Shallow Foundation Consolidation Settlement - 3-story c1927 house possibly undersized chimney footin

[EngineerEtcetera]

Smart People,

I’m working on renovation of 3-story c1927 house (unfinished walkout basement, street and upper level finished, 30 ft x 30 ft footprint). Basement is to be finished, including lowering by 18in (underpinning) to increase ceiling height from 6ft 8in to 8ft 0in (incl flooring).

House has central chimney. Chimney footing appears to rest on 2ft to 4ft of ML / sandy clay loam native soil over deep SP / loamy sand native soil (40ft + likely - based on familiarity with soils local to site). The seasonal high water table is probably 20ft + below footing.

Numbers I’m working with are:

DL_chimney: 46kips
DL_foor: 15kips (appears chimney acts as structural column - assessment phase)
LL_floor: 45kips (as above)
DL+LL: 106 kips
q_a: 2000psf (prescriptive bearing capacity)
A_fe: 20.4sf (existing footing)

So, A_fr = (106E3lb)(1sf/2000lb) = 53sf (required footing)

A_fe/A_fr = 0.38sf/sf (38%) ... required footing area significantly less than existing

I’m tracking this is all simplistic prescriptive bearing capacity (possibly 3x +/- safety factor) and without regards to expected / potential / allowable settlement.

I plan to check soils during underpinning excavation to adjust footing design as required, but I’d rather not disturb existing conditions unnecessarily.

So, question is ... after 94 yrs, is there any practical reason to expect footing area needs to be increased beyond existing if there is no foreseeable increase of DL/LL?

Any and all input appreciated!

 

[msquared48]

I would lean to the side of no, disregarding any forming issues.

Additionally, the further you excavate down, the greater the allowable bearing should be.

Mike McCann, PE, SE (WA, HI)

 

[EngineerEtcetera]

Mike - Thanks very much!

 

[r13]

I suspect the footing was sized for the chimney only. Agree with msquared48.

 

[emmgjld]

If the existing structure, as a whole, has behaved well, then the footing sizes and pad sizes are probably fine.
I always look carefully at the interior chimney stack pad sizes, in I often find them to be somewhat oversized, in relation to the rest of the foundation footings. I have had to reduce some interior pads, due to excessive settlement of the exterior foundation footings, compared to the interior chimney stack pads.

 

[EngineerEtcetera]

Current plan is slight extension of footings on new pour - likely 2in.

Thanks again!

 

[r13]

Like stacked footing?

 

[EngineerEtcetera]

I'm not familiar with "stacked footing" terminology. Current plan is to excavate 3ft wide x 18in deep sections underneath existing footing and pour/drypack in a 1,3,5,2,4 sequence.

 

[dik]

Is there any signs of movement? The footing is such that failure will be time dependent. Is it possible to leave it and take remedial action if there is a 'change'?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[EngineerEtcetera]

Only visible settlement (minor) appears to have reached relative terminal stage/completion at least 50 yrs ago. I'm also distributing load in that area somewhat more than originally, so I don't see that as an issue. I'm tracking you on time dependent / primary / secondary consolidation, which was my original inquiry.

Thanks!

 

[dik]

Can you talk to the AHJ and 'just leave it'? I've worked on projects where there is a Caveat on the property that it be reviewed initially annually, 3 years, 5 years... extending to decades (a schedule prepared) and then not at all. The Caveat was prepared because the AHJ was not thrilled about the issue being 'left'... the problem was a masonry wall moving (a little dicier), not a footing...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[EngineerEtcetera]

I'm not tracking 'just leave it'. AHJ basically said as I'm a PE taking responsibility that pretty much whatever I decide to do is fine and if there is something unusual, then just to give him calcs to keep on file. I'm only underpinning to lower basement. There are no settlement issues affecting structural integrity that I've detected and none that affect appearance/use.

 

[dik]

Fine... I'd likely not reinforce... with the understanding that it be monitored by the owner... that was the reason for the Caveat (attached to the property). Failure will not likely be catastrophic.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik