By James D. Lake, NFPA
Sprinkler Protection in EarthquakesEngineering
Chapter 9 in the National Fire Protection Association's Standard 13 addresses structural issues related to the installation of fire protection piping systems in buildings and facilities. Section 9.3 s...
Chapter 9 in the National Fire Protection Association’s Standard 13 addresses structural issues related to the installation of fire protection piping systems in buildings and facilities. Section 9.3 specifically addresses the protection of sprinkler piping where it may be subjected to earthquakes.
Sprinkler systems need to be protected during seismic events, of course, so that they can help extinguish any fires that might occur.
For the new 2007 edition of NFPA 13, the seismic requirements of chapter 9 were updated to ensure that the standard meets or exceeds the requirements of the American Society of Civil Engineers’ standard, ASCE 7, “Minimum Design Loads for Buildings and Other Structures.” The ASCE standard is adopted by reference in the current building codes of U.S. states, though not in the National Building Code of Canada.1
Seismic zone maps removed
One of the first notable changes to Section 9.3 is the removal of the seismic zone maps that were included as examples in the Annex of previous editions of NFPA 13. These were removed to make it clear that NFPA 13 does not address the issue of where earthquake protection of sprinkler systems is to be provided. The standard only describes how to protect sprinkler systems against earthquake damage once the decision has been made. The decision to require protection of sprinkler systems or other mechanical systems against earthquake forces is generally stated within building codes or is determined by other authorities having jurisdiction. The decision is based on the past seismic activity within an area and the potential for future earthquakes.
Changes for flexible couplings
One objective of providing protection for sprinkler systems during earthquakes is to provide the necessary flexibility to alleviate stresses that can develop due to differential building movement.
A key location where differential building movement occurs is between two storeys of a building. This differential movement, known as “storey drift,” is the maximum lateral displacement within a storey, that is, the displacement of one floor relative to the floor below.
The maximum storey drift is limited by building codes based on the type of construction, since some construction types are more flexible than others. Sprinkler systems are intentionally designed with a high degree of flexibility to address this storey drift. Most significant in addressing storey drift in NFPA 13 is the requirement for two flexible couplings per floor on a riser. This allows the riser to accommodate more storey drift than is contemplated for even the most flexible of buildings.
The requirements for two flexible couplings per floor was first included in the standard in 1994. In the 2007 edition, a further change in wording permits the flexible coupling to be located on the horizontal portion of the tie-in pipes. The figure “Flexible Coupling on Horizontal Portion of Tie-In” was added (see opposite).
Another change has been introduced regarding couplings for drops to hose lines, in-rack sprinklers and other portions of systems. Flexible couplings have been required at the tops of drops since the 1983 edition. In 1996 an additional flexible coupling was required at the bottom of drops supplying hose lines, rack sprinklers and mezzanines. The 2007 edition further adds a requirement for an additional flexible coupling for drops with additional supports on the rack. This coupling must be within 610 mm above the uppermost drop support, or within 610 mm above the bottom of a drop where no support is provided. The figure “Flexible Coupling for Drops” (see opposite) identifies the location of couplings and supports.
The final change pertaining to flexibility relates to seismic separation assemblies. The 2007 edition now recognizes the use of flexible fittings or flexible hose for this purpose.
New limits for brace spacing
Sway bracing, is provided for automatic sprinkler systems to prevent the piping system moving excessively, which can lead to hangers being pulled out and the fittings fracturing. Prior to the 2007 edition of NFPA 13, all loads were at allowable stress levels, with the exception of the buckling loads that served as the basis of maximum horizontal load tables. In the 2007 edition, the allowable loads in these tables were reduced to add a factor of safety appropriate to the use of allowable stress design.
Lateral sway bracing is required for system piping greater than 65 mm. For branch line and other piping smaller than 65 mm, bracing is not required. Branch lines 50 mm in diameter and smaller are considered to have enough flexibility to avoid being damaged under their own momentum. However, in areas with high seismic loads bracing may be required for branch lines in order to reduce the loads in the brace’s zone of influence.
System mains, on the other hand, may also carry the loads of adjacent branch lines and therefore must be braced, regardless of size. The 2007 edition of NFPA 13 clarified that bracing is not required for 65-mm starter pieces, that is, the first sections of pipe on branch lines from a cross main, provided they do not exceed 3.6 m in length.
A major change to the 2007 edition of NFPA 13 is to limit the maximum lateral sway for brace spacing. The limit is based on the size of the pipe being braced and the load determined by the zone of influence method. The traditional maximum brace spacing of 12.2 m still applies, but in situations involving high lateral forces or heavy branch lines, the maximum spacing may need to be reduced. This reduced spacing is detailed in a table. The change recognizes the fact that maximum lateral brace spacing is related to the beam strength of the piping, and it represents an attempt to limit maximum pipe deflection as well as pipe stress.
In previous editions of the standard the last lateral brace was permitted to be up to 6.1 m from the end of the pipe. This requirement was dramatically lowered to 1.8 m in the 2007 edition with the intention of eliminating the potential for large cantilevered loads caused by branch lines beyond the brace.
Clarification on restraint for branch lines
Piping restraint is a means of holding piping components in place to a lesser degree than by the use of bracing. The restraint of the ends of all branch lines has been required since the 1987 edition of NFPA 13, but intermediate restraint along the lengths of the branch lines was not required until the 1991 edition, and then only where the movement of the branch lines could damage sprinklers through their impact against the building structure, equipment or finish materials.
The spacing of such restraint was set at 9.1 m, but the requirement was considered highly subjective, since it would depend in many cases on the relative fragility of ceilings and sprinklers.
The 2007 edition eliminates this subjectivity by requiring intermediate restraint for all branch lines, other than those that are supported by short rods and those that are that are laterally braced, that is, branch lines 65 mm and larger.
The maximum spacing of restraints is based on the seismic coefficient of the area and ranges from 8.2 to 16.8 m, depending on the size of pipe. The allowable spacing between restraints is reduced as the pipe gets smaller and as the ground accelerations increase.
James Lake is a senior fire protection specialist with the National Fire Protection Association (NFPA) in Quincy, Massachusetts and staff liaison for the technical committees on automatic sprinklers.