What sort of soil pressure checks do I need to do for footing?

[kk88818]

Hi, everyone. I am designing a silo footing, say 18 ft x 18 ft, and 2 foot thick. There is a 58 foot tall silo on my footing. Forces acting on my footing include the dead and live loads of the silo. I have wind and seismic loads on my silo, too, resulting in base shear and overturning moments on my footing.

I am checking the adequacy of the footing, and so far have checked stability and sliding. F.S. for stability is >1.5 and for sliding is >2, so both are OK.

My question is what sort of soil pressure checks do I need to do on my footing? What load cases do I need to consider? So far, I have checked DL + LL. I understand I need to do DL + WL, and DL + EQ, too, is that correct?

Also, what is the purpose of calculating the eccentricity, e, locating the kern, and then finding the values for the coefficients C1 and C2?

kk88818
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[kslee1000]

1. Correct. I remember the allowable load is different for the transient loadings (watch out for uplift).
2. Minimize the unfavorable effects due to bending and rotation of the footing. (no idea about C1 & C2 coefs)

 

[structuralengr89]

The first thing you need when designing a footing is an allowable soil pressure. This is typically where the geotechnical engineer comes in. Unless you have an extensive background designing footings in a particular area, you should contact a local geotech to do a boring or two and get you that info.

The eccentricity (M/P) will determine the soil pressure distribution under the footing.
If e =M/P <L/6, the soil pressure is trapezoidal.

If e =M/P =L/6, the soil pressure is triangular

If e >L/6, the soil pressure is triangular

Get a good structural concrete textbook and look up footings under eccentric column loads.

 

[kk88818]

Thank you.

 

[civilperson]

All load combinations shown in ASCE 7-05, (Chapter 2), must be checked for each component of the structure. "0.6D + W + H" may control for soil/footing design.