Seismic design of fire suppression piping systems
- Learn about NFPA 13 and other relevant codes, which discuss seismic design of fire suppression systems.
- Understand the combination of flexibility and rigidity to protect the fire suppression piping.
- Learn about the calculation procedure using the “zone of influence” concept to determine the location and strength of lateral, longitudinal, and four-way sway braces for fire suppression system piping.
The subject of seismic design of fire suppression systems has been incorporated in NFPA Standard 13: Standard for the Installation of Sprinkler Systems since 1947. The design guidance contained in NFPA 13 for seismic bracing of sprinkler piping has also been adopted by NFPA 14: Standard for the Installation of Standpipe and Hose; NFPA 15: Standard for the Installation of Water Spray Fixed Fire Protection Systems; NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection; and NFPA 2001: Standard for the Installation of Clean Agent Fire Suppression Systems.
The design guidance has evolved since the earlier versions of the standard. The most notable updates were made based on lessons learned in the San Fernando, Calif., earthquake in 1971; the Loma Prieta, Calif., earthquake in 1989, and the Northridge, Calif., earthquake in 1994. A study performed by the Pacific Fire Rating Bureau (PFRB) of 973 sprinklered buildings involved in the San Fernando earthquake led the PFRB to conclude that “if a sprinklered building fared well, so did the sprinkler system.”
The knowledge gained from previous seismic events has led to continuous modifications to the NFPA 13 design guidance for sprinkler systems. These modifications have been aimed at avoiding significant damage to sprinkler systems and permitting sprinkler systems to remain functional following an earthquake. The seismic design provisions contained in the 2010 edition of NFPA 13 have been coordinated with the provisions of the National Earthquake Hazard Reduction Program (NEHRP) and SEI/ASCE 7, Minimum Design Loads for Buildings and Other Structures.
NFPA 13 Chapter 9.3, Protection of Piping Against Damage Where Subject to Earthquakes, sets out the requirements for seismic bracing and restraints of sprinkler systems. It is important to note that although this chapter gives guidance for how a system should be designed, it does not specify the geographic locations where seismic design must be incorporated into a sprinkler system design. This is done by other documents, typically local building codes.
In the past, only sprinkler systems located in areas that were perceived to be subject to high frequencies of damaging earthquakes (typically in California) were designed to resist damage due to seismic events. However, the International Building Code (IBC) has now published maps that give guidance regarding the maximum predicted peak ground acceleration due to earthquakes. In addition, the U.S. Geological Survey (USGS) publishes maps of the United States that show the areas with a 2% probability of exceedance of the peak ground acceleration in a 50-year period (see Figure 1). Such a map indicates that there are areas in the central and eastern United States in which the peak ground acceleration expected is similar to areas in California, which are considered to be high earthquake hazard areas.
NFPA 13-2013 edition sets forth design guidance. Section 18.104.22.168 allows the use of alternative methods of providing earthquake protection of sprinkler systems based on a seismic analysis certified by a registered professional engineer such that system performance will be at least equal to that of the building structure under expected seismic forces.
Appendix A Section A.9.3.1 states the goal of the design guidance in NFPA 13: “Sprinkler systems are protected against earthquake damage by means of the following: (1) Stresses that would develop in the piping due to differential building movement are minimized through the use of flexible joints or clearances. (2) Bracing is used to keep the piping fairly rigid when supported from a building component expected to move as a unit, such as a ceiling.”
To provide the necessary combination of flexibility and rigidity to protect the piping, NFPA 13 stipulates the following design guidance:
- Provide clearance where needed.
- Install longitudinal and lateral sway bracing at prescribed intervals on the piping system to provide support against horizontal movement caused by ground movement.
- Install flexible couplings at certain locations to allow for movement where it is expected to occur.
- Install four-way (combined longitudinal and lateral) bracing at the top of risers serving sprinkler systems. This requirement does not apply to riser nipples, which supply branch lines or sprigs up from branch lines to serve individual sprinklers at a higher elevation.
- Install seismic separation assemblies where the sprinkler system crosses building seismic separation joints at ground level and above.
- Install sway braces designed with sufficient strength to withstand the anticipated forces caused by the horizontal movement.
- Restrain branch lines against vertical movement. It should be noted that “restraint” is considered to be required to resist a lesser degree of loads than “bracing.” “Restraint” of branch lines against vertical movement is a relatively recent addition to the standard, and was added after the performance of sprinkler systems in the Loma Prieta and Northridge, Calif., earthquakes was evaluated.
- Install restraints on C-type clamps so that they will not slip off a building structural member due to horizontal movement.
Where passing through platforms, foundations, walls, or floors, sprinkler pipe from 1- to 3.5-in. in diameter is required to have clearances provided by holes that are 2 in. greater in diameter than the pipes. Where passing through platforms, foundations, walls, or floors, sprinkler pipe from 4 in. and larger in diameter is required to have clearances provided by holes that are 4 in. greater in diameter than the pipes.
