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Possible mistakes with post footings

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  1. #1
    Novice
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    Feb 2012
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    Default Possible mistakes with post footings

    Hi everyone - hoping I can get some advice before I proceed any further. Also hoping that the answer is not taking a jackhammer to my new footings but if that's what must happen then so be it

    I am building a deck - really 3 decks, as it is over some sloping land. The decks share footings, but have their own post stirrups/anchors.
    Deck A is just under 9 sqm, is roughly rectangular, and will support a pergola; Deck B is just under 10 sqm and has 5 sides. Deck C is much smaller (more of a walkway). All decks average about 0.5m off the ground.

    At this stage, I have cemented the corner supports in of all decks, with intermediate post holes still to be dug. The holes are 900 deep and 400 wide for deck A, and the front of deck B has footings 600 deep and 300 wide.

    bad-me-1.jpgbad-me-2.jpg

    The main two issues that I am concerned about at the moment, preventing me from continuing are:

    1. All I could find at my local hardware were Pryda 300mm high wind post anchors (they had nothing larger), and 200mm full stirrups (200 is the size of the post, whereas the 300 is the size of the entire unit). They will support 90x90 posts.

    Initially, I decided to use the high wind post supports for the 4 corner posts which will support the pergola, but after embedding the first 2 (at the back of deck A), I decided to use the stirrups at the front. Both are pictured (the timber is just for a very low retaining wall (under 200mm).

    My concern is the size of the supports. The 300mm anchor is 200 out of the concrete so only 100 in it - is this too small? I could not find any info on when to use which size.

    2. Is there any problem with 2 stirrups sharing a footing?

    Any comments and suggestions would be gratefully received!

    Many thanks
    Z

  2. #2
    Old Chippy 6K
    Join Date
    Mar 2007
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    Canberra
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    71
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    6,582

    Default

    Those pics do not seem to match your words - the stirrups appear to be just a few mm above the concrete? Suggest you provide a sketch/plan and some more pics.
    Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.

  3. #3
    Novice
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    Feb 2012
    Location
    Brisbane
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    35

    Default Embedment depths and anchor choice

    Hi Bloss, and thanks for your reply. I'll try explain better this time!

    My question is really about the 'embedment depth' of both types of anchor. The specs matching my images are attached here.

    I used the smallest of the high wind anchors - the 300, and the 200 post of the full stirrup.

    I know I should have allowed ~70mm above for termites, but when I installed them I chose to rather sink as much as I could! This means about 110 of the high wind kind is in the concrete, and of the stirrup, about 170.

    So my question is whether the size of the supports themselves are sufficient, and whether the embedment depths are OK. Assume for now the rest of the deck design is properly designed according to tables - the info on the supports indicates that the loads are limited by the capacity of the posts, and I will have 10 posts to a 10sqm deck, 6 extending to a simple pergola without a roof.


    I'm not sure how you would choose between sizes of the anchors besides wind uplift, or pest/water protection? Is the anchor is really just a means to connect to the concrete (and counter wind uplift), and the concrete footing providing the support?

    Hope that is sufficient for an opinion - I will try to scan the sketches as soon as I can. Thanks for your time.

    Cheers!
    Z
    Attached Thumbnails Attached Thumbnails screen-shot-2012-02-20-6.26.12-pm.png   screen-shot-2012-02-20-6.44.48-pm.png   screen-shot-2012-02-20-7.16.30-pm.png  

  4. #4
    Senior Member
    Join Date
    Dec 2011
    Location
    Brisbane
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    382

    Default

    I think if you use them to the instruction then they are fine. However, I'm not a fan of them hence used galvanised posts instead. They are not much more expensive and you can buried part of the post and reduce the need for bracing. Neater look.

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