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RE: Live load reduction and load combinations

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Bruce,
 
    From what I see in the code, you are applying the factors correctly. Under the load combinations, IBC says that you can add 3/4 of two variable loads to the dead load as long as they aren't less than one of loads by itself. The live load reduction factor applies to members with large tributary areas, because the whole area isn't likely to fully loaded. The 3/4 factor is applied to two variable loads because they aren't likely to both be full strength simultaneously. For instance, what is the probability that the footing will see the full 50 psf live load over the entire 2,000 sq ft of floor and have the design snow load at the same time? It appears to me that the controlling load case could actually be D+L, rather than D+.75*(L+Lr or S) unless your roof tributary area is greater than your floor tributary area or your snow load is 17 psf or greater.
 
Wesley C. Werner, EIT
-----Original Message-----
From: Bruce Holcomb [mailto:bholcomb(--nospam--at)brpae.com]
Sent: Tuesday, December 21, 2004 3:20 PM
To: seaint(--nospam--at)seaint.org
Subject: Live load reduction and load combinations

I just had a discussion with a co-worker and want to be sure of how I am designing.  I am designing an interior footing supporting a column with a large tributary area (2000 sq. ft.).  My reduced floor live load is 50% of the unreduced live load (100 psf in a non-assembly area).  I am using a reduced roof live load of 12 psf.  When I check Dead + Live + Roof Live, my load combination is D+3/4(L+Lr).  I am checking other combinations too, such as D+3/4(L+S).  I?m using IBC 2003.

My question is, can I apply the ¾ load combination factor on the reduced floor live load and the reduced roof live load (or snow load) My co-worker says that I am effectively using less than the minimum reduced live load which is 50% of the unreduced live load (3/4*0.5*L=0.375*L).  But the way I read it, these are two separate issues and I am not violating the code.  First, you calculate the reduced live load, checking the maximum reduction, then you put that load in the load combinations and design the footing for the result.

I may simply be having a brain-lapse, so please be kind? I also have had a lot of sugar? cookies, cakes, etc., even a honey baked ham today!

 

Bruce D. Holcomb, P.E., S.E.

Structural Engineer

Member AISC, SEAKM

Butler, Rosenbury & Partners

300 S. Jefferson, Suite 505

Springfield, MO 65806

ph. 417-865-6100

fax 417-865-6102

www.brpae.com

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