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
*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