When a geotechnical report is unavailable for seismic design, which site class should be selected?

In seismic design, when a geotechnical report is unavailable, it is recommended to select Site Class D. Site Class D is typically characterized by stiff clay or dense granular soils, which can have moderate shear wave velocities and are considered to have a more predictable response to seismic loading compared to softer or more problematic site classes.

Choosing Site Class D in the absence of specific geotechnical information is a conservative approach because it accounts for potential soil conditions that may lead to more predictable seismic response. In contrast, Site Class A represents hard rock, which is less common in most areas, while Site Class B typically refers to more stable soils than those found in Site Class D. Site Class E, on the other hand, indicates very soft soils or organic clays, which could lead to significant amplification of seismic waves but cannot be confidently assumed without a geotechnical analysis.

Therefore, selecting Site Class D in this scenario ensures a balance between safety and practicality, addressing the uncertainties that arise from the lack of detailed site-specific data.