Part 3 - Problem 02: A site class of D is assumed for this part. Using ATC hazard tool parameters such as S1, Ss are to be obtained. However, in the tool drop-down menu there are two site class D options (stiff soil-default). The two options provide the same Ss & S1 values, but the other parameters’ value are different. So what’s the difference between site class D(stiff soil) & site class D (default). And how is it possible that they both provide the same values for parameters Ss and S1 but different values for the other parameters?

Question

Part 3 - Problem 02: A site class of D is assumed for this part. Using ATC hazard tool parameters such as S1, Ss are to be obtained. However, in the tool drop-down menu there are two site class D options (stiff soil-default). The two options provide the same Ss & S1 values, but the other parameters’ value are different. So what’s the difference between site class D(stiff soil) & site class D (default). And how is it possible that they both  provide the same values for parameters Ss and S1 but different values for the other  parameters?

Accepted Answer

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.