Determining the need for seismic protection for a sprinkler system and designing the protection used to be a straightforward task. The designer would look on an Earthquake Zone Map in NFPA 13 to see if the system was supposed to have the protection, apply the loads based on a generally accepted percentage (force factor) of the weight of the system and choose the size and location of the seismic protection components.
With the introduction of model building codes, additional variables come into play. They include soil characteristics, building structural design and layout, building occupancy, and ground acceleration. These are the same variables that determine the need and level of seismic protection for the building itself. This can lead to a sprinkler system in one building needing seismic protection and another system in a different building in the immediate area not needing it.
The codes of concern are the International Building Code (IBC) and the Building Construction and Safety Code (aka NFPA 5000).
2000 Edition of IBC
The 2000 Edition of the IBC stated that if the seismic protection was designed to NFPA 13, and that those forces and displacements used to design the protection “…when multiplied by a factor of 1.4 are determined to be not less than that prescribed by…” the IBC, then adequate seismic protection would be provided. What this means plainly is that the IBC design is 1.4 times as great as the NFPA 13 design. So to meet IBC requirements using NFPA 13 requirements the designer must calculate and design per NFPA 13, apply the 1.4 factor to all calculations, calculate the IBC requirements, and compare the two. It may make better sense to use IBC from the start.
The IBC mentioned a “seismic design displacement calculated in accordance with NFPA 13,” though the latter does not have a provision for this design. What NFPA 13 prescribes are the when and where of flexible couplings on a system and the amount of clearances around piping. Calculations to determine the adequacy of these items are not required.
NFPA 5000 references American Society of Civil Engineers (ASCE) Standard 7, Minimum Design Loads for Buildings and Other Structures. In turn, this standard states that designing the seismic protection for the sprinkler system using NFPA 13 is a suitable alternative.
2003 Edition of IBC
Like NFPA 5000, the 2003 Edition of the IBC references ASCE 7 but with some modifications and exclusions. What’s interesting is that these modifications and exclusions specifically prevent the use of NFPA 13 to design the seismic protection for the sprinkler system (in fact, it was written as a double exclusion).
The chief concern is the use the force factor. NFPA 13 uses a force factor of 0.5 (that is, the seismic force on the sprinkler system is one-half of the weight of pipe and the water that it holds). In most areas the force factor is more than what is needed and the protection would be over designed. But in some areas, the protection would be under designed compared to the building code criteria using the variables previously mentioned. NFPA 13 does not take these additional variables into account.
As a result, the NFPA 13 Technical Committee involved with seismic protection issued an interim amendment to address the issue of the force factor. Unfortunately, the changes in IBC 2003 to exclude NFPA 13 continue, though IBC plans to eliminate the exclusion in their next edition now that force factor has been addressed in NFPA 13.
For reference, FM Global recommends a minimum force factor of 0.5 for those areas they deem needing seismic protection (Zone 150 or less per their maps in Data Sheet 1-2); they also recommend contacting the local building code authorities if they deem an even higher force factor is needed. So although FM Global retains a conservative design it has provisions to allow for protection beyond it standards.
In conclusion, you should be knowledgeable about the particular building code being used in your area. If you have further questions please feel free to contact Risk Logic.