Seismic Design for Fire Sprinkler Systems

Part 3: Practical Example for Designing and Sizing Seismic Bracing and Components.
by Steven Scandaliato, SET

In the previous articles of this series [PS&D September/October 2005 and January/February 2006] 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, 9-inch 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 water-filled 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

 
where

 = seismic design force;

 = component amplification factor (1.0 for sprinkler systems);

 = design short period spectral acceleration (from maps or software);

 = component operating weight (850 pounds for our example);

 = component response modification factor (3.5 for sprinkler systems);

 = component importance factor (1.5 for sprinkler systems);

 = height of attachment point with respect to the base; and

 = average roof height with respect to the base.

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  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 . 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 send me an e-mail (steven@scanfire.net). 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!”

Steven Scandaliato is a principal partner with Scandaliato Design Group Inc. and a NICET IV Certified Senior Engineering Technician. He has more than 20 years experience in fire protection design and engineering. He serves on the Technical Advisory Committee for the American Fire Sprinkler Association and is a member of the NFPA 13 and 5000/101 Committees. For more information or to comment on this article, e-mail articles@psdmagazine.org.