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Thread: Deck Pier/Footing Size and Tributary Loads

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    Administrator jar546's Avatar
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    Deck Pier/Footing Size and Tributary Loads

    Around here, we are often lucky to get soil compaction tests for commercial projects of 2,000 psf. For residential, it is beyond rare that we have tests performed (I did have 1 person pay for one within the last year) just to find out it was 1,500 psf. We could have just assumed a 1,500 psf bearing capacity right from the IRC and saved him money, but that is another story.

    We consistently get deck designs that have undersized piers/footings. I usually tell people to just download the DCA-6 manual and go from there but some of the deck designs are more complicated than the simple tables that they have making it hard to use realistically.

    Even when we go over the requirements for simple residential stuff, it is still not what we find when we get out there.

    Yesterday's inspection for a pre-pour with 12" Sono-Tube was as follows:

    Rectangle deck 16 x 32, long axis with the house.

    They had planned 4 - 12" sono tubes to support the beam.

    The tributary load for that worked out to 3,200# per pier. Not even close.

    This is based on a live-load of 40# per deck requirements and a 10# dead load.

    I was not a very popular guy yesterday. I even had to explain that a 12" sonotube is only about 75% of a square foot.

    Do you guys have the same problem? It seems like most inspectors in our area just don't give a crap.
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    Sawhorse Francis Vineyard's Avatar
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    We have more of a concern with footings not deep enough on slopes than undersized. Historically there has not been a failure of simple spanned decks with plain footings 6 inches thick. For 2-tier decks I use the calculator but most of the time they are submitted 24 inches square and 10 inches thick regardless.

    It may be the inspectors don't know how to use a calculator or as you say don't care.


    Francis
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    Sawhorse TheCommish's Avatar
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    Decks have allways scared me, I have seen a few fall off. I have attached my excel caculator for sono size, the unfinished draft om my guidance for decks (based on the DAC manual) and a deck templete drawing, it is amazing how many cant draw a simple deck. well I was going to but the files are to large

    Any hints group?

    I recomend spread footings for any deck that there is a slight chance of a roof being put on and require spread footing for anything that has a roof even the 6' farmers porch.

    I find that the center pier of anyhing over 10x10 the center tube need to be at least 16" diamater

    I tell customers it is easyer to fix it on paper than have to take a do over.
    Code compliance and inspections, leads to non-events


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    I keep my spreadsheets here; timbertoolbox.com I'd be happy to stick it there and you could link to it if you're comfortable.

    I've never been comfortable with an unfooted sonotube, I can't recall a plan that didn't at least start with a 2x2x8" w/2 #4 bars ea way. Personally I prefer to bury a .60 6x6 post from footing to top of rail and avoid hinges. The decking will need replacing well before the post at which point the frame is being scrutinized again.

    One of the form companies is touting "code approved" for it's forms in the glossy mags and there is an ESR that acknowledges that the form is made to hold concrete. I'm seeing houses on the net going up on these and it's "code approved", just like the ad says.

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    When manufacturers claim their product is "code approved" because they have an ESR they do not understand what an evaluation report is and they do not understand that the only approval that counts is the approval of the local building official.

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    Oh, the manufacturer knows exactly what they are doing when they print that ad.
    But the other side of that is that we do not have good prescriptive guidelines for typical pier situations.

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    Administrator jar546's Avatar
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    Another issue we contend with is when they want to frame a roof over an existing deck. Depending on what elevation they are building at, we have a ground snow load that varies from 40-55.

    I make them dig up at least one pier to see what size it is. They are usually ****ed about this because most of the time it is nothing more than a 4x4 in the ground with sakrete poured around it. Something we don't allow
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    16x32 is a pretty big deck.
    Plan review needs to catch it up front. 4 piers may not work, 5 or 6 might.
    It's up to the designer whether to go 4 bigger piers or more of the 12" circular (or a design that utilizes soil friction against the posts).
    Should not get to the construction phase when there is so much delta on the bearing capacity.
    (PE)ach
    some days are just that..

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    Sawhorse TheCommish's Avatar
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    DRP
    nice site, can i link it on my web page? http://townofcharlton.net/buildinginspect.htm
    i wll send you my excel caculator if you pm me
    Code compliance and inspections, leads to non-events


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    Sure, feel free, check your PM's.

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    Sawhorse Francis Vineyard's Avatar
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    Quote Originally Posted by jar546 View Post
    Another issue we contend with is when they want to frame a roof over an existing deck. Depending on what elevation they are building at, we have a ground snow load that varies from 40-55.

    I make them dig up at least one pier to see what size it is. They are usually ****ed about this because most of the time it is nothing more than a 4x4 in the ground with sakrete poured around it. Something we don't allow
    Could place the new roof column footings on outside perimenter of the deck footings.


    Francis
    ACCESS TO VIRGINIA CODES: 2009& 2012 Virginia Codes

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    Administrator jar546's Avatar
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    Quote Originally Posted by Francis Vineyard View Post
    Could place the new roof column footings on outside perimenter of the deck footings.


