Steel column footing under slab

[Alan CA]

Hi everyone,

I saw this detail in one residential buildings in Ontario. It's a concrete footing of a tube steel column. The footing is located under the concrete slab in the basement. Between the slab and the footing, gravel was placed. Why wasn't the footing poured right at the slab level, extending under it as it's thicker, or beneath the slab with no gap in between the two? There would be no gravels needed to fill this gap!
What do you think about this arrangement? What are the concerns here?

 

[dik]

None, it's done often. By putting the gravel 'buffer' between the slab and the footing, it is less likely that the edge of the footing will 'telegraph' a crack on the slab surface of the shape of the footing.

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

-Dik

 

[dik]

I should have noted that it's often done with a concrete pier on top of the footing so the steel column is not exposed to the soil.

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

-Dik

 

[JAE]

We use a coal tar epoxy coating on the columns and base plates below the slab for this condition.

Agree with dik on the telegraphed crack concern - also many slabs perform better with a uniform layer of granular base underneath which also serves as a capillary water break to help keep moisture below from wicking up into the floor slab and damaging floor finishes.

 

[driftLimiter]

I'm working on a job where they did this and the slab still cracked like crazy. The best detail is a column blockout with concrete poured from TOS to TOF.

I'm fairly certain that the detail is subject to cracking due to differential movement and will inevitably happen.

 

[dik]

JAE... have to be careful. Many coal tar epoxies are carcinogenic...

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

-Dik

 

[dik]

I keep the water out of the slab by using PEVB... compacted Gran A doesn't free drain very well...

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

-Dik

 

[JAE]

have to be careful. Many coal tar epoxies are carcinogenic...

I've gotta quick pouring the epoxy on my pancakes...

 

[dik]

Just a caution... my work is designing connections for metal fabricators... whenever the EOR specs tar modified epoxies... I add the note... for fasteners I stipulate that cadmium plated fasteners shall not be used... just personal quirks... I have a few of them...

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

-Dik

 

[Alan CA]

Thank you everyone for your insights and replies.. All highly appreciated.

dik, speaking of fasteners, no anchoring of the metal column plates were installed at this site by the way. The argument is always that "meh, it's a residential house with no significant lateral loads"

 

[dik]

I don't do a lot of housing... but, I'd still use anchors...

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

-Dik