Mat foundation

[Nikkib]

During the proposal phase of my current project, the engineer completed the proposal documents was talked into turning a 24" mat slab into a 12" monolithic slab-on-grade and exterior grade beam type of system. There are also thickened areas under column footings. I took over the project recently and was beginning to draw a few typical details and came up with an issue. If the slab-on-grade is monolithic with the column footings and the exterior wall footing, how do you conceal the base plates? I have worked on mat foundations before where there was actually a layer of sub-base above the mat and a separate slab. In that situation the columns went through the slab down to the mat and an isolation joint was placed in the slab around the column. It was suggested that we simply recess the base plate and have an isolation pour around it, but I am concerned that it invites the structural slab to crack at those locations. Has anyone done anything similar or have any good ideas other than adding sub-base and an additional slab? Thanks!

 

[KootK]

I've recessed columns before. No cracking issues that I'm aware of. Just keep an eye on punching shear with the reduced depth at the recess.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

Why do you have to conceal the base plates? The system you described is commonly used for pre-engineered metal buildings (PEMB's), where base plates are not recessed or concealed.

 

[CBSE]

Like this. Do a diamond shape enclosure where you fill the pocket back in with concrete. Run control joints off the corners of the enclosure.

 

[Guastavino]

^CBSE,

DO you have a bond breaker between the slab poured over the top of the column footing to allow the slab to shrink?

 

[CBSE]

I typically specify On my foundation page to use 15lb felt or something similar between pad footings and slabs. I stopped putting it in my detail because contractors would miss it. It's now a standout note on my foundation plans along with some other stuff such as galvanized fasteners at all PT wood connections.

 

[Nikkib]

It is for an office building so the base plates need to be covered. I was not the engineer who selected the system during proposal so I am stuck with it. The column footings and the slab will be monolithic similar to a mat except not a constant thickness. With that being said, typical isolation joints around the columns with joints in the slab at the corners is not an option because there isn't really a separate slab to joint, it is the mat foundation.

 

[RPMG]

I'm with Ron in that I wouldn't conceal the base plates. I would put the anchors between the flanges, and have small base plates similar to PEMB's.

But it wounds like you've made up your mind about recessing the columns without adding joints. If you use steel fibers, you can specify that the vendor do the concrete design, as in dosages and fiber type. Then you can coordinate with them and benefit from their expertise.

 

[CBSE]

NikkiB, you can still do the detail similar to what I posted with a monolithic pour if you are recessing the column anyways.