Retaining forces on residential footing stemwall in crawlspace

[jdgengineer]

When you have a relatively deep crawlspace below grade (say 42" tall, but not a true basement) how do you typically handle the retaining pressures on the stemwall? For basements in our area we typically design cantilevered retaining walls rather than rely on lateral support at the top of the wall. However, in this case I presume it would be preferred to rely on the lateral support at the top of the foundation wall.

Section R404.1.1 of the IRC states that an engineered design is only required where the unbalanced backfill exceeds 4' in height and the top and bottom of the foundation wall does not have lateral support. Therefore, do you just ignore the forces where the unbalanced height is less than 4'?

In a typical crawlspace, we do not have a slab on grade to resist the lateral thrust at the base of the footing. Sometimes, we do have a 2" thick rat slab. We would typically consider this non-structural but do others consider this lateral restraint? We could deepen or widen the footing at the base to resist sliding and/or overturning similar to a typical retaining wall but that could lead to pretty large foundations.

The elevation of our floor is often close to grade so the floor joist sometimes hang from the mudsill with top flange hangers and the floor sheathing is directly nailed to the mudsill. In these instances with perpendicular joists I think the restraint is probably there, but what do you often do with parallel conditions? Blocking by a couple of joist bays? Some geotechs require us to design soil as undrained with 80 pcf lateral pressure with an additional 8H to convert to at rest pressures. Presuming a 12" deep footing below crawlspace depth and designing the lateral pressure to the bottom of the footing (42" unbalanced backfill + 12" footing depth = 54" height), this could have a lateral reaction at the bottom of 620plf and at the top of 351plf. This seems like a high force to resolve into the floor framing and a high sliding force at the base of the footing.

How do you all handle these situations?

 

[TRAK.Structural]

I would definitely use the floor framing for lateral bracing at the top, add blocking where the framing is parallel to the wall.

IRC has tables for CMU size, grouting, and rebar based on retained height.

I don't like 4 feet of unbalanced backfill with no grout, but they are built like that in my area a lot....

 

[jdgengineer]

I would definitely use the floor framing for lateral bracing at the top, add blocking where the framing is parallel to the wall.

IRC has tables for CMU size, grouting, and rebar based on retained height.

I don't like 4 feet of unbalanced backfill with no grout, but they are built like that in my area a lot....

Thanks for the input. We will be using a concrete stemwall that will be reinforced and can take the retaining forces. I understand the approach conceptually, but really my question is how others handle the sliding force at the base and the reaction into the floor quantatively. Without a slab at the base to resolve the sliding, the sliding force would require a fairly deep footing / key. At ~3'-6"- 4'-0" unbalanced fill height do you calculate these forces or just provide some amount of modest bracing and hand wave it away? The numbers I provided above are quite high and would be challenging to work on paper with typical standard details.

We could try and brace the foundation walls with perpendicular footings, buttresses, etc. but given the building configuration the footing horizontal span may be ~22' in some areas and this would require additional footings with stemwalls to provide this bracing.

 

[Ron247]

I would use the floor framing as a brace. In my area, we use the floor for basements also. Never had any problems provided the construction sequence is correct.

If you excavate, pour your strip footing and backfill the excavation after the wall is laid, it will probably work by the numbers. Not backfilling is the biggest problem I run into because it ponds a lot of water inside the crawl space perimeter and allows for more potential lateral movement. I would make them backfill before the floor is built, or they will never do it. I have been in several crawl spaces similar to what you describe.

Crawl space drainage and ventilation would be my main concerns since exterior grade is higher than the crawl space. The height of fill to me is not that much of an issue from a force standpoint. If HVAC ducts are in the crawl space, it is an even bigger issue.

At this small height, grouting is not that much of a strength issue, but it does help reduce long-term water intrusion. Even though there may be a code requirement or even a calculation saying I need dowels, I tend to use at least a #4 dowel at 4' on center in a grouted cell. I just hate the idea of nothing mechanical connection footing to wall but also love to hear the contractor tell me how no one else requires them.

Another item I run into is when they do not connect steps in the foundation elevation together with reinforcing. I routinely find uneven settlement (i.e. cracking) at the foundation step if there is no vertical Z-bars.

 

[TRAK.Structural]

Thanks for the input. We will be using a concrete stemwall that will be reinforced and can take the retaining forces. I understand the approach conceptually, but really my question is how others handle the sliding force at the base and the reaction into the floor quantatively. Without a slab at the base to resolve the sliding, the sliding force would require a fairly deep footing / key. At ~3'-6"- 4'-0" unbalanced fill height do you calculate these forces or just provide some amount of modest bracing and hand wave it away? The numbers I provided above are quite high and would be challenging to work on paper with typical standard details.

We could try and brace the foundation walls with perpendicular footings, buttresses, etc. but given the building configuration the footing horizontal span may be ~22' in some areas and this would require additional footings with stemwalls to provide this bracing.

I'm not aware of any silver bullets for sliding resistance.

Use sliding friction, passive pressures, and perpendicular resisting elements in combination to resist the sliding at the base of the wall. Hopefully you have some decent gravity loads to help with friction. You might also try to sharpen the pencil on your lateral earth pressures if possible to reduce the driving side loads.

 

[XR250]

It is hard for me to believe sliding is an issue here. Are you adding the weight of the house on top?
But, yeah, the floor framing can take this although your loads seem way too high for that much backfill.

 

[jdgengineer]

It is hard for me to believe sliding is an issue here. Are you adding the weight of the house on top?
But, yeah, the floor framing can take this although your loads seem way too high for that much backfill.

It's potentially at-rest clay pressures that we are working with. The local jurisdiction does not always allow subdrains (it's complicated).

I'm not worried about global sliding, more of the bottom of the footing kicking in with a sliding failure. I am accounting for some dead weight of the house, but depending on the footing line there is not significant tributary loading (rafters / floor joists parallel).

I think I may just thicken the ratslab to 3-4" to provide the slab sliding restraint or add some subterranean tie beams to resolve the sliding force to the other side.

 

[swooneyWOODSTRUCT]

when the joists are parallel, and the house isn't heavy enough to provide enough sliding resistance via friction (or surface area/cohesion if clay soil), I put a PT ledger on the bottom of the stemwall and brace it back to the underside of the floor (to blocking between joists)

 

[jdgengineer]

when the joists are parallel, and the house isn't heavy enough to provide enough sliding resistance via friction (or surface area/cohesion if clay soil), I put a PT ledger on the bottom of the stemwall and brace it back to the underside of the floor (to blocking between joists)

That's a creative idea, although I'd be a little concerned about a hump in the floor if the braces were actually engaged to resist sliding.