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Seismic Design For Fire Sprinkler Systems – Part 3b: Practical Example

May 29th, 2009

Part 3: Practical Example for Designing and Sizing Seismic Bracing and Components.

Continued from Seismic Design For Fire Sprinkler Systems – Part 3a: Practical Example

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.

Seismic 3 Figure 5

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.

Seismic 3 Figure 6

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).

Seismic 3 Figure 7

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

Seismic 3 Formula

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!”