I'm in the same situation, and not familiar with API 650 details.

1) is Sp expressed as acceleration in m^2/s, or is it a percentage of gravity?

2) should I select the Site class according to Asce (earthwuale type= mapped) even if I'm using a site acceleration? Or should I select "site specific" (but in this case, even if I insert a huge Sp, my analysis is passing anyway..).
I think that "mapped" has to be selected.

Sorry for the "basic level" questions, I'm not really expert on this code and in using Tank too..