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

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 1c: Determining the Seismic Design Category of a Building

Seismic Design Part 1

A Word About Responsibility
Prior to the introduction of the IBC, contract specifications were usually the vehicle used to require seismic restraint. Engineers would add language to the specifications indicating “earthquake bracing shall be provided per NFPA 13.” This usually meant the contractor would multiply the predetermined force factor by the weight of water-filled pipe in a zone of influence to size the braces. However, the method has changed; you now must take several variables and steps to evaluate and determine whether seismic protection is needed and, if so, the data required to properly size the components that will be used. This is the “how” in the process, which I will look at in the second article of this series.

Before we go any further, I believe a discussion regarding responsibility is warranted. Just like every other aspect of sprinkler system design, the criteria for seismic should be determined and provided to the contractors by the engineer of record. This certainly does not mean that contractors are not capable of learning this process and applying it correctly. They have been taking on the liability and exposure for the majority of the design criteria from the beginning. However, it is time that the engineers who have decided to practice in the discipline of fire protection take on the responsibility that goes with it. I am sure that many of you are rolling your eyes and beginning to complain about how all this is going to affect you. But before you do, let me point out that while going through the learning curve, I discovered something that will most likely help you digest this. Are you ready? Here it is: The structural engineers have been figuring this out as part of their design process for years. Just like many other items that fall under the engineer’s responsibility, the information needed in the course of this process is available from the other design team members (the structural engineer) at the time that the construction documents are prepared. So you see, it really should not take that much effort to determine a very important part of the required design criteria that the engineer of record should be providing.

As I said, meeting the installation requirements for seismic components in a sprinkler system is costly, and the matter needs to be given serious consideration during the bidding process. Therefore, the information needed, namely the “if” and the force factor to be used, should be included with the rest of the information that is required in the owner’s certificate found in NFPA 13 Chapter 4.3.

I think you’ll agree that this is an important process and one that will take some time to become familiar with. Whether you are in Orlando, Fla., the plains of West Texas, Boise, Idaho, or Yuma, Ariz., the evaluation of seismic protection is required. It is the design professional’s job to determine the Seismic Design Category that is assigned to a building, as well as provide the force factor that should be used if seismic protection is required, a process I will explain in the second part of this series.

Continued at Seismic Design For Fire Sprinkler Systems – Part 2a: The Objective of Seismic Restraint

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