Local thickening of a slab on grade

[gmd255]

Hello, I havent done this before so Im asking for some guidance and suggestions.

When Im dealing with slabs on grade I choose thickness (ussually 250 mm thick for residental structures) and this is sufficient most of the time.
But in this case Im dealing with large single loads (from columns) that are acting on a slab on grade so shear breakthrough - punching shear is a problem.

Is it allright to locally increase thickness of a slab as shown in view a-a in attached image?

Should I reinforce this thickened part as a spread footing?

Is this expensive to do in a field?

 

[TehMightyEngineer]

Is it allright to locally increase thickness of a slab as shown in view a-a in attached image?

Yes

Should I reinforce this thickened part as a spread footing?

I would.

Is this expensive to do in a field?

No


Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[JoelTXCive]

Here's a detail for a holddown anchor that is located in the 4" thick section of a ribbed slab. You could just calc the required spread footing reinforcement and modify.

 

[JAE]

The main issue (beyond the pure strength design) on thickened slabs-on-grade is that under the higher pressure from walls and columns, that portion of the thickened slab will deflect differently than the relatively unloaded slab adjacent to them. So you may get slab cracking due to the differential movements. In most cases these are minor but depending on the stiffness of the subbase material you could have a very pronounced crack and vertical offset at the crack.

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[greznik91]

JAE, but is there a better / another option for this? I dont see one...

 

[JAE]

The other option would be to construct independent footings under columns and walls where the top of footing is located at the bottom of the slab.
The walls and/or columns are then separated by expansion joint material to allow the footings to settle somewhat independently from the slab.

However, I say "somewhat" because if a footing settles, the earth under it and adjacent to it will also be affected by that settlement and the slab may still settle down as well.
The slab cracking would just be less severe.

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