Part 1: Using the Seismic Design Category to determine the need
for earthquake bracing.
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
thosesilent 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.
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 shortperiod 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.)
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.
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.
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:
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).
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.
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.
Part 2: The Fundamentals of Seismic Design and the Design
Features Involved.
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.
Couplings
The first element is couplings. The general idea is to provide rigid couplings throughout the
system except at locations where the piping is installed vertically. In fact, if flexible couplings
are installed on piping running horizontally, a lateral sway brace is required to be included
within 24 inches of the coupling. (Please note that this applies only to piping that is 2½
inches and larger.) So it stands to reason that you do not want to install flexible couplings
anywhere other than where they are required.
Following are the coupling requirements as listed in NFPA 13 (2003). (Nos. 2 and 4 are taken
from the 2002 edition.)
1. Within 24 inches (610 millimeters) of the top and bottom of all risers, unless the
following provisions are met:
a) In risers less than 3 feet (0.9 meter) in length, flexible couplings are
permitted to be omitted.
b) In risers 3-7 feet (0.9-2.1 meters) in length, one flexible coupling is
adequate.
2. Within 12 inches (305 millimeters) above and within 24 inches (610 millimeters)
below the floor in multistory buildings. When the flexible coupling below the floor is
above the tie-in main to the main supplying that floor, a flexible coupling shall be
provided on the vertical portion of the tie-in piping.
3. On both sides of concrete or masonry walls within 1 foot (0.3 meter) of the wall
surface, unless clearance is provided in accordance with Section 9.3.4.
4. Within 24 inches (610 millimeters) of building expansion joints.
5. Within 24 inches (610 millimeters) of the top and bottom of drops to hose lines, rack
sprinklers, and mezzanines, regardless of pipe size.
6. Within 24 inches (610 millimeters) of the top of drops exceeding 15 feet (4.6 meters)
in length to portions of systems supplying more than one sprinkler, regardless of
pipe size.
7. Above and below any intermediate points of support for a riser or other vertical pipe.
It is the practice in my company to include a sheet note on the drawings that says, “All
couplings shall be rigid type unless noted otherwise.” In the design of the system, we use
some type of symbol designation to indicate that the couplings are to be flexible. The
coupling requirements are usually stricter in inrack sprinkler systems, standpipe systems,
systems that are multilevel, and riser assemblies.
Seismic Separation
The second element involved is seismic separation. Building separation is a critical aspect of
design for structural engineers. The building codes require buildings to be structurally
separated once they reach a specific length and/or square footage. Where a building is
separated, no part of the structure is connected at that point. In other words, while the
building may appear to be one complete structure, it is structurally separate such that the
two parts move independently of each other.
You usually can identify this occurrence by reviewing the structural drawings. You will find
two column grid bubbles that are very close together, usually 12 inches apart. You will see
two beams or other structural members running side-by-side, parallel to each other for the
entire width of the building. If you look at the details you will see that no part of the
structure at that point is connected. From the foundation up through the roof, the two parts
are completely separate. The only thing that makes the building appear whole is the siding
and roof coating that are applied.
A separation should not be confused with a building expansion joint. While an expansion joint
is designed to allow the building to move, it certainly does not provide the magnitude of
movement that a separation is designed to allow. Expansion joints also have coupling
requirements, but NFPA 13 requires a specific type of assembly to be used with building
separation. Many contractors and designers have seen pictures of this assembly, but I have
found that few have investigated its purpose or actually used it.
This section includes only one statement, but its effects are far reaching. In fact, this one
requirement can completely dictate the type of piping configuration you will use for the
system. If this section is overlooked during the estimating process, complying with
the requirement in the field most likely will use up most of the profit. This section requires
that separation assemblies with flexible fittings be installed, regardless of size, where piping
crosses building seismic separation joints.
Figure 1 Gridded System
The magnitude of this requirement is best explained by considering a gridded system. This
type of piping configuration involves the installation of a primary main on one side of the
building
and
a
secondary
main
on
the
opposite
side.
