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rudius
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# Posted: 8 Apr 2018 02:16pm - Edited by: rudius
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I'm trying to wrap my head around the various building methods and I have some questions.
First, piers:
When building on piers with girders, how do people prevent lateral/racking movement, from wind or earthquake, on the girders? If you're using triple 2x10 or 2x12 that's a tall beam that could lean over and most simpson brackets don't seem like they could hold it if the building really shifted.
Second, skids:
A bed of compacted crushed rock seems to be common, with pressure treated 6x8 or triple built up beams as skids. However, this only puts your joists/building 8" or so off the ground. So are all the joists pressure treated as well? How are people protecting the portion of the building that's less than 18" high (which seems to be the minimum height you want to have non pressure treated stuff).
Thanks in advance. And yes I realize that a full poured footing or a pad is also an option. Just trying to understand how the above work.
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KinAlberta
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# Posted: 8 Apr 2018 05:12pm - Edited by: KinAlberta
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We have two cabins (on a lake) built in the 1950s (24x24, 24x30). Both on blocks in the ground (thin turf then clay soil with random rocks below the surface and about 5' and 8' respectively above the lake level/water table) and then sitting on mostly modest 24' 3ply beams. These girders may be toe nailed to the floor joists but otherwise there is nothing substantial. Both have held up remarkably well with minimal movement.
We've never experienced any measurable heaving. However letting eavestroughs to dump rather straight on the ground at two corners of the larger cabin allowed the ground to soften and a bit of settling has occured.
Besides now needing a bit of levelling this design presents a few problems (over a half-century later)
1) I can't get under the main cabin to insulate or foam the floor.
2) I can't get under the cabin to repair/reinforce/strengthen or add additional beams. So I can't now easily raise the cabin.
3) I can't now install slate or tile flooring because of potential movement
Assuming no high water table, the important thing in such a design I think is to have even freezing because any uneven water accumulation can cause heaving. A gravel pad would be great if it channeled water away from the perimeter. The ground under the middle of the cabin will be dry unless water flows under it.
I really do wish higher blocks or 2x12s or 2x14s had been used to allow underside access. Diagonal braces could easily stabilize the building. Or simply bolt through extra plys at key points making sure that these short pieces touch the block and the joists. Then they can't tip over.
Using such larger dimension wood would greatly increase the rigidity of the whole building and spread the load far better than just laminated 2x8s or so.
Screw in piers would be wonderful.
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Just
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# Posted: 8 Apr 2018 06:16pm
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How big a cabin are you thinking ?
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rudius
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# Posted: 8 Apr 2018 08:20pm - Edited by: rudius
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Maybe 15 x 20 or a little smaller. But it's a very theoretical project. Mostly just trying to understand how people navigate the potential problems that I see when people do choose to build on concrete piers or on skids.
I will say, it doesn't freeze in my area, so if I ever build I am fortunate. No frost heave. No snow load.
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Just
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# Posted: 8 Apr 2018 09:48pm
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I like to use 2 laminated 6x6 skids supported every 6 ft with some sort of concrete piers , there size being limited by soil conditions. I use pt rim joist and cover the bottom of the floor with 1/2in pt plywood . I have built a few and had good luck . I uses hurricane strapping and ground lags to hold down the cabin .
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rudius
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# Posted: 8 Apr 2018 10:57pm
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I thought skids go on the ground, girders are on piers?
You have any photos of your setups?
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Just
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# Posted: 9 Apr 2018 08:38am
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You maybe correct I just build them.
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rudius
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# Posted: 9 Apr 2018 09:24am
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Thanks. That first picture starts to get a little close to my first question about building on piers. Did you do anything to prevent lateral movement on the girders? How are they fastened?
You mentioned hurricane ties (which I’m familiar with and have used to connect joists to beams), and ground lags, which I’m not familiar with.
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Just
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# Posted: 9 Apr 2018 12:23pm
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The cement blocks are filled with cement , the Beams are not attached to the blocks , the beams are nailed to the joists only . The straping is attached to 4cement blocks buried 3 ft deep one at each corner and up-to the rafters.
