Effective Bearing Area

[someengineer]

Hello,

I am designing some shallow footings that rest on granular fill with vertical load and moment applied to them. Someone proposed to me that there is a limit to determining the effective area of the footing based on a maximum eccentricity of less than B/6 (or L/6). My understanding is that limiting this eccentricity to less than B/6 ensures that the footing has a trapezoidal load distribution and there will be no areas of zero bearing pressure on the footing.

Is there a reason that you cannot have an eccentricity larger than B/6? I have heard it is not recommended, however as long as there is no overall overturning on the footing as a result and the bearing pressure (considering effective area) is within the allowable limit, why can't you have the eccentricity exceed B/6?

Thanks!

 

[someengineer]

As an addendum to this - I know there has been much discussion regarding factors to be used to foundation calculations and I hope I don't open a can of worms, but would it be correct to assume to use unfactored loads to determine the eccentricity?

 

[Doctormo]

The middle third of the footing criteria (e < B/6) goes back to a triangular pressure diagram and no negative heel pressure as you noted. It is probably a good idea as a general rule but not necessary. There may be more structure deformation when pushing eccentricity limits though.

AASHTO LRFD has gone the way of using factored loads and calculating eccentricity from them which is kind of silly in my opinion. The criteria has been changed to B/4 or equivalent which no longer represents anything other than numbers. I think it is B/3 on rock also. Why would one want to calculate a dimension with a factored load is beyond me as the balance of forces is distorted. They seem to understand that it would be silly to calculate settlement with factored loads but eccentricity is ok.

I gave up a while back and just do what the code requires. Stop thinking and just do it is what LRFD becomes many times.