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Seismic Design For Fire Sprinkler Systems – Part 1c: Determining the Seismic Design Category of a Building

January 23rd, 2009

Part 1: Using the Seismic Design Category to determine the need for earthquake bracing.

Continued from Seismic Design For Fire Sprinkler Systems – Part 1b: IBC Requirements and Exemptions

Determining the Seismic Design Category of a Building
So how do you know if seismic protection is required? The process begins with assigning a Seismic Use Group to the building. This classification can be found by using IBC Table 1604.5. (The relevant portion of this table is found in Table 1.) The second part of the initial process involves an evaluation of ground motion. This can be determined using a general procedure or a site-specific one. The only exception to this is if the Site Class is determined to be F. This class mandates the site-specific procedure be used.

Seismic Use Group Classifications Table 1

Using the general procedure, two maximum earthquake spectral response accelerations (short term and long term) must be considered as discussed. Remember that both time periods must be evaluated separately. A Site Class of A through F then is determined based upon the soil at the site per IBC Table 1616.5.1.1. This step is very important because a building’s Site Class directly dictates whether or not it has to be designed for seismic. Keep in mind that you can use the specific Site Class value from the table, or, if this information is not readily available for some reason, you are allowed to default to Site Class D. However, this classification more than likely will require you to provide seismic protection so do not be too quick in deciding to use this option. A quick call to the structural or civil engineer on the design team should provide this information.

As I noted previously, seismic protection for sprinkler systems can be costly. For example, a Site Class A allows a reduction of the spectral response acceleration values, which possibly would result in exempting seismic protection. The response values are adjusted based on the effects of the Site Class using formulas in IBC Sections 1615.1.2 and 1615.1.3:

Seismic Formula

Using the design response accelerations and Seismic Use Group, Tables 1616.3(1) and 1616.3(2) yield the Seismic Design Category (see Table 2).

Seismic Design Categories Table 2

Again, this must be evaluated for both the short- and long-term accelerations. These categories also use designations A through F. The most severe Seismic Design Category of the two time periods is used. The last step is determining whether seismic protection is required based on the assigned Seismic Design Category.

Now, if your head is in a tailspin at this point, don’t feel left out. Many of us have had to perform the process several times before grasping it. To help you understand this process, I’ve listed the steps below.

Steps to Determining the Seismic Design Category

Continued at Seismic Design For Fire Sprinkler Systems – Part 1d: A Word About Responsibility