Purpose of thickened edge for concrete slabs on grade? 2

[jochav52802]

Good Afternoon,

I'd appreciate guidance on what the purpose is of a thickened edge for slabs on grade as well as what the design process looks like.

Many thanks for your help!

 

[Celt83]

To extend the point of bearing for the edge of the slab down below the frost line to attempt to mitigate frost heave effects.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[jayrod12]

we've also used them when we have a wall bearing down on the edge of a slab. Localized thickening to support the wall load is generally cheaper than thickening the entire slab.

 

[SlideRuleEra]

Another reason, reduce the chance that erosion will undermine the slab.

http://www.SlideRuleEra.net

 

[dhengr]

Jochav52802:
A thickened slab edge usually won’t solve a frost problem, if such problem exists, that needs a deeper edge beam, or wall and ftg. The edge of a slab is generally its weakest point since it has no added/adjacent support from surrounding slab on one edge. The thickened edge allows you to place a couple cont. rebars along the edge to make it act as a bit of a grade beam and to tie the entire edge together in tension, against any cracking potential. High point loads on the edge, column loads or wheel loads and the like are particularly detrimental and need some ability to be distributed. All corners, outstanding and reentrant/interior are also weak spots and need some special attention to detail and reinforcing.

 

[wannabeSE]

Some (many?) engineers specify thickened slab edges to prevent slab curling.

 

[Agent666]

Also required if you have expansive soils, to cutoff moisture loss/gain under the slab and minimise seasonal fluctuations in moisture content to minimise vertical movement of the foundations/slab.

Sometimes you also have a difference in levels to exterior ground, or need a minimum step to natural ground fir weatherproofing purposes.

 

[slickdeals]

Another vote for curling. However, there's another school of thought that suggests providing the thickened edge prevents the slab from shrinking. There are some recommendation that talk about a sloped transition (say thickening from 8" at edge x 1' long to a 5" thickness over 5') to minimize the effect of restraint that a abrupt thickened slab would provide.

I typically only provide them at slab edges and try to avoid them at interior locations.

 

[Ron247]

I assume you mean Slab on Grade for a carport, driveway, walkway, patio etc and not Slab on Grade that has a significant structure on it. For the former:
[ul]
[li]prevents erosion under the slab[/li]
[li]helps keep the edge from cracking/breaking so easy; especially the corner[/li]
[li]helps reduce curling[/li]
[li]helps with frost heave if it is deep enough[/li]
[li]provides a small foundation for light loads[/li]
[/ul]

 

[Phil1934]

The 12" minimum for footings prevents burrowing animals undermining them

 

[Agent666]

3' minimum would prevent authorities burrowing under to find all those missing architects....

 

[BridgeSmith]

I had assumed it was done primarily to distribute concentrated loads from a wall on top of it, such as at jack studs around wide openings in a bearing wall. Erosion/undermining would seem to be another good reason to do it.

 

[BridgeSmith]

Shhh Agent666, you never know who might be reading...