In an earthquake, shaking makes sprinkler pipe sway relative to the building and strains hangers and pipe. Seismic bracing makes the piping move together with the structure it hangs from, so the system stays intact and can keep working after the event.
Why bracing matters
Seismic provisions aim to avoid significant damage to sprinkler systems and keep them functional. A study of 973 sprinklered buildings in the 1971 San Fernando earthquake found that if a sprinklered building fared well, so did its sprinkler system. Lessons from the 1971, 1989 (Loma Prieta) and 1994 (Northridge) California earthquakes shaped later NFPA 13 revisions; a lack of longitudinal bracing is cited as a common condition linked to leakage after Northridge.
Three kinds of brace
- Lateral braces resist movement perpendicular to the pipe run.
- Longitudinal braces resist movement parallel to the pipe run.
- Four-way braces combine both and are used at the top of risers, where there is no lateral/longitudinal distinction. With cable this means four cables spaced 90 degrees apart.
What gets braced
- Risers: four-way bracing.
- Feed and cross mains: lateral and longitudinal bracing, plus bracing at changes of direction.
- Branch lines 2.5 in. and larger: lateral bracing. Smaller branch lines are generally restrained rather than braced.
- Seismic separation assemblies also need bracing.
Spacing limits quoted by published summaries
- Lateral braces on mains: no more than 40 ft apart on center, with the last brace within 6 ft of the pipe end.
- Longitudinal braces on feed and cross mains: no more than 80 ft apart, with one within 40 ft of a pipe end or direction change.
- Riser four-way braces: within 3 ft of the riser top (with an exception for short risers), then no more than 25 ft apart.
Zone of influence and brace load
Each brace carries the load from the pipe in its zone of influence, which generally extends to the midpoint between adjacent braces. The seismic load is based on the weight of the water-filled pipe times a seismic coefficient; where the site value is not known, a default coefficient of 0.5 is used. A brace is sized as an assembly, so its weakest component governs, and its capacity falls as the angle approaches vertical.
Cable or rigid?
- Rigid braces are stiff steel members that resist both compression and tension. They must be cut to length and very long members can buckle.
- Cable braces resist in tension only, so two cables are needed to match one rigid brace. Cable can be cut to fit any space, and each cable must carry the full seismic load.
Braces must attach to structure that can take the extra seismic force. The whole assembly must be listed for its maximum load, and C-type clamps cannot be used to attach braces.
Flexibility and clearance
Bracing handles inertial sway. Differential movement between parts of a building is handled by flexible couplings, seismic separation assemblies and clearance. Pipes 1 to 3.5 in. need penetration holes 2 in. larger than the pipe; pipes 4 in. and larger need holes 4 in. larger. Clearance is not needed through frangible construction without a fire rating, but is required where the construction is a fire barrier.
Branch line restraint
Branch lines need less than full bracing. Accepted methods include listed sway brace assemblies, wrap-around U-hooks and No. 12 wire rated at 440 lb.
Related pages
See our seismic sway bracing and FM-approved seismic pipe hangers pages, the seismic bracing selector and the 3D brace demo.
Sources
- NFPA 13 seismic bracing requirements (QRFS)
- Seismic design for fire sprinklers, part 2 (QRFS)
- Seismic design of fire suppression piping systems (CSE)