Novice question about piers in a slab

[racerboy]

My neighbor is extending his garage/shop out by 8’ on one side and 8’ on the back. I was watching him spread out the gravel the other day for the slab and I noticed these big round ‘sleeves’ that went about 3’ deep. I asked him what they were and he said these were where the beams would go to support the walls/roof. He didn’t call them piers, but I did a bit of research and I think that is what they are called. Anyway, I couldn’t watch the whole process, but later that day, the cement truck came and I saw them pouring the concrete over the gravel. I didn’t see them pour it into the ‘sleeves’, but I’m assuming this all took place at the same time. My question is: if I decide to go with a slab for a concrete building (26x14) instead of a shed, do I need these piers as well? I did more reading on these and it seems that it is usually a slab OR a pier/beam foundation, not necessarily both.

 

[ConCretin]

It does seem to be an odd detail. It's not unusual to thicken a slab at the edges or where a column will land but I'm not sure what your neighbor was looking to achieve.

If the slab is built on a proper base, the bearing capacity of the soils won't be any better 3' down than they are at the surface. If the intent is to get below the frost line, it's not going be very effective if the rest of the slab 'floats'.

I have seen the use of integral piers where certain types of expansive soils are present but they are accompanied by a grid of grade beams that span between them to support the slab.

I suspect your neighbor assumed he needed additional support for the structure above but didn't really understand how to execute it.

 

[billconner]

So LL, are you saying you recommend against a slab on grade in a unheated post frame garage, where frost line is 36 or 48" below grade? Does that leave any answer for a slab on grade floor in unheated buildings other than SFPF? I thought it made sense to not connect slab to post frame structure. I would guess that ultimately success depends on soil type and drainage.

 

[duneslider]

The reason for going deep (36") is typically for frost reasons. Why this would be the case in the middle of a structure I can't say and suspect what LL said is true and they just didn't understand what was needed.

Typically, load bearing points are called out by an engineer. It sounds more like this person is shooting from the hip and doesn't have specs called out by the engineer.

 

[ConCretin]

So LL, are you saying you recommend against a slab on grade in a unheated post frame garage, where frost line is 36 or 48" below grade? Does that leave any answer for a slab on grade floor in unheated buildings other than SFPF? I thought it made sense to not connect slab to post frame structure. I would guess that ultimately success depends on soil type and drainage.

Not at all. When it comes to frost, the major concern is differential movement. I'm not an expert on pole structures but I suspect they do move a bit with the frost regardless of how deep you set the poles. The slab will move too but differently from the structure. This is why, as you say, It's best not to connect the two and let them move independently of one another.

I do like the idea of thickening the edge of a floating slab a little around the edges and adding a couple rebars even if it's not load bearing to provide a ring beam of sorts to hold the slab together and provide some support where you drive in. I just don't see the sense in extending it to the frost line.

Isolating the slab from the structure has the added benefit of allowing it to contract without restraint during curing thus minimizing shrinkage cracks.

 

[billconner]

I must be misunderstanding the arrangement. I see a floor slab not unlike in a post frame building and separate - not connected - piers for columns like in post frame. The sequence is unusual but it does allow a single pour for slab and posts. I of course assume the "sleeves" - sonotube? - extending to or above top of slab.

A picture would have helped.

 

[theoldwizard1]

If the slab is built on a proper base, the bearing capacity of the soils won't be any better 3' down than they are at the surface.

First, for newbies, a vertical piece of a building is typically called a pot or a column or a stud. Beams are heavy load carrying horizontal members and should place their load directly on to a post/column/pier. Joists are also horizontal and rest on top of a beam and are lighter load carrying members.

Much of what you are asking depends on local building codes and whether you are building a typical "stick" building (2x4 studs) or post and beam. Stick built is the most common. In the Midwest/Northern areas a foundation wall going down below the frost line is required. The studs transfer their load to this.

It sounds like your neighbor is doing post and beam construction. Again, depending on local codes, the post might be set direct into a dirt hole with or without a concrete "cookie" at the bottom. Sometime a reinforced concrete section is added to the part of the post that goes below ground (perma-column) or "pier" is poured into a round form. In all of these cases, they need to go below the frost line.

 

[ConCretin]

I must be misunderstanding the arrangement. I see a floor slab not unlike in a post frame building and separate - not connected - piers for columns like in post frame. The sequence is unusual but it does allow a single pour for slab and posts. I of course assume the "sleeves" - sonotube? - extending to or above top of slab.

A picture would have helped.

I didn't consider that......I assumed he augered some holes under his slab and filled them with concrete monolithically with the slab. That would be a whole different thing and might actually make more sense.

 

[theoldwizard1]

Not at all. When it comes to frost, the major concern is differential movement. I'm not an expert on pole structures but I suspect they do move a bit with the frost regardless of how deep you set the poles.

I would hope NOT ! If it does the piers were inadequate and/or inadequate gifts and corner bracing.

 

[ConCretin]

I would hope NOT ! If it does the piers were inadequate and/or inadequate gifts and corner bracing.

It's is not uncommon for frost to 'squeeze' piers and posts and lift them. I'm certain the same is true for the posts on a pole structure. It might not be noticeable to the naked eye but movement is inevitable when the soil freezes around the posts. This is why tapered precast piers or big foot bases under soon-tube piers are increasingly common. All the wood bracing in the world isn't going to stop the force exerted by freezing water.

 

[billconner]

Last time I put in piers with sonotube, there was a much larger base well below frost line. Where I live now, the soil is very well drained - sandy and slope to a creek a few hundred feet away. All I see suggesting is not foundations perform the same in every location.

If the op's neighbor has an existing well braced building and is setting columns on the piers - do we even know there is more than a roof planned? - it could be smartly designed. Too little known to know but I like the discussion. Thank you!