    Francis
    Absolutely, which is what we often recommend.
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    Sawhorse Builder Bob's Avatar
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    Often times a quick checklist made avaialble to the public will help.... not a design guide but a generic checklist with most common violoations/errors/omissions on it.
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    Sawhorse
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    We fight it here a fair bit, we have decent soil, I figure most of them at 2000#psf they usually are pretty close. They can add more piers, hog out the bottom of the holes, or put bigger tubes in.....
    Please support Jim Brown, Deputy Building Official from Gillette, Wyoming for ICC Board of Directors.
    My questions and answers are based on some bastardized hodge podge starting with the 2003 I-codes amended by the state of CT in 2005 and 2009 including the 2005 NEC ....and as of today.....The 2011 Connecticut Amendments to the 2009 International Energy Conservation Code portion of the 2005 State Building Code has been posted to the State Building Inspector’s website

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    Quote Originally Posted by jar546 View Post
    Do you guys have the same problem? It seems like most inspectors in our area just don't give a crap.
    Eveeeeery day! ...and we even make sure they are sized correctly at plan submittal. We get a lot of, "i've been doing this for twenty years and haven't needed anything bigger than an 8" footing." They often balk at the frost protection requirement when attached to a house.

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    Are we only concerned with vertical loads?
    Are there conditions on where the column is hinged and how it is footed?
    A picture of a pier several feet tall in a basement with a post on it supporting a girder drew comments the other day. If I have a 12" pier 4' tall into and top flush with grade and then place the post on it then there is an assumption that something is somehow different. Now start extending that post to pier connection into the air, when does your discomfort rise or does it?

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    Administrator jar546's Avatar
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    Quote Originally Posted by DRP View Post
    Are we only concerned with vertical loads?
    Are there conditions on where the column is hinged and how it is footed?
    A picture of a pier several feet tall in a basement with a post on it supporting a girder drew comments the other day. If I have a 12" pier 4' tall into and top flush with grade and then place the post on it then there is an assumption that something is somehow different. Now start extending that post to pier connection into the air, when does your discomfort rise or does it?
    My post was specific to footing sizes and did not address other issues. All other issues are also dealt with but this was specific to vertical loads. Don't forget that the 2009 IRC improved the requirements for lateral loads with R502.2.2.3 and ledger attachment addresses both vertical and lateral loading under R502.2
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    Quote Originally Posted by jar546 View Post
    Another issue we contend with is when they want to frame a roof over an existing deck. Depending on what elevation they are building at, we have a ground snow load that varies from 40-55. I make them dig up at least one pier to see what size it is. They are usually ****ed about this because most of the time it is nothing more than a 4x4 in the ground with sakrete poured around it. Something we don't allow
    SSTD 10-99 has buried column requirements with and without concrete. They're for uplift, but similar resistance will occur in the other direction.

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    Jar546:
    Maybe one of the things you should do in permitting a deck or porch, is to make a list of a few things like a potential future roof over the deck or porch, if this is a regular problem, so you can make a determination on the lower structure, posts, piers and footing, and make this part of the file for future reference. And, at the same time they sign off on this, I would explain the need for larger structure now, for these future loads, otherwise forever hold your peace. This list might include any other areas which are typical problem areas in your experience. In working with a client at the beginning of a design project, I certainly want to know all the potential load conditions. We can’t design for or accommodate conditions we don’t know about. I would also question a little the intended use of such a large deck, large parties, dancing, etc. for increased LL considerations, joist flexibility (vibration/deflection) and for lateral load considerations. The 16' long deck joists are pretty long and they will require some special attention at the ledger at the bldg. Also, on a deck with the outer handrail 16' from the bldg. ledger beam, where a lot of wild action can occur, I would pay some special attention to lateral stability out there.

    Given the 16' x 32' deck and 4 posts with 12" sonotubes, I get a larger footing load than your 3.2 kips per footing. You have over simplified your tributary area calc., thus (8' x 32')(40 + 10)/4 = 3,200 lbs., but it’s not quite that simple. The two outer corner posts have a lesser load, about 2 kips and the inner two posts have loads of about 4.8 kips, these are just off the top of my head proportions, no actual calcs., and a 4' deep pier weighs another 500 lbs. But, if the outer beam is moved in under the deck a foot or 18" which is common, and the outer corner posts are moved in so the beam cantilevers a foot or 18", also common, we could actually run some reaction calcs., and the posts would be even more highly loaded. The 12" dia. sonotube is most likely o.k. from a column strength standpoint, but the footing sized must be sized for the loads and soil bearing capacity. The 12" sonotubed could be belled out at the bottom to pick up bearing area. The biggest warning about these piers or footings is that they bear on undisturbed (virgin) soil, or at least well recompacted, at the bottom of the hole. They should not be cast on 6" of loose crap, or they become settlement problems. I would also like to see some vertical rebar in these type piers.

    DCA-6 is certainly a good general knowledge guide for deck design, but you still have to read all the footnotes and read between the lines and do some interpolation at times. Like all the tables in the IRC, they cover the simplest, most basic conditions conservatively, and can be easily abused if you don’t have a reasonable idea of what’s really involved in a complete structural design.

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