Foundation problem (need some creative ideas)

mldrenen · July 14, 2010, 03:01:42 PM

I've started a concrete pier foundation for a small storage shed/cabin that i'm building on land that i hope to someday build a house on.

initially, i wanted to build a foundation similar to ones recommended in john's plans ("concrete tube pier" http://www.countryplans.com/foundation/index.html ). however, after speaking with a local contractor, he suggested i just save some money and hassle and put it blocks, seeing as it is only a shed.

i already had holes dug, and sonotubes purchased, so i figured i'd split the difference. i'd pour the piers, and just lay the shed on top, letting gravity do all the work of holding the building in place. now i'm not so sure that's a good idea.

my site is slightly sloped, and the seasonal high water table is roughly 24". i'm also on a hilltop, and get some wind. i'm getting nervous about my plan.

i've only poured two piers, and would like to get some advice about the best way to continue. should some sort of "j" bolt for the remaining piers? how can i brace my piers to withstand any lateral forces? is there anything i can do about the two piers that i already poured, or should i leave them alone so as not to interfere with their structural integrity?

Erin · July 14, 2010, 03:07:56 PM

You shouldn't really have to worry about lateral pressure. The benefit of this type of foundation, afterall.
But you do want some way of tying your building down to your foundation...
J-bolts would be a good way to do this (and tie your sill down with the bolts, right?)

July 14, 2010, 03:28:12 PM

Quote from: Erin on July 14, 2010, 03:07:56 PM

J-bolts would be a good way to do this (and tie your sill down with the bolts, right?)

that's what i was thinking. i'm wondering how much of a problem it would be if i didn't have those j-bolts in the two piers already poured.

Erin · July 14, 2010, 03:38:33 PM

I wouldn't do it.

Get a hammer drill (or even a standard drill and masonry bit, considering how green your concrete still is) and drill in some bolts. Either with expansion sleeves, or epoxy them in.

Don_P · July 14, 2010, 09:33:02 PM

Park a car on the dining room table and give it a healthy shove. A building is a sail, the wind provides the lateral pressure. Measure the square footage of the sail, vertical projection of wall and roof, multiply by pressure at design wind speed. If you want to guess... 15 pounds per square foot or more for most places , and decide if that lateral load needs to be worried about. Using shallow unbraced piers of any appreciable height is something I would be concerned about.

July 14, 2010, 10:16:19 PM

You should get a bracket in the piers you have but you might consider doing the rest w/ PT 6x6 posts on a gravel or concrete footer rather than tube piers. That would give you the chance to cross brace to the beams and joists for the other piers.

How tall above grave is the tallest pier and how deep does it go below grade?

mldrenen · July 15, 2010, 07:29:15 AM

Quote from: John Raabe on July 14, 2010, 10:16:19 PM
You should get a bracket in the piers you have but you might consider doing the rest w/ PT 6x6 posts on a gravel or concrete footer rather than tube piers. That would give you the chance to cross brace to the beams and joists for the other piers.

How tall above grave is the tallest pier and how deep does it go below grade?

tallest is probably 16" above grade, and 30" below grade (sitting on bedrock).

i had a conversation with my brother, who happens to be a contractor, and he recommended that i finish pouring the tubes the same way as i poured the first two. when all are poured, snap my square lines across them, and hammer-drill some holes where i want to sink some bolts. he uses bolts with expansion sleeves, mounts some PT 4x4's, and builds on top of that.

does that sound reasonable? if i use 12"-16" 4x4 posts, will that leave me enough surface to brace against lateral forces?

mldrenen · July 15, 2010, 07:31:12 AM

nm

JRR · July 15, 2010, 08:14:10 AM

On the existing poured posts, I would install a custom external clamp. If you use bright metal chain to wrap, a couple of wraps, around the post ... and bright metal bolts and washers to tie the chain end together (don't use the very "end" links) you will have a very effective anchor that will bite the posts very securely. From the chain-anchor you can tie down the sills or whatever ... think more chain and bolts. Be use to install the system so that there is downward "pre-load" from the sill all the way down to the post/pier. You could drill thru a link or two and install concrete anchors ....tying the chain to the concrete if it will make you sleep easier.

July 15, 2010, 08:37:58 AM

Quote
Park a car on the dining room table and give it a healthy shove. A building is a sail, the wind provides the lateral pressure. Measure the square footage of the sail, vertical projection of wall and roof, multiply by pressure at design wind speed. If you want to guess... 15 pounds per square foot or more for most places , and decide if that lateral load needs to be worried about. Using shallow unbraced piers of any appreciable height is something I would be concerned about.

Oh.
I must have been misreading the OP.
I thought he was worried about lateral pressure on his foundation. Wind sheer, of course, is why he wants to tie his building down, though!

Quotehe uses bolts with expansion sleeves, mounts some PT 4x4's, and builds on top of that.

This is what I'd do, not to mention, what I've seen others do over the years...

So far as clamps or brackets though, I'm wondering why (how!) those instead of bolts. If a sill is going straight over the tops of his tubes, bolts would work better, wouldn't they?

Don_P · July 15, 2010, 09:03:55 AM

Quote from: Erin on July 15, 2010, 08:37:58 AM
I must have been misreading the OP.
I thought he was worried about lateral pressure on his foundation. Wind shear, of course, is why he wants to tie his building down, though!

We tie down to restrain uplift and brace to restrain lateral loads, wind/seismic

Erin · July 15, 2010, 09:25:17 AM

Well wind shear goes both ways, but I dunno....

I've seen enough old buildings shoved off their foundations due to wind, I'll tie down for lateral, too. lol

Don_P · July 15, 2010, 04:19:40 PM

You are exactly right

The hold down also transfers the lateral loads from the building above to the foundation, and then the foundation needs to be braced to withstand that lateral load. I was posting too fast on a break between runs. Got out a load of oak logs this morning and went back to meet the concrete truck to grout piers with hold down anchors. Our main lateral restraint/ bracing on this house is a root cellar/basement under ~1/4 of the building.