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3 columns, 1 footing

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I have found that, when you have unusual loadings on footings, if you turn 
the footing upside down (on your paper) and treat the soil pressure as the 
loading, and the column forces as reactions, the solution jumps out at you.

By finding the center of pressure of the column loadings, and, if you make 
this the centroid of your footing, you have dictated that the soil pressure 
will be uniform.  The only thing left to do is to find the moments and shears 
at the critical locations.


A. Roger Turk, P.E.(Structural)
Tucson, Arizona

John Nader wrote:

. >  I have a situation where three columns fall at 2 expansion joints that 
. > share one footing.  Will this approch be correct to analays the footing.  
. > The 3 columns look like an L.  The columns do not have equal loads. They 
. > are about 2' apart from each other.

. > 1.  Find the center of gravity between the columns.

. > 2.  Take the load from each column get the moment, P X the distance to the
. > center   of  gravity.

. > 3.  Add the moments and the Loads, and find e.

. > 4.  Calulate the footing with total P, total M and e.

. > Thanks in advance for your input.

. > John

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