A good option for those considering a pier foundation

[mgramann]

As many of you know, I am in the process of building in the Michigan Upper Peninsula.  I will be doing a wrap around porch on 2 sides,connected to the house with the ends supported by sonotubes.  Building this time of year adds a thick layer of frost to the mix, making the installation of these piers challenging.  One common option is to jackhammer then auger, but that is very time consuming.

One common issue with sonotubes in frost heavy areas is that they have been known to lift, even when installed to the depth dictated by local code.

My search revealed a very good option that solves both of these problems.  Rather than jackhammer and bore, this company uses a backhoe to dig trenches in line where the tubes are to be installed-much quicker than an auger, especially when frost is present.  They pour a full length footing at the bottom of the trench with rebar poking up where the tubes are to go.  They then set the tubes over this rebar, pour the piers and backfill when dry.  The footing links all of the tubes together, and prevents the tubes from lifting.

It definitely uses more concrete, but I suspect it is much stronger than a typical pier foundation.

I apologize if this information is a repost-but it was new to me, and the route I am taking with my build.

[Redoverfarm]

I do recall a few that have done that on the board.  I am not an engineer but it would appear that your lateral stability would be comprimised with the absence of undisturbed soild seperating.  If you could devise something in the mix to take it's place I would say you might be ok.   The major downside of pier foundation is that lateral stability and most using will brace to prevent.

[JRR]

To my mind, it is a superior plan.  Once the backfill is atop the footing section, that thing should go neither up or down!  If I ever did a pier foundation, I had already planned to go this route.

[MountainDon]

That is very much along the lines that is used by the folks who bring us the "raised floor"  This is an image of it being used in the South, for lifting above termites, so not much depth.



But it does raise the question of, why not build a full perimeter foundation since the trench is already dug? 

[Don_P]

I'll argue that there is never undisturbed soil around a sonotube... shovelling fill around a column provides zero lateral bracing. The point of rotation of a soil embedded pier is not at the groundline where people envision it, it is rotating around a point about 1/4 height off the bottom. Hook your truck to a stout fencepost in a dug or augered hole and watch how it comes out. It's rotating and failing below ground line. That lever of post with a horizontal load up top is effectively bracing against a lot shorter height of very questionable soil than most people realize.  (And that old farmer did tamp well around the fencepost).
Forget the soil as the brace, make a real brace some other way, a good project for an engineer.

The continuous footing is a code requirement and is hidden from view in most situations. When we see pictures on the net of what folks think are "pier and beam" foundations they are very often pier and grade beam foundations. That "footing" they are pouring is thick enough and has rebar top and bottom to form a horizontal beam that will not break between the piers as they deliver load. It is not a strip footing. I'll lay odds that is what we are seeing in Mt Don's pic. The piers in turn are reinforced with rebar that is tied with bent L's to the grade beam steel. It works and is another engineered solution. The closest code comes to that without engineering is to build a 4" thick wall between piers to provide lateral bracing to the piers. Personally, if the lateral conditions are not severe I'd be happy to see ~4' of wall out of each corner,  make the wall sections at least as long horizontally as they are tall. The bracing triangle that forms is now a 45 degree angle, reinforced or not, try to tip that over. A sonotube 12" wide and 4' tall forms a bracing triangle that is looking closer to an 89 degree angle, that's a tall order to keep upright even when reinforced. The trench is open, the footing is there, I'd build at least wall corners. Or do it correctly and build full walls. Then the perimeter of the floor is fully supported, the walls are over continuous support, the building becomes a strong box structure as opposed to a heavy box on a set of table legs. Sort of like rebuilding an engine on the kitchen table. It can work but don't bump the table hard.

[Redoverfarm]

I do recall a few that have done that on the board.  I am not an engineer but it would appear that your lateral stability would be comprimised with the absence of undisturbed soild seperating.  If you could devise something in the mix to take it's place I would say you might be ok.   The major downside of pier foundation is that lateral stability and most using will brace to prevent.

