Limiting Lateral Brace Spacing
New tables in the 2007 NFPA 13 limit pipe stresses and deflections.
NFPA Journal®, July/August 2008
As reported in this column a year ago, the interim amendment cycle following the issuance of the 2006 edition of the International Building Code (IBC) endorsed the earthquake protection provisions of NFPA 13, Installation of Sprinkler Systems. The amendment to IBC Section 1613.6.3 states, "Automatic fire sprinkler systems designed and installed in accordance with NFPA 13 (2007) shall be deemed to meet the requirements of Section 13.6.8 of ASCE 7." This endorses the NFPA 13 approach to meeting the seismic load and story drift requirements of the American Society of Civil Engineers’ ASCE Standard 7-05, Minimum Design Loads for Buildings and Other Structures.
Among the changes to the 2007 edition of NFPA 13 that merit this endorsement were new tables intended to limit piping stresses and deflections. Tables 188.8.131.52.2(a) and (b), titled "Maximum Load Fpw in Zone of Influence" for Schedule 10 and Schedule 40 steel pipe, respectively, limit the maximum load in the zone of influence for each lateral brace. The zone of influence for each brace includes the contributions of expected earthquake loads from branch lines and mains halfway to the next brace, and the allowable loads are smaller as brace spacing increases. Based on pipe size and schedule, the spacing between braces can range from 20 to 40 feet (6 to 12 meters), which for years has been used as the nominal brace spacing.
There is a basic logic to reducing brace spacing as loads increase, since the spacing of braces is related to the piping’s beam strength. The 1951 edition of NFPA 13 was the first to incorporate two important requirements: a brace spacing of "30 to 40 feet [9 to 12 meters]" and the stated intent to brace the system against a maximum expected earthquake force equal to half of gravity. For years, this intent to protect the system against earthquakes using a load of half the weight of the water-filled pipe was considered sufficiently conservative. And with a load equal to half the weight, an earthquake brace spacing about twice that of hanger spacing made sense. But the Loma Prieta and Northridge earthquakes showed that earthquake forces could exceed this value in high-risk earthquake areas. As a result, the 2007 edition of NFPA 13 reflects that the spacing of lateral braces must move closer to that of hangers as the assumed earthquake loads grow.
The only problem with the new tables is that they address only Schedule 10 and Schedule 40 steel pipe, leading some authorities having jurisdiction to question whether other types of pipe can be used in seismic areas. The Technical Committee on Hanging and Bracing is addressing this in proposed amendments, adding tables for copper tube and chlorinated polyvinyl chloride (CPVC) nonmetallic piping.
Perhaps even more urgent is clarification that the concepts can be applied to steel pipe in other pipe schedules. The committee is proposing a table for Schedule 5 steel pipe and will be asked to clarify that steel pipe between Schedule 40 and Schedule 10 can use the Schedule 10 table and that steel pipe between Schedule 10 and Schedule 5 can use the Schedule 5 table.
Although there is steel pipe marketed as Schedule 30 and Schedule 7, these are not official pipe schedules typically used for fire sprinkler piping, and these products cannot have their own brace spacing tables in NFPA 13. However, the use of the tables for the next-smaller, official pipe schedules should resolve all questions on the subject.
Russ Fleming, P.E., is the executive vice-president of the National Fire Sprinkler Association and a member of the NFPA Technical Correlating Committee on Automatic Sprinklers.