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1、The Evolution of Seismic Design of Fire Sprinkler SystemsRussell P. Fleming, P.E. Executive Vice President National Fire Sprinkler Association Patterson, NY Special fire sprinkler system installation guidance intended to guard against damage due to earthquakes first appeared in the North American sp
2、rinkler installation standard in 1947, and was largely based on experience in the Long Beach earthquake of 1930. At that time many building codes in use in the United States did not even address the subject of earthquakes. Insurance reports of the Long Beach earthquake had indicated 90 damaged sprin
3、kler systems. The first guidance, contained only in the appendix, called for 1 to 2 inches clearance around pipes, flexible couplings in risers and “some form of lateral and longitudinal bracing” for feed and cross mains. In the 1951 edition requirements were placed in the body of the standard for p
4、lacement of longitudinal and lateral braces with spacing indicated as “30 to 40 ft”, a maximum slenderness ratio of brace members of 200, and flexible couplings on risers where it was necessary to protect systems against earthquakes. The stated intent was to laterally brace for 50% of weight of wate
5、r-filled piping and attachments. In the decades since then deliberate efforts have been made to clarify the rules and improve protection within the text of NFPA 13 Installation of Sprinkler Systems, but always with the intent of addressing “how” systems are to be protected against earthquakes, not “
6、where” such protection is to be provided. It was recognized that building codes and other authorities made the basic decisions with regard to the location of earthquake-prone areas. Although the earliest guidance contained in NFPA 13 came from the insurance industry in the western states, the NFPA C
7、ommittee on Automatic Sprinklers formed an Earthquake Protection Subcommittee in 1985. One of the goals of this subcommittee was to develop a means to address the bracing of sprinkler systems in the same manner that the application of system hydraulics had been accomplished. Although a complex subje
8、ct, it was believed that many of the design aspects could be pre-engineered into tables and then properly applied by the technicians who normally lay out and detail fire sprinkler systems. The 1983 standard contained no information on the sizing or fastening of bracing relative to loads. A method wa
9、s proposed in 1984 whereby “zones of influence” could be evaluated to determine total loads for proposed brace locations. Tables were proposed for determining maximum brace loads and allowable fastener loads based on six different combinations of brace and fastener orientation and angle from vertica
10、l. This concept and the accompanying tables were accepted into the appendix of the standard in the 1987 edition, and in the 1989 edition the tables were expanded to nine combinations of orientation and angle and moved into the body of the standard. 2007 Structures Congress: New Horizons and Better P
11、ractices 2007 ASCECopyright ASCE 2007 Structures Congress 2007Downloaded 06 Mar 2009 to 202.118.74.100. Redistribution subject to ASCE license or copyright; see http:/www.ascelibrary.orgThe NFPA 13 Earthquake Protection Subcommittee was in place at the time of the Loma Prieta and Northridge earthqua
12、kes in 1989 and 1994, and held public hearings in cooperation with the National Fire Sprinkler Association and the Society of Fire Protection Engineers following those events in order to evaluate how fire sprinkler systems performed during the earthquakes ands to develop suggestions for improvements
13、 to the protection criteria within the standard.In general, the performance of fire sprinkler systems in those earthquakes was judged to be good, with a strong correlationbetween observed failures and departures from the rules of NFPA 13. One area acknowledged to be in need of improvement was the in
14、teraction of sprinklers and ceilings; impact of sprinklers against rigid ceilings resulted in a several high-profile incidents in which a number of sprinklers opened and water damage resulted. In 1999 the NFPA sprinkler project was reorganized. Responsibilities for the earthquake protection rules we
15、re given to a new NFPA Technical Committee on Hanging and Bracing of Water-Based Fire Protection Systems, one of four separate technical committees coordinated through a Technical Correlating Committee responsible for NFPA 13. This allowed the direct input of more earthquake engineering expertise in
16、to the standard development process. Major changes in building code requirements for earthquake protection have taken place in the past twenty years, mainly due to the National Earthquake Hazards Reduction Program (NEHRP) provisions developed with funding from the Federal Emergency Management Agency
17、.The NEHRP provisions initially found their way into the codes through direct adoption, and more recently through reference to the American Society of Civil Engineers standard ASCE/SEI 7. As a result, the goal of the committee that prepares NFPA 13 has evolved during the years. From the original goa
18、l of independently providing a means of protecting sprinkler systems against earthquake forces, the committee in recent years 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 transi
19、tion period taking place right now involving the seismic provisions of NFPA 13 and the 2003 edition of the International Building Code (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.