Don_P that was what I was referring to in that just the tubes themselves on the footing without some type of lateral bracing would/could cause a hinge point where the tubes lay on the footings. No doubt that a trench footing would be superior but then again any footings in addition to the sonotube pour would be better than none at all.  With the use of rebar for quite some time and discussions with several concrete contractors rebar has little shear strength in comparison to tensile strength.  I had used it to attach a 8' high 8" poured retaining wall to my house some years ago.  There was 5-6 pieces of #5 used as pins between the basement and the house.  Over time the soil and freeze/thaw pushed the wall snapping off the rebar until it lisp 5" from it's original plumb location. I never really put it into prospective until I though of a rebar cutter which only used a minimal amount of pressure to cut it.  It was since repaired with deadman and pushed back to it's original location.

I doubt that the depth of the sonotubs will be such to have that amount of pressure but put a sail (house) on top and the right conditions and it may lisp some w/o some bracing.  General practices here with sonotubes is not to place the tube completely to the bottom but to allow it to protrude up out of the ground to give a finished look and provide a form above ground.  They are placed just below the surface.  The hole created in digging the diameter is filled with concrete using the ground as a form for below grade with the exception of a small portion of the tube which initiates below the surface.  That way there is not filled area. The one disadvantage of sonotubes is that there is not anchor point but if the frost line is penetrated the tube pour will not heeve but the gound surround the tube.  Of course the soil conditions play a major role in the stability of the tubes and rebar is used in the conventional fashion. 

[mgramann]

What about this method used for a porch as I plan to? 

Another option was also suggested.  What about sonotube footings?  http://www.menards.com/main/building-materials/concrete-cement-masonry/forming-tubes/28-sonotube-square-concrete-footing/p-1398448-c-5652.htm

I'm trying to figure out what my best option is, knowing that a full wall for a porch is probably overkill, and likely not financially feasible for me.

The covered porch is on 2 sides, extending out 10ft from my 40x24 structure.

[flyingvan]

Around here, you don't have to look far to see porch floors sloping at funny angles away from the house.  The cabins sit on nice perimeter foundations then the porch columns sit on concrete plugs or piers; the water that runs off the porch roof softens the soil enough that after a decade or so the porch columns start to settle different than the main foundation

[mgramann]

That is what I hope to avoid, which is why tubes alone aren't an option.  Since I'm not supporting an entire house, I think the tube/footing combination might be best.

[Don_P]

Ahh, I read your op too fast. I don't mind at all piers or what you're doing if it is supporting a porch or deck. The strip footing can be thickened or widened if needed for the column loads. With a strip footing in place if the porch is ever enclosed and bracing perimeter walls are needed below, the footing is there.

Here's how I interpret the requirements and intent. If the main lateral force resisting system of the structure of the house, including the braced foundation walls, the braced exterior walls, floor and roof diaphragms form a laterally stable structure. Then non habitable structures can be attached to that solid frame. The floor of a deck forms a semi rigid diaphragm attached to the rigid house (the codes are making that more rigid and better connected to the core structure every cycle). Add a roof sheathed with panels and you have a rigid diphragm locked to the main building's lateral support. The pier is then resisting vertical load and the lateral is resisted by the diaphragms connected to the building. If the footing is adequately sized for gravity, you're done. I prefer to run a 6x6 all the way down to it to avoid a hinge at mid column between a pier and post, or run the concrete column from footing to girder in one unbroken piece. The horizontal is taken out by the floor deck, the load is supported by an adequate footing. I don't care if you backfill it at all or with popcorn, the soil is not the lateral force restraint. The deck is a horizontal beam connected to the rigid structure, that is the columns lateral restraint.   If the house is on piers, there is no laterally stable structure to attach to, that's where I was asking for some form of bracing wall segments.  As you get into stronger frost action, pouring against the hole gives the frost more to grab onto if it is trying to lift the pier, you want relatively slippery walls to frost depth. That doesn't seem to matter here, it does as you get into more cold.  Where I've suspected a porch migh become enclosed we've discussed it and sometimes have dug the trench and poured the strip footing then covered it. It's there and easy if they want to expand that way.

[mgramann]

I hear many different things.  Some recommend sonotube only.  Some recommend "big foot" sonotube footings.  The other recommendation was the full footing with the tubes.

Sonotube only is out.  It may work fine, but as flyingvan said, over time they could move.

