1

I am in the process of building a 19'x19' cabin that will be perching on 8' piers. I got a structural engineer draft for the foundation and piers and he recommended using nine 6"x6" posts with knee braces (side corners, side centers + one in the middle of structure) and stackable concrete piers (>2' under the ground and >1' over ground) for foundation. My builder asked if I would rather use them all the way to the base 8' up. My question is what would actually be a better solution?

My concerns are that concrete might crack and fail faster and I don't think there is an ability to attach any knee supports to stackable piers. However, it seems like the concrete all the way to the floor might be more sturdy and less prone to swaying during wind and such, but I am just guessing here. Cabin sketch

I am not considering deck in my question. 19'x19' are the main cabin dimensions.

2
  • Can you show a sketch, your description is a little confusing.
    – r13
    Commented Dec 27, 2021 at 17:48
  • Cabin sketch is attached now.
    – i--
    Commented Dec 27, 2021 at 18:36

3 Answers 3

3

If the contractor is proposing the scenario as below, you shall contact your structural engineer to re-sizing the EZ tube piers to withstand the lateral load. Without strengthening, your structure is unstable, as the piers are acting as cantilevers that can deflect/rotate a large amount.

The structure below is unstable.

enter image description here

The structure below is stable.

enter image description here

4
  • Thank you for the insight. I will stick with the wood posts, but will use 8x8" with slightly deeper and larger cilindrical piers for the foundation.
    – i--
    Commented Dec 30, 2021 at 16:12
  • I can see how diagonal bracing as shown could translate cantilever forces into vertical or horizontal ones. If the diagonal bracing is the entire solution here, then the question becomes whether the concrete piers should stop at ground level or rise up to the bottom of the diagonal braces or whether bracing to concrete is possible. So this answer is only partial. Or is there more to it than the bracing?
    – jay613
    Commented Dec 30, 2021 at 16:51
  • In other words, to whatever extent you have pure vertical support rising from the ground, where should concrete transition to wood?
    – jay613
    Commented Dec 30, 2021 at 16:52
  • 1
    @jay613 The main function of the diagonals is to keep the structure square (not to twist) when subjected to the lateral load. The small kickers are good for the prevalent wind load in the local, otherwise, full bay diagonals are to be used. Also, the diagonally braced framing has its own rigidity (with moment carrying capacity), so the posts can be simply supported on/attached to the pier. Extending the pier all the way up will raise the lateral stability concerns unless it WAS designed as a cantilever column, which is not. So any change requires blessing from the design engineer.
    – r13
    Commented Dec 30, 2021 at 18:58
1

Well, concrete is going to be stronger and won't rot compared to the 6x6 posts.

Tall vertical concrete needs support either via rebar or mesh during the pouring process. You will likely still need knee braces or else you run the risk of having the entire structure shift. I am not sure if these can be Tapcon'd to the concrete or if you need to plan for enough 6x6 height directly beneath the structure.

See https://www.youtube.com/watch?v=cZINeaDjisY to learn about why concrete needs reinforcement.


I cannot speak for the performance of the EZ-Tube product specifically but it looks like that tall rod will assist with lateral stresses.

1
1

If there is damage to any of the supports (concrete: cracks; wood: rotting), with wood it is relatively easy to fix: set up a temporary support, remove the old wood, install a new support attached to the concrete and to the building.

With concrete, you have a much tougher job:

  • Concrete is heavier to start with, and the support will likely be much thicker (and heavier again) than 6x6 wood.
  • Replacement requires digging out the old concrete, which will require more space, equipment and time.
  • A replacement concrete support will not be usable for until it has cured for quite some time (days? weeks?), so the temporary supports will have to stay in place for quite a while.

So the initial installation time may be comparable, since the concrete bases for wood supports will need time to cure before use and the building itself can wait however long is necessary before being placed on supports, repairs are a lot more complicated with concrete than with wood.

From the Q&A on the Home Depot link in the original question, here is an answer posted by the manufacturer:

Hi James, great question. Pier footings like EZ-TUBE should be buried to at least to the frost line but can come above grade to support a structure on a downslope as you describe. That said, if you need to come above grade more than one section (12 inches) it is better to use a 4x4 or 6x6 post to come up to the required level. I hope this helps answer your question. Good luck with your project. If you use EZ-TUBE please post photos when it's done.

which indicates that this particular product (which is easy to install and to replace as it consists of precast pieces instead of pouring onsite into a form) is not designed for above ground use. Poured concrete can be used for above-ground supports but then has the concerns I noted above.

2
  • 1
    You have read my mind on this. Plus it fits the aesthetic look I am going for better and I have the structural engineer sign off on this one, so I feel better taking the wood route. I can always use thicker posts and it would not be hard or expensive to even add additional support, like x bracing later if I feel it's needed.
    – i--
    Commented Dec 30, 2021 at 17:34
  • And thanks for supplemented home Depot faq addition. I wish I could upvote twice :)
    – i--
    Commented Dec 30, 2021 at 18:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.