20、 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 sa
21、me as other mechanical piping. However, sprinkler system piping is not arranged like other mechanical piping systems, and some of the rules of NFPA 13 have been specifically developed to prevent system damage during earthquake movement. For example, while most mechanical piping supported by hanger r
22、ods less than 12 in. (400 mm) in length is exempt from bracing under current codes, comparative experience of sprinkler systems with that exemption versus the NFPA exemption of only 6 in (150 m) was provided by the 1987 New Zealand earthquake. The 2007 Structures Congress: New Horizons and Better Pr
23、actices 2007 ASCECopyright ASCE 2007 Structures Congress 2007Downloaded 06 Mar 2009 to 202.118.74.100. Redistribution subject to ASCE license or copyright; see http:/www.ascelibrary.orggeneral conclusion was that systems installed in accordance with NFPA 13 withstood the earthquake, while those not
24、so designed failed. Over the past two decades there have been a numerous refinements in the language of NFPA 13 that reflect the lessons learned in earthquakes as well as the influence of the NEHRP provisions. These can be evaluated by looking at how the various sections of NFPA 13 have evolved in t
25、wenty key areas: 1. Applicability of NFPA 13 Earthquake Protection Provisions 1983 - Where subject to earthquakes 1996 - When sprinkler systemsare to be protected against damage from earthquakes 1999 - Where sprinkler systems or aboveground fire service mains are required to be protected against dam
26、age from earthquakes 2002 - Where water-based fire protection systems are required to be protected against damage from earthquakes 2. General Intent of Earthquake Protection Provisions 1983 - Minimize or prevent pipe breakage 1991 - Prevent pipe breakage 1999 - Protect against damage 3. Flexible Cou
27、plings - General 1983 - For piping 3 in. or larger 1994 - For piping 2 in. or larger; “Flexible listed pipe coupling” defined as “a listed coupling or fitting that allows axial displacement, rotation, and at least 1 degree of angular movement of the pipe without inducing harm on the pipe” (minimum 0
28、.5 degrees for 8-in. and larger) 2007 - Flexible couplings and clearance identified as addressing displacement due to story drift 4. Flexible Couplings for Risers in Multi-Story Buildings 1983 - At ceiling of each intermediate floor 1994 - Within 12 in. above and below the floor in multistory buildi
29、ngs such that the flexible coupling below the floor is below the main supplying that floor 2007 Structures Congress: New Horizons and Better Practices 2007 ASCECopyright ASCE 2007 Structures Congress 2007Downloaded 06 Mar 2009 to 202.118.74.100. Redistribution subject to ASCE license or copyright; s
30、ee http:/www.ascelibrary.org 1999 - Within 12 in. above and within 24 in. below the floor in multistory buildings; Additional coupling required on vertical portion of tie-in piping where tie-in is below the upper flexible coupling on the riser 2007 - Additional flexible coupling can be provided eith
31、er on the vertical portion of tie-in piping or within 24 in. on horizontal tie-in piping 5. Drops to Hose Lines, Sprinklers in Racks, and Portions of Systems 1983 - Fittings with flexible joints at the top of drops to hose lines (regardless of size); Swing joints assembled with flexible fittings on
32、drops to racks over 3 in. 1987 - Flexible couplings at the top of drops to hose lines (regardless of size); Swing joints with flexible fittings on drops to racks (regardless of size) 1989 - Flexible coupling at the top of drops to hose lines, rack sprinklers and mezzanines (regardless of size); Flex
33、ible coupling at the top of drops exceeding 15 ft. to sprinklers or portions of systems (regardless of size) 1991 - Flexible coupling within 24 in. of the ceiling at top of drops to hose lines, rack sprinklers and mezzanines (regardless of size); Flexible coupling within 24 in. of the ceiling at top
34、 of drops exceeding 15 ft. to portions of systems supplying more than one sprinkler (regardless of size) 1994 - Deleted “of the ceiling” 1996 - Added “at top and bottom” of drops supplying hose lines, rack sprinklers and mezzanines 2007 Additional flexible couplings for drops required within n 24 in
