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Weight Limit

Rickk | Posted in Construction Techniques on August 10, 2008 04:48am

HI All,I have a question. I am building an observation deck in my back yard,and have poured 4 footings 5′ deep with 1′ being set concrete and 6’x6’x16’PT pine sitting on each poured  footing x4plcs.  My Question is how much weight will each post support? Each post is 4′ in the ground.  Thanks Rickk

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  1. dovetail97128 | Aug 10, 2008 05:40pm | #1

    Before anyone can really give an accurate answer to your question they would need to know the soils bearing capacity on your site or else use a "minimum assumed bearing capacity".
    An answer can be given using an assumed bearing capacity but that varies by region. Here, where I am, that minimum assumed soils bearing capacity is considered to be 1500# psf.

    Some one from your area may be able to post what the assumed capacity is in your area.

    They can't get your Goat if you don't tell them where it is hidden.
    1. User avater
      Matt | Aug 10, 2008 07:30pm | #5

      2000 psf is minimum here.

      Is it red clay soil or what?

      Edited 8/10/2008 12:31 pm ET by Matt

      1. dovetail97128 | Aug 10, 2008 07:47pm | #6

        Clay is what we usually see unless you get up into the hills here, color varies.
        Soils in the county I have worked in for the last 35 years vary a lot. River bottoms near Willamette River are deep soils, areas not around that river bottom land are a real mix of clay , "weathered basalt", and in some places "hard basalt" rock.
        Each site can contain any of the above so code minimum and what structural engineers use when they do not have a soils test is the 1500# psf number.
        They can't get your Goat if you don't tell them where it is hidden.

        1. User avater
          Matt | Aug 10, 2008 10:00pm | #7

          sorry DT - I ment the "red clay" thing for the OP.  My bad.

          1. dovetail97128 | Aug 10, 2008 10:03pm | #8

            I should have known , even here we have heard tales of your "red clay" !!
            ;-)
            They can't get your Goat if you don't tell them where it is hidden.

          2. User avater
            Matt | Aug 10, 2008 10:28pm | #9

            Actually, red clay makes a very good bearing surface for footers as long as it is undisturbed.  .  My own house is built on it.  The footing contractor thought I was insane when I had him put three #5 rebar in the footers.

  2. frenchy | Aug 10, 2008 06:07pm | #2

    Rickk

      That also needs information such as what species wood will you be using and what size..

      My 6x6 white oak timbers will each support 10 tons.  Shorter would carry even more.. Please note these are actual not demensional sizes.. adjust accordingly for demensional sizes if purchased from a lumberyard etc..

  3. User avater
    BillHartmann | Aug 10, 2008 06:23pm | #3

    Why do you ask.

    Sounds like you are building nothing more than a free standing deck.

    The post aren't the limiting factor. Is what the joist & girders can support.

    And then the size of the footings.

    Look at these.

    http://www.awc.org/Publications/dca/dca6/dca6.pdf
    http://www.fairfaxcounty.gov/dpwes/publications/decks/details.pdf

    .
    .
    A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
  4. User avater
    Matt | Aug 10, 2008 07:29pm | #4

    Had a little trouble following you.

    How wide are the footings?  12"x12"?  18"x18"?  16"x14"?  etc.  Is it red clay soil or what?



    Edited 8/10/2008 3:01 pm ET by Matt

    1. Rickk | Aug 10, 2008 11:24pm | #10

      Matt the footings are 14 x14 x 12'' thick with the top of each 36'' below grade.The 6x6 Pressure Treated southern pine post set on top of each footing and are encased with concrete.The framing for the decking is PT 2x10 (band) bolted (staggared) with .375 x 5" SS lag bolts 3 into each 6x6  with oversized ss washers.The joist are 2x8 x approx.87" bolted thru the outer band with .375 x 5" lags w/ oversized ss washers and they sit on a 1.75x1.75 ledger strip nailed and glued to the innerside of same banding,as an extra measure I placed a PT 2x4 (2 are40" long & 2 are 80' long) on the outerside of each 6x6 under the outer banding and lag bolted these to the 6x6 post as well.I live in central Georgia and lots of red Clay..Hope yall can follow my ramblings.    Thanks Rickk

