16x32 cabin pier and beam support question

Started by brivox, April 26, 2017, 09:34:11 PM

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brivox

Hello everyone,

This is my first post to the forum, though I've benefited greatly from lurking around absorbing all the excellent work and advice displayed here. I relied heavily on these discussions in completing my dream of building my own 16x32 cabin in Northern VT, and I am now here with a question I'm hoping you all might be able to help with.

It appears I have made the rookie error of building my cabin on a pier and beam system without embedding bolts in the piers to allow for the addition of lateral bracing and I'm worried this has resulted in the foundation beams being allowed to begin to lean a bit as shown in this photo.



The structure is built on 15 piers, 3 rows of 5, with 24" footers. On each pier sits a Simpson bracket holding a beam made of tripled 2x12 PT, making 3 beams in all, 32' long each.  The boards are joined with nails staggered every few inches, but no thru-bolts or structural lags, which I'm also beginning to think might have been a mistake. The walls are 10' 2x6, with a 12/12 pitch roof. The cabin was built a couple summers ago, so it's been through two wet and snowy Vermont winters so far.

I initially thought the lean in the beam might have been caused by wind forces but I would have expected all the beams to have shifted in the same direction/degree but that doesn't appear to be what's happened. Instead, it looks like the outer beams are both leaning slightly toward the outside, away from each other. This led me to think perhaps it was a load problem pushing them apart, but really I'm not quite sure what's caused it, or what I might be able to do about it, or how urgent a problem it is. 

So that's what I'm hoping you all might be able to advise me on. 1) Any ideas what might be causing this? 2) If it's a lateral support issue is it possible to brace these beams without having existing attachment points on the piers? Any other ideas? 3) How urgent/dire is this?

I'm also wondering if you think these beams need to be skirted for protection from the elements? They're pressure treated, but do get quite wet during rainstorms.

Happy to provide any other information I might have missed.  Here's a few more photos.

Brion


akwoodchuck

Quick fix: run blocking in between the beams, perpendicular to them....every 4' or so should do, wedge em in tight and toenail to center beam, some steel angle and lags would be better than toenails at the outside beams.

Permanent fix: lift it up and put it on a better foundation.....
"The lyf so short, the craft so long to lerne."


Bob S.

Mountain Don put a pressure  treated stem wall under his cabin to protect against a problem like this.
  Bob

MountainDon

Just because something has been done and has not failed, doesn't mean it is good design.

brivox

Thanks for the responses everyone. Just a couple follow up questions. In MountanDon's example there were wood posts to affix the wall to, how would this work with concrete piers that run all the way up to the beams? How would the wall be fixed to the concrete piers?

Is this method meant to stabilize the piers from shifting laterally, the beams, or both? I'm not sure I've had a problem with the piers moving, but the beams do appear to be rolling outward.

If stabilizing the beams is the goal, is there a benefit to doing the wall versus blocking between the beams as akwoodchuck suggested?

If I were to go with the blocking between the beams would screwing into the ends of the blocking wood through the face of the beam be preferred? I imagine this would take some 6 or 7" screws, or is toenailing to the inside of the beam sufficient.

Thanks again for the assistance!

Brion


Don_P

This is the reason that joists and beams are required to be laterally braced to prevent rotation. I would probably use angle brackets between the blocking and the girder or saw the ends of the girders back and run a band across the ends of the girders... at this point with blocks on the inside face to prevent further rotation. If the girders are attached to the floor joists those tops aren't rolling out, you're missing something.
The rafters are also untied, that is generally not a good idea.

brivox

Thanks for that clarification Don_P.  I realize now how important that lateral bracing is. I mistakenly thought since I did concrete piers all the way up to the girders that the system would be significantly stronger than one built on posts, though posts appear to make this lateral bracing much easier.

The floor joists are nailed into rim joists to create the floor box which is sitting on top of the girders. The rim joists are toe nailed to the girders every 12" or so, but that might not be a strong enough connection to prevent the girders from turning out? Or, as you say, maybe I'm missing something. Perhaps the piers have shifted. I'll check if they're plumb next time I go up.  I think for now I'll go ahead and put in the blocking with angle brackets for connecting to the outer girders. How long should the lag into the girder be, and what would you use to attach the bracket to the blocking since I assume it's only 1 1/2" thick?

Also, Don_P, I had actually read here quite a bit about the necessity of rafter ties, and I do have them, just not on the loft side, the other 16' behind the point of view of the camera have 5 3"x8" hemlock rafter ties, each spaced about 3-4 feet apart. A structural engineer I spoke with said the loft joists should serve the same purpose since they are tied into the doubled 2x6 studs that I put in the loft area. The loft joists sit about 8' up a 10' wall, so they're holding the walls together there, which means the weak point is the 2' of stud above those loft joists. The engineer thought the doubled 2x6 studs in that area would be enough to counter the deflection forces in that 2'. Does that sound like a reasonable design?

Don_P

Well, you are drawing a conclusion that bracing one form of pier is easier than the other. If you look at prescriptive methods they use walls, rigid planes to support and brace the structure. These work better. When you step outside of that it is a good idea to have an engineer design that support and bracing.

The blocking between or rim across the end of beams or joists is to prevent rotation. That along with continuous support on the top edge of a beam is a requirement that is often ignored but again, that works. The attachment of blocking is simply to hold it in place.

My comment on rafter ties was because I saw what looks like at least 8' of untied rafters in the center of the building, where we usually see a building go swayback. If the engineer is happy and you are happy then that is fine. The last rafter table in the codebook shows attachment requirements for that connection on up to 2' centers. As you skip ties the forces need to be accounted for and resisted.

Hope that all makes sense.

akwoodchuck

Quote from: brivox on May 01, 2017, 06:10:01 PM


The floor joists are nailed into rim joists to create the floor box which is sitting on top of the girders. The rim joists are toe nailed to the girders every 12" or so, but that might not be a strong enough connection to prevent the girders from turning out? Or, as you say, maybe I'm missing something. Perhaps the piers have shifted. I'll check if they're plumb next time I go up.  I think for now I'll go ahead and put in the blocking with angle brackets for connecting to the outer girders. How long should the lag into the girder be, and what would you use to attach the bracket to the blocking since I assume it's only 1 1/2" thick?



I would just use Teco nails into the blocking and 1\4" x 4" lags into the beam...should be plenty of extra beef. Course it'll make it even harder to work under there but thems the joys of a pier foundation.... ;D
"The lyf so short, the craft so long to lerne."