Seismic Site Class (Nch or Su)

[nbr1]

Can someone please explain to me the following statement in the IBC:

"When the Su method is used and the Nch and Su criteria differ, select the category with the softer
soils (eg. use Site Class E instead of D)."

I have a pure clay site to bedrock at ~50-ft, Su method says site class C, N-method says site class D. Which should be used according to IBC CODE statement above???

 

[moe333]

That statement in IBC is for mixed soil profiles (clay and sand). You should use Site Class C since you have pure clay.

 

[nbr1]

Thanks Moe

 

[abih]

I know in some regions you use SPT in regardless materials (i.e., clays as well). When you need the undrained shear strength, you will rely on empirical formulae. So it is debatable if it is a Su or N method to get you a Site Class assignment.