Unsupported crawlspace footing

[toptobottom]

I have looked at a New Residential two-story home in PA. It has a CMU foundation with crawlspace and an attached garage w/frost walls to one side.


Details:

The crawlspace is about 5.5' tall with vapor barrier and crushed stone on top which is uneven and sparse (non-compacted).

Facing the house, the exterior grade at foundation varies (due to minor slope) with 36" above base of footing at left side foundation wall and 36" (left corner) to 60" (right corner) at front and rear walls. Garage is on right side.

Soils are clayey loam type.

No visible drainage pipe to daylight

No sump pump/pit in crawl nor was any moisture noted.

PA has adopted the 2003 IRC. 42" frostline footing depth required.


Situation:

Inside the crawlspace, the crushed stone is so sparse in many areas that the bottom edge of the footing is visible. The stone can easily be brushed away from the footing to the bottom edge inside the entire crawlspace.

Questions:

Is this a concern for lateral movement due to hydrostatic pressure or frost heave?

What are proper remedies, if needed, to prevent possible future cracking or inward foundation movement?

I do not believe seismic protection is required since this on the Northeastern state line.

 

[youngstructural]

Sorry, just a little more information needed for me to be clear about the situation: Are the walls backfilled? IE: Is this crawl space below ground level, and as such properly a very rough basement area? This would be a more severe situation in that case, as the base of the footings must have either a slab to push against, or be burried in a sufficient amount of soil & gravel to resist thrust of the soil pressure on the outside.

Is the crawl space heated? Again, makes a difference... And if it is heated, is it a reliable heating situation (ie: regularly used space) which is unlikely to be left unheated for more than 7 days?

Perhaps the outside wall was insulated with SM insulation? Very unlikely, but possible.

So as I see it you have two potential structural problems: Frost heave (as I believe you realize from your mentioning the min. frostline footing depth required) and the unrestrained footing against latteral thrust.

If I've understood and outlined your case correctly, there are a few possible solutions, but my number one choice would be to insulate with structural SM foam and finish the crawl space floor (ie: give it a slab).

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[toptobottom]

Thanks for your reply concerning my situation.

My answers to your questions in black.

BH.Eng Wrote:
Are the walls backfilled?

Yes

Is this crawl space below ground level, and as such properly a very rough basement area?

Yes, See the Detail: section of my original post as to height of grade/slope at this crawlspace foundation type.

This would be a more severe situation in that case, as the base of the footings must have either a slab to push against, or be burried in a sufficient amount of soil & gravel to resist thrust of the soil pressure on the outside.

There is absolutely no support at the entire interior side of this exposed footing. This was a flat excavation and footing forms were constructed/poured on top of flat surface, then finished with loose/sparse crushed stone inside the floor area. The Builder finished this project 6 months ago and the local building codes official signed off on this house. Builder stated to me that there was no intention of ever pouring a finish floor in this space and sees nothing wrong with it the way it is. I guess that's why he's just a builder!

Is the crawl space heated?

Yes, with a ceiling mounted space heater set at just enough to keep the plumbing from freezing

is it a reliable heating situation (ie: regularly used space)

Somewhat reliable, but would be unknown if unit was defective, since this area would have infrequent entry.

which is unlikely to be left unheated for more than 7 days?

It is possible to be unheated for 7 days or more but it is unknown what the lowest ambient temp could be reached assuming the heated living space above is still actively heated. We are in Zone 4 reaching temps of -3F (-17C), down to -30F (-34C) for a few days in a row.

Perhaps the outside wall was insulated with SM insulation? Very unlikely, but possible.

No Insulation outside or inside. Of course you knew that! It's typical of saving that almighty dollar!!!

So as I see it you have two potential structural problems: Frost heave (as I believe you realize from your mentioning the min. frostline footing depth required) and the unrestrained footing against latteral thrust.

Touche'. The reason of my posting here to confirm my initial thoughts.

If I've understood and outlined your case correctly, there are a few possible solutions, but my number one choice would be to insulate with structural SM foam and finish the crawl space floor (ie: give it a slab).

Which makes all the sense in the world. Thank you!

