metal building foundation on expansive soil 4

[DoubleStud]

I am trying to understand this detail that was done by another engineer. The building has been built. Now we are trying to add foundation to support overhead crane. So the grade beam spans from drilled pier to drilled pier. They put void box below the grade beam to eliminate uplift from the expansive soil below it. My question is, what will that do if the slab next to it is tied/poured over the beam? Wouldn't the slab going to push up? I have never done one of these before but if I were to detail it, I would just pour the slab over (or ledge) the grade beam. This way the grade beam will prevent the slab at the perimeter from settling, but will not prevent it from an uplift.

I am trying to figure out how I should do the drilled pier cap for my overhead crane. How do I transition from existing slab to the pile cap? Do I want the transition to be free to slide? Add expansion joint? But on the other hand I dont want water to seep through the joint and make the soil swell. Thoughts?

 

[JoelTXCive]

Could you draw/sketch what you would like to do on top of the existing detail.

Also, where is the paving/final grade located at outside the building. (is this where your pier cap is going to go?)

I'm not following what you want to to.

 

[DoubleStud]

Joel,

I was just questioning the original detail. I am not going to build anything on the top of existing grade beam. The new drilled pier will be a few feet from the existing grade beam. I was just wondering how I should transition from the existing slab on grade to my pier/cap. Do I want the existing slab to slide up and down? Or do I need to dowel it to my new cap?

 

[BAretired]

I agree that if the soil is expansive, the slab is bound to pull upward on the grade beam, although #3@18 will not cause a very large tie force. It might seem better to do it your way, but if that is an exterior section, a joint opening and closing between slab and beam, exposed to view, is not really acceptable either. The section does not indicate a tie between pier and grade beam, which seems unusual too, possibly just an omission.

Some soils are considered expansive when they are only slightly expansive. It would be of interest to learn how much expansion is expected.

 

[dik]

The slab should not be attached to the grade beam; there should be a flexcel joint, and the slab should be floating. If it is attached, then it should be a structured slab on voidform, too not a SOG.

I prefer using plastic foam void forms that are fabricated for that purpose. I've done a couple of reports, decades back, where the cardboard void has collapsed and the void filled with water which has frozen and caused heaving.

Prior to a commercial product, I used to use aerofoam 'skins' bundled because they have a slightly less compressive strength than aerofoam... a few psi over a square foot can be a lot of uplift.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[gte447f]

Perhaps the slab is indeed a structural slab designed and reinforced for uplift? But then, what would be the point of isolating the grade beam from the expansive soil below it? In other words, I guess I have the same questions as the OP about the detail.

 

[dik]

Very unlikely... uplift forces can be several times greater than service live load forces. I know it depends on the type of soil, but does anyone have an idea of the magnitude of these uplift forces?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BAretired]

I am trying to figure out how I should do the drilled pier cap for my overhead crane. How do I transition from existing slab to the pile cap? Do I want the transition to be free to slide? Add expansion joint? But on the other hand I dont want water to seep through the joint and make the soil swell. Thoughts?

Water seeping through the floor in sufficient volume to cause significant soil swell seems unlikely. In any event, it will be more easily remedied than damage due to the slab heaving and tearing away from the crane foundation. I suggest keeping the foundation for the overhead crane separate from the floor slab.

 

[DoubleStud]

Water seeping through the floor in sufficient volume to cause significant soil swell seems unlikely. In any event, it will be more easily remedied than damage due to the slab heaving and tearing away from the crane foundation. I suggest keeping the foundation for the overhead crane separate from the floor slab.

So you are saying to not dowel the pile cap into the existing slab? I was thinking about pouring the cap under the existing slab so that it creates a ledge for the existing slab to sit on. This way it will prevent it from settling and creating trip hazard. Then maybe put a void form below the cap. Maybe also put a water stop/swell stop at the corner of the ledge to prevent water seeping through? Thoughts?

 

[dik]

I would not support the slab on the pile cap/pier.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[DoubleStud]

dik, what is your reasoning?

 

[dik]

There is an isolation joint between the slab and the pile/pier. If the slab lifts or settles it won't crack. Expansive soils can generally expand or shrink depending on moisture. For round piles, I generally use sonotube forms to extend the pile and accommodate the anchor rods; sawcuts go to the centre of the pile. If a rectangular pier then sawcuts can go from centre to centre or edge to edge (pinwheel pattern).

The depth of the cap/pier would be greater than the length of the anchor rods.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

Something like... Anchor rods not shown, but they would fit within the pilecap.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[DoubleStud]

dik, but the slab is existing and the control joints are already placed. Are you saying to add more joints?

 

[DoubleStud]

also the pier will be round. Since the slab is existing (already poured), I am planning to make the cap as square because they will saw cut it on the existing slab.

 

[dik]

No... that was just a standard detail. If the slab is cast and the soil is cohesive (often for swelling soils) I'd just sawcut a small square to accommodate the pile and excavate without forming... maybe that's the reason for the concrete lip supporting the slab. Just advise the client that there could be some slight cracking of the slab.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BAretired]

[li]Anchor bolts should fit totally within the pile cap. They should be located accurately, not easily done during pile installation.[/li]

[li]The saw cut slab, covered with A.I.F.B. separates pier and slab. Cut off any protruding slab reinforcement.
[/li]

[li]Slab support is not necessary. Best to let the slab move freely up or down, but if it hangs up on projecting concrete, no big deal.[/li]

 

[dik]

Yup BART, your detail is pretty much what I do... I always stipulate that anchor rods be set using a template, and not be installed at the time of piling and that's the reason for the pile cap... Just in case, I always stipulate that anchor rods be weldable...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BAretired]

Yes dik, to do a good job during piling, without a template would be difficult.

 

[BAretired]

Some further thoughts about the original detail by the previous engineer:

[li]If the section occurs at an overhead door, the slab must be tied to the grade beam, and I would suggest the section below.
[/li]
[li]If the section occurs at an exterior wall, it may be preferable to separate the slab from the grade beam to allow vertical slab movement. Some folks prefer top of grade beam a few inches higher than top of slab to facilitate washing the floor without splashing the wall finish.[/li]

[li]Insulation of the grade beam would prevent frost heave, but wouldn't prevent soil expansion.[/li]

[li]Finally, six inch provision for soil expansion seems like a helluva lot. Is it necessary? How long has the building been in existence? Is there any evidence of such large slab movement?[/li]