Resultant Outside Middle Third of Tower Crane Footing 2

[cldea8]

I am designing a tower crane footing for the construction of a parking garage. As expected, I have a large moment on this footing. The resultant falls outside the middle third of my 18'x18'x5' thick footing. I know this is generally undesirable, however, I performed (and passed) the following checks:

*My allowable bearing capacity (on Geopiers) is 5000psf

*Overturning about the toe yields a Factor of Safety > 1.5

*M/P yields an eccentricity of ~5ft which is > L/6 (outside middle third).

*Because it is outside the middle third, I used the following bearing equation from Bowles 5th Ed.(zero stress for a distance then triangular load on bottom of footing): q = 2P/(3B(B/2-e))< qa (have seen this on several threads too)

*This resulted in a toe pressure less than 5ksf allowable.

*Flexure, Flexural Shear, 1&2 way Shear, Min Ast Satisfied


If I keep my resultant within the middle third, the footing grows to 23'x23' (which is about $4000 more in concrete).

Is this procedure acceptable? As expected summing moments about the resultant gives me a Factor of Safety of one. I would assume my factor of safety is equal to that of the bearing capacity (2-3)?

This text reads that there is no way to determine qa for a triangular load and that it is "prescribed" by the geotechnical engineer. However, some texts show triangular distribution when the resultant is within the kern.

Have I checked everything? Do I have instabilities? Is this procedure acceptable?

Thanks,

cld

 

[cldea8]

Me neither. Hence my post.

The geometry and stress distribution is quite simple if you assume P/A + Mc/I stress distribution. It is relating this to the allowable bearing capacity. I consulted the rock column engineer and he told me that the allowable bearing capacity (he is providing) is based on a uniform load distribution.

The general convention is to compute your toe stress and check it against the allowable bearing capacity. I just dug a little deeper into the soil structure interaction because I thought I was stepping outside of conventional analysis methods (by letting the resultant fall outside the middle third of footing)

This text (Bowles 5th Ed.) uses a more in depth approach that doesn't assume the soil is always linear-elastic and that stress redistribution takes place.

I would scan in the text and post it but don't want to violate copy rights

cldea8