Oddball Underpinning Condition
Oddball Underpinning Condition
(OP)
Attached, and shown below, is an atypical underpinning situation that I need to sort out for a one story light frame wood building. Some questions that I have include:
1) Can the system that I've shown be installed incrementally as regular underpinning would be?
2) Is there any way to hide the tie back anchors within the thickness of the wall somehow?
3) Is there some better way to do this? Maybe an L-shaped retaining wall installed as underpinning?
4) Should I do anything around the corners to address potential soil sloughing etc?
It's worth noting that the existing building will be demolished to the ground floor level before being rebuilt. Rebuilding the wall altogether is an option, albeit not preferred by the client.
1) Can the system that I've shown be installed incrementally as regular underpinning would be?
2) Is there any way to hide the tie back anchors within the thickness of the wall somehow?
3) Is there some better way to do this? Maybe an L-shaped retaining wall installed as underpinning?
4) Should I do anything around the corners to address potential soil sloughing etc?
It's worth noting that the existing building will be demolished to the ground floor level before being rebuilt. Rebuilding the wall altogether is an option, albeit not preferred by the client.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.






RE: Oddball Underpinning Condition
If in clay, normally underpinning is done in 4 foot alternating segments.
If in sand, sheeting is often required.
RE: Oddball Underpinning Condition
RE: Oddball Underpinning Condition
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Oddball Underpinning Condition
Mike McCann, PE, SE (WA)
RE: Oddball Underpinning Condition
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Oddball Underpinning Condition
RE: Oddball Underpinning Condition
RE: Oddball Underpinning Condition
RE: Oddball Underpinning Condition
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Oddball Underpinning Condition
The only weight you are adding to the new strip footing that is not seen by the existing strip footing is the weight of the wall, and you can just make your new strip footing a little wider to pick it up. I would not be that worried about the load eccentricity in this case. As jayrod mentioned, block the top of the new wall to the existing diaphragm.
I would also add short horizontal tie rebar dowels from the new to the existing walls.
Contractors and Architects live me...
Mike McCann, PE, SE (WA)
RE: Oddball Underpinning Condition
Mike McCann, PE, SE (WA)
RE: Oddball Underpinning Condition
RE: Oddball Underpinning Condition
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Oddball Underpinning Condition
With the hinge situation, you seem to have enough restraint to maintain stability if you actually design it as a hinge. If you can reasonably manage to dowel enough to transfer some shear across that point you can design this to work without making it continuous for moment or needing to install tie backs.
RE: Oddball Underpinning Condition
Figure out the depth of excavation required below the existing floor slab. Draw an underpinning pier about 3 feet wide, extending back behind the face of the basement wall, under the existing structure. Draw the earth pressure diagram and any lateral surcharge pressure diagrams behind the underpinning. Check overturning and sliding of the underpinning. If the safety factors are OK, you are good. If not, you will need to make the underpinning thicker than 3 feet, or allow the front face of the underpinning to project farther into the proposed basement, or you will need to install some type of permanent lateral support for the underpinning. This usually is expensive and causes the home owner to move the new basement away in order to eliminate the need for underpinning.
Sec. B-B just shows the return of the new basement wall at each end of the underpinned section. Again, this is no big deal EXCEPT that you may have a vertical excavation depth of ABOUT 10 or 11 feet (= 7 to 7.5 feet plus the depth of about 3 to 3.5 feet from OG outside the building) at each end of the Sec. A-A underpinning. You would then need to install a perpendicular underpinning return at each end of the Sec. A-A underpinning or install some temporary sheeting until you are far enough away from the existing building to allow a safely sloped open excavation that does not undermine the existing building.
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RE: Oddball Underpinning Condition
http://www.eng-tips.com/viewthread.cfm?qid=358995
RE: Oddball Underpinning Condition
1) Is this possible?
2) Is there any way to get rid of the temporary tie-back "remnant".
3) Could the underpinning and the new 8" wall be poured in one shot somehow, eliminating the temporary bracing?
