Seismic Design Category 101

15-Aug-2011Several times a year we get calls about understanding the Seismic Design Category (SDC). There are significant detailing ramifications of the different levels and since two of the variables in determining the SDC use letters, it just adds to the confusion in the understanding. Here's a brief little tutorial to explain the basic variables and how they work together based on IBC 2006.

The main variables for determining a building's Seismic Design Category are:
  • Occupancy Category (I, II, III, IV)
  • Mapped Acceleration Parameters (Ss and S1)
  • Site Class (A-F) Adjustment

If you wrote this as an equation you would get:
        Occupancy Category + Mapped Acceleration Parameters + Site Class Adjustment = Seismic Design Category

Occupancy Category:
This is strictly a function of the proposed use of the building.
    Category I - structures that represent low hazard to human life.
    Category II - any structure not falling into Category I, III or IV.
    Category III - structures that represent substantial hazard to human life.
    Category IV - structures designated as essential facilities.

Mapped Acceleration Parameters:
In most areas of the country, these two numbers are determined by the geographic location (latitude and longitude) of the project site (determined from something like Google Earth) and maps in ASCE 7 or a program. Their values represent a ground acceleration at the site as a percentage of gravity. One is called the short period (0.2 sec) acceleration Ss and the other the 1-second period acceleration S1. They are based on an assumed Site Class B.

Site Class:
This is determined by the specific soil properties found at the site and is usually provided by the geotechnical engineer for the project. Generally speaking, a Site Class A is associated with a site that has rock very close to the surface while Site Class F has soils which exhibit high liquefaction tendencies. A Site Class of D is permitted to be assumed in absence of a soils report unless the local authority determines Site Class E or F materials are present at the site. In our region, the common procedure for determining the Site Class is via blow counts from the boring logs. A site-specific shearwave velocity test is another and potentially more accurate way of determining the Site Class.

The Site Class is then used to make adjustments to the mapped acceleration parameters. The adjustments for both the short term (Ss) and 1-second (S1) accelerations are interpolated from charts in ASCE 7 based on the actual Site Class. This adjustment yields the final design values Sds and Sd1 (which are typically shown in Appendix B in the project documents or the structural drawings).

These modified accelerations together with the Occupancy Category are used to determine the Seismic Design Category from two charts in the Code. The highest value from the two charts is used to determine the Seismic Design Category for the project (although there is an exception for buildings that fall within certain criteria in which case only the first chart is used).

For example, suppose a project is located where Ss = .385g and S1 = .106g, the Site Class is assumed to be D and the Occupancy Category is III. From Ss, S1 and the Site Class adjustment come the design values of Sds = .383g and Sd1 = .167g. Entering the two design charts with these values and using the worst value reveals SDC = C.

So, in summary the Occupancy Category, geographic location and the Site Class of the project site determine the Seismic Design Category. Even though the Site Class uses a lettering system like the Seismic Design Category, it is only used to adjust the site accelerations.