ASCE 7-16 Default Site Class D vs. Site Class D

[davab]

Hi guys,

Do you guys understand the difference between Default Site Class D vs. Site Class D?

I went to SK Ghosh seminar and I believe Default Site Class D should be used if I do not have any soils report.

I am not sure where in the code this is stated.

Any thoughts?

 

[davab]

Silly me.. It says Section 11.4.3.

Can anyone explain this section and the difference?

I have a project in Oakland and the difference in SDS is 1.9g for Default Site Class D and 1.6g for Site Class D.

 

[r13]

I guess you determined site class D with a set of given soil characteristics. Without such information, you shall use default site class D as suggested by the code, which is more conservative, as you have found out.

 

[RPMG]

I haven't figured it out, but I know that it's about soil testing. Assuming now comes with a penalty, and I believe that there are seismic design categories where one is not permitted to assume the site class.

 

[Deker]

Site Class D (Default) applies when the properties of the soil aren't known with enough detail to determine the site class. Site Class D (not the default) applies when the geotechnical engineer verifies that the soil may be classified as such. The default option is meant to be conservative, but since the Fa and Fv values were modified in ASCE 7-16, it created a situation where Site Class D would be less conservative than Site Class C for values of Ss above 1.0. See the commentary from ASCE 7-16 below.

 

[davab]

Thanks guys!

Because Sms = Fa x Ss and Fa cannot be less than 1.2 if default site class D is selected per Section 11.4.3, it puts a penalty on the Sds. It is making all the sense now.

Down in Orange County, specifically in Irvine, there wasn't any increase. Up in Oakland, it went up almost 20%....