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RE: Live load reduction and load combinations

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Title: Live load reduction and load combinations
I would use the 3/4 number.  Wesley described the reasoning correctly.  The 3/4 is for multiple transient loads.  It is very unlikely that both peak conditions occur simultaneously.  It is even less likely that multiple transient and full uniform live load would occur simultaneously.  You must still check the condition with 100% reduced live load alone.  As already, I also believe this will control your design.
 
Just for kicks, look at the LRFD load set.  You will see that you are required to check one full transient load with one or more signficantly reduced transient loads.  The reduction ratio is even larger.  You go from 1.6 with LL to .5 Lr or S.  If you take .5/1.6 you get less than 1/3.  Would you design for unreduced LL in the LRFD set?  I know I wouldn't.
 
Happy Holidays,
Jake Watson, P.E.
Salt Lake City, UT
 
-----Original Message-----
From: Bruce Holcomb [mailto:bholcomb(--nospam--at)brpae.com]
Sent: Tuesday, December 21, 2004 1:20 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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