Floating slab on grade detail at exterior door 1

[gte447f]

For a floating slab on grade, what is best practice at an exterior door? I assume the slab edge should get extended to the outside face of the wall. Does the slab edge need to be thickened? If so, how much, or should it be turned down onto the strip footing/grade beam? It seems to me that if the slab is isolated from the footing/exterior wall at the typical perimeter section, then it should remain isolated from the strip footing at an exterior door? Do others agree or disagree? This is in an area where frost heave is not a big concern (frost depth typically 12" by code, but probably actually less than that). The slab is 5" thick. The strip footing is 1'-4" thick x 2'-6" wide and is 1'-4" below slab.

See attached sketch of basic alternatives that I am thinking about.

 

[GC_Hopi]

Depends on the what material you are using for the super structure. Don't limit yourself to one design. I usually go option #3 when there are stem walls. Stem wall to be held down at the doors and slab poured through opening. And for that case I usually detail the typical slab to be poured with a 1:1 turndown to bear on the fooing. Link Thats for a shallow frost depth.

 

[charliealphabravo]

I have wondered the same and look forward to the responses.

I think you either:
A. pour the slab to the outside face of the wall and accept some cracking due to horizontal and vertical movement or
B. you form an isolation joint at the inside face of the wall (as everywhere else) and risk the appearance of a sharp transition and tripping hazard at the door due to relative vertical movement.

I think most choose A and then provide adequate control joints near the door and proper compaction of the granular to ensure similar behavior between the slab and footing.

 

[WARose]

That first option (i.e. where it terminates prior to hitting the wall and does not turn down to the footing) is what I have detailed most often. (With a expansion joint call out and a joint sealant (at the top) between the wall and the slab.)

 

[gte447f]

WARose, that is what I like also for the typical exterior wall/foundation section, but what I am asking about is what do you then do at an exterior door location? I am leaning toward option #2 in the sketch (i.e. extend the slab through the door opening to align with outside face of wall, and thicken the slab edge locally at the door, but not thick enough to bear on the wall footing).

 

[gte447f]

GC_Hopi, I have seen it done the way you describe many times (with the slab edge turned down onto the footing at both the inside face of the wall and at the door opening). I think that is OK, but I think it makes the edge of slab more likely to settle with the wall footing more than the interior of the slab, possibly resulting in slab cracks from differential settlement?

 

[gte447f]

charliealphabravo, if you choose your option A would you thicken the slab edge locally at the door opening (like my option #2 in the sketch)?

 

[WARose]

WARose, that is what I like also for the typical exterior wall/foundation section, but what I am asking about is what do you then do at an exterior door location?

Why do anything different at the door?

 

[gte447f]

WARose, I assume you either pour the slab to the outside face of the wall in the door opening, or leave a foundation/stem wall below the door opening, right? Sounds like maybe you leave a stem wall below the door opening if you don't do anything different with the slab.

 

[WARose]

I do that first option all the way around. Whether there is a door, not a door, or whatever.

About the only time I think about connecting the two (or having a turn down to the footing) is if there is going to be heavy vehicle traffic through that "door". (I.e. it's like a garage door.)

 

[charliealphabravo]

I actually prefer WARose's approach (my Option B) and would hope to see it for substantial commercial and industrial construction. In that case I think you need to be prepared to provide a plate or cap across the isolation joint to accommodate and spread out the effect of any vertical movement.

For residential and small commercial you would pour to the outside face of the wall and maybe even monolithic with the footing. In that case I don't see any real difference between OPs option 2 and 3 as long as adequate control joints are provided.

OP, in option 1 what is between the underside of the slab and the top of the footing?

 

[WARose]

If the concern here is a tripping hazard from differential settlement......you've got a tripping hazard anyway at most doors from the sill plate (and it's cover).

You can't idiot proof your building.

 

[KootK]

I see a similar condition all the time in my jurisdiction but with 4' frost walls. We're always dealing with expansive clays. Most of he geotechnical reports here will specify floating slabs but with a downturn & dowels at the exterior doors. It's a bit of a contradiction which, presumably, is why you've posed the question. I think that iy simply comes down to this: aesthetics and door function at these locations trump concerns over slab heaving. I don't think that much metal energy goes into the geometry of the downturn. 1:1 with a minimum overall thickness of 8" etc. As short as your walls are, extending the downturn to the footing makes sense to me. Performance wise, I see that solution as identical to the one with the mini wall between downturn and footing.

