Concrete Anchor / Through bolt length, and how to get them level!

Discussion in 'Builders' Talk' started by Matt J, Apr 29, 2020.

  1. Matt J

    Matt J Member

    Hi all!

    I hope this is the right area of the forum.

    I am building a pergola and decking over a concrete slab and I’m using post bolt downs anchored with throughbolts into the concrete.

    there’s two thinks bugging me I’m hoping you guys might have some advice on?

    1) the first is other than the holes in the post bolt down dictating what size M8/10/12 etc to go with how do I determine the length of bolts to buy? It’s for a 2.5ish m high 150x150mm post. I know they all have recommended embedment depths which I understand, but how do I know if I should go for a m12 95mm or a m12 180mm for example? Is there a calculation to the the correct length?

    2) the second thing on my mind is what is the best way to get the post bold downs level and therefore the posts plum? It was ‘unofficially’ suggested that I could put a few washers under bolts to shim up a side that’s low and get it level but if there a best practice for this?

    thanks very much for any comments!
     
  2. spannerw

    spannerw Active Member

    1 Would depend on depth of concrete slab you are bolting into.
    2 Unofficial way sounds good to me.
     
    Matt J likes this.
  3. Matt J

    Matt J Member

    Ok great thanks @spannerw. The slab is around 400mm deep in total. Do you just get the longest bolts you can, but ensure the embedment depth still leaves a little bit of concrete at the bottom of the hole. Say if the slab was 200m you’d go for a 160/180mm bolt?
     
  4. CharlMil

    CharlMil Member

    I would leave at least 100mm before the bottom of the concrete. As depending on how close to the edges you’re bolting, you run the risk of weakening the structure going all the way through.

    the bolts are strong enough to hold the wood and the frame itself will be more self supporting when it’s fully attached in square shape. (No wobble if you get what I mean)

    as for the plumb, how out is your base? It should naturally have a slight fall for drainage. If it’s only minor, surely there’s enough adjustment inside the floor bracket before you clamp/screw the wood down to make them plumb? If not, then Washers will work, just make sure they’re man enough.
     
    Matt J likes this.
  5. Matt J

    Matt J Member

    Got it very helpful thanks @CharlMil. And yes I get what you mean regarding having the frame with the struts etc to stop any movement. I haven’t started to fit the bolt downs yet but I just know it will need a bit of tinkering as the stab isn’t the flattest of slabs!
     
  6. CharlMil

    CharlMil Member


    I did comment on your other post too.. what is your final base going to be? The existing slab?
     
    Matt J likes this.

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