The
mains
are
connected
with a series of branch lines that run perpendicular to each main (see Figure 1). Since
seismic separation applies to all pipe sizes, a seismic separation assembly is required at
every location that these grid branch lines cross a required separation. If you look at what
this involves, you will better understand what is at stake (see Figure 2). Six 90-degree ells
added to each branch line will be included in the hydraulic calculations, and their presence
most likely will increase the branch-line size at least one size, making the system even more
expensive.
The only currently known alternative to this assembly is a fitting assembly called a
Metraloop, which provides the same movement in a more feasible manner. While the NFPA
13 assembly can take out as much as 5 feet or more depending on size, the Metraloop
provides a more compact and easy-to-install alternative. While a grid usually is considered
the most cost-effective piping configuration, you also should consider a series of center-feed,
tree-type systems requiring only the bulk feed main to cross the separation once, rather
than several times as with a gridded system. Remember: If you use the Metraloop, flexible
couplings are required for its connection to the piping.
Clearance
The third design element involved with seismic restraint is clearance. This feature includes
provisions for piping that penetrates specifically concrete and/or masonry floor/ceiling and
wall assemblies. Do not confuse this with penetrations through rated assemblies that are
framed with wood or steel studs with gypsum board. This section has nothing to do with
assembly ratings or the requirements for sleeves or fire caulking. Those are usually a
function of other specification requirements and should not be in this section of your
specification or drawings notes.
Like separation, this feature is simple but very expensive. This section requires a specific
nominal annular space to be provided around the pipe penetrating the assembly. A 1-inch
annular space is required around 1-3- inch pipe. A 2-inch space is required around pipes that
are 4 inches and larger. Core drilling a 10-inch-diameter hole for a 6-inch pipe is not
something most fire protection contractors are very eager to do. This process can be quite
involved, and the cost of core drilling is tied directly to the size of the hole.
However, there is a less expensive way to accomplish this penetration. You will recall that I
previously mentioned that flexible couplings also could be used as a solution for clearance
requirements. This is where couplings prove their worth. In lieu of large clearances, the
standard allows for a flexible coupling to be installed on either side of the assembly within 12
inches of the face of the penetration. By providing these couplings, standard hole diameters
may be used. My experience is that contractors prefer this method to providing the larger
holes.
This section applies to all pipe sizes, so, like the separation requirements, consideration of
the piping configuration is important. It is usually better to penetrate once into a concreteor masonry-assembly room with main piping and then create a smaller tree-type system
than it is to penetrate several smaller holes into the space simply to maintain uniformity.
A prudent plumbing designer would discuss these types of design features with the architect
during the design development phase to try to minimize the amount and/or configuration of
these assemblies as well as the overall sprinkler system cost. Doing so also may help you
gain a level of favor with the installing contractor.
Sway Bracing
The fourth and most commonly referenced seismic restraint design feature is sway bracing.
Unlike in other plumbing systems, the water and pipe that comprise fire protection systems
are lifesaving features. While the majority will never activate, fire sprinkler systems must
perform when needed or people and property will suffer. With that in mind, it becomes
obvious why the bracing of fire sprinkler systems has its own rules for spacing, location, and
force factor criterion.
The process for laying out sway bracing starts much like that for laying out sprinkler heads.
There are three types of braces: lateral, longitudinal, and 4-way. Lateral bracing is required
to be spaced at a maximum of 40 feet between braces. We also are required to install a
brace within 20 feet of each end of the run of main, which is half the allowable distance
between braces. Finally, we must have a brace on the first piece of pipe on each end of the
main. Figure 3 depicts an example of lateral bracing.
When applying the rules to each run of main piping, you’ll want to try to maximize the
distance between braces as much as possible. However, remember to leave room for the
braces to be moved in either direction in case actual field conditions inhibit the fitter’s ability
to install the brace at the location shown on the drawing. Also, as the distance between
braces grows, so does the total weight that each brace will be required to resist. If you are in
a high seismic category or if the site soil or building importance dictates a high force factor,
maximizing the spacing may not be cost effective.
Once the lateral braces are located, you lay out the longitudinal braces. The maximum
spacing for these braces is 80 feet. As with lateral braces, you are required to install a
longitudinal brace within half the allowable distance between braces, meaning you must have
one brace within 40 feet of each end of the run of main. Normally there will be fewer
longitudinal braces than lateral.
