Seismic Design Category v. Soil classification`


In evaluating Seismic design category (SDC) and soil classification I am finding similarities between the two. The IBC Ch.16 provides the tables for the two as separate but there seems to be similarities. Can you clarify what is the difference in SDC and soil classification tables and where they are applicable?

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Hi @areyes & welcome to the ARE Community!

@coachlizschneider do you mind answering this question?

Thanks for the question @areyes!
There are some similarities between the two and your soil type will help you determine your site class which plays a role in determining your Seismic Design Category.

To start with Soil Classifications, classifications are broken down into four main categories; gravel, sand, silt and clay, and organic soils. For gravel and sand, you have poorly graded and well graded categories. And then you have classifications representing the different combinations like Silty Sands. These classifications will be used to determine the bearing capacity of your soil and the overall quality of the soil for construction. For more info on soil classifications you can refer to Fundamentals of Building Construction by Edward Allen.

These classifications will help you to determine your site class. Site class can range from A to F, with A representing hard rock and F representing unstable soil. site class along with your occupancy and the location of your building in relationship to fault lines will determine your Seismic Design Category.

Your seismic design category can range from A for structures with low seismic risk to F for structures with the highest seismic risk. This will be used to determine the structure of your building. For more info on Seismic Design Categories you can refer to Earthquake Resistant Design Concepts by FEMA.

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Thanks for the information!