The difference in choosing site class D (stiff soil) and the site class D (default) is stated in ASCE 7-16 section 11.4.3:”where the soil properties are not known in sufficient detail to determine the site class, Site Class D … shall be used unless the authority having jurisdiction or geotechnical data determine that Site Class E or F soils are present at the site”. The “default” selection (based on insufficient data) is typically more conservative in its values than the “stiff soil” selection (based on the available data). Because shear wave velocity data is available near the project site, it can be assumed that we have sufficient data.