The final bracing that is required is referred to as 4-way bracing. Industry terminology for
this feature has been diluted, so for the purpose of clarification, 4-way bracing is not where
both a lateral and longitudinal brace are located. Rather it is a bracing assembly that is used
to restrict the movement of pipe that is installed in a vertical position such as the riser piping
at the fire service entry into the building. As you can see in Figure 4, this bracing usually is
installed in the horizontal position and has specific attachments that are designed to meet
the intended installation configurations. The brace must be located within 24 inches of the
top of the riser.
Like many of the requirements of this standard, nuances and exceptions can be applied. Both
lateral and longitudinal braces can serve each other’s purpose if located within 24 inches of
the end of the run of main (see Figure 5). Notice that the 4-way brace can be considered as
the longitudinal brace as well. As a matter of design, I usually first lay out the bracing for
each run of main independently, and then go back and consider the relocation of the braces
at each end of the mains as a whole to apply these alternatives. Some designers have been
taught to simply install a 4-way brace at every change of direction if sway bracing is
required. Not only is this wrong, it is very expensive and does not accomplish the goal of
seismic design. Bracing layout needs to be done with consideration of total weight and the
ability of the fitter to actually have ceiling space to install the brace.
For example, in ceiling areas with an excessive amount of ductwork above the piping, it will
be very difficult to run the sway brace up to the top chord of the structural member. If you
have maximized the spacing, little can be done. Whereas if you have allowed for this
condition ahead of time, the fitter can relocate the brace further down the main in one
direction or the other without compromising the ability of the hanger to carry the weight that
it was designed to resist. While it is not cheap, adding a brace to cut down the spacing is
much less expensive than having field personnel trying to figure out how to make it work.
It is my hope that you see the importance of the “how” of the process of seismic design of
fire sprinkler systems. As with any engineered system, especially life safety systems,
understanding the overall goal and applying the standards by which we are intended to meet
these goals is very important. Remember: Vince Lombardi said, “Excellence is achieved by
mastering the fundamentals.”
Part 3: Practical Example for Designing and Sizing Seismic
Bracing and Components.
In the previous articles of this series I discussed the “if” and the “how” of seismic design for
fire sprinkler systems. Let’s now take a look at an actual design and apply this knowledge in
a practical example. For the sake of size and complexity, I’ll use a basic design; however,
keep in mind the basics should be applied to each design no matter how complex it might be.
To begin, I recommend that you print out Figure 1 that will be referenced throughout. This is
our basic system design. Using the step-by-step guideline that was provided in the first
article, assume that this system falls into a seismic category C-F. Remember, if the building
has been classified as an A or B it is exempt from seismic design.
It is the job of the engineer of record not only to designate the seismic category but also, if
required, to provide the force factor that shall be used. This force factor now is going to be
used to help in sizing the seismic bracing that is part of the overall seismic design. Seismic
bracing is only one of the five design features that must be provided when doing seismic
design for a system. While bracing is the most common, remember that the system must
have rigid and flexible couplings located in specific locations, separation or expansion
components at specific locations, and clearance provided as specific locations. Further,
restraint for branchlines must be considered as well.
Using the example let’s first locate the lateral bracing that is required. (Remember, the
requirements for lateral brace location are found in NFPA 13: Standard for the Installation of
Sprinkler Systems Chapter 9.3.5.3.) The braces are spaced a maximum of 40 feet apart from
each other with a brace required within the first 20 feet from each end of the run of main
being considered. This is half the allowable distance between braces. Also, a brace must be
located on the first piece of pipe from each end. This may sound confusing but considering
that steel pipe comes in 21-foot, 24-foot, and 25-foot lengths, putting a brace on the first
piece of pipe and within the first 20 feet of each end is not that hard to grasp.
However, let us say the first piece of pipe on a run of main is 14 feet long. Then the first
brace has to be located within that first 14 feet. It cannot be located after that somewhere in
the next 6 feet. Locate the braces on the cross main first. We will deal with the bulk main
last. This main is 97 feet, 7 inches long from end to the last branchline on the end. If you
divide 97 feet, 7 inches by 40 feet (maximum distance between braces) you can determine
the minimum number of braces needed. Keep in mind that this is the minimum.
