Eccentric load bearing capacity VS overturning check

[LearnerN]

I'm considering the design of a footing for a 25' tall, 24" OD process tower that only weighs 7500 pounds. It'll be on a large footing mainly due to the overturning effect of wind loading. However, I'm trying to figure out how to get the eccentric loading bearing pressure calculation to work out for the foundation. I have P=8k and M=30k-ft...overturning checks fine, but bearing pressure calcs are making this have a huge footing to get enough bearing contact. I know it has to have a huge footing since it's lightly loaded anyway, so just a little applied moment will create uplift under some of the footing. Is there any alternative to having such a massive footing for such a small tower?

I recently considered a similar footing but for a heavier similar tower. However, I think this smaller tower will have to have a larger footing due to the eccentric loading issue.

 

[BAretired]

BAretired, the point where I'm disagreeing is that you can't factor in the weight of the footing for a bearing pressure calculation - it's already assumed per the definition of "net bearing pressure" per the quotations I provided above from Braja Das. However, I think you can add the difference between the concrete and soil unit weights as an additional vertical load.

You are misreading the intent of what Das said. You must include the weight of concrete in your calculation. Why wouldn't you? That is the load that the soil/footing interface experiences. If the maximum resulting pressure exceeds permissible soil pressure plus overburden, you have to try again.

In the case at hand, I assumed that the top of footing was at grade. The maximum resulting pressure was found to be 1428 psf based on a moment of 30'k. If the moment was taken at grade, it should have been increased to account for the one foot footing thickness and I neglected that.

I'm pleased that you got your numbers to work out but you still need to understand the concept which Das discussed in his book. Read it again.

BA