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gotdurt
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# Posted: 5 Mar 2025 12:46pm - Edited by: gotdurt
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I am building a 12'x32' cabin on piers in central Texas; I'll be using tube forms on gneiss bedrock (similar to granite), with 6x6 posts on the concrete, (3)2X12 job-built beams on the posts, with 2x10 joists at 12" OC. 3 of the pier posts will be connected directly to the rock at the high points (no concrete).
I'm having trouble figuring out the connections between:
1) The concrete and the posts; I would like to use threaded/adjustable brackets, like these:
URL
But, I'm having a hard time fining any information regarding load handling, etc. I'm open to other options, but conc-to-post connectors are a bit overwhelming, and many appear to just be for patio posts, not pier specific.
2) The posts and the bedrock (at the 3 points without concrete); again, the threaded brackets above seems like a good idea, as I can drill into the rock with a hammer drill and insert the bracket, but I'm not finding much to go by here.
3) The posts and the triple 2x12; I'd like to use bolts if possible, in case I ever decide to move/remove the cabin.
I'd appreciate any experience some of you might have with these connection types, or other ideas...
Sketchup rendering for visualization (except I used concrete piers at all 10 points for ease)
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Grizzlyman
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# Posted: 5 Mar 2025 03:58pm - Edited by: Grizzlyman
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I used 5/8 (or maybe 3/4?) threaded rod bent like a j bolt inside the concrete forms to affix Simpson 6x6 post bases to the concrete. Then after it was in place buzzed the excess rod off with an angle grinder. There is a gap built into in them so they do stand-off from the concrete. I think this is easier than the standoff base you have here. Also they can be moved around slightly before bolting down so they can be adjusted- unlike the bases you have here that have to be perfectly placed. To connect post and beam I used Simpson 6x6 brackets.
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gcrank1
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# Posted: 5 Mar 2025 05:38pm
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Do you really need to have it high off the grade? Sure would simplify if your beams were down on the pads.
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Brettny
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# Posted: 5 Mar 2025 05:49pm
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In TX and only a 12x32 I would just put it on blocks right up to the girders. 12in on center 2x10 floor joists is overkill. Your going to pull your girders in 8-12in anyway so the span on a 12' wide building is prob more like 10'.
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DRP
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# Posted: 5 Mar 2025 06:31pm
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How deep is it to the rock?
As drawn that is an unbraced stack of loosely connected parts; a tall narrow concrete post pinned to a rock and a tall narrow wood post, which is pinned to a tall narrow girder pinned to a floor diaphragm. Not a very stable foundation.
2x8 joists can span up to 12' on 16" centers. Joists can overhang the girder up to the depth of the joist prescriptively, perhaps further if you look at the cantilever tables.
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gotdurt
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# Posted: 5 Mar 2025 07:08pm - Edited by: gotdurt
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Quoting: Grizzlyman I used 5/8 (or maybe 3/4?) threaded rod bent like a j bolt inside the concrete forms to affix Simpson 6x6 post bases to the concrete. Then after it was in place buzzed the excess rod off with an angle grinder. There is a gap built into in them so they do stand-off from the concrete. I think this is easier than the standoff base you have here. Also they can be moved around slightly before bolting down so they can be adjusted- unlike the bases you have here that have to be perfectly placed. To connect post and beam I used Simpson 6x6 brackets.
Those are the same bases that are usually used under covered patio posts without much weight; I'd be concerned about how it performs under the weight of a complete building, especially one that is basically 2-story... how are yours holding up?
Quoting: gcrank1 Do you really need to have it high off the grade? Sure would simplify if your beams were down on the pads.
This is a grossly oversimplified drawing... It's actually a complex site; the left side will be on sloping gneiss bedrock that slopes down to earth, the right side will be dug down to the bedrock. The rear left 2 contact points will actually be on the bedrock; no pads. There is also a seasonal creek that is nearby, as well, so I want to keep it off the ground.
Quoting: Brettny In TX and only a 12x32 I would just put it on blocks right up to the girders. 12in on center 2x10 floor joists is overkill. Your going to pull your girders in 8-12in anyway so the span on a 12' wide building is prob more like 10'.
