Replies

1. 
   Fred_Matthews | Jun 30, 1999 10:43am | #2

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   nice graphic Matt-I used to live in Raleigh, I assume you're in Wake Co, NC. In simple terms, the footing bearing area is proportional to the tributary area, tributary area is just the contribution of loads to any given support. But it is dangerous that for a given tributary load a given footing size applies. Footing size is determined by bearing capacity of the soil. When in doubt about bearing capacity, consult a professional.

2. 
   Guest_ | Jun 30, 1999 12:35pm | #3

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   Matt, nice graphics!could you let me know what software you used to generate the drawing? e-mail me if you could, Thanks, BTW , I too am an HFHer in Concord,NH. Q. Why do you distribute the loads to each post( 1,2&3) so unevenly? I was under the impression that the load was distributed evenly along a simple beam unless there is another applied load.Doesn't post #2 support the same load as the other posts?

   Geoff

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   Guest_ | Jul 01, 1999 02:02am | #4

   *I am quite sure that the center post (of a 3 post beam) bears 2x the weight of the outer two unless the deck is cantelevered out over the outer 2 posts. This is what is indicated by the dotted lines in the drawing. I'm sure there is someone out there (a pe?) who can verify it one way or another.

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   Guest_ | Jul 01, 1999 03:26am | #5

   *Fred:Right you are. The supportable tributary load is ultimately determined by the soil bearing capacity of the soil below the footing. Many rookies don't understand that, given consistent soil (no layers of unsuitable soil), the frost line determines the depth of the footing, and the width (more soil area under the footing is better) and to a lesser degree the thickness of the concrete determines the load bearing capacity. They think that a deeper footing is a better footing - when actually the width of the hole and the thickness of the poured concrete is what makes the difference. I've seen 'em dig footings that were shaped like wedges - deep and small at the bottom - WRONG!Baring bad soil - (in my area, usually dark brown and mushy or light grayish marine clay) the soil bearing capacity is well within what is needed to support the properly sized concrete footing.Here is some info from our CABO code book which can provide some rules of thumb:Clay, Sandy clay, silty clay support 2000 PSF.Sand, silly sand, silty gravel and clay gravel support 3000 PSFSandy gravel or gravel supports 5000 PSF.Etc.Rather than pay a professional 2, 3 or 5 hundred bucks to test your soil, I'd depend on the building inspector to do his job. For those who are not familiar with this process, in our area, the inspector probes the newly dug footings with a ~ 3/8" metal rod, checking to see how easily the rod penetrates the bottom of the footing. In simpler terms, once again in my area, if it's easy to dig, it very possibly bad soil or has been previously excavated. If there is doubt, make the footing wider.In DML's original post he stated that his building code requires a 12" x 12" footing for 40 sq ft tributary load. Assuming 30 PSF Live + Dead load x 40 sq feet would mean that the soil below the 1 square foot footing would need to support 1200 pounds. Comparing 1200 #s to the above soil type list, anything but bad soil will do.Of course, the safest (least liable) route is to tell him to go get a PE. On the other hand, this is the internet! This is free advice! We have no liability! LOLDML - where do you live, and what is the soil like around there? What model code (CABO, BOCA, etc) Is used? Also, could you make sense of the drawing I posted?

3. 
   Guest_ | Jul 06, 1999 06:43pm | #6

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   Our building code determines sizes of footings for decks based on tributary area in square feet. For 36 square feet, the footing would be 8"X16"X6 inches, for 40 square feet, 12"X12"X6" etc.

   How is tributary area determined? Is is span between posts supporting the girder multiplied by length of joists between girders?

   Thanks!

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   Guest_ | Jun 30, 1999 01:42am | #1

   *Here is a basic drawing I made some time back to explain tributary area...Let me know if you have trouble viewing it.