Seismic Bracing for Fire Fighting System
Protecting fire sprinkler systems from earthquakes is particularly crucial for several reasons. NFPA provides guidelines for the protection of fire sprinkler systems against earthquakes. NFPA 13 explains how seismic protections for fire sprinkler systems should be installed when they are needed. You can protect fire sprinkler systems from earthquakes by making certain components more rigid. This is accomplished through seismic bracing of fire sprinkler systems. As a building moves during an earthquake, non-structural components can experience strong inertial forces. In other words, they shake and sway violently, sustaining damage and potentially failing. The solution is rigidity. To prevent non-structural components from shaking and swaying relative to the building, they are firmly attached to structural components that are expected to move as a unit during an earthquake. For example, a pipe suspended from the ceiling might receive extra supports so that it always moves with the ceiling during an earthquake. This technique is known as seismic bracing, or sway bracing. NFPA 13 requires seismic bracing for fire sprinkler risers, main lines, and branch lines 2.5 inches or larger. Pipe smaller than these areas is generally more flexible and requires only vertical restraint rather than bracing.
The principles of Seismic Bracing
Seismic bracing resists horizontal motion. Braces are installed to resist both lateral (perpendicular to the pipe) and longitudinal (parallel to the pipe) swaying. Vertical motion is not usually a concern. Seismic braces, like pipe hangers, must be attached to the building structure so that fire sprinkler pipe and other non-structural components can move as a unit with the building. The structural members to which seismic braces are mounted must be able to withstand the anticipated seismic forces. The seismic load is the inertia force on the structure. Inertial Forces when the shaking of buildings causes pipe to shake relative to the building, stressing pipe hangers and the pipes themselves. All feed and cross mains, and branch lines 2.5” or greater in diameter must be braced against lateral (perpendicular to the pipe) and longitudinal (parallel to the pipe) motion.
Types of Seismic Bracing
There are two kinds of seismic braces: rigid braces and cable (tension-only) braces. Both rigid and cable braces counter swaying motion by strongly attaching pipe to structural members of the building.
Rigid Seismic Bracing
Rigid bracing is, as the name indicates, a stiff and unbending piece of equipment. These braces are almost always comprised of steel. The core advantage of this stiff bracing is that it resists motion in two directions (through compression and tension). The big disadvantage of rigid bracing goes along with its advantage: a rigid length of steel has to be cut to the proper length to run from the structural anchor point to its attachment at the pipes.
For easier installation, rigid brace systems often use hinged attachments. The attachments bolt into both the structure and the non-structural equipment. Pivot points where the attachments connect to the brace member allow them to be easily placed at the correct angle. Note that all bolts and screws used to mount rigid braces must be listed and approved for the purpose and seismic load.
Seismic Cable Bracing
Seismic cable bracing is made of steel cable that’s listed for use as seismic bracing. Since it’s somewhat flexible, it resists motion in only one direction, and it is sometimes referred to as “tension bracing” for this reason. This means that to do the same job as a stiff brace, two seismic cable braces are needed. But cable bracing has a huge advantage over rigid bracing: it’s basically unlimited in length, and it can be easily cut to fit any space.
The engineering behind proper Sway Brace Design
To properly install cable sway braces in your fire sprinkler system, you need to understand the following:
Seismic load and how it’s calculated
The way the seismic load is distributed through zones of influence (ZOI)
The maximum allowable loads of hardware in the system
Where sway braces must be placed.
Understanding Seismic Loads
Remember, seismic braces resist horizontal forces. Seismic braces guard against this by tightly securing pipe to structural members, protecting against lateral (perpendicular to the run of the pipe) and longitudinal (parallel to the run of the pipe) shaking. The fundamental job of seismic braces is to resist the seismic load a system will experience during an earthquake. Seismic load is a measure of force—the force of a fire sprinkler system shaking back and forth during an earthquake.
The specific variation of this formula for calculating seismic loads is found in section 18.104.22.168 of NFPA 13:
Fpw = Cp x Wp
Wp is the weight of the water-filled pipes multiplied by 1.15,
Cp is the seismic coefficient, and
Fpw is the horizontal force
Calculating weight for Seismic Loads (Wp)
The mass portion of this calculation— Wp—is easy to understand. It is the adjusted weight of the water-filled pipe. Don’t forget to include the weight of branch lines in your calculations.
When calculating the water-filled weight of pipe, NFPA 13 requires a slight correction to account for the weight of miscellaneous hardware attached to the pipe—sprinkler heads, valves, etc. To make the correction, simply multiply the water-filled weight by 1.15 (22.214.171.124). Again, the final value is represented as Wp.
Calculating acceleration for Seismic Loads (Cp)
NFPA 13 lists values for Cp, the seismic coefficient, which are used to model ground acceleration for seismic load calculations. One Cp value applies for your whole building—and the bigger Cp, the more severe an earthquake that you have to prepare for.
