Protection of sprinklers against
damage by earthquakes — Part 1
By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu
The last two editions, 2007 and 2010 of NFPA 13 contain significant changes to the way fire sprinkler systems are designed to provide protection from seismic events. The changes were brought about to bring the provisions of NFPA 13 into alignment with ASCE/SEI 7-05 Minimum Design Loads for Buildings and Other Structures, specifically ASCE/SEI 7-05 Chapter 13 Seismic Design Requirements for Nonstructural Components. As a result, sprinkler system earthquake protection that is in compliance with the 2007 or later editions of NFPA 13 is equivalent to compliance with ASCE/SEI 7-05 Chapter 13 and hence in compliance with IBC Chapter 16.
When is earthquake protection required? The answer is not found in NFPA 13. As with most installation standards, NFPA 13 does not tell us when something is required, it tells us how to design and install it when it is required by others.
For the most part in the US those “others” will be jurisdictions enforcing the International Building. We will use the 2009 edition of the IBC and 2010 edition of NFPA 13 in this discussion. The IBC permits seismic protection to comply with ASCE/SEI 7-05 Minimum Design Loads for Buildings and Other Structures. A discussion as to how to determine when earthquake protection is required can get quite complicated. At the risk of oversimplification, earthquake protection for sprinkler systems is required for Seismic Design Categories C, D, E and F. It is noted that mechanical systems with a component importance factor of 1.0 do not require protection. However, ASCE/SEI paragraph 13.1.3 assigns life safety systems, including fire sprinkler systems, a component importance factor of 1.5. Therefore, sprinkler systems in SDC C must be protected. To determine the Seismic Design Category one needs to go through the following steps:
1) Determine the building Occupancy Category from IBC Table 1604.5. There are four categories I, II, IIII, and IV based on increasing life safety importance. Category IV includes essential facilities, such as, hospitals with surgery or emergency room functions, fire stations, air traffic control towers, critical national defense facilities and power generation and utilities required to support essential facilities.
Category III occupancies include buildings with large occupant loads such as large schools and public assembly occupancies, hospitals and utilities that are not Category IV. Category I facilities are those with a low hazard to life, and Category II facilities are those that are not I, III or IV.
2) Determine the Spectral Response Accelerations for short period (Ss), and long period (S1). This can be done using seismic maps published in the IBC. As an alternative it can be calculated using maps or software that can be downloaded from the Java Ground Motion Parameter Calculator found on the U.S. Geological Survey website at: http://earthquake.usgs.gov/hazards/designmaps/
javacalc.php.
3) Determine the Site Class building location. The classes are defined in IBC Table 1613.5.2. This information may be available from the project soils engineer or structural engineer. There are 6 classes A to F with A having the best soil condition geotechnically and F being the worst. If it is not known what site class applies and the building official approves then Site Class D may be used.
4) Determine Site Coefficient Fa from IBC Table 1613.5.3(1), which is based on the Mapped Spectral Response Acceleration at Short Period (Ss) and Site Coefficient Fv from IBC Table 1613.5.3(2) which is based on the Mapped Spectral Response Acceleration at One-Second Period (S1). These coefficients can also be calculated using the software from USGS.
5) a) From the SS and S1 values determine the SMS and SM1 using IBC equations 16-36 and 16-37.
b) From the SMS and SM1 values determine the SDS and SD1 using IBC equations 16-38 and 16-39.
6) Determine the Seismic Design Categories from Table 1613.5.6(1) Seismic Design Category Based On Short-Period Response Accelerations and Table 1613.5.6(2) Seismic Design Category Based On 1-Second Period Response Acceleration. Apply the most severe category.
On second thought maybe you should just ask a structural engineer for the SDC. If your head is not hurting yet let’s talk about how to protect sprinkler systems when earthquake protection is required. The approach to protect sprinkler systems from seismic events involves both 1) measures to protect the sprinkler system from displacement due to story drift 2) measures to keep the piping fairly rigid when supported by a wall, floor or roof that is expected to move as a unit.
Story drift is the maximum lateral displacement expected between two floors. This is accommodated by providing flexibility in piping near points where the building components are expected to move differently, such as at wall/floor connections and by providing adequate clearances for piping penetrating floors and walls.
NFPA 13 paragraph 9.3.2.3 provides the following requirements for flexible couplings:
“9.3.2.3 Systems having more flexible couplings than required by this section shall be provided with additional sway bracing as required in 9.3.5.3.8. The flexible couplings shall be installed as follows:
(1) Within 24 in. (610 mm) of the top and bottom of all risers, unless the following provisions are met:
(a) In risers less than 3 ft (0.9 m) in length, flexible couplings are permitted to be omitted.
(b) In risers 3 ft to 7 ft (0.9 m to 2.1 m) in length, one flexible coupling is adequate.
(2) Within 12 in. (305 mm) above and within 24 in. (610 mm) below the floor in multistory buildings. When the flexible coupling below the floor is above the tie-in main to the main supplying that floor, a flexible coupling shall be provided in accordance with one of the following:
(a) On the horizontal portion within 24 in. (610 mm) of the tie-in where the tie-in is horizontal
(b) On the vertical portion of the tie-in where the tie-in incorporates a riser
(3) On both sides of concrete or masonry walls within 1 ft (305 mm) of the wall surface, unless clearance is provided in accordance with 9.3.4
(4) Within 24 in. (610 mm) of building expansion joints
(5) Within 24 in. (610 mm) of the top of drops exceeding 15 ft (4.6 m) in length to portions of systems supplying more than one sprinkler, regardless of pipe size
(6) Within 24 in. (610 mm) above and 24 in. (610 mm) below any intermediate points of support for a riser or other vertical pipe.”
There are also flexible coupling requirements for drops to hose lines, rack sprinklers and mezzanines.
Note that in the first statement of 9.3.2.3 that any flexible couplings provided in the system in addition to these requirements must be provided with additional sway bracing. For this reason where other couplings are used they should listed or approved rigid couplings to avoid the requirement for additional sway bracing.
The general requirements for piping clearances is that for pipes less than 4-inches in diameter the clearance opening must have a diameter at least 2-inches larger than the pipe and for pipes 4-inches and larges the diameter shall be 4-inces larger than the pipe. There are similar clearance requirements for pipe sleeved penetrations, in which case the pipe sleeve must have a nominal diameter of 2-inches greater that the pipes less than 4-inches, and a nominal diameter of 4-inches greater than pipe 4-inches or large in diameter.
No clearance is required for piping passing through gypsum wallboard partitions and similar materials that are not required to have a fire resistance rating.
Experience shows that these large clearances are a bit onerous to contractors. An alternative would be to provide flexible couplings within one feet of each side of a wall penetration per 9.3.2.3(3), Then only normal pipe clearances are required.
Also, piping must be at least 2 inches away from structural components not supporting the pipe.
A third method of providing system flexibility is through the use of seismic separation assemblies. These assemblies are required where piping cross building seismic separation joints above the ground level. Rather than use the assemblies made from six elbows connected with flexible couplings as depicted in Figure A-9.3.3(a) of NFPA 13, you may wish to use some of the listed or approved assemblies using looped steel braided pipe to accomplish the needed flexibility. Do not confuse the seismic separation joint with a building expansion joint. Again, ask the structural what type of joint it is. Also, these devices require a great deal of space so be sure to plan ahead.
In the next article we will discuss sway bracing requirements.
Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer and past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates, Inc. He can be reached via email at SDannaway@ssdafire.com.