If it were spring, and I had time to wait for material, I would considering going with the big foot, and bore the holes.  Unfortunately, the turn around time on ordering them is 18 days.  The contractor needs them sooner.

Since I'm getting a full trench anyway, I think the full footing makes sense.  It means extra concrete, and an extra trip charge, but I think it is just enough overkill to stand the test of time.  It is also much more affordable than a full wall.

One question regarding that footing.  Is it worth making a "pretty" form for, or better just to let the concrete follow the form of the trench?  It would save time, and the footing would be in direct contact with "undisturbed" soil.

[Don_P]

We pour against the soil, square shovel it crisp and square. The rebar is supported on chairs and is no closer than 3" to the edges.

[Redoverfarm]

I hear many different things.  Some recommend sonotube only.  Some recommend "big foot" sonotube footings.  The other recommendation was the full footing with the tubes.

Sonotube only is out.  It may work fine, but as flyingvan said, over time they could move.

If it were spring, and I had time to wait for material, I would considering going with the big foot, and bore the holes.  Unfortunately, the turn around time on ordering them is 18 days.  The contractor needs them sooner.

Since I'm getting a full trench anyway, I think the full footing makes sense.  It means extra concrete, and an extra trip charge, but I think it is just enough overkill to stand the test of time.  It is also much more affordable than a full wall.

One question regarding that footing.  Is it worth making a "pretty" form for, or better just to let the concrete follow the form of the trench?  It would save time, and the footing would be in direct contact with "undisturbed" soil.

No one is going to see it As long as you have the necessary width & depth.  I have formed up footings in circumstances where your footing happens to rip wide while being excavated because of rock or shale.  This was only done for financial reasons to keep the cost down.  Doesn't seem like much but if 75% of the footings is 6-10" wider than need be that is a chunk of change at $145 a yard for concrete. 

The undistrubed soil in a footing would be a concern for what is under it not necessairly on the sides.  If the soil is disturbed under the footing then it would allow the footing to settle and possibly crack.  Everything above it would also settle and you can end up with cracked drywall, windows not working correctly, doors sticking and etc...

[mgramann]

We pour against the soil, square shovel it crisp and square. The rebar is supported on chairs and is no closer than 3" to the edges.

Thanks.  To save money I may end up doing some of this myself.  How much rebar would you recommend?  My though was 2 lines in the footings, and 2 lines projecting upward into the 12" tubes.

Also, do you think it would be possible to perform all of this in 1 pour?  Dig the trenches, suspend the tubes, set the rebar, pour the footings, then fill the tubes?  Or do you think the concrete would "equalize" out of the bottom of the tubes?  Perhaps that is taking too much on for 1 pour, especially for a beginner.

[Don_P]

Actually, it would not flow out the bottom if poured in stages. When you pour steps or something with an open bottom do it in stages. For this, first, I would not let the driver add any water to the mix, don't increase the slump. Then pour the footings full to the bottoms of the tubes, put a little in the tubes but not so much it is pushing out. Keep moving down the line doing that. When that phase is complete go back to the beginning which is stiffening up and begin to fill the tubes. Take a scrap of rebar and consolidate the pours together at the base of the tube. if it begins to flow move on and leave it alone you can remove around the tube later if you need to, let that lift solidify some while continuing down the line. You may be able to do the tubes in one lift or it may take several. ICF's are poured this way as well. If you filled them full wall height in one lift they would blow out. Keep making rounds and rod the lifts together.  I prefer at least 3 bars in the tube forming a wide triangle (think of the steel forming a "tripod" type support inside as opposed to a single wimpy rod or a flat resisting plane formed between 2 rods) but leaving at least 2" of steel cover above grade. They should have a hook bent in the bottoms of them, otherwise if there is a tipping moment it'll just withdraw the rebar out of the footing, put a hook in the end and it's locked in the footing, tie it to the pair of footing bars and it's really locked.