This minimum clearance will also be required where pipe sleeves are used where the piping passes through these structures.
Clearance is not required for piping passing through frangible construction such as gypsum board walls or ceilings if those walls or ceilings are not required to have a fire resistance rating. The rationale for the lack of clearance in this case is that the frangible construction does not have sufficient strength to interfere with the movement of the pipe, and although the frangible construction will be damaged, the piping will not. If the frangible construction is part of a fire resistant barrier, then the damage to the fire barrier cannot be accepted because it would compromise the fire protection scheme for the building and clearance must be provided. This is a common issue where sprinklers from a piping system above a ceiling penetrate a gypsum board ceiling to provide sprinkler coverage below the ceiling.
Longitudinal and lateral sway bracing
The purpose of longitudinal sway bracing is to counteract forces that are parallel to the direction of piping; lateral sway bracing counteracts forces that are perpendicular to the direction of the piping. The sway bracing is intended to prevent excessive movement of the system piping.
With some exceptions, bracing is required at:
- The top of the system riser
- All system feed mains and cross mains, regardless of size
- On branch lines 2.5 in. in diameter and larger (lateral bracing only).
Per NFPA 13, longitudinal sway braces are required to be located on feed mains and cross mains, spaced at a maximum of 80 ft on center. They may be required to be spaced closer together depending on the sway bracing calculations. Per NFPA 13, the distance between the last brace and the end of the pipe is not allowed to exceed 40 ft.
Per NFPA 13, lateral sway braces are required to be located on feed mains, cross mains, and branch lines 2.5 in. in diameter or larger, spaced at a maximum of 40 ft on center. The last lateral brace is required to be within 6 ft of the end of the pipe.
Zone of influence load calculation
The design of sway bracing is typically performed using a technique referred to as the “zone of influence.” This is the portion of the system that the brace is intended to protect against movement. This “zone of influence” is the piping served by the brace to the point at which the next brace will take over resistance to movement. In practice, the zone of influence of each of the two braces will generally extend to the midpoint between the braces.
Figure 2 shows a simplified sprinkler system piping configuration. Figure 3 shows a typical location of lateral, longitudinal, and four-way braces that meet the requirements of NFPA 13. Figure 4 shows a simplified example of the “zone of influence” concept.
The load that sway bracing must counteract in a brace’s “zone of influence” is the total weight of pipe in the zone filled with water multiplied by 1.15 to account for valves and fittings on the pipe multiplied by the seismic coefficient.
The load that must be counteracted by the brace is calculated by adding the weight of all piping filled with water, which may be taken from tables such as Table A.22.214.171.124 in NFPA 13 (2013 edition). The total weight of all segments of pipe filled with water added together and multiplied by 1.15 is the load in the “zone of influence.”
The horizontal force (Fpw) acting on the brace is calculated by multiplying the seismic coefficient (Cp) by the weight (Wp) calculated following the equation below.
Fpw = CpWp
The coefficient of acceleration (Cp) used in the calculation procedure is assumed to be Cp = 0.5 unless specified to be a lower value by the authority having jurisdiction (AHJ) or a lower or higher value is derived from the ground motion parameter (SS) at the site. This ground motion parameter has been developed by the USGS based on research of previous earthquake events and modeling techniques. The values of SS are available on the USGS website. This website offers a tool that enables the user to select a site on an interactive map and the tool will indicate the SS factor that is applicable to that site. For example, Aon Fire Protection Engineering’s office has an SS factor of 0.125 g. Based on Table 126.96.36.199.3 in NFPA 13, a ground motion parameter value of 0.33 or less has a Cp coefficient of 0.35.
Using the example in Figure 4, the horizontal load that is required to be counteracted by lateral brace 2 would be calculated as shown in Table 1.
Based on this example, the bracing assembly is required to counteract a horizontal load of 1,032 lbs.
Based on Table 188.8.131.52.2(a) from NFPA 13, the maximum load (Fpw), which is allowed in the zone of influence for a 6-in. schedule 10 steel pipe with lateral bracing spaced 40 ft on center, is 1,900 lbs. The sprinkler system load calculated above is within this limit; therefore, the design of the brace meets this requirement. If the maximum load exceeded the limit, additional braces would be required to reduce the “zone of influence” to a smaller size.
Each brace is sized by using a similar calculation procedure to verify that it is properly sized.
Design of the brace assembly
The brace assembly consists of the sway brace, the brace fittings (which attach the brace to the pipe and to the structural member), and the fastener. The entire assembly must be designed to resist the load anticipated, and the strength of the assembly is no greater than the strength of the weakest portion of the assembly. For example, if the brace can resist a maximum load of 900 lbs, the brace attachments can resist a maximum load of 1,000 lbs, and the fastener can resist a maximum load of 800 lbs, the assembly can resist a maximum load of only 800 lbs. In addition, the ability of the brace to resist horizontal load is dependent on the angle at which the brace is installed, because the assembly is resisting the horizontal load.
