Eccentricity - Actual e vs. Calculated e
Eccentricity - Actual e vs. Calculated e
(OP)
I have an exterior concrete spread footing, say 10' x 10'. The thickness is 3', say. My column is 2' from the edge of the footing. Therefore, my actual eccentricity, e is 10'/2 - 2' = 3. Now, I have been told that my eccentricity - the calculated eccentricity, that is - can be less than the actual one if I take the weight of the footing into account. The formula for the calculated e, is
M/(Summation of P), i.e. Pe/(P + Weight of footing). Is this formula correct? Most of time, we see e as being just M/P. Does anyone know where this formula is from? lease provide references.
If you look at it, if the weight of footing is not considered, i.e. if Weight of footing is zero, that e is M/P. However, if the weight of the footing is considered, then the calculated value of e becomes smaller the more the weight of the footing is.
Hope someone can verify the accuracy of the formual, and provide some further insight into this. Thanks so much.
M/(Summation of P), i.e. Pe/(P + Weight of footing). Is this formula correct? Most of time, we see e as being just M/P. Does anyone know where this formula is from? lease provide references.
If you look at it, if the weight of footing is not considered, i.e. if Weight of footing is zero, that e is M/P. However, if the weight of the footing is considered, then the calculated value of e becomes smaller the more the weight of the footing is.
Hope someone can verify the accuracy of the formual, and provide some further insight into this. Thanks so much.





RE: Eccentricity - Actual e vs. Calculated e
RE: Eccentricity - Actual e vs. Calculated e
This is correct from first principles - Newtons third law to be precise.
"For every action there is an equal and opposite reaction"
Think of the stages that load is applied.
1. dig a hole - removes load of soil from hole.
2. Fill hole with concrete - applies a uniform load to soil.
3. apply eccentric column load - applies eccentric pressures on soil giving a triangular load.
The final bearing pressure is the combination of all three but generally 1. is ignored.
RE: Eccentricity - Actual e vs. Calculated e
e' = e*P/(W+P)
If the footing is submerged below the water table, W would be the submerged weight of footing.
BA