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ICC
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# Posted: 9 Apr 2018 04:23pm
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Quoting: rudius I'm trying to wrap my head around the various building methods and I have some questions. First, piers:
Most on pier builders don't give enough thought to wind or seismic activity. You are right in that those 2x10, 2x12 built up girders can roll when things go wrong. One way to help prevent that would be to hang the joists between the girders using joist hangers. But even then you still have the main problem which is the piers can move in the ground; up, down, sideways, rotation. If you read the Simpson data sheets you will see the common brackets are designed to prevent uplift, hardly ever lateral rotational forces.
Quoting: rudius I'm trying to wrap my head around the various building methods and I have some questions. Second, skids:
Again, you are right, any wood that is 18" or closer measured at the lower edge or surface must be PT to meet codes. That can mean the subflooring too. Plus for PT the fasteners and any other metal like hangers, clips must be the approved HD galvanized type.
Some folks have used the mobile home type of anchors to try to make up for the loss of uplift protection that a proper foundation supplies. Better than nothing I suppose, but you still have the deficiencies of piers or just skids.
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cspot
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# Posted: 9 Apr 2018 05:05pm - Edited by: cspot
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Ours is on Piers with girders. We have 3 rows of 6 piers and we x braced on all 3 rows in between the last 3 piers as seen in the pics. We also braced going in the other direction between the last 3 piers. It is very solid.
 26273727407_6bcd18c97f_z by cspot12, on Flickr
I believe that our girders are a double 2x12. Our posts are 6x6's which were notched for the 2x12's to set on. We also had a some 2x12 running in between the girders. Since we were building on the weekend these were partly to keep the girders lined up prior to putting the floor joists on. Our floor joists set on top of the girders. We toe-nailed then into the 2x12 and then also put straps from the girder to each joist.
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rudius
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# Posted: 9 Apr 2018 09:25pm
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Quoting: ICC You are right in that those 2x10, 2x12 built up girders can roll when things go wrong. One way to help prevent that would be to hang the joists between the girders using joist hangers. But even then you still have the main problem which is the piers can move in the ground; up, down, sideways, rotation. If you read the Simpson data sheets you will see the common brackets are designed to prevent uplift, hardly ever lateral rotational forces.
The piers moving independently in the ground is a whole other issue, but I get why some locations would choose to use them.
Hanging the joists between them does help. One could also do some blocking I suppose, hanging a few false joists. Are there brackets you know of that prevent rotational force? I've seen many triple 2x10 girders standing on edge in these builds and I'm just wondering what they've done, or could have done, to prevent lateral rotation.
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ICC
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# Posted: 9 Apr 2018 10:43pm
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There are some HD Simpson brackets designed to be embedded in fresh concrete. I forget the name off hand. Assorted witdths IIRC. Something like $225 eachd IIRC. Most diy builders don't give it a thought or believe "it'll only happen to the other guy". Mostly they may be right on that.
You could also build a shear wall at both ends of the cabin girders. I know someone whom I helped to retrofit shear walls under his pier and girder foundation. That was much more work than it would have taken to do it right before the cabin went up. Shear wall is framed like a regular wall and sheathed with 3/4 PT plywood and firmly nailed to the piers and floor joists.
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rudius
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# Posted: 9 Apr 2018 11:22pm
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It's a gamble, for sure. And a costly one if it happens to not work out.
Diagonal bracing seems like a good compromise for those using piers and girders. Say you have 2x10 joists on top of 3x2x10 girders, brace diagonally at 30 deg from the joists to the inside of the girders, using short pieces of 2x10s. Nail the braces flat against the joists and attach them to the girder with joist hangers.
The shear wall is an interesting idea.
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ICC
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# Posted: 10 Apr 2018 10:24am
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Quoting: rudius The shear wall is an interesting idea.
They are best when engineered, but generally, they are nailed every 3 inches around the edges and 12" in the field. Nail length, diameter, head size all affect strength as does the sheathing and the framing lumber. Most often the nail spec sets the miminum penetration into the framing lumber to be 12x the diamtyere of the nail with 8D or 18D being commonly spec'd. Clipped head air gun nails not permitted as the heads pull through easier. Overdriving nails, common with some air guns, also weakens the joint.
My point of all the above is to try and make one realize the fasteners are often the weak point when bracing is used. A few nails at the ends of some diagonally placed 2x members often are much weaker than the diy person thinks.
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