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[SEAOC] Re: Seismic Design of Foundations

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I agree that the building code is pretty vague about foundations.  
This is an area that SEAOC has been planning to start looking into.

Meanwhile, my own poor 2 cents worth is that, if you can really 
predict what the strength and stiffness of the column, grade beam, 
foundation, etc. really is, and provide a grade beam that will be 
stronger than the column (so that the hinging will occur in the 
column, then it doesn't really matter whether you provide ductile 
detailing or not.  For all other cases, however (and this includes 
everything that I do), you have to expect that a mechanism may occur 
in the grade bem.  So you had better make the grade beam stronger in 
shear than in fleure, and you had better provide closely-spaced ties 
in the area where the hinging will occur.

For the toe of a shear wall, my opinion is worth even less than 2 
cents. One approach might be to provide a structural strength that is 
greater than the ultimate strength of the soil (which is 2 to 3 times 
greater than the "allowable" soil pressure).  But that may be 
excessive - it may be better to cause the hinge in the footing, where 
the ultimate strength is more predictable.  Perhaps someone else has 
an opinion .......?

> When designing a foundation of a seismic lateral resisting system, is it necessary to 
> design the foundation using all the concrete seismic design provisions?
> 1. Moment resisting frame, & you are counting on some fixity at the base.  Do your grade 
> beams have to have the same details as a flexural member of a concrete SMRF?
> 2. Shear wall system.  For overturning, you need to extend the foundation beyond the end 
> of the shear wall a several feet.  For the extension, must you use the same details as a 
> concrete SMRF?
> Let's hear what the current practice is.
> James Lord, S.E.
> jlord(--nospam--at)
> ...
Martin Johnson
EQE International, Inc.