NFPA 13 requires that a value called short-period response parameter, or Ss, be used to determine Cp. Put as simply as possible, Ss represents the acceleration felt by a building during the peak ground acceleration of an earthquake. Don’t worry, you don’t have to calculate this value either.
Distributing Seismic Loads – Zones of Influence (ZOI)
To design seismic a seismic bracing system, we have to consider the zone of influence (ZOI) for each brace we install. A brace’s zone of influence is all of the pipe for which it is responsible during an earthquake. A brace’s zone of influence includes all branch lines and other pipe connected to the braced pipe.
As mentioned, the use of longitudinal bracing on branch lines allows them to be excluded from the ZOI of their main’s brace. After all, such bracing is in the same direction or dimension as the main’s bracing. And the zone of influence for longitudinal braces does not include branch lines.
Example for sprinkler system load calculations for each ZOI
Noting maximum allowable loads
Every component of a sway brace for fire sprinkler systems, including the brace, anchors, fittings, pipe, and structure, have maximum allowable loads. This is the maximum amount of force that any part of the brace can handle. If the seismic load exceeds the maximum allowable load for the brace, the brace is not suitable and could fail during an earthquake. And the weakest part of a brace determines the maximum allowable load.
Lateral Sway Bracing Design
The system piping shall be braced to resist both lateral and longitudinal horizontal seismic loads and to prevent vertical motion resulting from seismic loads.
Lateral sway bracing shall be provided on all feed and cross mains regardless of size and all branch lines and other piping with a diameter of 2-1/2 in (65 mm) and larger.
The spacing shall not exceed a maximum interval of 40 ft (12.2 m) on centre.
The distance between the last brace and the end of the pipe shall not exceed 6 ft (1.8 m).
The last length of pipe at the end of a feed or cross main shall be provided with a lateral brace.
Longitudinal Sway Bracing Design
Longitudinal sway bracing spaced at a maximum of 80 ft (24. 4m) on centre shall be provided for feed and cross mains.
The distance between the last brace and the end of the pipe shall not exceed 40 ft (12.2m).
Examples of Lateral and Longitudinal Brace Installations
Tops of riser exceeding 3 ft (1 m) in length shall be provided with a four-way brace.
When a four-way brace at the top of a riser is attached on the horizontal piping, it shall be within 24 in (610 mm) of the centreline of the riser and the loads for that brace shall include both the vertical and horizontal pipe.
The distance between four-way braces for risers shall not exceed 25 ft ( 7.6 m).
Four-way bracing shall not be required where risers penetrate intermediate floors in multi-storey buildings.
Sway Brace Installation
Sway bracing shall be tight
For individual braces, the slenderness ratio (l/r) shall not exceed 300, where l is the length of the brace and r is the least radius of gyration.
Where threaded pipe is used as part of sway brace assembly, it shall not be less than schedule 30.
All parts and fittings of a brace shall lie in a straight line to avoid eccentric loadings on fittings and fasteners.
For tension-only braces, two tension-only brace components opposing each other must be installed at each lateral or longitudinal brace location.
The seismic load determined shall not exceed the lesser of maximum allowable loads.
BRANCH LINE RESTRAINTS
A branch line restraint is different than a sway brace or a hanger when it comes to certification and testing. A branch line restraint must be Underwriters Laboratories (UL) listed to perform its function, but its function is not clearly defined by National Fire Protection Association (NFPA) 13. Per NFPA 13 (2013) section 126.96.36.199 states, “A restraint is considered a lesser degree of resisting loads than bracing.” This, in turn, has led to some confusion throughout the building and construction industry.
NFPA guidelines in relation to branch line restraints
NFPA 13 goes on to define exactly how to comply with this “lesser degree” requirement, by listing five standard installation practices (1-5 below) that are generally accepted. This leaves the door open for what is defined as “other approved means” (option 6 below).**
1) Listed Sway Brace Assembly
2) Wraparound U-Hook satisfying the requirements of 188.8.131.52.11
3) No. 12, 440 lb. (200 kg) wire installed at least 45 degrees from the vertical plane and anchored on both sides of the pipe.
4) CPVC hangers utilizing two points of attachment
5) Hanger not less than 45 degrees from vertical installed within 6 in. (152 mm) of the vertical hanger arranged for restraint against upward movement, provided it is utilized such that l/r does not exceed 400, where the rod shall extend to the pipe or have a surge clip installed.
6) Other approved means
**Source; NFPA 13 (2013) Sec 9.3.6.
Branch Line Restraints - Exceptions
We can avoid branch line restraint where the branch lines are supported by rods less than 6 inch (152 mm) long measured between the top of the pipe and the point of attachment to the building structure, the requirements of 184.108.40.206 through 220.127.116.11 shall not apply and the additional restraint shall not be required for the branch lines.
**Source; NFPA 13 (2010) Sec 18.104.22.168