'Footing/ Base Slab/foundation loss of contact theory'

[LearningStructEngg]

Hi all, Can anyone briefly explain me how footing loss of contact theory works? I am currently working on design of base slab and while doing bearing pressure check, the eccentricity exceeds L/6, and the structure is is subjected to tension I understood the procedure of calculating the edge pressure (i.e. P = 2R/3By) but am unable to understand the theory behind it? Can someone explain it in brief or provide a better way to understand it?
Thank You in advance!!

 

[BAretired]

[li]When load is concentric, soil pressure is P/A where P is load and A is area of footing.[/li]

[li]When eccentricity 'e' is within the kern of the footing, i.e. the middle third, soil pressure is P/A + Pey/I where y is distance from c.g. and I is moment of inertia of the footing area.[/li]

[li]When e = L/6 soil pressure is zero at one end and 2P/A at the other end. Average pressure is P/A.[/li]

[li]When e exceeds L/6, the effective area of footing is reduced, otherwise there would be tension between soil and footing, which cannot exist.
Distance from load to edge of footing = L/2-e.
Effective area of footing = A[sub]red[/sub] = 3(L/2-e)W where W is Width of footing
Average pressure = P/A[sub]red[/sub]
Min. pressure = 0.
Max. pressure = 2P/A[sub]red[/sub].[/li]

 

[LearningStructEngg]

Thank you BAretired.

 

[BAretired]

You are welcome, LearningSE.