|Changes to the 2007 Edition of NFPA 13|
Issue 9: Top 10 Changes in the 2007 Edition of NFPA 13
By Chris Dubay, P.E.
Now that the 2007 edition of NFPA 13 has been published, it is a good time to review some of the changes that directly affect the fire protection engineering community. This article will review ten specific changes; these changes by no means represent an all-encompassing list of changes but have been selected to highlight some of the major areas of change.
Chris Dubay is with the National Fire Protection Association.
1"Evaluation of Sprinkler Performance in Protecting Gondola-Type Shelf Storage," Fire Protection Research Foundation, Quincy, MA, 2006.
Portions reproduced with permission from NFPA 13, Installation of Sprinkler Systems, copyright © 2007, National Fire Protection Association, Quincy, MA, 2007. This reprinted material is not the complete and official position of the National Fire Protection Association on the referenced subject which is represented only by the standard in its entirety.
Seismic Changes in NFPA 13
By Russ Fleming, P.E.
One of the most significant areas of change within the 2007 edition of NFPA 131 is related to requirements for earthquake protection. NFPA 13 has included provisions for the protection of sprinkler systems against earthquakes since 1947, a time at which many building codes didn't even address the subject. But major changes in building code requirements for earthquake protection have come in the past 20 years with funding from the Federal Emergency Management Agency, leading to the development of the National Earthquake Hazards Reduction Program2 (NEHRP) provisions. The NEHRP provisions initially found their way into the codes through direct adoption, and now through reference to the American Society of Civil Engineer's standard ASCE/SEI 7.3 The 2007 edition of NFPA 13 includes major changes intended to correlate with the NEHRP/ASCE/SEI provisions.
The NFPA Committee on Hanging and Bracing of Water-Based Fire Protection Systems has been trying to keep pace with a moving target as successive editions of ASCE/SEI 7 address protection of architectural and mechanical systems. There is a difficult transition period taking place right now involving the seismic provisions of NFPA 13 and the 2003 edition of the International Building Code4® (IBC). Because of the reference to ASCE/SEI 7, the 2003 IBC does not contain the detailed seismic requirements that had been included in the 2000 edition of the IBC. Although general reference to the 1999 edition of NFPA 13 is contained in the 2003 IBC, a section of ASCE/SEI 7 that made special reference to the use of NFPA 13 for earthquake protection was omitted during the adoption process, leading some to believe that sprinkler piping must be protected the same as other mechanical piping. However, sprinkler system piping is not arranged like other mechanical piping systems, and some of the requirements of NFPA 13 have been specifically developed to prevent system damage during earthquake movement.
Several structural engineers who helped write the NEHRP provisions have been participating in the NFPA 13 amendment process, trying to eliminate areas of conflict. Among the areas of concern were the fastener requirements for bracing, specifically anchorage allowances that were outdated. These were addressed with a TIA (emergency amendment) to the 2002 edition of NFPA 13 that became effective August 6, 2003. That TIA can be viewed at the nfpa.org Web site and is officially part of the 2002 edition of the sprinkler standard, but was not issued on the 1999 edition referenced by the 2003 IBC.
The 2003 TIA to NFPA 13 reduced the allowable fastener loads to match current industry standards, limited the maximum spacing of lateral braces to 40 ft (12.2 m) on center and provided clarification of how NFPA 13 could be used in conjunction with the seismic design force formula contained in ASCE/SEI 7 for use with nonstructural components:
Fp = 0.4 ap SDS Wp (1+2(z/h)) / (Rp / Ip)
The design acceleration, SDS, for any part of the United States is found in widely available maps developed by the U.S. Geological Survey (USGS), and some of the other factors are directly assigned for fire sprinkler systems in ASCE/SEI 7. While the importance factor, Ip, has always been 1.5 for sprinkler systems, assigned values for some of the other factors changed between the 2002 and 2005 editions of ASCE/SEI 7. The TIA issued on the 2002 edition of NFPA 13 suggested the use of component response modification factor, Rp, of 3.5, correlating with the 2002 edition of ASCE/SEI 7, which also specified a component amplification factor ap of 1.0. In the 2005 edition of ASCE/SEI 7, the Rp for a steel piping system (with joints made by something other than welding or brazing) is 4.5, and the ap is 2.5 (regardless of how joints are made).
