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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 1b: IBC Requirements and Exemptions

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 1a: The Seismic Shift

IBC Requirements and Exemptions
Now that we are working in this new era, you must understand the “if” of the requirement before discussing the “how.” The information used to determine design standards includes data that is collected and tested by the National Earthquake Hazard Reduction Program (NEHRP). IBC, NFPA 5000: Building Construction and Safety Code, and others all use the data collected by this organization to create the criteria that should be followed.

Let’s first take a look at how the IBC deals with seismic. The text about earthquake protection in the IBC is based in large part on criteria found in ASCE 7. This separate document is published by the American Society of Civil Engineers. It includes design criteria for seismic restraint of architectural, mechanical, and electrical components and systems. The first edition of IBC in 2000 introduced the requirement for seismic design for fire sprinklers but did not directly reference ASCE 7 at that time. IBC Section 1614.1 states, “Every structure, and portion thereof, shall as a minimum, be designed and constructed to resist the effects of earthquake motions and assigned a Seismic Design Category as set forth in Section 1616.3. Structures determined to be in Seismic Design Category A need only comply with Section 1616.4.” The 2003 edition kept this requirement in place but revised the exemptions that followed.

The first exemption says, “Structures designed in accordance with the provisions of Sections 9.1 through 9.6, 9.13 and 9.14 of ASCE 7 shall be permitted.” This exemption allows the use of ASCE 7 in lieu of IBC.

The referenced sections that deal specifically with fire sprinklers are found in the body of Section 9.6. Within this section are six exemptions that detail when seismic is not required. It is within these six exemptions that the “if” can be determined. The first exemption allows you to exclude all aforementioned components if the Seismic Design Category is A. The second allows architectural components that are in a Seismic Design Category B with some exceptions concerning parapets and wall types. The third exemption is where fire sprinkler systems are addressed. This exemption allows mechanical and electrical components that are a Seismic Design Category B to be excluded. This section will prove to be the most referenced section in the process. After several years of dealing with this process, I have found that the majority of the country will be classified as a Seismic Design Category B. The fourth exemption appears to affect fire sprinkler systems as well. It allows mechanical and electrical components that have a Seismic Design Category C to be excluded; however, they must have an Importance Factor (Ip) that is equal to 1.0. Fire sprinkler systems have been assigned an Ip of 1.5 (ASCE 7-9.6 1.5) because they are considered life safety systems. Therefore, this exemption cannot be applied. The fifth and sixth exemptions, while applying to mechanical and electrical components, both include a requirement for an Ip equaling 1.0, meaning fire sprinklers are not allowed to be excluded.

Five other exemptions in IBC Section 1614.1 can be applied if the first one does not apply. The second exemption states, “Detached one- and two-family dwellings as applicable in Section 101.2 in Seismic Design Categories A, B and C, or located where the mapped short-period spectral response acceleration, Ss, is less than 0.4g, are exempt from the requirements of Sections 1613 through 1622.” No specific language in NFPA 13D: Standard for the Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes requires seismic design for structures of this type; however, be advised, this does not include multifamily structures. NFPA 13R: Standard for the Installation of Sprinkler Systems in Residential Occupancies Up to and Including Four Stories in Height requires systems to follow the requirements of NFPA 13 in their entirety. There are no exceptions to this. If your project is a one- or two-family detached dwelling, seismic design is not required. However, if you are working on a multifamily structure that most likely falls into a R1 type of occupancy, it will be subject to seismic design if it cannot meet any of the other exemptions.

The third IBC Section 1614.1 exemption states, “The seismic-force-resisting system of wood frame buildings that conform to the provisions of Section 2308 are not required to be analyzed as specified in Section 1616.1.” This exemption deals more with the structure itself rather than the portions thereof. I doubt this section ever could be applied in an effort to exempt fire sprinkler systems. The fourth exemption states, “Agricultural storage structures intended only for incidental human occupancy are exempt from the requirements of Sections 1613 through 1623.” The reasoning behind this exemption seems self-explanatory. Obviously these types of structures would have a very low occupancy load and most likely would not require very extensive life safety systems. Hence, it stands to reason that system protection would be minimal.

The fifth and sixth exemptions are really the only other viable exemptions where seismic design for fire sprinkler systems is allowed to be excluded. Exemption five allows you to use the seismic maps that are included in Section 1615. “Structures located where mapped short-period spectral response acceleration, Ss, determined in accordance with Section 1615.1 is less than or equal to 0.15g and where the mapped spectral response acceleration at 1-second period, S1, determined in accordance with Section 1615.1 is less than or equal to 0.04g shall be categorized as Seismic Design Category A. Seismic Design Category A structures need only comply with Section 1616.4.” The contour lines shown on these maps are based on two different time periods. Without delving too deep into the world of seismology, we will accept these maps as a guide to determining the anticipated g-forces that are expected over a given time period.

Finally, exemption number six allows you to use a calculation procedure to determine the values to be compared with the allowed minimums. It states, “Structures located where the short-period design spectral response acceleration, SDS, determined in accordance with Section 1615.1, is less than or equal to 0.167g and the design spectral response acceleration at 1-second period, SD1, determined in accordance with Section 1615.1, is less than or equal to 0.067g, shall be categorized as Seismic Design Category A and need only comply with Section 1616.4.”

According to these two exemptions, if you look at the two different maps—short period and long period—and interpolate your location on each, and the values you determine are less than those listed in these exceptions respectively, then you do not have to provide seismic
restraint for the system.

For example, consider a single-story office building being built in Tampa, Fla. You would look at the short and long-term spectral response maps, IBC Figures 1615-1 and 1615-2, and interpolate as exactly as possible the closest g-force percentage. (Keep in mind that these values are presented as percentages. This will become useful when we actually do the calculations.)
Seismic Figure 1
The maps in the code book itself are very small and somewhat difficult to read. Several resources are available that provide these maps as .dwf files, which are a type of AutoCAD viewing file similar to a .pdf or Acrobat file. Most manufacturers that provide components that are listed for seismic restraint have these files available. A software program available through the International Code Council also provides a more useful and accurate way to evaluate these maps. You also can purchase the maps as one large foldout that has the short-term period on one side and the long-term period on the other. I highly recommend
this investment.

Figure 1 is an enlargement from the electronic version of the map that came from ICC. It helps you better determine the short- and long-term values for our example. Figure 1 is the short-term map. The upper contour line is 10 (the number is not visible) and the lower contour line is 5 as shown.

As you can see, Tampa, which is in Hillsborough County, falls nearly between the short period response percentages lines of 5 and 10. Depending on the project’s location in the county, you can further define, or interpolate, between these two lines; however, for the sake of this example we are going to assume a value of 7.5.

The long-period map (Figure 2) has contour lines 2 and 4 showing in the same area (2 is the lower contour line and 4 is the upper contour line). Here again, you can interpolate between the contour lines or simply choose the higher of the two.

Keep in mind that you must satisfy both the short- and long-term values in order to use the fifth exemption.

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

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