Site class is a code-based classification of ground conditions used in some seismic design procedures.
Definition
Site class categorizes ground conditions for use in certain seismic design procedures. The classification is based on defined soil or rock properties over a specified depth, and determining it may require geotechnical data or investigation. The applicable code sets the method and input requirements, while a qualified professional interprets the information available for the site. Firms may have different geotechnical scopes depending on existing data, access, and project requirements, so the basis and limitations should be recorded. Site class can influence how seismic hazard information is applied in design. It is not a general soil description, a complete geotechnical evaluation, or a direct measure of how a structure will perform in an earthquake. Owners can coordinate the investigation and design schedule around the information needed to classify the site. The governing code and project-specific conditions determine what classification method applies.
How owners use it
- Check whether available subsurface data cover the depth and properties needed by the applicable method.
- Coordinate geotechnical investigation scope and timing with structural design milestones.
- Example: 3 borings planned + 2 additional borings = 5 borings in the proposed investigation scope.
- Record the data source and classification basis in project criteria for design coordination.
