biaxial ftg analysis

huangyhg

biaxial ftg analysis
if you have a footing with moments in both directions at the column base, i have to that the moment in both directions is going to impact the maximum bearing pressure.  
i am using enercalc, but i am disagreeing with the methodology.  for moment about the x-axis, the pressures change for the top and bottom of the footing - this makes sense.  for moment about the y-axis, the pressures change for the left and right of the footing - this also makes sense.  
once these moments act together, however, you can't just consider them independently anymore, can you?  that is what enercalc seems to be doing.  
e.g.  if i have a positive moment about x-axis, the pressure at the top of the footing increases.  add in a positive moment about y-axis and the pressure at the right side of the footing increases.  because the top of the footing has increased pressure (along the entire footing length) from moment about x-axis, i would think this would be added to the pressure on the right side of the footing.  is this not done with footing design?
it will obviously be a very small portion of the footing that sees this maximum bearing pressure, but there could be a large portion of the footing that is seeing a larger bearing pressure than given by doing the two axes independently and not accounting for the interaction.  
any ideas/opinions?
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you can see the new methodology for biaxial footing design in bowles 6th edition.  previously, i used aashto or arema (which is where this graphical method was first used).
regards,
qshake
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arghhhh!  i said bowles but meant das' 6th edition.   
regards,
qshake
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first to point out, i am not aware of any of the computer program is capable of handling bi-axial stresses on foundations. for practical purpose, if the source of stresses is due largely to transient loads, i wouldn't worry about if the stresses are within limits. however, for bi-axial stresses caused by odd geometries, irregular foundation layout, and special external loadings, i would do a hand cal (using spreadsheet)rather than relying on the computer programs.
are you checking bearing or designing the footing?
if checking bearing, check if the loads are specified as simultaneous.  if checking the footing, you'd normally figure moments about one axis or the other, but the moment at right angles to it wouldn't affect the total moment about a cross section.
jstephen-
i am actually checking whether a contractor's "oops" in the field is ok.  i was checking bearing pressure - the loads are simultaneous (from bending about both axes of the column base).
i was also checking the footing strength.  i understand you only consider one axis at a time for the footing, but i would presumable use the largest bearing pressure in my calculation to get the mu in my footing at the column face.  if i only considered one axis at a time for the footing design, i would be using an artificially low ultimate bearing pressure acting on the footing.  am i missing something?
qshake-
i have das 5th edition.  there is a little discussion on moments about both axes, but it relates more to calculating the ultimate bearing capacity, not to figuring out the actual max bearing capacity.  does the 6th edition have something more substantial?
you need to calculate the pressure distribution resulting from the moments acting simultaneously, if that's how they would occur in real life.  
you will get a maxium pressure at one corner, a minimum at another and two in betweens.  another method is to look at effective bearing area which is based on the actual footing dimensions b and l and reduced according to the amount of eccentricity in each direction.  i believe das discusses both of these.
i wouldn't just check the moments independently unless they will only ever occur independently.