Several factors must be considered when determining how many braces actually are needed.
For instance, if it is an exposed system with the piping near the roof deck or structure above,
the bracing usually is spaced to its maximum as long as weight is not an issue, which we will
see when we are sizing the braces. However, if the system is feeding pendants or is several
feet lower than the structure, braces more than likely will need to be added to find locations
to attach to the structure. When systems are hung lower other systems such as HVAC,
electrical, and plumbing usually are above it, which makes it more difficult to locate a place
where the braces can reach the top of the structure.
Hence: 97.58/40 = 2.4395. This means the minimum number of lateral braces required is 3.
For the sake of example, consider this system to be unobstructed to structure. Our example
ends up with something that looks like Figure 2. As you can see, the approximate locations
fall into the allowances given in NFPA 13 Chapter 9.3.5.3. Again, if the starting pieces on
each end where less than 20feet, the brace would need to be located somewhere on that
first piece. Notice that the distances from the braces on each end to the middle brace both
are within the 40 feet maximum.
The second step is locating the longitudinal braces. The requirements for longitudinal braces
can be found in NFPA 13 Chapter 9.3.5.4. Again, we will concentrate on the cross main first.
As you may recall, longitudinal braces affect only the main itself and do not have anything to
do with the branchlines. Also, size is not an issue. The cross main could be 8 inches or 1
inch. Either way, longitudinal bracing is required.
The spacing requirements for longitudinal bracing are double that of the lateral bracing. The
maximum spacing is 80 feet with a brace required within the first 40 feet, which is half the
allowable distance between braces. To find the minimum number of longitudinal braces
divide 97 feet, 7 inches by 80feet. Hence: 97.58/80 = 1.21975. So a minimum two braces
are necessary to meet the requirements of NFPA 13 Chapter 9.3.5.4. Locate the longitudinal
braces on the example layout. Remember that there must be a brace within the first 40 feet
of each end of the run of main. As you can see in Figure 3, two braces are adequate. Notice
the amount of over spacing. This is advantageous because it allows the fitters plenty of
distance to relocate the braces from one end to the other in case obstructions are
encountered yet still stay within the limits allowed.
Now that the lateral and longitudinal braces are located on this run of main, attention can be
given to the bulk main feeding this cross main. As was previously described, lay out the
lateral and longitudinal bracing for this run of main. It should look something like Figure 4.
The overall length of this bulk main is 35 feet, 9 inches, so the minimum number of lateral
braces required is one since it is less than 40 feet in overall length. The brace must be
located within the first 20 feet of each end and must be on the first piece of pipe from each
end.
A common question raised here is what to do about the 11-foot, 9-inch piece of pipe. If we
put one brace within 20 feet of the system riser symbol, we have nothing on the 11-foot, 9inch piece on the other end. Technically speaking, that is correct; however, given the fact
that the entire run is less than 40 feet and the brace is located within 20 feet of each end, it
generally is understood that the amount of weight will not be such that one brace cannot
adequately provide the support required. In such a case it is recommended to locate the
brace as close to center as possible so the weight is distributed as equally as possible.
The required longitudinal brace is also a single brace since the overall distance of the main is
less than 80 feet. This brace also can be located toward the middle of the run so the weight
is distributed equally. When a lateral and longitudinal brace end up relatively near each
other, it is usually cost effective to use bracing components that are made specifically to
accommodate both braces. This is one example where the vocabulary gets diluted, so be
careful. This is not a 4-way brace as described in Part 2 of this series. Rather, it is a
combination brace that allows for support in both the lateral and longitudinal directions.
Notice that the symbols are not crossed but rather two individual symbols side by side. This
is done on purpose because it can be confused with the next brace that we are going to
locate, which is a 4-way brace.