Our craft always winds up being an exercise in compromise. It's rare that all performance and cost objectives can be simultaneously met idealistically.

 

[gte447f]

Thanks for everyone's input and feedback. KootK, you are right about the reason I am posing this question... I am of the opinion that it is better to isolate (float) the slab from the foundation if possible, but then it seems contradictory to this theory to then connect the slab to the foundation at doorways. Nevertheless, I think that is the direction that I am moving with my thinking on this. I will probably continue to show a floating slab with an expansion joint between the slab and the typical exterior wall, and then show a downturn thickened slab edge bearing on either a short foundation stem wall (frost wall I guess you guys and/or gals call it up north) or directly on the footing. In the grand scheme of things I don't think it is hugely important, but there are several issues at play like potential for differential settlement/movement between the slab and foundation and door and slab performance, and it does seem to be an exercise in compromise.

 

[DaveAtkins]

What I normally do is...none of the above.

My standard detail looks like Option 2, but the interior slab only laps onto half of the foundation wall. The exterior slab (the "stoop") laps onto the other half of the foundation wall.

DaveAtkins

 

[gte447f]

Thanks Dave.

 

[emmgjld]

My first question is ... Why was the floating Slab recommended?
Low Expansive soils?
High Expansive Soils?
Compressible/Collapsible Soils?
Anticipated Seasonal slab movement?
Anticipated Economies (either real life savings or just being cheap)?
Protection against intense cold?

 

[gte447f]

emmgjld, a floating slab was not recommended specifically for this particular project, but some building engineers, myself included most of the time, think that a slab on grade should basically always be isolated from walls, columns, footings, etc. on principle because of the possibility that the movements of these structural elements will be different from the slab on grade due to differences in support conditions, loading, and environment. I routinely make exceptions to this principle, mostly for lightly loaded stick built structures with a monolithic thickened slab edge as the perimeter footing, but this is just because of pressure from the construction trade where the monolithic slab style of footing is more popular with lighter, lower-end buildings in my area. I usually design for southern US climates so frost heave is not a big concern. Neither is expansive clay usually a concern. Anticipated movement would be from differential settlement. Do you not think that it is necessary to isolate slabs from structural elements, or do you prefer them structurally connected?

 

[BridgeSmith]

I don't have a recommendation, just some random thoughts about the options presented.

Any way you go about it, if you're doing it because you're expecting excessive settlement of the slab relative to the wall foundation, the solution is likely to be less than ideal. Minimizing the settlement would seem a prudent step regardless of the configuration chosen.

If you extend the slab through the doorway and let it float, how do you not end up with a gap under the door? The door frame is on the foundation, and if the slab sinks, there will be a gap under the door. However, if you extend the foundation across the doorway, and stop the slab at the inside face, you have the potential for lip at the doorway. It would seem the choice between those 2 options would depend on what's under the door. Is there a raised threshold at the door frame, or just a sweep on the bottom of the door? If you have a raised threshold, then there's lip at the door already, so any small change due to settlement is inconsequential - it's already a bit of a step up to the doorway. If it's just flat through the doorway (or sloped to the outside for drainage, which would be advisable), then you have to accommodate any settlement with an adjustable sweep on the bottom of the door (unless we're talking about an overhead door, in which case there is no issue).

The third option is with the slab supported by the foundation, which doesn't cause any problems for the door, but could cause issues with the slab, if the slab settles. Then you have potential cracking issues, as well as possible issues with any rainwater that makes it way around or under the door running toward the middle of the floor. The cracking could likely be mitigated with thickening and/or reinforcement in the slab, but the potential for puddles in front of the doorway could be a trickier issue. Providing a positive slope greater than the potential settlement could be an option, but possibly difficult to execute. Careful detailing to minimize potential for water to get in through the doorway may work, but I've never seen that approach be successful in the long term.

 

[BUGGAR]

I've found that with commercial buildings, they always have threshold requirements that the Architect doesn't know at the time. I depress the footing wall and hold the slab back to the inside edge of the wall so it is fully floating, and later pour back whatever fancy threshold concrete is required. Sometimes, we don't place this concrete until all door hardware is on site. It's a little extra work but frequently saves the day.