Introduction to Seismic Protection for Sprinkler Systems
We rely on a sprinkler system within a building to protect both the building and occupants during a fire. After a building is subject to a seismic event, such as an earthquake, there is an increased possibility for a fire to occur. For a building that is in an area subject to seismic activity, it is crucial to provide seismic protection to ensure the sprinkler system installed remains capable of protecting the building and occupants after a seismic event.
What does Seismic Protection Include?
Chapter 18 of the 2019 edition of NFPA 13, Standard for the Installation of Sprinkler Systems, provides requirements for the general protection of fire sprinkler systems against damage resulting from seismic activity. The requirements are based on a comprehensive approach of providing not only bracing but also flexible connections, pipe clearance to structural members, pipe restraint, and restrictions on the types of hangers and fasteners used.
It is the intent of NFPA 13 for the sprinkler system to coordinate movement from a seismic event with the building structure. The sprinkler system and the building are to move as one unit, which traditionally is accomplished with sway braces connecting the system piping and the structure. Where the building is expected to move, the sprinkler system must be able to accommodate that motion. For example, where there is a seismic joint within a building that can create differential (or opposing) movement of the structure, the sprinkler system also must have a seismic joint so that the system is not damaged from the movement. Additionally, proper clearance around the piping needs to be provided to allow for movement, this includes when piping is run through walls and partitions.
Example of a Seismic Loop at a Seismic Joint. (Courtesy of Anvil International)
When is Seismic Protection Required?
Not all areas are subject to seismic activity or expect significant seismic forces if an event were to occur. As such, not all fire sprinkler systems require seismic bracing. Usually, the local or state building code will drive the need for seismic bracing. The authority having jurisdiction and the system designers and/or technicians should agree early in the project whether protection against seismic damage will be required.
A fire sprinkler system requiring seismic protection is held in place within a building with some combination of sway bracing, restraint hangers, and pipe stands. These components should be laid out in such a manner that all potential horizontal movement expected from an earthquake is controlled.
Successful bracing of fire protection systems involves determining the appropriate force factors, tentative placement of sway bracing, determining the loads to braces, and verifying the loads can be carried by the sway brace components.
Sway bracing is provided to prevent excessive movement of system piping. Shifting of large pipe because of an earthquake has led to the pull-out of hangers and the fracture of fittings. With some exceptions, seismic bracing is required at the following locations:
- Top of the system riser
- All feed and cross mains or other piping regardless of size
- Branch lines 2½ in. (65 mm) in diameter and larger (lateral bracing only)
NFPA 13 contains requirements for both lateral (perpendicular to the piping) and longitudinal (parallel to the piping) horizontal braces (shown below). The maximum spacing of lateral braces of 40 ft (12 m) is based on the strength of the piping as a beam under the uniform load of its expected horizontal “weight.” Longitudinal braces are required at a maximum spacing of 80 ft (24 m).
Lateral Sway Bracing (Drawings courtesy of AFCON)
Longitudinal Sway Bracing (Drawings courtesy of AFCON)
Lateral and longitudinal braces shown above are “two-way” braces, that is, they prevent piping from moving back and forth in a single direction. Four-way bracing, shown below, requires the simultaneous application of lateral and longitudinal bracing.
Two Typical Four-Way Sway Brace Assemblies (Drawings courtesy of AFCON)
As with system hangers, sway braces must be attached directly to the building structure. Fasteners and structural elements at the points of connection must be adequate to handle the expected loads.
Sway Bracing Calculations
Sway braces need to be designed to withstand forces in tension and compression of the system. To properly size and space the braces, it is necessary for the designer to complete a calculation (sample calculation form shown below). This calculation includes:
- Determining the seismic coefficient Cp
- Choosing the brace shape and size based on the system piping and the structural members that will support the braces
- Determining the total load tentatively applied to each brace by the water filled system piping in the “zone of influence” (see examples of load distribution to bracing below)
- Determining if the total expected loads are less than the maximums permitted
- Checking to see if the fasteners connecting the braces to the structural members are adequate to support the expected loads.
Sample Seismic Bracing Calculations (NFPA 13)
Examples of Load Distribution to Bracing (NFPA 13)
A sprinkler system installed in a building that is subject to seismic activity may require seismic bracing by the local or state building code . If required, Chapter 18 of NFPA 13 outlines requirements for the seismic bracing, which includes bracing, flexible connections, pipe clearance to structural members, pipe restraint, and restrictions on the types of hangers and fasteners used. The design of required seismic bracing must also include calculations based on the specific piping to be installed in the system. I hope this blog served as a good introduction into seismic bracing, are you interested in seeing a deeper dive into the specific calculations? Have you completed these calculations for a project of your own? Let us know in the comments below. IF you would like to learn more about seismic protection, take a look at the commentary and supplemental information found in chapter 18 of the NFPA 13 Handbook.
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