35、. above the uppermost drop support or, where no drop support is provided, within 24 in. above the bottom of the drop 6. Flexible Couplings for Expansion Joints 1983 - On one side of building expansion joints 1987 - At or near building expansion joints; Seismic separation assembly needed for seismic
36、separation joints 1999 Within 24 in. of building expansion joints 7. Seismic Separation Assemblies 2007 Structures Congress: New Horizons and Better Practices 2007 ASCECopyright ASCE 2007 Structures Congress 2007Downloaded 06 Mar 2009 to 202.118.74.100. Redistribution subject to ASCE license or copy
37、right; see http:/www.ascelibrary.org1987 - Swing joints assembled with flexible fittings required where piping crosses seismic joints; Figure added to appendix 1989 - Added “regardless of size” 1991 - Designated as “seismic separation assemblies with flexible fittings”; Added applicability only “abo
38、ve ground level” 2007 Expanded to “flexible fittings or flexible hose” and specified minimum capability of allowing movement to accommodate closing of the separation, opening of separation to twice normal size, and movement relative to the separation in the other two dimensions in an amount equal to
39、 the separation distance 8. Clearances 1983 - Clearance through walls, floors, platforms and foundations 1 in. all sides through 3-in pipe, 2 in. all sides 4-inch and larger; Exception added to permit pipe sleeves of nominal diameter 2 in. larger through 3-in. pipe (4 in. larger for 4-inch and large
40、r pipe); Another exception added for pipe passing through gypsum board or equally frangible construction not required to have a fire-resistance rating 1985 - Flexible joints permitted to substitute for clearance for pipe entering a basement wall with ground water conditions 1987 - Flexible couplings
41、 or swing joints within one foot of both sides permitted to substitute for clearance in any wall 1994 - Flexible coupling within 1 ft. of each side accepted as alternative for walls, platforms and foundations 2002 - Clearance dimensions to be considered nominal; Clearance not required for horizontal
42、 piping passing perpendicularly through successive studs or joists that form a wall/ceiling assembly; Clearance not required for nonmetallic pipe having flexibility equal to flexible couplings within 1 ft. of both sides. 9. Sway Bracing - General 1983 - Piping to be tied to the structure for minimum
43、 relative movement, but allowing for expansion, and differential movement within and between structures; Tops of risers secured against drifting in any direction using 4-way brace 1987 Zone of influence method added to appendix with maximum load tables for pipe, angles, rods and flats based on three
44、 ranges of angle from vertical and maximum slenderness ratio of 200 2007 Structures Congress: New Horizons and Better Practices 2007 ASCECopyright ASCE 2007 Structures Congress 2007Downloaded 06 Mar 2009 to 202.118.74.100. Redistribution subject to ASCE license or copyright; see http:/www.ascelibrar
45、y.org1989 Table for maximum brace loads moved to body of standard 1994 - System piping to be supported to resist both lateral and longitudinal horizontal loads 1996 - System piping to be supported to resist both lateral and longitudinal horizontal loads and vertical loads; Each run of pipe between a
46、 change in direction to be provided with both lateral and longitudinal bracing; Exception for runs less than 12 ft. in length supported by braces on adjacent runs; Tension-only bracing allowed if listed for the application 2002 - Clarification that 4-way bracing is required at the tops of risers exc
47、eeding 3 ft in length; Maximum vertical distance between 4-way braces limited to 25 ft 2007 - Braces and restraints identified as possible obstructions to sprinkler discharge 10. Sway Bracing - Loads 1983 - Sway bracing to withstand a force in tension or compression equivalent to not less than half
48、the weight of water-filled piping 1989 - Assigned load table added. Alternative permitted for zone of influence method: For lateral braces - all branch lines and mains within zone For longitudinal braces - all mains within zone 1994 - Multipliers permitted for horizontal force factor Fp = 0.5 Wp where use of ot