4) The underpinning will be done in 4' segments which means that the horizontal steel in the underpinning will be discontinued every four feet. The new wall poured above would be poured in one shot potentially. I feel as though this might be an issue for shrinkage cracking. Perhaps the way to go is to just warn the architect and suggest furring and drywall on the wall face.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Oddball Underpinning Condition
#3 - Usually you would want to drypack and shim the top of your underpinning usually which may preclude a one pour deal. Although since you are demo'ing most of the building you may not need much pre-loading so might be possible to count on the head to to fill in all voids and full bearing.
RE: Oddball Underpinning Condition
#2. Depends on the loading obviously, but if you excavate out in 4ft segments you might be able to get rid of the raker by making the new footing large enough to work as a gravity wall. I will often assume some cohesion (real or apparent) in the soil at least for the temporary condition. It exists or you wouldn't be able to make the cut to begin with. If the soil behind it can't dry out I regularly count on it for temporary conditions. 50 psf cohesion goes a long long way.
#3. You would have to cut off the existing footing on the way down, but I do not see why not.
#4. I always have the contractor drive rebar into the soil each side. You are not going to get a full Codified lap splice, but you get continuity.
RE: Oddball Underpinning Condition
#3 -> I suppose it could, PE Inc will know.
#4 -> Usually underpins are unreinforced however in this case, if you are using the vertical well to resist lateral forces then you most likely need the vertical bars. You could probably omit the horizontal bars.
I'd be inclined to extend the underpin underneath the existing slab until sliding/overturning is eliminated. Then you don't need the vertical wall. Typically you would be worried about undermining the existing slab, but in this case, who cares? It's going to be demo'd if I understand correctly.
EIT
www.HowToEngineer.com
RE: Oddball Underpinning Condition
www.PeirceEngineering.com
RE: Oddball Underpinning Condition
I have to confess that, in the past, I haven't given any explicit consideration to the lateral stability of underpinning. My bad. The sketch below shows my understanding of the statics of the situation based on the posts above. Some additional questions:
1) Are we sure about this cohesion business? It's great but a little scary.
2) Is cohesion something that can be calculated from other soil properties? Or conservatively estimated? Or do I need to seek geotechnical help?
3) FOS the same as for retaining walls? 1.5 on OT and 2.0 on Sliding?
Several folks have inquired as to why I think that I need lateral bracing in the temporary condition. My plan was to do the underpinning in 4' segments but pour the upper wall element as one continuous pour after all of the underpinning is done. I thought that would be more construction friendly. However, it would yield a condition where all of the underpinning would be in place but none of the upper wall would be. Working under the perhaps incorrect assumption that the underpinning would not be stable on its own, the "hinge" between the underpinning and the existing footing would require bracing. If I go that route, I'll query the contractor as to whether or not they'd prefer to do the upper wall in 4' segments and forgo the lateral bracing.
I'm not sure that I follow. What is pre-loading in this context and how does it make it easier to count on the head to fill in the voids?
So this is horizontal rebar dowels driven into the dirt on each side of a segment that ultimately becomes a pseudo-lap for the horizontal reinforcing in adjacent segments? Clever.
Trust me, I read it carefully. I know that you have considerable expertise in this arena. I've been unable to get the unbranded statics to work so far. However, it's getting closer with each iteration!
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Oddball Underpinning Condition
You're forces/statics are on the right track. Solve for moments about the toe, check sliding and bearing pressure (I see bearing pressure get neglected often for some reason). Use dead load from above, dead load of soil on top of your concrete mass and your concrete mass as resisting vs your lateral pressure.
As for factor of safety, typically I see 1.5 for OT and sliding.
Place underpin in 4' segment. Pour to within 3" of existing. Set overnight. Drypack void, tamp into place.
EIT
www.HowToEngineer.com
RE: Oddball Underpinning Condition
The attached photo is exactly what you are trying to do except that intermittent underpinning piers were used instead of continuous piers. The other difference may be that my building is heavier than your building (see my original response). Going deeper with your underpinning probably will not help because the sliding will get worse.
Try using Coulomb's active earth pressure coefficient instead of Rankine. Use reasonable soil properties, not overly conservative values. High unit weight usually means higher phi angle for granular soils. Therefore a high unit weight with a low phi angle may be too conservative for a granular soil. While I also usually ignore cohesion, 50 psf is next to nothing. If the soil has cohesion, perhaps more that 50 psf is reasonable?
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