Seismic design of fire suppression piping systems
Fire protection engineers should use the guidance of NFPA 13 when designing fire suppression piping in nonresidential buildings.
- 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 188.8.131.52 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.