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RE: Enlarging a footing

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What about retaining a geotechnical engineer to do a test pit and some analysis to determine what bearing capacity is reasonable?  Perhaps you can avoid the changes outright (depending on what kind of demand you are dealing with).

 

-----Original Message-----
From: Gerard Madden, SE [mailto:gmadden(--nospam--at)maddengine.com]
Sent:
Monday, September 08, 2003 1:43 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Enlarging a footing

 

Robert’s step by step account for the load re-distribution is what I was thinking as well. Once the existing footing deflects beyond it’s current deformation, the new footings start to help out and do the work.

 

The footing in question is about 40 feet long and the width increase is necessary because there is no soils report and I have to assume 1000 psf soil pressure by code. It is very likely the soil’s capacity is more than twice this amount based on back calculating the loads and original footing sizes.

 

This footing is being enlarged because it is under a shearwall and I am strengthening the wall system for a larger seismic demand. This is also a voluntary action by the owner, his goal is to take care of the easily correctable deficiencies with a minimal disruption to daily operations within the building.

 

Yes, I was adding new full depth concrete on either side. I think that compressed air sandblasting and/or bush hammering would be suitable as Jim suggested. I’m not sure if this classifies as “a lot” – I think this about 1 or 2 days work (for 1-2 persons) for this amount of footing – but I could be wrong.

 

Thanks for the responses so far, keep them coming if you have more to add.

 

-gerard

 

 

 

 

 

-----Original Message-----
From: Robert Rogers [mailto:RRogers(--nospam--at)lorwil.com]
Sent:
Monday, September 08, 2003 10:20 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Enlarging a footing

 

Bill....you do have a good point....

 

I guess we could all look at this in terms of risk (probability & consequences)....just like those insurance folks....

 

If you add to the footings & the new load is not picked-up right away, then the existing footing capacity is exceeded.....(perhaps both the the strength of the concrete in bending & shear as well as soil bearing capacity).....the question then becomes.....what happens next.....you can assess the safety factors associated with the concrete design....and estimate the increased settlement due to additional loads.....but once you have the slightest settlement / failure mode, then the portions of the new footing pickup (you hope..unless compaction or in-situ soil conditions are suspect)......its all very qualitative in discussion (not so much quantitative).....

 

Add in all the unknowns and its very much a judgment issue........

 

I guess, in my mind, you need to assess the consequences if your engineering judgment (the new footing additions picking-up the additional load) proves to be different that what you thought (trying to assign a probability to that is almost impossible)......if you're carrying loads that have few redundant paths or pose lots of risk to the inhabitants (such as an assembly area, etc.) then you may want to be very careful with your judgment call....

 

Robert

Robert C. Rogers, PE

-----Original Message-----
From: Bill Allen [mailto:T.W.Allen(--nospam--at)cox.net]
Sent:
Monday, September 08, 2003 12:59 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Enlarging a footing

I totally agree with Jim’s concern.

 

I understand the desire not to disturb the existing soil, but the new section of the footing will be disturbed and the amount of load it picks up will be suspect IMO.

 

Underpinning might not be so bad. You could do it in four foot sections, alternating sections and doing it in two phases. In the first phase, leave a section of bar protruding from each end to lap with bars in the 2nd phase.

 

Regards,

 

T. William (Bill) Allen, S.E. (CA #2607)

V/F (949) 248-8588

San Juan Capistrano, CA

http://members.cox.net/ballense/

 

-----Original Message-----
From: Jim Persing [mailto:jpersing(--nospam--at)fhoarch.com]
Sent:
Monday, September 08, 2003 9:45 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Enlarging a footing

 

I always specify sandblasting or bush hammering to 1/4" amplitude.  But it can be spendy if you have a lot of footing to do.  Developing transverse bending moment stresses is not especially nice, either.  And you never know what the difference in true bearing values are with freshly compacted soil adjacent to a footing that has been there and settled for a length of time.

 

Jim Persing, PE

-----Original Message-----
From: Gerard Madden, SE [mailto:gmadden(--nospam--at)maddengine.com]
Sent:
Monday, September 08, 2003 9:09 AM
To: seaint(--nospam--at)seaint.org
Subject: Enlarging a footing

I am trying to enlarge an existing strip footing by increasing its width. I have proposed to place new concrete on either side of the existing footing thus doubling its width. The existing footing is 2ft widex2ft-6in deep and I have called for shear friction dowels on each side (2 rows, ea. Side) to share the bearing with the new concrete on either side.

 

The plan checker is questioning the surface preparation of the existing concrete and how that is going to be achieved. He wants underpinning – since the existing footing has not failed or shown any signs of settlement, I think increased bearing is an appropriate solution rather than deepening the foundation. Underpinning would require shoring and an increase in cost and possible disruption of service for the tenant. Underpinning would also change the distribution of shear and moment in the existing footing, it’s capacity may not be up to snuff.

 

Can anyone recommend a dry method of surface preparation to enlarge the footing as I described? The concern is hydraulic methods of surface preparation would saturate the soil.

 

Thanks,

-gerard