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Web Exclusive: Seismic Changes to NFPA 13 [Part 3]
by Christian Dubay, P.E.
January 11, 2010
Major technical changes are contained within the 2007 edition of NFPA 13 relating to seismic protection of automatic sprinkler systems.
Editor’s Notes: This article originally appeared in the Jan. 2007 issue of Sprinkler Age. Parts 1 and 2 of this series originally appeared in the October and December 2006 issues of Sprinkler Age. Reprints of Parts 1 and 2 of this series appeared in the January and July 2009 issues of pme.
This is the final part in this series, which addresses the major technical changes that are contained within the 2007 edition of NFPA 13 relating to seismic protection of automatic sprinkler systems. This series does not cover each and every change or review the existing requirements that have been contained in previous editions of NFPA 13. The 2007 edition made specific seismic changes in nine specific areas. Each of these areas will be discussed in detail with the final text of NFPA 13 included in quotation marks.
All figures, table and except reproduced with permission from NFPA 13-2007, Installation of Sprinker Systems, copyright © 2006 NFPA. This excerpt does not represent the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.
9. Restraint of Branch Lines
The final area where significant changes were made was in the area of restraint of branch lines. In previous editions of NFPA 13, restraint was only required for branch lines where impact or damage was possible due to movement of the system piping during the seismic event. However, in updating the requirements of NFPA 13 to ensure compliance with ASCE 7, it was determined that restraint is required for all branch lines.
Two new figures [see Figures A.9.3.6.1(5)(a) and (b)Hangers Used in Combination] were added to illustrate another acceptable method of branch line restraint. Section 9.3.6.1(5) permits the use of two generic listed hangers to be installed as required by 9.3.6.1(5), where combined the hangers restrain the vertical and horizontal movement.
“(5)*A hanger not less than 45 degrees from vertical installed within 6 in. (152 mm) of the vertical hanger arranged for restraint against upward movement, provided it is utilized such that l/r does not exceed 300, where the rod shall extend to the pipe or have a surge clip installed”
Section 9.3.6.4 provides the maximum distance between branch line restraints and is based upon the Cp for the system, which was previously discussed. Specifically, Table 9.3.6.4 relates the Cp to the maximum spacing of restraints.
“9.3.6.4* Branch lines shall be laterally restrained at intervals not exceeding those specified in Table 9.3.6.4 based on branch line diameter and the value of Cp.
A.9.3.6.4 Modern seismic codes require branch lines to be restrained, both to limit interaction of the pipe with other portions of the structure and to limit stresses in the pipes to permissible limits. The maximum spacing between restraints is dependent on the seismic coefficient, Cp, as shown in Table 9.3.6.4. Table 9.3.6.4 has been limited to 2 in. (50 mm) lines and smaller, because branch lines 21/2 in. (65 mm) or larger are required to be seismically braced.”
See Table 9.3.6.4 Maximum Spacing of Branch Line Restraints (feet).
Section 9.3.6.5 was added to the 2007 edition of NFPA 13 to align the allowances for branch lines supported by hanger utilizing short rods. Historically, NFPA 13 has provided an allowance to omit lateral bracing where supported by hangers utilizing rods less than 6 in. Since restraint is a lesser requirement than bracing, the committee determined that the same scenario would also omit the requirements for restraint of branch lines, where the branch lines are supported by hangers utilizing rods less than 6 in. in accordance with Section 9.3.6.5.
“9.3.6.5 Where the branch lines are supported by rods less than 6 in. (152 mm) long measured between the top of the pipe and the point of attachment to the building structure, the requirements of 9.3.6.1 through 9.3.6.4 shall not apply and additional restraint shall not be required for the branch lines.”
While these changes represent a significant shift in the design criteria of NFPA 13, they ensure that NFPA 13 will remain the referenced standard for sprinkler system design, including seismic design requirements. These changes ensure that NFPA 13 meets or exceeds the requirements of ASCE 7 and provides a direct reference from the model building codes.
Christian Dubay, P.E.
Christian Dubay, P.E., is vice president and chief engineer at the National Fire Protection Association and the editor of the Automatic Sprinkler Systems Handbook. Dubay holds a bachelor of science degree in fire protection engineering from the University of Maryland. He is a registered professional engineer in the State of Connecticut and is a member of the Society of Fire Protection Engineers.
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