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RE: Tributary Area

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I hadn’t noticed 1607.6 before.  For my case, the two methods give nearly identical reductions: 0.90 for Section 1607.5 vs 0.88 for 1607.6.  Thanks.  


Nels Roselund, SE

South San Gabriel, CA


From: S. Gordin [mailto:mailbox(--nospam--at)]
Sent: Thursday, August 11, 2005 11:21 AM
To: Seaint@Seaint. Org
Subject: Re: Tributary Area




UBC1607.5: A=area supported by the member, i.e., 1/2 of the span of the purlin for the edge beam, A>150.  

LL reduction for A>150 sq. ft. is R=0.08*(A-150).


For A=300 ==>R=12%




UBC1607.6 (ASCE7 similar): AI=influence area, twice the tributary area for the beam,  i.e., AI=2*A, AI>400;

Reduced live load: L={0.25+15/[sqrt(AI)]}.


For AI=600 ==> L=0.86 (effective LL reduction R=14%).



Both methods give similar results, but appropriate formulas and widths should be used.




V. Steve Gordin, PhD
Registered Structural Engineer
Irvine CA




----- Original Message -----

Sent: Thursday, August 11, 2005 9:20 AM

Subject: Tributary Area


What is the precise definition of Tributary Area for figuring live load reduction?  The UBC defines it as the area of floor or roof supported by the member.  If a beam supports one edge of a simple-span floor, is the tributary areas the span of the beam times width of the floor, or is it half that area because the beam supports only half the floor [the other half is supported on the other edge]?  I’ve seen it done both ways, and have done it both ways myself.


Nels Roselund, SE

South San Gabriel, CA