Soil Stress Calculation

The soil stress at any point beneath a spread footing is calculated according to the following equation:

q = \frac{Σ P}{A} \pm \frac{Σ M_{maj} C_{\min}}{I_{\min}} \pm \frac{Σ M_{\min} C_{maj}}{I_{\max}}

Equation 4-3

which assumes that the footing is in contact with the soil at all points on the footing.

Soil Stress due to Eccentrically Located Column or Bi-axial moment on Column

For footings with possible biaxial moment from column moments and/or a column not centered on the footing, an iterative analysis is used. The spread footing with biaxial moments is analyzed using a closed form analysis of the soil stress under the footing. The method considers the region (if there is any) of the footing that is experiencing uplift and accounts for it when calculating the soil stress. The tension area is then ignored when calculating the soil equivalent resultant force and its location. The location of the resultant force and the total force from the soil is checked against the location of the resultant force and the total force from the footing. If either value is off by more than 1% a new iteration is performed with a modified soil pressure until the magnitude and location of the resultant forces are within the tolerance.

The location of the resultant load on the footing from all the loads on the footing including the column moment is calculated as follows (when there is no uplift on the footing):

e_{maj} = \frac{Σ M_{maj}}{Σ P}

and

e_{\min} = \frac{Σ M_{\min}}{Σ P}

Equation 4-4