One more question if I may.

If the footings had rebar (twins) and block walls with pinned rod/grouted cavities every X amount of feet, would there still be a likelihood of cracks or movement? would this situation be acceptable?

 

[youngstructural]

I assume you're getting at the footing being designed as a lateral beam, and thus capable of taking the thrust of the wall due to the backfill without needing it's own restraint. Yes, it is possible, but requires a number of specific design checks that are not all that easy. In fact, particularly old foundations (usually pre 1950s) often worked this way, and did in fact work, even without reinforcing bars in the strip footing. What's actually occuring is that the load on the foundation wall is upping the friction (both mechanical and true coeficition of fricion. or mu, based types), which give the wall sufficient resistance against the thrust of the soil. This system can be (and often is) stable so long as it is left all on it's own without external "stimuli". Earthquakes, design (or near design) strength storm winds, house renovations, leaking water mains, etc, etc, etc, can all cause catastrophic failure with a system like that.

Back to your actual question: IF reinforce footing as a horizontal beam and IF the masonry units were given a doweled shear connection into the footing, could it work? Yes, but I doubt that's what's been done. The design would be unusual, but not implausible. I would not design a structure like this, since you still need to have the horizontal beam be capable of spanning between perpendicular walls in case of EQ loads. Not a good solution, not a good construction.

Oh, and yes, unless the wall as well was reinforced with steel (or perhaps fibres) I would think that you're running a greater risk of cracking. Also, it would be absolutely essential to have PROPER backfill and big-O (drainage) right down at the base of the footing for this wall to work as constructed.

I would, having said all that, design a foundation/basement like this in one (and only one) situation: If it was on bedrock. Since you're on clay-loam and worried about frost-heave, I'm pretty sure I can set aside the bedrock idea...

One last thought: I've never heard of a forensic engineering expert who didn't insist that collapses (or non-catastrophic failures) are always due to a string of conditions, situation, occurances or coincidences. I would think that (bearing in mind I haven't actually seen your situation) what you have here is step one of a three, four or five degree of failure system.

Hope that helps,
Good luck,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

One more thing, before I forget: How wide are these basement walls? Remember that with odd and frequently returning footprints you can get away with alot more. It's too bad it's not poured concrete, since although I DON'T remember the numbers off the top of my head, my gut (and very quick back of the envelope calcs) tells me that a 200mm (8") wall would span ~2.5m or 3m (8' to 10') unreinforced, if well restrained by it's footing. This is thanks to the fact that concrete does have some tension strength (if small) and if you have a moderate unit weight of soil, proper free-draining backfill and a good big-O, you should not develop additional ice-thrust. I have specified foundations with very little re-bar (simply two 10M top and bottom for a little beam action) in cases where the foundation had alot of jigs & jags. This obviously does not apply for a big box type foundation, and is somewhat more rare because the excavation is so much larger or more complex, and either way expensive. If you have horzontal reinforcing (even just steel ladders), you might have a stable situation with close returns.

Check one thing before calling for remedial works: You are laterally supported at the top and sides, with partial restraint at the bottom. Consider a yield-line solution for your walls with these conditions, and determine an appropriate moment capacity for your wall if the CMUs are nominally reinforced with your code's minimum vertical and horizontal steels. From that you can altually determine the maximum horizontal span permissible. Your walls are short, so that's going to be a big help.

Failing all of that, you can always present your client with the possibility of getting a Geotech's help for the lateral restraint of the footing... If it's effective with just a slight shear-key into the clayey loam, you've actually got a situation that works, if the wall is capable of restraining the backfill along it's 5.5' vertical span.

Again, good luck,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[toptobottom]

B.Eng (Carleton),

I agree with you it is doubted that special reinforcement design was in place for this ordinary residential construction.

Thanks again for insight on my follow-up questions. Lots of great information.

Igor Smetaniuk
Glen Spey, NY

 

[youngstructural]

Keep in mind that either way, you're still no good against frost-heave.... Something needs to be done about that, and I don't think there is realistically any option other than insulating.

Please let me know how it's turns out... And I don't think you're client is going to like the cost of the insulation!

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...