A builder I work with is looking at converting a crawlspace to a full basement here in Colorado (snow, wind control loading). The attached concept is what he gave me and I am a little leary of the feasibility of this detail actually working. He doesn't want to go full height as there will be no access to pour concrete.

Initially I am only going to check the feasibility of this detail working structurally before I OK this method. To do this, I am looking at approaching the lower wall like a cantilever retaining wall with an additional lateral and vertical force from the bottom of the other wall. Am not going to try and attempt any moment transfer. Am looking at doweling the bottom of the existing wall into the new wall for vertical forces.

Does anyone see any pitfalls to this detail up front? Has anyone seen this situation dealt with more effectively?

Cody Geisendorfer, PE
www.quality-engineering.biz

Sorry, now attached...

Cody Geisendorfer, PE
www.quality-engineering.biz

• http://files.engineering.com/getfile.aspx?folder=d8288c6c-5101-4883-b630-c2

Your contractor is OBVIOUSLY NOT a structural engineer. Lots of problems here.

The location of maximum moment in the wall is right at the "connection" shown, if you can call it that. This will not work.

Where is the existing wall footing in the detail? One should bve there

The wall needs to be full height with a new footing, and connected to the floor diaphragm above. An interior placed free standing wall is possible, but not recommended due to the large eccentric footing that would be required.

Hire a structural engineer to detail it properly.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

As stated above, would NOT to try and transfer moment, just horizontal and vertical forces as if the upper wall would be pinned-pinned. I agree that there is probably a footer there and the contractor just didn't find it. I know I would need to add a cantilered footer at the inside wall regardless.

Is there any safe method anyone has seen that would effectively keep the existing wall (and footer) in place? I have heard of just pouring a wall under the existing footing the thickness of the existing footing but then there is a lot of moment to handle that way, basically underpinning with concrete.

BTW - I would be the Structural EOR, just haven't ran into this one before.

Cody Geisendorfer, PE
www.quality-engineering.biz

Also, this is the contractor's concept, he knows that what is designed will be a lot different that what he shows.

Cody Geisendorfer, PE
www.quality-engineering.biz

M^2 brings up most of what I would say... In addition - do the floor joists have to get longer?? and how so??

The outer wall is existing, inner wall is proposed to create a full basement. So joist span would not change. I have seen this done by offsetting the interior wall enough to minimize the impact from the adjacent footer above (1:1 slope) but there are constraints such as hallways in the basement that would prohibit this in many areas.

Any other ideas besides replacing the existing crawlspace wall with a full height basement wall?

Cody Geisendorfer, PE
www.quality-engineering.biz

jack up the house and infill the with CMU above the existing foundation wall.

Thanks CANEIT, that is definitely an option.

Cody Geisendorfer, PE
www.quality-engineering.biz

Is it an old URM house with brick/stone foundation walls or concrete? How are you going to resolve the earth pressures with a pin at mid height of the wall?

I have done this with sequential excavation but normally on old URM houses where the foundation walls are 16"+ thick.

Ok, so went to observe the crawlspace/house and also swung by the building department prior to putting together a proposal for engineering. Turns out that the house was originally designed as a full basement and appeared that the reason that it was built as a crawlspace is due to high groundwater and a lake close by. There was water within 6" of the crawlspace floor in the dry well. Told the contractor that I would have to have a soils engineer recommend subsurface drainage requirements which at the minimum would probably be interior and exterior drain tile systems.

So if the homeowner still wants to go forward, told him that upon geotechnical recommendations that allow a full basement along with the extensive drainage, would also have to install a retaining wall at the minimum inside the crawlspace along with any other geotech recommendations.

Also, no footer under the wall - the contractor dug all around and couldn't find caissons either. Still can't wrap my head around no footers for this unless they hit sandy gravel at some point as the crawlspace was definitely a clay soil. They also had a second story addition over the garage built which was approved by another engineer.

Contractor is going to cancel the project because it will cost at least twice as much as he thought it would, which doesn't hurt my feelings.

Cody Geisendorfer, PE
www.quality-engineering.biz