Applying Multiple Modifiers
“I have a project that has a large exterior combustible canopy that has a combustible concealed space as described in 8.15.1.2.5. The canopy has a slope greater than 2 in 12 and will be protected with a dry system at ordinary hazard (Group 2). Based on 11.2.3.2.4 and 11.2.3.2.5, the required remote area would be 2,535 sq. ft. Then 11.2.3.1.5.2 comes into play and increases the remote area to 3,000 sq. ft. Both sections 11.2.3.2.4 and 11.2.3.2.5 state that the area of sprinkler operation shall be increased by 30 percent without revising the density. Section 11.2.3.2.1.2 indicates when using Figure 11.2.3.1.1, the calculations shall satisfy any single point on the appropriate density/area curve. Could 0.17 gpm/sq. ft. over 3,000 sq. ft. be used for hydraulic calculations or does the dry system and sloped ceiling supersede 11.2.3.2.1.2 where 0.2 gpm/sq. ft. density would be required?” We have reviewed NFPA 13, 2016 edition as the applicable standard. Our informal interpretation is that a lower density can be used. Granted, both sections 11.2.3.2.4 and 11.2.3.2.5 state that the area of sprinkler operation shall be increased by 30 percent without revising the density. However, 11.2.3.2.7.2 says that if the building has unsprinklered combustible concealed spaces, the rules of 11.2.3.1.4 shall be applied after all other modifications have been made. Figure 11.2.3.1.1 will allow you to use the density applicable to 3,000 sq. ft. of 0.17 gpm/sq. ft. But that’s too far up the curve since you still have to satisfy the requirements of 11.2.3.2.4 and 11.2.3.2.5 for the starting size of 3,000 sq. ft. (making the end result 0.17 gpm/sq. ft. over 5,070 sq. ft. at a flow of 862 gpm - ignoring discharge overages). You technically can move up the curve to 1,776 sq. ft., when increased by 30 percent for the dry system (making it 2,308.8 sq. ft.) and then again for the slope the final area is 3,001 sq. ft. As directed by 11.2.3.2.7.2 you then apply 11.2.3.1.4. The allowed starting density at 1,776 sq. ft. is approximately 0.193 gpm/sq. ft. resulting in a system demand of 579 gpm over 3,000 sq. ft. If you simply elected to use 0.2 gpm/sq. ft. over 1,500 sq. ft., the initial modifiers would have made it 0.2 gpm/sq. ft. over 2,535 sq. ft. You then apply 11.2.3.1.4 making it 0.2 gpm/sq. ft. over 3,000 sq. ft. This would result in a system demand of 600 gpm. Going up the curve certainly seems like a lot of work and potential questions from other parties for a savings of 21 gpm. As such, you can be conservative and apply the higher density of 0.2 gpm/sq. ft. over 3,000 sq. ft. ------------ Technical Update is prepared by the Technical Services Dept. of the AFSA: Roland Huggins, a PE registered in fire protection engineering, Vice President of Engineering and Technical Services; and Tom Wellen, a PE registered in fire protection engineering. This is provided with the understanding that the AFSA assumes no liability for this opinion or actions taken on it and they are not to be considered the official position of the NFPA or its technical committees. Copyright 2016, American Fire Sprinkler Association. All Rights Reserved. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.firesprinkler.org/private.cgi/sprinklernotes-firesprinkler.org/attachments/20161122/79c5b318/attachment.htm>
