Posts Tagged ‘Earthquake’

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

January 27th, 2009

Part 2: The Fundamentals of Seismic Design and the Design Features Involved.

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

Seismic Design Part 2

In the first part of this series, I discussed the “if” aspect of seismic design for fire sprinkler systems. The article reviewed International Building Code (2003) Section 1614 where the requirement for seismic design is made and each of the six exemptions to this requirement. Now it is time to discuss how to actually do this in your sprinkler system designs.

Let’s first review the process thus far. IBC Section 1621 references a document called ASCE 7, which is published by the American Society of Civil Engineers and used by structural and civil engineers for building component design criteria, among other things. ASCE 7 Chapter 9.6, “Architectural, Mechanical and Electrical Components and Systems,” is where the exemption for fire sprinklers is found if the Seismic Category as determined in IBC is an A or B. (Remember that fire sprinkler systems in Seismic Category C cannot be exempt from the seismic restraint requirement because they are considered life safety systems and therefore are given a higher rating than standard mechanical and electrical systems.) Having determined that seismic design is required, the “how” of the process begins.

A Word About Terminology
While almost everyone is familiar with the concept of sway bracing, it is important to standardize the language of this design process. For years specifying engineers and other entities have referred to seismic design by simply stating “provide earthquake bracing as required” or “sway bracing shall be provided as required in NFPA 13 [Standard for the Installation of Sprinkler Systems]” or “when bracing is required, it shall be installed per NFPA 13.”

I must stress that you immediately remove any such canned or standardized language in your company’s specifications. Such vague wording is very misleading. Seismic design for fire sprinkler systems includes several components in addition to bracing. While bracing is one of the most familiar methods, it certainly does not provide the necessary restraint for a system to meet the level of performance intended.

The Objective of Seismic Restraint
Understanding the purpose behind seismic design is the next step in the process. As with other aspects of sprinkler system design, plenty of gray areas make following the rules difficult. I believe that a designer must understand the overall objective behind a code or standard to better provide a solution for those times when the rules do not readily apply.

The objective of seismic design for a fire sprinkler system is twofold. The first goal is to minimize stresses in piping by providing flexibility and clearances at points where the building is expected to move during an earthquake. The second is to minimize damaging forces by keeping the piping fairly rigid when supported by a building component expected to move as a unit during an earthquake, such as a floor/ceiling assembly. The idea is to design a system that gives and moves as the building is designed to move. You want the system rigid where the building is rigid and flexible where the building is flexible. According to the standards, the
systems attached to the structure of the building all should work together as one unit.

That being the case, let’s look at each element required to make this happen. NFPA 13 Chapter 9.3 is where all the standard installation requirements for seismic design can be found. The chapter is organized by each required category: couplings, separation, clearance, and sway bracing.

Continued at Seismic Design For Fire Sprinkler Systems – Part 2b: Couplings and Seismic Separation

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

Seismic Design For Fire Sprinkler Systems – Part 1a: The Seismic Shift

January 23rd, 2009

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

Plumbing Systems & Design Magazine
By now, the majority of jurisdictions across the country is using, or at the very least has had some exposure to, the International Building Code (IBC). Although many of its requirements are identical to the codes that those of us in the engineering disciplines were using prior to
its adoption, a few revisions quietly made their way into mainstream design requirements and unfortunately have made their presence known in very expensive ways. One of those silent revisions concerns seismic design for fire sprinkler systems.

I know many of you in the plumbing and mechanical design disciplines probably are saying to yourselves, “I have been doing this for years. What’s the big deal?” Well, if you read on, you will learn that this seismic stuff is a very big deal.

The Seismic Shift
Seismic design for fire sprinkler systems historically has been governed by building codes that were not very specific regarding the requirements for seismic restraint. In fact, the need for earthquake bracing has been fairly clear and isolated in large part because, up until the last eight to 10 years, the majority of fire sprinkler systems was designed using performance specifications rather than installation specifications. As such, the design criteria were left up to the fire protection contractor.

Almost all performance specifications contain language such as “design and install per NFPA 13;” therefore, fire sprinkler contractors were using NFPA 13: Standard for the Installation of Fire Sprinkler Systems to determine what to include in the design of the system. NFPA 13 never was intended to dictate “if” seismic design was required in a system. It always has been and still is the standard for “how” to install seismic components when they are required. Normally this requirement comes from the adopted building code by which the project is governed. The requirement also can come from the local authority having jurisdiction or the client’s insurance company.

When contractors design and install per NFPA 13, it typically means consulting the seismic map that many contractors use as an indicator of the likelihood of a seismic event taking place in the location in which they are working. Based on this map, contractors decide whether or not to install earthquake bracing. For example, California is a Zone 4, which is the worst case; if a contractor sees that he is in a Zone 0, 1, or even 2, he most likely will decide to do nothing about seismic design. The fact that an earthquake never had occurred in the city and that the AHJ never had required seismic design often confirms the perception that seismic does not need to be included.

For many of you this may seem crazy; for others it may be perfectly logical. How you feel about the process most likely depends on where you live and practice. What is so amazing is that the previous building standards, including the Uniform Building Code and the Building Officials and Code Administrators, never intended for fire sprinkler systems to be exempt from seismic requirements. They were just vague about the extent to which the design was to be implemented. Since the specifications were not giving any definite guidance, the inclusion of seismic design was very isolated.

This is not the case any more. A slow but deliberate metamorphosis has been taking place in the industry, and FS (Fire Sprinkler) sheets are making their way into construction documents across the country. Engineers are beginning to take responsibility for aspects of the installation portion of the design, as well as the criteria, including
seismic, by which the system is to be installed.

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