Before moving on, let’s examine a couple of options that can be considered with the layout
that we now have. First, as indicated in NFPA 13 Chapters 9.3.5.3 and 9.3.5.4, the bracing
used in one direction on one run of main can be counted as the opposite type of brace for an
adjacent main in the perpendicular direction. For example, if we locate a lateral brace within
24 inches of the end of the bulk main on the 11-foot, 9-inch piece of pipe, it could be
counted as the first longitudinal brace on the cross main. Figure 5 shows how this would
affect the layout. Notice that the first longitudinal brace was eliminated because the next
longitudinal brace required can be up to 80 feet away. However, you also can see that we
really did not gain anything in terms of the number of braces needed. In this case we simply
traded one brace location for another. Furthermore, we now have loaded the brace at the
11-foot, 9-inch piece with much more weight than the original brace location, which means
we could end up having to size this brace much larger than the original brace location. This is
a great example of how brace location is somewhat subjective and, if properly done, can
offer a fair amount of flexibility to accomplish the overall goal of seismic design.
The last brace to be located is the 4-way brace. This is a bracing configuration that provides
support in all directions specifically for vertical pieces of pipe. The requirements for 4-way
bracing can be found in NFPA 13 Chapter 9.3.5.5: Risers. The requirements found here
include a maximum 25 feet between braces, so if the riser piece being braced is longer than
25 feet, two braces must be installed, one of which must be located within 24 inches of the
top. Remember also that size is not a consideration here. It applies to all pipe sizes. Further,
the 4-way brace can be counted as the longitudinal brace for the first run of pipe coming off
the top of the riser. In the case of our example, we could eliminate the longitudinal brace
that is in the middle of the run of bulk main because we are within 40 feet of each end, and
only one is required. The total weight on the brace is the same regardless, so this is a good
option to use. Figure 6 shows how the design is affected.
Now that the bracing layout is complete, we can begin to determine the sizing of the
components. This process is very similar to that of establishing the area of operation or
remote area when performing hydraulic calculations. The total force that the system must
resist is a function of the weight of the water-filled piping times the force factor that has
been assigned by the engineer of record (see Part 1 of the series). As shown in Figure 7, we
have clouded the piping that will be assigned to each of the lateral and longitudinal braces.
Notice how it is equally distributed. Obviously, since there is an odd number of branchlines,
one of the lateral braces needs to carry the weight of one extra branchline. The terminology
used to describe these areas is referred to as the zone of influence or ZOI. Examples of how
the ZOI is located for different system configurations can be found in NFPA 13 Figure
A.9.3.5.6(a).
Once the ZOI has been established, determining the total weight for each zone is next. To
calculate the total weight for a zone you add the weights of all pieces of pipe (as if filled with
water). Several resources are available for this information. Most of the pipe manufacturers
have these values for every pipe type they make. NFPA 13 provides the values for Schedule
10 and Schedule 40 in Table A.9.3.5.6. If you are using one of the specialty pipe sizes
equivalent to Schedule 7, the values are found in the manufacturer’s data sheets. The
conservative approach is to use the weights of Schedule 10 and 40 regardless of the actual
pipe type used.
For the purpose of this example, let’s examine the most demanding zone, which is ZOI No.
1. Since the branchlines are typical, determine the weight for one of them and multiply by
four. The weight of one branchline is approximately 133 pounds, so the total is 532 pounds.
The total weight for the main piping that is part of this zone is 35.0 feet = 3-inch pipe ≈ 320
pounds. Thus, the total load for ZOI No. 1 lateral brace is Wp ≈ 850 pounds.
As mentioned previously, the resultant design load is a function of the total weight of waterfilled pipe times the force factor expressed as Fp. It is important to note that the Fp from the
International Building Code (IBC) and the Fp from the map found in NFPA 13 vary greatly,
which causes a lot of confusion. For years, NFPA 13 has allowed a default value of 0.5 to be
used. This is where the phrase “half the weight of water-filled pipe” came from. When IBC
2000 began to be adopted, many designers realized this discrepancy and have taken a
completely different attitude toward seismic design of sprinkler systems. For example, the Fp
for California based on NFPA 13 is 0.4. Using IBC and ASCE 7: Minimum Design Loads for
Buildings and Other Structures, the Fp for California can be 1.35 or more. My advice is to use
the IBC/ASCE 7 formula. It is obvious that the 0.4 that NFPA 13 offers is substantially lower
than IBC and the default value, while greater than the value for California, will grossly
oversize the components for areas like Colorado Springs, which can be as low as 0.17. As a
reminder, the formula from ASCE 7 is expressed as
For the sake of our example I have used numbers that represent a location in Jonesboro,
Ark., zip code 72404. Using the formulas referenced in IBC from Part 1 of this series we end
up with a SSD of 1.317g. Since we have not considered height of the building in this
example, we drop of the last part of the equation. Therefore, Fp = (0.4 x 1.0 x 1.317 x
850)/(3.5/1.5) = 192. If we use the default value of 0.5 we would end up with a resultant
weight of 425 pounds (Fp = 850 x 0.5). That is a difference of more than 220 percent.
