Slab on ground on foundation 1

[AValdes]

Hello everyone,

We have a big Warehouse with superficial square 2ft deep footings. What are your thoughts about having an 8" slab on ground poured directly over the footings? Do you have any Code / Manual requiring any depth of sand or anything else between footing and slab?

I appreciate your help!

 

[Lomarandil]

You're right to want some separating material, to reduce the risk of SOG cracking due to differential support conditions.

What the magnitude of that material is highly dependent on what the subgrade looks like (where the slab and footing do not overlap).

 

[milkshakelake]

I don't have much experience with compressible soils. But for good sands, you can just pour it on top. I don't see what can go wrong. Use reinforcement to deal with differential settlement.

If differential settlement is not an issue and your soils are great, your shrinkage reinforcement will still probably be rebar instead of welded wire mesh. It will have a bit of flexural capacity. Also, there is an old-ish slab on grade manual, but I forgot what it's called.

 

[ads0221]

What specifically is the 8" slab supporting within the warehouse?

 

[hokie66]

Separate them. Differential settlement should not be an issue over 2 ft with adequate preparation. However, the footings are likely to cause restraint shrinkage cracking in the slab if separation is inadequate.

 

[AValdes]

Thanks very much, everyone! At the end there was enough space for 1" of sand between footing and SOG (I am not sure that will help in any way but made the owner happy).

 

[Alistair_Heaton]

its quite common to do this as a mono pour including the footings in some parts of the world.

floating slab foundation its sometimes referred to or raft . Aircraft hangers are done with it.

You can do heated and unheated. Insulated and uninsulated.

 

[dik]

It depends on the soil. You can have the footings integral with a 'stiffened slab'. The slab could be 5" or 6" or whatever, with the edges thickened to a 1'-6" x 1'-6" edge stiffener. Footings and edge stiffener formed by the soil, not formwork. Something like... You can even include extruded polystyrene insulation in areas where freezing occurs.

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

-Dik

 

[Alistair_Heaton]

The diagram above is missing the reinforcement along the bottom of the footings which is then linked up to the upper grid.

The other advantage is that the whole thing resists movement. If one side wants to go down there is push up due lack of places of soil to go and suction on the other side preventing lift. Hence why they are referred to as rafts.

And the whole building moves as one if it does move. You want to level things up again its relatively easily done.

Belt foundations as they call them where I am tend to be a knife cutting into the soil if there is subsidence. Which means corners go or sides. The raft you can just pump under the corner to stop it but there is no structural damage above the slab raft.

I believe you can count it as a shear diaphragm for lateral loads on the base transmitted to the anchors for the base of the columns.

 

[dik]

The reinforcing is shown on the floor plan for the footings...

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

-Dik

 

[Greenalleycat]

There is no one right answer to this but in general I'd say you don't -need- to have any particular separating layer

However, the devil is in the detail to give advice relevant to your specific context - is it a new or existing building?
Is this slab 'structural' (i.e. tied into the footings/lateral elements) or is it a wearing surface only?
Is there heavy machinery (e.g., forklifts)?
How is moisture managed (where do you put your polythene layer)? Where is the groundwater level?
What are the underlying soils?
Is it a particularly wet or dry climate?
Will you be reinforcing the slab (I hope so)?

 

[canwesteng]

I would have either cast monolithically with the slab (assuming a reinforced slab), or for plain concrete SoG cast the footing to top of slab, and have an isolation joint between footing and slab (probably harder to get vapour barrier in the right place). Not clear what the sand break is achieving in this case.

 

[dik]

From the details above, it is monolithic and formwork is created by the excavated soil except for the perimeter.

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

-Dik

 

[Alistair_Heaton]

It's strange how this setup is pretty much standard in some countries but literally 100m away across the border it's deemed highly dangerous and unprofessional to even suggest

 

[dik]

It's generally determined by the type of soil. Regarding the border issue, it's likely jurisdictional.

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

-Dik

 

[Alistair_Heaton]

its virtually exactly the same EU design codes.

Just one side from architects through to tradesmen are very against it and will refuse to do it and the other its basically the starting plan until things mean it can't be used.

 

[dik]

Why against it? It can often be a viable foundation type and can be very economical. I've done dozens of these (maybe hundreds; I quit counting after the first bunch).

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

-Dik

 

[Alistair_Heaton]

I really don't know apart from resistance to change.

My barn was done by myself with one.

If it had been up to the first two architect's I should have dug it out or cut round the footings so the middle was floating.

The final architect was alot younger and went for it. His mate who signed it off structurally spent a couple of weeks researching that it was actually done alot in some countries. Spent half a day going through my calcs with me.

The pair of them are using it on other projects now but using a building crew from Poland who came back from UK and didn't really fit back in at home and do these international design team project's.

 

[Greenalleycat]

I'm confused @dik and @Alistair...what are people in your jurisdiction scared about?

Industrial structures here use a variety of different foundations and it's common to have both attached and floating slab designs

 

[dik]

I'm not aware of any... but, it can be a local issue. I've often used them on soils that are unsuitable for other foundations, mainly poor soils with low bearing pressure or Soil modulus. They seem to hold up very well.

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

-Dik