Seismic Design Category Explained

April 4, 2016

by Stanley N. Fuller, PE, SE

 

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Seismic Design Category

No single measure is more central to seismic design than the “Seismic Design Category” (SDC).  What is it?  How is it found?  And what does it mean to a building?  We’ll take a little time here to answer these questions.

 

What is the “Seismic Design Category”?

SDC is an indicator of how much attention must be paid to the seismic design and construction of a building.  It ranges from “A” to “F”.  Buildings with an SDC of “A” must be designed for seismic forces, but do not require any special seismic attention.  Buildings with an SDC of “F” require a tremendous amount of seismic care and attention.

 

How is the Seismic Design Category Determined?

Three parameters determine the SDC: geographic location, underlying soils, and building use.  First, geographic location drives SDC.  Many regions have a low seismicity.  Others have a relatively high seismicity, including California, coastal South Carolina, and the Mississippi basin between Memphis and St. Louis.  The second driver of SDC is the underlying soils.  This is often confusing because soil types are also determined with an “A” through “F” rating system.  An “A” soil is one which is founded on very shallow, hard bedrock, which is advantageous to seismic design.  An “F” class soil is one that is relatively soft and tends to amplify seismic forces.  Most soils in Ohio are classified as type “C” or “D”.  The third factor in determining seismic design is the building use.  For the purposes of SDC, there are two types of buildings: typical structures (offices, apartment buildings, etc.), and those that must be functional after a seismic event (hospitals, fire stations, etc.).

 

What does the Seismic Design Category mean to a building?

As alluded to earlier, the SDC indicates the amount of attention that must go into the seismic design and construction of a building.  This affects the types of structural systems permitted, the amount of design and detailing involved, and the requirements for non-structural components.  First, buildings with a low SDC have very few restrictions on the type of structure that may be used.  Conversely, buildings with a high SDC have more stringent restrictions on the types of structural systems that may be used.  These buildings must be designed to yield in a specific manner to seismic forces.  Second, low SDC buildings can be analyzed to resist seismic forces with common analytical methods.  High SDC buildings require more complex analyses, and need significantly more design time.  Finally, seismic forces affect all parts of a building.  This includes additional bracing of ceiling grids, and specialized struts or cables for mechanical, plumbing, and electrical work.  These requirements have significant cost implications and should be discussed early in the project if anticipated to be an issue.

 

We hope you’ve gleaned valuable information from this article.  If you have any questions, fee free to contact our office.