The design of eccentrically loaded foundations differs from that of conventional foundations, in which the column load is applied at the center of the foundation.
A normal foundation becomes an eccentrically loaded foundation when there is an eccentricity in the loading or when a bending moment occurs at the connection between the foundation and the column.
Eccentricity can occur in one or both directions. The eccentrically loaded foundation must be constructed accordingly.
The following equations can be used to determine the maximum and minimum pressure under the foundation.
P Max = Q/BL + 6M/B 2 M
Minimum P = Q/BL – 6M/B 2 M
Where Q is the vertical load, M is the moment at the foundation, B is the width of the foundation and L is the length of the foundation.
The following figure shows the pressure fluctuations under the foundation.
If the eccentricity is < B/6 ist, ist der Mindestdruck immer kleiner als Null. Wenn jedoch and > B/6, the minimum pressure is as shown in the figure above.
It is important that the maximum pressure under the foundation due to eccentric loading is not greater than the allowable bearing pressure.
For eccentrically loaded foundations, the eccentricity can be calculated from the following equation.
e = M/Q
If the eccentricity is known, the above equation can be simplified as follows.
P Max = Q/BL (1 + 6e/B)
Minimum P = Q/BL (1 – 6e/B)
- If e = B/6, P Minimum becomes zero and with a further increase in eccentricity (e > B/6) a negative pressure is created in the soil.
- This is about creating tensions on the ground. In this situation, there is a disconnection between the soil and the foundation.
When planning, we take into account the bearing capacity of the soil. We can check whether there are tensions in the foundation and whether the pressure exerted on the ground (maximum pressure) exceeds the allowable load capacity at that level.
If eccentricities exist in both directions, a similar procedure should be used to determine the effective foundation area and the maximum pressure under the foundation. For more information on the design of eccentrically loaded foundations, see the book “Principles of Foundation Engineering”.
The article shallow foundation failure For more information about foundation defects, see the following.
