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Overburden Versus Pedestal

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How does the module handle overburden load in the footprint of a pedestal?

 

The General Footing module will not apply overburden pressure on the footprint of the pedestal.

 

To prove this to yourself, you can follow these steps:

 

1.Start with a General Footing calc that has both a normally sized pedestal (including a non-zero height) and an applied overburden load.  Make note of the bearing pressure.
2.Remove the overburden load and note that the bearing pressure changes as expected.  This proves that the program is definitely doing something with the overburden load, and if you were interested enough, you could work through the numbers and prove that they are correct.  But here is a faster way to get a comfort level that the module is not applying overburden load in the footprint of the pedestal.
3.Continue to work with the same calculation, and revise the plan dimensions of the pedestal so it is equal to the plan area of the footing itself, and provide a non-zero pedestal height.  This makes it so there is no area on which the overburden load can act.  Note the bearing pressure for this configuration. 
4.Revise the overburden pressure to zero and note that the bearing pressure does not change.  This proves that the program is only applying the overburden pressure to the area that is NOT already occupied by the pedestal.

 

This is a GOOD thing in a situation where the pedestal is high and comes all the way (or almost all the way) to finished grade.  But it could produce some unconservative results if the pedestal was relatively short and was buried well below finish grade.  So it is important to be aware of how the module is handling this situation.