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CONC - Footing Design

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Mark Ahlskog, PE asks:

I have a couple of footing design questions . . . . .

My response is as follows:

1. When you have large moment combined with moderate axial compression, your bearing pressure analysis would show the footing to be under partial contact, as you suggest -- that is, if neglect the soil self weight. But, in such cases, I tend to include the soil overburden as an applied load, too. This additional compression would add a constant compressive bearing pressure diagram to the partial-contact bearing pressure diagram. Depending upon how large is the applied moment and how deep you locate the foundation, the resulting pressure may be trapezoidal and not show any net "tension" now. For controlling the pressures, when using net allowable bearing pressures in such a case, I would add the soil pressure to our allowable bearing pressure values and get the gross allowable values. I then check the computed pressures against this gross allowable value.

When computing rebars, we can use the same pressure diagram (but of course factoring it this time). The only difference is that we have to include the soil overburden pressure as well as the bearing pressure, in our bending moment computations.

I would feel that it would be unrealistic not to consider the soil load, unless perhaps when the back-fill material is in a very loose state.

2. When using the strength design method, I tend to apply the load factors to the applied loads and get what are called pseudo-pressures. As an alternate, applying the load factors, separately to unfactored pressure diagrams, obtained load-by-load, would obviously give us the same result. But if we simultaneously apply the various loads, falling under different categories (dead and earthquake, for instance) and then apply the some kind of an "average" load factor, I agree that we would not get the same result. And I feel that doing that would not be consistent with the spirit of current ACI code. 

By the way, in this regard, I have a question too. What unfcatored load combinations you use, when checking pressures against allowable values. Do we have an unfcatored load combination, parallel to each factored load combination prescribed by the code? I have difficulty in finding a parallel for (0.9 D + 1.3 W). I would appreciate help in this regard.

3. Your last question has been bothering me too, as I have not come across a reference that discusses the problem of how to design for shear along two orthogonal axes. Using a vector sum would work obviously fine with a circular column but then what about rectangular columns? It appears that some kind of an interaction concept would have to be used, like the one we use in Bresler's Reciprocal method for Biaxial bending in columns. But this is just loud thinking. Perhaps someone else can educate me, too, on this issue. 


Rizwan Mirza