Seismic site class

[Okiryu]

Hi, Based on ASCE7 and IBC, I determine my seismic site class based on SPT N values. My question is: how do you consider the N value when you have refusal with a penetration less than 12 inches. For example, we normally consider refusal if we get 50 blows. If for example, we get 50 blows and 6 inches of penetration, how do you consider the N value when calculating the site class?
For my above example, do you consider 50 or do you extrapolate to 12 inches and consider 100 as the N value for site class calculations.

From my above question, depending on how do you consider the N value for refusal, it may have large impact in the site class.

Anyways, in my area, when classifying the site based on SPT N-values, I have only to options for site class: D or E. This is because in my area is very difficult to have an average N value larger than 50 for the 30m of top soils to classify the site as B. Of course, if I have sites prone to liquefaction, the class may be F.

Thanks.

 

[Okiryu]

I was checking the ASCE 7-10 code and found that for site classification calculations you can use a maximum N value of 100 blows/ft (305 blows/meter), which is around 92 blows/30 cm if we want to translate it to SPT metric language.

So, I think that you can use values greater than 50 for N-values in the equations for site classification (Equations 20.4-2 and 20.4-3) from ASCE 7-10.

The above is based on the paragraph indicated just below the equation 20.4-3 of the ASCE 7-10 code.

 

[numerical1]

Also, one more thing we should remember is that defining the seismic category/class of a site is not completely a mathematical exercise involving shear wave velocity measurements or SPT numbers. It does, however, require good judgment as well.

 

[Okiryu]

Numerical1, I agree that good judgment is always necessary. Just wanted to go thru the theorical process to proof site class determination. However, at the same time, it may be hard to proof some engineering results without calculations, unless you have quite large number of years of experience, which unfortunately is not my case.