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Re: live load to resist uplift from snow load?

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Sorry, I was being incredulous that another designer would want to do such a thing, not directing it at you - one of the pitfalls of email. I assumed that you were of the same opinion that the thought was a crazy idea, and were looking for something in writing to back you up.  I was offering up my response to the designer, not berating you. I do a large chunk of my work for design-build contractors, and they know they'll get the same response from me as I gave below if they suggest something like that.  They want to see that they can't do it in black and white, and I've had similar problems with the code (ex: the IRC does not address full height studs, and all references are to single studs, regardless of width or height).

I know I've lost a few clients to the engineer down the road who will ignore the engineering and wet seal anything that gets put in front of her/him.  It burns me a bit, and I've mentioned the more egregious violations (in passing) to inspectors so that they know which parts of a structure are substandard for when they do their framing inspections. I also have a little mental file for those buildings, as I expect to see them in about 5-10 years as a consulting job for unhappy owners/sellers.
Jordan


Haan, Scott M POA wrote:
Jordan:
 
Engineers tend to be really sanctimonious when they are not working for a design build contractor and they hear about how low a peer who is can stoop. 
 
I told the guy that asked me about it to tell the designer to just do it.  I also found the part in the live load section that says for continuous members to load alternate spans IBC 1607.10   and said to tell them to just load the live load on the cantilever.
 
Respectfully,
Scott


From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com]
Sent: Monday, July 31, 2006 1:01 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: live load to resist uplift from snow load?

You've got to be kidding.  Live load in the uplift case is always zero, assuming there isn't a negative live load case to consider. 

The case you could use from ASCE7-02/ASD is 2.4.1 (3) - D + H + F + (Lr or S or R). Note that there is no live load in that equation, and the quasi-live Roof Live is not combined with Snow.

That and it makes no reasonable engineering sense.  Heck, I'd go so far as to say the D in that equation should be run with a custom "light" dead load so as not to over-consider the balancing effect of an otherwise conservative dead load.
Jordan


Haan, Scott M POA wrote:
I was asked about using live load to resist uplift from snow load. The situation is wood trusses spanning 30 something feet sitting on the cantilevered ends of wood joists.  The designer wants to use live load so that the interior end of the joists do not need an uplift connection and the ends of the beam supporting cantilevered joists does not need an uplift connection.
 
 It seems intuitive to me that you should not use live load to resist uplift from snow because live load is make-believe and can be removed by definition and the live load and snow load would not necessarily occur at the same time.
 
I cannot find a code section or notes next to load combinations  that says you cannot do this.  However, I seem to remember a local home builders floor that started to bow up before  the 2 feet of stacked newspapers could be put on the cantilevered joists.
 
Does the code allow you to use live load to resist uplift from snow?  Does anyone ever use live load to resist uplift from snow or other live load for that matter?
 
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