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# Re: Tributary Area

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Tributary Area
• From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
• Date: Thu, 11 Aug 2005 14:32:21 -0500

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.
This whole tributary area thing drives me nuts. The way Mother Nature distributes statically indeterminate loading is by stiffness: Load follows the hard points. Support a rectangular panel with two soft beams on opposite sides and two stiff beams on the other sides and load is pretty much split between the stiff beams. Make the panel stiffer than the supporting beams and load goes to the corners. I'd expect that's even more true for ultimate loading where one or two of the supporting beams goes plastic and can't support any more than the limit load. I rally have a tough time seeing how all the micrometrically precise calculation that goes into figuring loading from tributary areas even comes close to reality.
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That said, the barn my brother rebuilt was probably designed using tributary areas to figure out load paths, and it's been sound for 100 years, and probably will be for another 100. It'd sure make me nervous, but not nervous enough to postpone a bike trip up there this month.
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Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
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.......................................| John Sedgwick, Spotsylvania 1864)
```http://www.skypoint.com/~chrisw/

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