I've used 2x8s on similar spans before (minimum IRC span), but they are too springy for my liking. Also, this will be basically 2-story, so there's a good bit of weight on the 12" cantilever ends.
Quoting: DRP How deep is it to the rock?
It varies, see my comment to gcrank1 above... the left side is above grade, the right sidewill only be a couple of feet of digging.
Quoting: DRP As drawn that is an unbraced stack of loosely connected parts; a tall narrow concrete post pinned to a rock and a tall narrow wood post, which is pinned to a tall narrow girder pinned to a floor diaphragm. Not a very stable foundation.
This isn't from the plan, just a very simplified illustration to show the stack for this forum post. It will be fully braced and fastened where it is more than 12" off the ground.
Quoting: DRP 2x8 joists can span up to 12' on 16" centers. Joists can overhang the girder up to the depth of the joist prescriptively, perhaps further if you look at the cantilever tables.
2x8s at that span is a bit springy, also the the canti'd ends will be supporting essentially a 2-story structure, which I wouldn't want on 2x8s supporting that with a 12" canti.
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Grizzlyman
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# Posted: 5 Mar 2025 08:03pm
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gotdurt
https://ssttoolbox.widen.net/view/pdf/ifyvoa2zmy/C-C-2024_p070-071.pdf?t.download=tru e
https://www.strongtie.com/retrofitpostbases_postbases/aba_base/p/aba
I believe they are rated for like 10-13,000 lbs each.
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spencerin
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# Posted: 5 Mar 2025 11:21pm
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Why not embed the 6 x 6s in the concrete? You can put the posts in plastic sleeves to provide a moisture barrier against the concrete. Putting the posts on those bases will require additional shoring up of the posts against lateral forces, which would otherwise easily topple the strucutre. If you ever need to move the cabin, you could just cut the wood at concrete level.....
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gotdurt
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# Posted: 6 Mar 2025 06:30pm
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Quoting: spencerin Why not embed the 6 x 6s in the concrete?
1/2 of the piers will be pinned to exposed bedrock, the other 1/2 wouldn't go deep enough before hitting bedrock (about 2'+/-).
Quoting: spencerin Putting the posts on those bases will require additional shoring up of the posts against lateral forces
Yes, they will need bracing.
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gcrank1
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# Posted: 6 Mar 2025 08:01pm
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Unless you really need access underneath and 1/2 will be on bedrock do short piers there and taller ones for the rest. Imo it would be a simpler and stronger base than using the posts.
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spencerin
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# Posted: 6 Mar 2025 11:12pm
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Yep, you can get 6 x 6s up to 8' long, maybe even longer if you search wide enough.
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gotdurt
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# Posted: 9 Mar 2025 06:19pm
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Quoting: gcrank1 Unless you really need access underneath and 1/2 will be on bedrock do short piers there and taller ones for the rest. Imo it would be a simpler and stronger base than using the posts.
There will be 2 points where the girder will be mounted directly to the bedrock, with no pier; from that high point to the lowest point, it would put the longest pier at about 3' high, from the natural grade.
I will want a comfortable crawl space under the end with the plumbing, which will be end/corner with the tallest pier.
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Brettny
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# Posted: 10 Mar 2025 06:04am
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Measure the height between your head and the seat your sitting on. That would be my bare minimum if you ever need to get under the cabin again.
Also since some are on bed rock they sell a concrete anchoring epoxy in a cawlk tube for about $20 I would drill and anchor each post. Or you could use blocks from the bedrock and not worry about racking and rotting.
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Grizzlyman
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# Posted: 10 Mar 2025 11:45am
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Here’s what mine looks like. I poured footings on the short side even on bedrock- This way there is still a little room to get under it. I’ve done some plumbing on the short side and it’s not bad at all. The taller side is only about 3’ of elevation but when you add up the 10” for the beams it’s fairly comfortable to be under.
