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RE: Footing design for uplift

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Jordan, I am not sure if 45 deg is in the safe side for the problem. As
in the retaining wall analysis, the soil failure plane is not at that
angle. Note that geotechs uses K, coefficient for lateral pressures, in
their formulas. Bowles in 'Foundation Analysis and Design' shows the
shape of the failure zone, and that shape is a curve. For this curve the
average slope appears to be greater than the slope with 45 deg. I prefer
60 deg respect to the horizon as a easy and conservative approach.

Jorge

-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com] 
Sent: Friday, January 12, 2007 9:18 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Footing design for uplift

Jorge, you must have a lot more geotechs than we do and owners far more 
willing to spend money on soil work. 45 degrees is easy to calculate and

doesn't rely on soil properties which are approximate at best.  For 
slabs, I use the 45 degree number and if that doesn't work I might try 
yield line. Usually I just add more concrete - it's cheap. I can add at 
least 8000-10,000lbs of uplift resistance to a building for what an hour

of my time costs.

Jordan



Jimenez, Jorge A. wrote:
> In the 'Structural Engineering Handbook', Gaylord & Gaylord, 3rd
Edition
> there are a discussion about this issue. The text gives a formula that
> have the soil angle for internal friction and coefficient for lateral
> pressure, among other variables.
>
> Jorge Jimenez, PE
>
> -----Original Message-----
> From: Javier Encinas [mailto:jencinas(--nospam--at)asdipsoft.com] 
> Sent: Thursday, January 11, 2007 9:17 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Footing design for uplift
>
> Sometimes in hurricane-prone zones the controling load in footing
design
> is
> uplift. However, the text books are silent about methods to estimate
the
> selfweight to counteract the uplift force. In addition of the concrete
> footing we have also the soil cover and the slab on grade. What angle
of
> the
> cone is appropriate (45 degrees?), and what is the tributary area of
the
> slab that helps to keep the footing in place? Any good reference on
the
> subject, or full scale tests? Thanks in advance,
>
> Javier Encinas, PE
>
>
>   

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