SQUARE FOOTING DESIGN

[gotlboys]

I am trying to find out how to design a square footing where the column is slightly off the footing center to any direction. Say it is located near property line (at corner). Can the analysis be the same as it is for square footing given that soil pressure is maximum at smaller sides?

 

[spieng89]

Analysis should be similar as if the footing was subjected to just a moment with the column load (P) being offset distance (e) resulting in an overturning moment of P*e. You'll end up with a trapezoidal soil pressure diagram. Just make sure the "e" falls within the kern (B/6) so that there is no net tension in your footing i.e. ineffective pad bearing area.

 

[jayrod12]

Search for eccentric footing. There should be countless threads on this. There are more than one way to skin this cat.

 

[msquared48]

You can design it as a normal footing and take out the eccentricity with a grade beam or two if the soil pressures become too great.

Mike McCann, PE, SE (WA)

 

[Okiryu]

Mike, can you expand on the approach about using grade beams to reduce eccentricity? I am assuming that additional grade beams can reduce the moment in the footing, but not sure how to calculate the forces and application locations of the grade beams. My work is more related to geotechnical, so it is nice to hear opinions from structural engineers for foundation design. Thanks!

 

[PanosDK]

A grade beam approach goes as follows:
Let us assume that you have a physical eccentricity of 0.5 ft and an axial compression of 200 kips. So, you need a moment of 0.5 ft x 200 kips = 100 ft-k to counter this eccentricity. You must then add a grade beam that connects this footing with a column/footing located in the opposite side of the physical eccentricity (i.e. towards the inside of your plan view if the footing of interest is close to the property line). The grade beam must be capable of carrying the moment of 100 ft-kips. The grade beam will also transfer a moment of 1/2 x 100 ft = 50 ft to the opposite column generating a smaller eccentricity (in the opposite direction) to that column. Typically this should not be an issue (smaller moment, larger axial compression to an interior column). However, it is best if you extend the grade beam to the next column, which will now have to carry a moment of 1/2 x 50 ft-k, and so no. The idea behind this approach is that your footing tends to rotate toward the side of eccentricity. This tendency activates the resistance of the grade beam, which now develops a counter moment, thus reducing the effects of the physical eccentricity.

 

[Okiryu]

PanosDK, thanks for the explanation. That was clear and makes sense to me.

 

[PanosDK]

I am glad that I could help. I consider myself both a structural and a geotechnical engineer, and I enjoy very much discussing issues on the interface (or interaction) of the two.

http://www.qsystemsengineering.net