I am designing pipe support foundation using a spreadsheet. Since there are lots of iteration, I thought I need a quick guide of finding the optimum foundation size by a given load. And the famous :
`P/A + 6M/(bd^2)`
is really necessary.
How quickly important is this? For example, we have an allowable soil bearing capacity of 50 kpa only. This means we are only allowed to stress the soil at that limit. Given a concentrated load, and lateral load applied on top of footing and an initial sizing of footing, we can then compute the Max pressure of footing at the edge by using formula above.
Ex: `Py=60KN; Px=50KN`
`e=3m("top of pedestal to bottom of footing")`
`B=5m; D=4m`
Thus,
`P/A=60/(5*4)=3Kpa; (6M)/(b*(d)^2)=(6*50*3)/(5*4^2)=11.25Kpa`
And we have maximum soil pressure of 3+11.25 = 14.25kpa only which is way below than our allowable soil bearing capacity of 50 kpa. Using my calculator to prepare an equation with variables; i can easily manipulate or optimize the footing to lessen the dimension at the same time not exceeding `50kpa`.
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