      1. Piffin | Aug 11, 2008 02:50am | #11

        You have a bearing surface of 1.36 sq ft for each of those four pods. or 5.4total bearing area.At 2000#/ft soil bearing capacity, that will hold about eleven thousand pounds. Friction on the sides of the piers will add to that figure.The weak point in your structure as I see it is the 3/8" lags holding perimeter framing to the posts if they are not notched into the post. If I read the description right, you only have three lags holding the whole assemby up at each corner. 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

      2. User avater
        Matt | Aug 11, 2008 03:56am | #12

        Sounds like it is built quite strong.

        As far as answering your question it is the 14"x14" dimension I was interested in.  As long as the clay is undisturbed and not in a low wet area the 14"x14" red clay footing footprint will support at least 2700#s figuring it at 2000 PSF.     As far as the 36" deep, you really didn't need to go that far - did you?  Unless you had some soft stuff to get through to get to firm soil.  The depth of the footing doesn't make it bear more weight as long as it is below the frost line and is to firm soil. 

        Looking in my building code book a footing of that size will support a tributary area of about 55 sq ft.  If you tell me how big the thing is and how many posts there are I'll tell you haw to figure that.  Some of it depends on what your local (state) code require as far as PSF rating for a deck like that.  Normally it would be 40psf, 50psf or 60psf.  That is live and dead load combined.  Dead load refers to the building materials and generally 10psf is a number for that portion.

        As far as the concrete is concerned as long as it is thick enough and again, on firm soil, it in not the weak link by a long shot.  The soil is measured in PSF.  The concrete in PSI, so let's say, even if you got a weak mix of 2000 PSI that would mean there is roughly 60,000 #s of bearing capacity under each of the 6x6 (5.5"x5.5"x2000) posts.  A footing like that would normally never need to be more than 10" thick. 

        As far as the bearing capacity of the posts, again they are not the weak link.  The guy above who started talking about species of wood doesn't know what he is talking about - as is often the case.  Sure some species are stronger but who cares if a post supports 7000 or 8000#s when the associated footing only supports 2700#s.  BTW - the 7k and 8k numbers were off the cuff.  I don't have a chart for this but these numbers are at least in the ballpark.  (anyone have a chart?)

        I really can't see your connections from here so I don't really know about them but you said: >> The framing for the decking is PT 2x10 (band) bolted (staggared) with .375 x 5" SS lag bolts 3 into each 6x6  with oversized ss washers.<< .375" diameter is kinda small for a structural application but the key question is do the 2x10s sit on top of or are they notched into the posts?  Also be aware that SS bolts are not as strong as regular HD steel bolts.

         

        1. Piffin | Aug 11, 2008 05:20am | #13

          He has four footings and posts, presumably one at each corner.But he has not stated the area of the deck supported.In reviewing, I see he describes some 2x4s under the band joists that I take to be vertical attached to the post instead of notching the post itself. So the bearing there has a few more lags than just the three in the joist. I think he is needing some more lags 

           

          Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          1. User avater
            Matt | Aug 11, 2008 05:48am | #14

            It's not clear to me what the post to beam(hopefully) connection is either.  We are required to use 5/8" bolts for anything that is structural, and can't use lags on much of anything on a deck except maybe rail posts.

            OTOH, in another thread Blue and I were talking about how over the top some codes are.

            BTW - although my previous post showed up an hr after yours, and covered some of the same stuff, I basically typed it, went and watched the Olympics for a while, came back added a bit more, spell checked and then posted.  So, in effect we were typing at the same time... sorta....

          2. Piffin | Aug 11, 2008 06:35am | #15

            and saying about the same thing, whilst the OP sleeps soundly;)Curious why he wants to know now after the fact... 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          3. Rickk | Aug 11, 2008 01:11pm | #16

            Thanks all for the input, I thought the lags were a bit too small. I will change those next. If I can figure out how to post a photo I will. Thanks again for all the info.     Later Rickk

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