Tables 184.108.40.206.8(a), (b) and (c) contained in NFPA 13 give values for the loads that braces can withstand based upon their type, their size, their length to radius of gyration (l/r) ratio, and the brace angle. As an example, per Table 220.127.116.11.7 (b), a 1.25-in. schedule 40 pipe up to 9 ft long installed at an angle of 30 to 44 deg from the vertical could be used as a sway brace for a maximum horizontal load of up to 1,254 lbs.
Manufacturers of braces should be consulted to verify that the brace attachments have sufficient strength for the maximum load that will be encountered.
Where sway bracing assemblies are used, the assemblies are required to be listed by a nationally recognized testing laboratory for a maximum load rating.
Figure 18.104.22.168.1 in NFPA 13 lists the maximum loads for various types of fasteners to structures. The figure indicates that a 0.5-in. diameter unfinished steel bolt installed perpendicular to a steel mounting surface with the brace installed at an angle of 30 to 44 deg from the vertical can be used for a maximum load of 1,600 lbs.
C-type clamps, including the beam and large flange clamps, with or without restraining clamps, are not permitted to be used to attach braces to the building structure. Powder-driven fasteners are not permitted to be used to attach braces to the building structure unless they have been specifically listed for service in resisting lateral loads in areas subject to earthquakes.
Restraint of branch lines and hangers
Branch lines are required to be restrained to resist vertical movement. This can be achieved by a listed sway brace assembly; a wrap-around U-hook; No. 12 440-lb wire installed at least 45 deg from the vertical plane and anchored on both sides of the pipe; a hanger not less than 45 deg from vertical installed within 6 in. of the vertical hanger arranged for restraint against upward movement; or other approved means.
The spacing of these restraints varies based on the pipe size, the seismic coefficient (Cp), and the type of pipe. For the example indicated above, based on NFPA 13 Table 22.214.171.124(a), the 2-in. steel branch lines have a maximum spacing of 53 ft between restraints.
According to NFPA 13 Section 126.96.36.199, where the branch lines are supported by rods less than 6 in. in length from the top of the pipe to the point of attachment to the building structure, additional restraint is not required for these branch lines.
Sprigs up to supply sprinklers that are 4 ft or longer are required to be restrained against lateral movement.
Where seismic protection is provided, C-type clamps are required to be equipped with a restraining strap or the strap is required to be through-bolted or secured by a self-tapping screw. This is to ensure that the C-type clamp does not slip off the beam due to horizontal movement of the pipe.
Future codes and requirements
In previous years, the design of fire suppression systems to counteract seismic forces has been primarily confined to areas that have had serious earthquakes in the past, notably in California. However, the building codes have evolved to recognize that there are many areas in the United States outside of California that may also be subject to damaging earthquakes. Because of this evolution, design professionals who specify fire suppression systems in areas that have not traditionally been considered to be subject to earthquakes may now have to be aware of the seismic design requirements contained in NFPA 13. It is common for specifying engineers to specify that a sprinkler system meet the requirements of NFPA 13 and then to review the submitted drawings. The drawings that are submitted may be required to provide sway bracing to resist seismic loads. It may then be the responsibility of the specifying engineer to ensure that the submitted plans meet the requirements of NFPA 13, including the design of sway bracing to resist seismic loads.
The design of sprinkler system piping in an area where seismic activity is such that a local building code or the AHJ requires that the system be designed to resist horizontal movement due to an earthquake can be achieved by following the design guidance contained in NFPA 13. This guidance includes:
- Clearance where needed to ensure that the piping is not damaged by building movement
- Longitudinal and lateral sway bracing at prescribed intervals on the piping system to provide support against horizontal movement
- Flexible couplings to allow for movement where it is expected to occur
- Four-way (combined longitudinal and lateral) bracing at the top of risers
- Seismic separation assemblies where the sprinkler system crosses building seismic separation joints at ground level and above
- Sway braces designed with sufficient strength to withstand the anticipated forces caused by horizontal movement
- Branch lines restrained against vertical movement
- Hangers designed with restraints to ensure that the hangers do not slip off the building structural member to which they are attached.
Design guidance is contained in NFPA 13 Chapter 9.3. The design guidance is subject to change as improved design methods are developed. NFPA 13 is currently in the public comment period for revisions to the standard that will be voted upon by the association for incorporation into the 2016 edition of the standard. There are a number of proposals currently under consideration that may affect the design of sway bracing if they are adopted.
Joseph H. Talbert, PE, ARM, is a project manager at Aon Fire Protection Engineering. His expertise includes fire suppression system design, fire alarm and fire detection system design, building and fire code consulting, and risk management.