Underpinning Brick Party Wall - Stability
Underpinning Brick Party Wall - Stability
(OP)
See attached rough sketch. This is a four story brick party wall (3 wythe) shared by two rowhouses. Currently they have approximately matching cellar levels and a floor height of about 8ft. On our project we will be lowering the cellar slab by 4 feet and also bringing the floor above down by the same amount. This means 4 feet of underpinning at the party wall.
My concern/question is the stability of the wall with the underpinning. It essentially has a hinge at midheight. A check of P/A + Mc/I with a fairly conservative value of P (i.e. no live load, light estimate of dead loads) shows no tension on the wall face by a factor of about 1.5.
I've seen this done many times but I don't feel very comfortable with it. The alternative, building a 'liner wall' inboard of the underpinning is an expensive proposition and I don't want to force it unless it's definitely required.
Any thoughts on this? All other basement walls on this and surrounding properties are brick masonry, and probably rely on a similar principle of the P overcoming the flexural forces.
My concern/question is the stability of the wall with the underpinning. It essentially has a hinge at midheight. A check of P/A + Mc/I with a fairly conservative value of P (i.e. no live load, light estimate of dead loads) shows no tension on the wall face by a factor of about 1.5.
I've seen this done many times but I don't feel very comfortable with it. The alternative, building a 'liner wall' inboard of the underpinning is an expensive proposition and I don't want to force it unless it's definitely required.
Any thoughts on this? All other basement walls on this and surrounding properties are brick masonry, and probably rely on a similar principle of the P overcoming the flexural forces.






RE: Underpinning Brick Party Wall - Stability
Working with old structures can often have unanticipated issues.
Dik
RE: Underpinning Brick Party Wall - Stability
RE: Underpinning Brick Party Wall - Stability
www.PeirceEngineering.com
RE: Underpinning Brick Party Wall - Stability
RE: Underpinning Brick Party Wall - Stability
RE: Underpinning Brick Party Wall - Stability
shobroco - I'm not sure we're allowed to anchor under the neighbor's property and count on it for a permanent condition.
PEinc - I took a look through your presentation, I see similar stuff happening here in NYC. I agree that these buildings are precarious. The question is what is an appropriate check and reasonable factor of safety. If we were lowering the slab by 1 foot would you still be concerned? I am in favor putting a new liner wall in front of the underpinning and original wall - the pushback, as usual, is that this is done all the time with no problems. I don't want to be overly conservative, if the calcs show that it works and it's done often where is a reasonable line to draw?
RE: Underpinning Brick Party Wall - Stability
As a compromise solution, could you utilise some of the ground-bearing capacity on your side by underpinning with rc, and casting a toe on your side, like a retaining wall. If your calcs show their would be no tension anyway (by a reasonable margin), this might give a little extra support. (see sketch)
RE: Underpinning Brick Party Wall - Stability
RE: Underpinning Brick Party Wall - Stability
1. I do not like the four to fve feet of soil on one side of the wall and the inherent knuckle joint. Just does not feel right regardless of the no tension result. I would add embedded ties from the wall to the existing slab, epoxied into a cut slot at intrvals before the underpinning process began.
2. The diagram implies no spread footing for the brick wallk. Seems like there is an excellent chance that the footing is overstressing the soil here, even in the current situation, unless it is on rock, which I doubt.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Underpinning Brick Party Wall - Stability
Another idea for a "compromise":
1. Do your underpinning as you have shown...but,
2. At some interval, include a series of vertical embedded steel tubes or small WF shapes that would extend from your
lower concrete up inside a cut vertical slot in the brick to the new floor above.
This could be set inside the concrete and brick so there would be no pilaster sticking out...
or you could build out your new concrete 4 to 6 inches inside of the brick and extend the vertical members just inside the existing brick.
3. This provides vertical wall girts that would provide a limited amount of redundancy in the wall system....
4. Your original design has no redundancy...if the dead load is lower than you think and the soil load is higher,
you have only the limited P/A + My/I safety factor to fall back on.
With the vertical girts you have an additional load path.
Another idea that just occurred to me would be to add vertical strap ties on the inside face of the wall. Any "failure" of your system would be tension on your side of the wall so the straps could be used as tension-side reinforcement.
RE: Underpinning Brick Party Wall - Stability
Whatever method is chosen, the only dead load which I would use as contributing would be the wall itself. The assumption should be that one rowhouse or the other could be demolished or otherwise removed without compromising the wall's integrity.
RE: Underpinning Brick Party Wall - Stability
What you are showing in your sketches is done all the time. You are underpinning for a depth of only about 5 feet. This is minor when supporting a four story party wall. The dead load of the party wall and floors, combined with the weight of the underpinning piers, should be enough to prevent overturning and sliding of both the underpinning and the foundation wall above the underpinning. You are removing only about 6 inches to a foot of soil from in front of the botton of the existing foundation wall. Therefore, the foundation wall should be stable above the underpinning piers. Because you are not removing any floor slabs from either side of the party wall, the party wall should remain laterally stable. The attached photo shows just what you are trying to do, except that there are no floor slabs framing into the front side of the party wall.
www.PeirceEngineering.com
RE: Underpinning Brick Party Wall - Stability
RE: Underpinning Brick Party Wall - Stability
I still don't love it but I've come around to living with leaving it as just underpinning. The party walls themselves are in very good condition, this is a very well maintained row-house. A full wall survey will be done and any problems will be repointed or repaired as required. For dead load on the wall I have counted wall self weight and only a total dead load of 7.5psf for floors, no live load. I agree that I am increasing the unbraced height of the floor above, but I am ok with this based on the condition of the walls and calculation. The new framing will use bolted ledgers which will provide some tension capacity in addition to the compression capacity.
PEInc - Not sure what you mean about removing only 6 inches of soil. The existing walls go down about 6 inches, and then have an approximately 12 inch deep stone footing (based on test pits).
Like I said, I still don't like it, but I've grilled a couple of SoE engineers and it seems like they are very comfortable with it and the calcs do work out.
RE: Underpinning Brick Party Wall - Stability
You added an amount for the floors; would it still work without them, ie just relying on the weight of the wall?
RE: Underpinning Brick Party Wall - Stability
The bolted ledgers go against the principle of firecut joists, the idea of which is to allow the floor to collapse in a fire without pulling the party wall with it. Whether or not you believe in that concept, the building you are working on was built with that in mind.
Are you or your client involving the owner of the other rowhouse? You only own the half of that party wall to the centre line, so whatever is done needs agreement by both sides.
RE: Underpinning Brick Party Wall - Stability
Mostly I was just rethinking the underpinning approach that is usually done and whether or not it was reasonable, and what factor of safety people were comfortable with. Based on the variety of answers it seems like there is a lot of variation on this one.
Tony1851 - I didn't apply any reductions to dead load. I went with straight loads and demands to get a feel for the factor of safety.