[UK4X4]

"Two pours . . .    Concrete would pour out of the footing if you tried to pour tubes at the same time"

Don_P articulated perfectly

heres an pictured example- of a single pour -walls -footings -waffle beams all in one

reply 36

http://countryplans.com/smf/index.php?topic=9588.25

My front deck and the porch is all part of the whole foundation - nothing seperate all one unit to stop seperate movement.

yes cost is involved- do it once- do it right

or at least as good as you can- if you tied the lower footings into the house foundation with rebar and a 45deg concrete fill and then used posts or pillars from that up to the deck you'd be pretty well fixed.

the 45deg would be the reverse of my front porch ears

If its existing - then you'd need to drill and resin in rebar to link the footings together

[bayview]

Thanks.  To save money I may end up doing some of this myself.  How much rebar would you recommend?  My though was 2 lines in the footings, and 2 lines projecting upward into the 12" tubes.

Also, do you think it would be possible to perform all of this in 1 pour?  Dig the trenches, suspend the tubes, set the rebar, pour the footings, then fill the tubes?  Or do you think the concrete would "equalize" out of the bottom of the tubes?  Perhaps that is taking too much on for 1 pour, especially for a beginner.

I stand corrected!  One pour if you have an experienced crew . . .

mgramann was suggesting he was going to do it himself.    Seems like a lot of work for a beginner to do in one pour. . .

Uhh, how deep and wide were you going to make the footing?   Maybe someone can suggest size of footings, size of rebar, and how much rebar for the footing.

/.

[Don_P]

There are a couple of upsides to pouring it all at once, there is no cold joint at the base of the pier, and there won't be a trip charge for that second light load. What cement does as it is setting, from the moment water is added, is try to grow needlelike crystals that go in every direction and interlock into a solid. Once you stop mixing these needles begin to lengthen and knit the concrete first into a mush and then into a solid. What you're doing by running around the pour is letting the slush begin to knit up enough to support weight without flowing. When you rod the new lift into the old you're breaking up, mixing and reforming crystals across that break in the pour. When you see the surface of a slab flaking and dusting off with lots of surface cracks it is often the result of someone trowelling too enthusiastically. The intent there is to break up the surface crystals into smaller ones, pack and densify the surface while smoothing it then get off it and let it reknit. When they trowel till it's set there is little if any crystalline knitting going on, they've pounded it back to dust rather than letting it form a rock.

For a strip footing 8x16" with 2 strands of #4 rebar is a pretty standard detail. For spot footings in a ripped trench I've also had them dig a 2' wide trench, dam off 2' long footing areas and pour them with 2 bars each way in the pad #.  As an aside the number in rebar is eigths of an inch, a #4 is 4/8" or 1/2" diameter. And a story, a helper and I went to pick up a 20' section of 4x4 steel tubing I had found at the salvage yard and as we went to put it on the trailer my helper just about buckled under the load. It's about 12 lbs/foot which isn't light but come on. I ribbed him a little bit. We got it back to my place and went to unload and it happened again but this time we dropped it to the ground. That was when we heard something inside. Fishing around a bit we pulled out a 10' section of #12 rebar someone had slid in his end of the tube... another 50 lbs.  I allowed that I sure appreciated him smuggling that piece out for me like that  . It's reinforcing a suspended fireplace hearth now, that thing isn't going anywhere.

[Redoverfarm]

Might check around at your supplier for salvaged. In salvaged I mean some that was ordered and never picked up.  I was luckily to find a supplier had a stack that was mixed from #4 to #9 that was ordered in and never picked up by the customer.  It was something that he didn't normally sell.  I got all for $4 a stick.  Never really priced #9 but I would imagine that price would not cover half the original price.  A little bit harder to work with in that there was no bending only cut and tie.  Normally I use 12-16" grid but this larger I stretched it out to 24".  But I doubt that my basement and footers are going anywhere anytime soon.  Only problem with using such large diameter in a 4" floor is being able to embed it midcenter of the pour.

The largest I have used was 1-1/2" which was also salvage from an industrial build site.  I used it in my 12'X50' bridge which has a 7" concrete deck.

Earlier you had mentioned form for your footings.  You can see on my footing there was certain areas that had to be narrowed because of the shell they gained to almost 3' in width.  The upper portion of the picture wasn't enough to worry about but the lower I definitely had to form it.

http://countryplans.com/smf/index.php?topic=3613.msg40670#msg40670