Concrete Slab On Grade with Haunch

[jseng9]

Our firm provided structural engineering services for a new two story steel framed building supported on ordinary conventional reinforced concrete spread footings and strip footings. At the first floor, the owner chose not to pour the concrete slab on grade as it's going to be retail and they wanted to avoid trenching the slab for underground plumbing. It is finally time to fill the space and as part of the tenant fit outs the owner is proposing to "value engineer" the haunch out of the project. When I say haunch, I am referring to a localized thickening of the slab on grade at the perimeter of the new slab where it meets the existing foundation walls.

I argue that the haunch provides additional cover for the reinforcement when you go to dowel in to the existing reinforced concrete strip footing from the new cast in place slab on grade. This cover will potentially prevent future frost heave in my opinion, saving the owner money over time as it lessens the chance of the slab cracking and prevents exterior doors from becoming inoperable.

Can anyone think of additional advantages of the haunch detail? This is, after all, one of our typical details and I would prefer not to change our detail unless there is a legitimate reason the haunch is not necessary. Thanks!

 

[skeletron]

The haunch is great structurally but a pain in terms of grading. You need to grade the site to slab depth, then scratch out around the perimeter. Then you need to fit the insulation (if you have that) and match/seal the cuts of the slope for the haunch. It can be done, but it's work which equals labour which equals cost. Long term performance is a main advantage with a slab without a haunch. Installation of the dowels is easier because you have more depth to play with, thus lessening the chance of hitting wall rebar and/or not having enough cover (if you did not have the haunch).

 

[phamENG]

We call that a "turn down." I've only used it where the slab is the primary foundation element and the turn down is located under the exterior wall. For slabs "floating" inside the building and bounded by foundation walls (usually CMU here but often concrete in other locations), it's just slab thickness all the way to an expansion joint. We don't dowel it in as loading between the slab and building foundation are so different, fixed slabs always crack from differential movement. Of course, that's largely a function of local sub surface features and soil types. You may not have that problem.

 

[r13]

I agree more with phamENG. If the slab is enclosed by foundation element, the dowel in should be kept to a minimum, and can be achieved through a few pre-installed bars/hair pins from the perimeter element, though the form worker does not like it.

 

[bones206]

One disadvantage of a haunch/turndown is that it resists the slab's desire to shrink, which increases the likelihood of cracking. Some people use them to combat curling at the slab edges, but I don't think that benefit outweighs the drawbacks.

I usually detail SOG's to be as free from restraint as possible. No dowels if I can avoid them.

I don't really see how a haunch protects against frost. I only really use them when I want some scour protection on exterior exposed slabs. And I try to use shallow slopes to reduce the shrinkage restraint.

 

[wannabeSE]

Often times the thickened slab edge is specified to prevent curling which is moot if it is also dowelled into the walls. I’m surprised they aren’t trying to VE the dowels. Since it doesn’t sound like a structural slab, I would educate the client as best as possible and let them decide.

 

[phamENG]

Is slab curling a big problem where you are? It can usually be managed by proper vapor barrier detailing and curing methods during construction. But, again, that can vary with weather and local climate.

 

[r13]

I think curling can be prevented by better protection during curing period, if temperature variation is significant in the area of application.

 

[bones206]

Agreed. Good curing is the best medicine and good medicine can cure a lot of headaches.

 

[jseng9]

Thanks all for the very helpful responses!