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RE: Enlarging a footing[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Enlarging a footing
- From: "Gerard Madden, SE" <gmadden(--nospam--at)maddengine.com>
- Date: Mon, 8 Sep 2003 13:44:19 -0700
Unfortunately, I also need the concrete for uplift mass resistance. So either way, I need the concrete, but perhaps save a little if the bearing capacity was higher. I don’t know if the geotech report would offset the price of labor to roughup and clean the sides of the footing.
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).
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.
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 C. Rogers, PE
- RE: Enlarging a footing
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- RE: Enlarging a footing
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