While the 2002 edition of ASCE/SEI 7 was referenced for use by the 2003 editions of the IBC and NFPA 50005® building codes, the 2005 edition of ASCE/SEI 7 is referenced by the 2006 editions of those codes. As such, compliance with the 2003 or earlier editions of these codes would produce an ap/Rp ratio of 1.0/3.5 = 0.29, whereas compliance with the 2006 editions of these codes produces a ratio of 2.5/4.5 = 0.55, resulting in significantly higher forces. To some extent this has been offset by the clarification that the calculated design force can be reduced by a factor of 1.4 due to the fact that ASCE/SEI 7 is based on strength design, whereas NFPA 13 uses allowable stress design. Prior to the 2007 edition, all loads in NFPA 13 were at allowable stress levels with the exception of the buckling loads for brace members. In the 2007 edition, tables that contained the allowable loads on braces have been reduced to add a factor of safety appropriate to the use of allowable stress design.
A simplified approach to determining seismic forces on sprinkler systems has also been developed for the 2007 edition of NFPA 13, involving a number of conservative assumptions. For example, the approach assumes poor soil conditions, leading to higher earthquake forces on the piping. The user of the standard always has the option to determine lateral forces in accordance with the actual equation of ASCE/SEI 7, but the simplified approach allows the determination of loads without the use of the equation. Table 126.96.36.199.2 contains a series of "seismic coefficients", factors that are simply applied to the design acceleration, SDS, taken from the USGS maps to arrive at the earthquake design-force levels.
Because of the changes in ASCE/SEI 7, AHJs and others should be aware that the appropriate criteria for use in conjunction with the 2000 and 2003 editions of the model building codes are those found in the 2002 edition of NFPA 13, including the TIA. The appropriate criteria to be used with the 2006 editions of the model building codes are found in the 2007 edition of NFPA 13. Designers and AHJs do need to be aware that the building code references a specific edition of the standard. Generally, this does not preclude designers from using newer editions, especially if such revisions work better technically, as long as the use of newer editions is specifically documented and justified.
The new changes to the 2007 edition of NFPA 13 bring the standard fully in line with the latest thinking of the earthquake experts. In fact, the 2007 edition of NFPA 13 was unanimously approved as "deemed to comply" with the seismic requirements of ASCE/SEI 7 without exception at the IBC Structural Committee Hearings in Orlando in September of 2006.
The 2007 edition of NFPA 13 also contains a number of individual changes of significance relative to the installation of seismic bracing and restraint, including the following:
One of the issues not addressed within NFPA 13 that is an important design consideration relates to clearance around sprinklers penetrating ceilings. The 2005 ASCE/SEI 7 as adopted by the 2006 building codes requires a 1-in. clearance around sprinkler penetrations of ceiling membranes in high-risk seismic areas. Some sprinkler manufacturers have developed larger escutcheon plates to cover these holes; flexible drops can be used as an alternative.
The special earthquake protection criteria of NFPA 13 have been developed over decades, making use of both laboratory findings and real earthquake experience. The new changes bring NFPA 13 into conformance with the latest standards in the field of earthquake engineering. Attempts to bypass the NFPA 13 approach and treat sprinkler system piping like other mechanical systems is not advised, since it will produce the worst of both worlds: more cost with less actual protection against earthquakes.
Russ Fleming is with the National Fire Sprinkler Association.
1NFPA 13. Installation of Sprinkler Systems, National Fire Protection Association, Quincy, MA, 2007.
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