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gotdurt
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# Posted: 10 Mar 2025 11:47am - Edited by: gotdurt
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Quoting: Brettny Also since some are on bed rock they sell a concrete anchoring epoxy in a cawlk tube for about $20 I would drill and anchor each post. Or you could use blocks from the bedrock and not worry about racking and rotting.
And this brings us back around to the original purpose of my post; what to use for connections... Drilling and anchoring is a given, just looking for specific connectors/brackets/bases for 6x6 to rock and 6x6 to concrete that others have had experience with (like the bracket link I posted and the base that Grizzlyman used).
Quoting: Grizzlyman Here’s what mine looks like. I poured footings on the short side even on bedrock- This way there is still a little room to get under it. I’ve done some plumbing on the short side and it’s not bad at all. The taller side is only about 3’ of elevation but when you add up the 10” for the beams it’s fairly comfortable to be under. IMG_7921.jpeg IMG_7921.jpeg
This looks like a fairly similar situation to mine...
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Brettny
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# Posted: 10 Mar 2025 06:19pm
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Quoting: gotdurt rotting. And this brings us back around to the original purpose of my post; what to use for connections... Drilling and anchoring is a given, just looking for specific connectors/brackets/bases for 6x6 to rock and 6x6 to concrete that others have had experience with (like the bracket link I posted and the base that Grizzlyman used
Ops...if your attaching a bracket use threaded rod. If your attaching cement to the rock rebar would work fine.
I would really recommend useing concrete blocks with rebar sticking up in the holes then filling them with concrete.
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DRP
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# Posted: 11 Mar 2025 08:11pm
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Larry at the building supply handed me the new Simpson catalog. At lunch yesterday it flopped open to these moment resisting bases, cool! Most bases are just a pin with no rotational resistance at all.
https://www.strongtie.com/standoffpostbases_castinplacepostbases/mpb_base/p/mpbz
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MtnDon
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# Posted: 11 Mar 2025 08:55pm
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Very nice! Bases that can help hold a post in a vertical position. What an idea.
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gotdurt
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# Posted: 11 Mar 2025 10:21pm
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Quoting: Brettny I would really recommend useing concrete blocks with rebar sticking up in the holes then filling them with concrete.
Not really the aesthetic I'm wanting, but it sure would make things a little easier.
Very interesting... not much margin of error for placement though. It says for free-standing patios and pergolas, I wonder if it would be suitable for a cabin...
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gcrank1
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# Posted: 11 Mar 2025 10:28pm
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Im thinking 'pergola' strong isn't what Id want for a cabin
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Brettny
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# Posted: 12 Mar 2025 05:48am
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Have you looked into those post based price?
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DRP
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# Posted: 12 Mar 2025 08:07am
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Quoting: gotdurt Very interesting... not much margin of error for placement though. It says for free-standing patios and pergolas, I wonder if it would be suitable for a cabin...
Quoting: gcrank1 Im thinking 'pergola' strong isn't what Id want for a cabin
Quoting: Brettny Have you looked into those post based price?
As politely as possible, nothing proposed thus far is suitable for a habitable structure 
This is I think the first time we've had useable strength numbers. Pier type foundations fall outside of prescriptive building codes, having been stricken years ago due to high failure rates. The current section on piers says "designed according to accepted engineering practice" or something similar. Which amounts to, quantify load(s) and provide resistance.
Price... careful there, price as compared to a legitimate alternative or in comparison to some inadequate notion. I'm thinking price in comparison to the prescriptive solution, which is what I do most times. Most of the time prescriptive is the norm for a reason, it is the easiest or lowest cost way of achieving the goal above. Generally engineered solutions are not the cheapest solution to the problem, sometimes, but usually when I call him I'm trying to solve a unique problem not a money problem. For instance when the client says "Not really the aesthetic I'm wanting" That is when we call the engineer and let them hash it out.
In other words I find a prescriptive solution, or, I find or have done, an engineered solution.
My first question in an engineered scenario of this is then, what is the lateral load on the structure? Does the proposed solution provide adequate resistance? If we, or someone proposing some notion, cannot answer that, then its time to call in someone who can.
Which leads back to, I'd bet it is cheaper to do it prescriptively.
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