Slab on Grade dowels - geotechnical or structural

[Echo26]

Why is it that most geotechnical reports do not state any recommendations on dowelling slab on grade to grade beams/foundation walls? Is it a standard that they have to be dowelled except for some townhouses? Is dowelling the slabs more of a structural or geotechnical recommendation?

 

[TigerGuy]

Depending on your site, the slab may need to move independent of the foundation. A foundation may be loaded to 1,000 kPa, while the slab is loaded to 75 kPa. If there are any expansive soils beneath the building footprint, the slab will move easier than the foundation. If there is a structural connection between the two, the slab will crack.

 

[Harbringer]

Why would a geotech tell a structural engineer how to reinforce concrete?

 

[TigerGuy]

Harbringer, for the same reasons some structurals tell the geotech how to analyze soils. Our reports always defer to the structural engineer on slab reinforcement. However, if we feel there is a valid reason to avoid a structural connection between two members of a building (slab and foundation) we will state it in the report.

 

[Echo26]

Thank you for your reply TigerGuy!

If you don't have medium to high plastic clay/expansive soil, you won't really need to dowel the slab on grade to your foundations then except probably at door locations where you want to keep the slab and foundation moving together to prevent the doors from jamming.

But if you have medium to high plastic clay/expansive soil, it'll probably be recommended to have structural slab instead to minimize cracking or replace the subgrade with engineered fill approved by the geotechnical engineer.

So we're saying doweling of slab on grade to foundations is more of a structural/architectural recommendation based on geotechnical recommendation regarding the soil conditions (i.e. presence of medium to high plastic clay/expansive soils).

 

[TLHS]

I wouldn't say that either of those things are necessarily true in a general sense. There are lots of reasons you may want to float your slab and foundations separately or join them. You might have different movements for a variety of reasons or you might want to transfer loads one direction of the other for a variety of reasons. You might have a local practice for light residential that's something like you describe, though.

And yeah, the driver for this would be the structural people, but it's one of the many foundation and ground areas where it's really a combined system. Most of the time the structural engineer would work this specific item out on their own, but there are times when it's a discussion.