FM Global DS 2-8: Earthquake Protection for Sprinkler Systems

In a major earthquake, a logistics warehouse loses its sprinkler system before the shaking even stops: the main cracks where it crosses a structural movement joint with no seismic separation, and water floods the floor for hours. Had there been a fire, the system would have been unable to do anything. FM Global DS 2-8 sets out how sprinkler pipework is protected against seismic movement: sway brace type and spacing, flexible couplings and movement joint crossings.

Seismic Design Category

FM Global classifies the structure by seismic design category, applied in parallel with national seismic codes:

• SDC A–B: low seismicity — minimum bracing sufficient
• SDC C: moderate seismicity — full lateral and longitudinal bracing
• SDC D–F: high seismicity — the full package: sway bracing, flexible connections and separation joints

Across most of the Turkish seismic zones, and particularly along the Marmara corridor and the Aegean coast, the higher categories apply in practice.

Sway Bracing

• Lateral brace: perpendicular to the pipe in the horizontal plane. Required at intervals of up to about 12.2 m on mains and 14 m on branch lines, for DN65 and larger.
• Longitudinal brace: along the pipe axis, at intervals of up to about 24.4 m on mains.
• Four-way brace: lateral and longitudinal combined at critical points — riser to main connections, tees and branch points.
• Size threshold: pipe of DN50 and below does not require sway bracing, although hangers must still be of seismic type.
• Brace angle: between 30 and 60 degrees to the horizontal; outside that range the load distribution is inadequate.

Flexible Connections at Movement Points

Structural movement joints, separations between buildings and points subject to differential settlement all require flexible connections:

• Seismic separation: pipework crossing a structural movement joint needs an expansion loop or flexible coupling
• Riser movement: risers hung from double supports with flexible couplings top and bottom
• Pump connections: flexible connections on pump suction and discharge, coordinated with DS 3-7
• Buried tank: a flexible coupling between tank and pump absorbs differential settlement

FM Approved Components

• Sway brace components — tube, fittings and fasteners — as a single FM Approved package
• Anchors: FM Approved expansion anchors or through-bolts for the structural connection
• Pipe attachments: U-bolts or proprietary clamps, FM Approved
• Cable bracing: permitted in some cases, but only as an FM Approved kit

A Recurring and Expensive Omission

A pattern seen repeatedly: a sprinkler system is installed to NFPA 13 and EN 12845, but the seismic detailing is left out under schedule or budget pressure. When a significant earthquake occurs, structural movement cracks the main at a large diameter connection, production stops for days or weeks, and the insurer declines the claim on the basis of non-compliance with the applicable seismic provisions. Retrofitting the bracing afterwards costs several times what it would have cost to install during construction — and that is before the business interruption.

Quick Checklist

• Seismic design category established for the structure
• Lateral bracing within about 12 m and longitudinal within about 24 m for DN65 and larger
• Four-way bracing at risers, tees and branch points
• Brace angles between 30 and 60 degrees
• Flexible couplings at every structural movement joint
• Flexible connections on pump suction and discharge
• All bracing and anchors FM Approved
• The full package applied in high seismic zones

Frequently Asked Questions

Why does sprinkler pipework need seismic bracing at all?

Because pipework is heavy, suspended and relatively rigid, while the building moves in an earthquake. Without bracing the pipe swings, impacts structure and equipment, and fails at its most rigid points. The result is a system that floods the building and is unavailable for the fire that often follows an earthquake.

What is the difference between lateral and longitudinal bracing?

Lateral bracing resists movement perpendicular to the pipe and is required at closer intervals, around 12 m on mains. Longitudinal bracing resists movement along the pipe axis and is required at wider intervals, around 24 m. Critical points such as riser connections and tees need a four-way brace providing both.

Why do flexible couplings matter at movement joints?

A structural movement joint exists precisely because the two sides of the building will move relative to each other. Rigid pipe crossing that joint will be sheared. A flexible coupling or expansion loop absorbs the differential movement, and its absence is one of the most common causes of post-earthquake sprinkler failure.

Can seismic bracing be added after installation?

It can, but at several times the cost of installing it during construction, because access is obstructed by the completed building services and the work has to be done around an operating facility. It is also a common reason for an insurance claim to be declined, so the economic case for doing it upfront is overwhelming.

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