Finally one more factor is involved before we can size the components: the system
component factor. This is a percentage that has been assigned by NFPA 13 to account for the
fittings and sprinkler heads on the main and branchline piping within the ZOI. If you are
using the 1999 edition of NFPA 13, the factor is allowed to be revised; however, if you are
using the 2002 edition it shall be 15 percent, or 1.15 times Fp. This raises our value from
192 pounds to approximately 221 pounds.
Now that we have the force we can proceed with sizing the components. The most common
type of brace material is steel pipe. However, several other popular materials are available.
Tension cable is another method for bracing that provides the required resistance while
sometimes being easier to install than rigid pipe and structural attachments. In the case of
using cable or any other material not listed in the tables of NFPA 13, the manufacturers’
values must be used. As seen in NFPA 13 Table 9.3.5.8.9(a-c) several options are available
depending on the type of brace, such as pipe, steel angles, steel flats, and rods.
Another very important factor is the angle at which the brace is situated in relation to the
pipe it is bracing and the structure to which it is attached. For lateral bracing, the brace gets
stronger as it nears 0 degrees. Notice that as the slenderness ratio grows, the maximum
length of the component lengthens. Take 1-inch pipe for example. When the brace is 3 feet,
6 inches long it can withstand loads as high as 12,242 pounds when the angle is reduced
from 90 degrees to 60 degrees or more. Obviously the brace can resist more weight the
closer to perpendicular it is. Also, the longer the brace is, the less weight it can resist.
When sizing braces it is important to consider the reality of the project. Again, you want to
provide as much flexibility with the actual installation as possible, so sizing the brace based
on the worst-angle orientation (30 degrees to 44 degrees) is prudent. The length is a
function of the type of system you are designing, that being its location with regard to
structure. For instance, if the system is in a warehouse, the bracing most likely will be
relatively short since the system piping will be located high in the structure. On the other
hand, in a hospital the system will be located right above the ceiling with several
obstructions above it and several feet from the top of the structure where the anchoring
attachment must be located. If we assume the warehouse situation, our lateral brace could
be 1-inch pipe up to 10 feet, 6 inches long. We can conclude that 221 pounds is well under
the allowable 786 pounds.
After determining the brace type and size, the final step is to determine the type of
attachment. NFPA 13 Figure 9.3.5.9.1 provides the available attachment arrangements and
corresponding attachment types, which usually are dictated by the type of structure. Several
more
types
of
listed
seismic
attachments
are
available
from
the
major
hanger
manufacturers, and I strongly recommend that you reference them in this process as well.
Even in the past few years, several new types of attachments have become available,
making it easier to deal with the types of structures being built today.
The previous steps then are used to determine the longitudinal braces as well. Remember,
the only weight considered for longitudinal braces are the main piping. Branchlines are not
considered. Also, in case your situation does not fall into the scope of the tables listed in
NFPA 13, the standard does allow for other types of methods and materials as long as the
system is designed by a registered professional engineer.
I hope that this series of articles has helped you understand seismic design for fire sprinkler
systems. If you need further explanation, feel free to contact me. I also recommend
contacting the local sales representative for the major hanger manufacturers. Several of
them are represented on the hanging and bracing committee of NFPA 13 and surely would be
able to answer specific issues as they arise. Three familiar manufactures (Tolco, Afcon, and
Loos) offer software that streamlines this process. Two of them even integrate with
AutoCAD.
Finally, let me remind those of you who have decided to practice fire protection engineering.
You are responsible for the design criteria of a life safety system. While toilets and drains
and warm and cold air are important, they are not life safety systems. Take this seriously
and do your homework. Keeping a sprinkler system in place and able to perform in the case
of a seismic event is of utmost importance. Like I always tell myself, “Just do it right!”