection checks. The paper "Serviceability Limit States Under Wind Load" by Larry Griffis (Engineering Journal, 3rd Quarter, 1993) recommends using a 10-year MRI vice 50 years for strength purposes. Table C6-7 in ASCE 7-05 provides conversion factors for mean recurrence intervals other than 50 years.