Eccentrically Loaded Foundations
The design of eccentrically loaded foundations is not the same as the conventional foundation where the column load is applied at the center of the footing.
Normal footing becomes eccentrically load footing when there is an eccentricity in the loading or when there is a bending moment at the connection of footing and the column.
The eccentricity could be in one direction or in both directions. Desing of the eccentrically loaded footing shall be done accordingly.
The following equations can be used to find the maximum and the minimum pressure under the foundation.
qmax = Q/BL + 6M/B2L
qmin = Q/BL – 6M/B2L
Where, Q – vertical load, M – moment on the foundation, B – width of the foundation, L – length of the foundation
The following figure indicates the pressure variation under the foundation.
When the eccentricity, e < B/6, the minimum pressure is grader than zero always. However, when e > B/6, minimum pressure will be as indicated in the above figure.
The important factor is that the maximum pressure under the foundation due to the eccentric loading shall not be greater than the allowable bearing pressure.
The eccentricity can be calculated from the following equation for eccentrically loaded foundations.
e = M /Q
when eccentricity is known the above equation can be simplified as follows.
qmax = Q/BL (1 + 6e/B)
qmin = Q/BL (1 – 6e/B)
- When e = B/6, qmin becomes zero, and the further increase of the eccentricity ( e > B/6), negative pressure will be generated on the soil.
- It is the development of tension in soil. In this situation, there will be a separation between the soil and the footing.
During the design, we have the bearing capacity of the soil. We can check whether there is tension in the foundation and whether the applied pressure (maximum pressure) on the soil exceeds the allowable bearing capacity at that level.
Further, when there are eccentricities in both directions, a similar process shall be followed to find the effective area of the foundation and the maximum pressure under the foundation. Further information on the design of eccentrically loaded footing could be reffed for the book principle of foundation engineering.
The article shallow foundation failures could be referred to for more information on foundation failures.
