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  1. #1
    Join Date
    Jun 2013
    Location
    near Paris, France
    Posts
    4

    Question Pergola footing shear forces?

    Hi all!

    This is my first post - found the forum after reading about its presence in an article in TOH mag. Happy to be here!

    Anyway, we've moved into a 'new' old house (foundations laid in the 1680s), and are planning on putting a pergola over our (cement) porch, which sits just off the kitchen at the back of the house. The kitchen and porch are above grade, and the porch itself is actually the roof of a workshop which is attached to the basement level (walkout) of the house. The floor of the porch is about 8" thick, although I am unsure how it was poured, if it's reinforced, etc.

    To support the pergola (~5 m x 3 m) we are going to use a ledger board attached to the exterior (stone) wall of the house. I am not worried about length of bolts for attaching the ledger as the walls are about 2.5 feet thick so I'll have PLENTY of support.

    However for supporting the other side of the pergola, we are going to put down 6x6 posts anchored into these sort of 'top hat' footings that will be bolted into the cement floor (http://www.clicfixation.com/f-serie-...boulonner.html) with anchor bolts. However! The bolts themselves, due to the short depth of the floor slab, will not be able to be more than 4" in length.

    My question, therefore, is: will 4x4" anchor bolts per post (3 posts total, one every 150cm) be enough to keep wind shear from ripping the posts and the bolts out of the floor slab?

    Here's a picture of the side of the house for reference: https://pbs.twimg.com/media/BMdY6JGCIAADJ4S.jpg:large

  2. #2
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,768

    Default Re: Pergola footing shear forces?

    In a word, yes, BUT, the top of the pergola will be a deciding factor. You will need to have corner bracing in every corner so that the pergola cannot be racked by the winds. If racking occurs from the top, then the bolts at the base can be pulled out. If the top cannot be racked, then the only forces on the bolts at the base are the sheer forces and the bolts should be up to that.

    I would recommend that you use Stainless Steel bolts for long term durability.

    Beautiful house BTW.

    You have another option if you wish and if you have access to the ceiling directly under each bolt and that would be to use machine bolts that are long enough to go all the way through the slab plus about an inch (2.5cm) and use a large diameter washer (aka "body washer"), a lock washer and a nut. This will show in the workshop if you don't mind.

    Another suggestion not related, after you are done, get 4 concrete stepping stones for each post, 12"x12"x2" thick (30 cm X 30 cm X 5 cm thick). Stand them vertically to form a box at the base of each post, glue the edges with concrete glue. Then fill with potting soil and small plants to hide those ugly steel anchors.
    Last edited by keith3267; 06-13-2013 at 03:14 PM. Reason: add info

  3. #3
    Join Date
    Jun 2013
    Location
    near Paris, France
    Posts
    4

    Question Re: Pergola footing shear forces?

    Thanks, Keith! See in-line for a few responses:

    Quote Originally Posted by keith3267 View Post
    In a word, yes, BUT, the top of the pergola will be a deciding factor. You will need to have corner bracing in every corner so that the pergola cannot be racked by the winds. If racking occurs from the top, then the bolts at the base can be pulled out. If the top cannot be racked, then the only forces on the bolts at the base are the sheer forces and the bolts should be up to that.
    I was planning on adding mortis-and-tenoned braces with pegs through the tenon, wood-framed-house style. I suppose that would work?

    Quote Originally Posted by keith3267 View Post
    I would recommend that you use Stainless Steel bolts for long term durability.

    Beautiful house BTW.
    Thanks on both accounts. We're super, super happy with it so far, it's way less humid than our previous house (avg. humidity 75-80%, versus 60% in the new one) and is actually ventilated. I will say that I always get a laugh out of the TOH guys talking about 'international building code' which to me, living in France, sounds a lot like 'world series' - practiced only in the US. You should see the stairs in some houses, for example... Or even the steps leading up to the Eiffel Tower train station:

    Quote Originally Posted by keith3267 View Post
    You have another option if you wish and if you have access to the ceiling directly under each bolt and that would be to use machine bolts that are long enough to go all the way through the slab plus about an inch (2.5cm) and use a large diameter washer (aka "body washer"), a lock washer and a nut. This will show in the workshop if you don't mind.
    I had considered this. But, I'm worried about humidity and ice melt getting down in there and disintegrating the concrete with freeze/thaw. I suppose a bit of silicone around the top part of the washer would work?

    Quote Originally Posted by keith3267 View Post
    Another suggestion not related, after you are done, get 4 concrete stepping stones for each post, 12"x12"x2" thick (30 cm X 30 cm X 5 cm thick). Stand them vertically to form a box at the base of each post, glue the edges with concrete glue. Then fill with potting soil and small plants to hide those ugly steel anchors.
    No concerns about rot here as long as it's PT then, right? But it would preclude something like Cedar I suppose?

    I had otherwise thought of doing a craftsman-style stone-faced column thinger around the base of each post to get the same look. Might be too busy though. We'll see. Thanks again!

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