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RE: Enlarging a footing

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Have you looked at "hilical anchors"?  They can develope both gravity loading 
and uplift loading.

--
> 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.
>  
> -gerard
>  
> -----Original Message-----
> From: Eric Ober [mailto:eober(--nospam--at)holbertapple.com] 
> Sent: Monday, September 08, 2003 11:17 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Enlarging a 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).
>  
> -----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
>  


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