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 breaks where it crosses a building seismic joint with no seismic separation assembly, 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 (Earthquake Protection for Water-Based Fire Protection Systems, October 2017, Interim Revision April 2025) sets out how sprinkler pipework is protected against seismic movement: sway brace locations and sizing, flexible couplings, clearances and seismic joint crossings.

DS 2-8 applies in FM 50-year through 500-year earthquake zones as defined in DS 1-2, Earthquakes (Section 1.0). Its recommendations serve seven goals: bracing, flexibility, clearance, hangers, connections, anchorage and design verification (2.1).

Design Load: the G Factor (2.2.1.2)

• The horizontal seismic load at each brace location is H = G · Wp, where Wp is the weight of water-filled piping within the brace’s zone of influence (2.2.1.2.1, 2.2.1.2.3).
• G is at least 0.754 · S_DS on an allowable stress design (ASD) basis. Where S_DS is not available, the generic values are G = 1.0 in FM 50-year zones, 0.7 in 100-year zones and 0.4 in 250- and 500-year zones; a higher value applies if the local building code requires it (2.2.1.2.2, p. 9).
• FM classifies sites by the FM earthquake zones of DS 1-2, not by the ASCE 7 seismic design categories. The mapping of national seismic zones to FM zones is in DS 1-2, which is not among the sources used for this article.

Where Braces Go (2.2.1.1)

• Risers: regardless of size, a four-way brace within 0.6 m of the top of each riser; in multistorey buildings, a four-way brace within 0.6 m of each floor level; intermediate braces at no more than 7.6 m (2.2.1.1.2).
• Vertical mains: four-way braces at top and bottom where 1.8 m or longer (2.2.1.1.3).
• Horizontal feed and cross mains: regardless of size; lateral and longitudinal braces within 0.6 m of changes of direction with runs of 1.8 m or more; at ends, lateral within 1.8 m and longitudinal within 12.2 m; on straight runs, lateral at no more than 12.2 m and longitudinal at no more than 24.4 m (2.2.1.1.4, p. 6). U-hangers and short-rod hangers do not replace bracing on mains.
• Branch lines: lateral bracing on DN65 and larger branch lines longer than 6.1 m; the first lateral brace 3.1–12.2 m from the cross main, the last within 1.8 m of a dead end, and no more than 12.2 m apart on straight runs. Branch lines under DN100 hung on rods shorter than 150 mm with no more than 13 mm between pipe and rod need no lateral bracing. Longitudinal bracing on DN65 and larger branch lines longer than 12.2 m, at no more than 24.4 m (2.2.1.1.5, p. 7).
• Small branch lines: dead-end branch lines under DN65 get a short hanger or a lateral restraint sized for 150 lb (0.67 kN) within 1.8 m of the end (2.2.1.1.6).

How Braces Are Built (2.2.1.3)

• Angle: diagonal members on horizontal pipe at least 30° from vertical, preferably at least 45°; on risers the four-way brace members at 90° to each other and 45° to the axes, or otherwise 30–60°; with two diagonal braces the angles differ by no more than 15° (2.2.1.3.5.2).
• Slenderness: l/r ≤ 200 for tension-compression braces and ≤ 300 for tension-only braces, which rules out cable bracing (2.2.1.3.5.3–2.2.1.3.5.4).
• Uplift: with a single diagonal brace, if VF = H / tan Θ − ½ Wp is greater than zero, increase the angle or add a vertical member (2.2.1.3.5.6).
• Attachments: FM Approved components or positive mechanical connections such as U-bolts, visually verifiable (2.2.1.3.6.2). In concrete, no powder-driven fasteners; anchors listed for seismic use with edge distance at least 12 bolt diameters and embedment at least 6 bolt diameters. In steel, at least 2.65 mm thickness, and no C-clamps or powder-driven fasteners.
• Hangers (2.2.1.8): in addition to DS 2-0, no powder-driven fasteners; C-clamp hangers with retaining straps; every other hanger on gridded branch lines and the last hanger on dead-end branch lines of a type that prevents upward movement.

Flexibility (2.2.1.4)

• Flexible couplings within 0.6 m of the top and bottom of each riser connected to underground piping; the top one may be omitted on systems welded from the riser through the cross mains (2.2.1.4.3.1).
• In multistorey buildings, a flexible coupling within 0.3 m of each floor; where there is no clearance through the slab, both above and below the floor (2.2.1.4.3.3).
• A seismic separation assembly, FM Approved flexible pipe loop or equivalent wherever piping crosses a building seismic joint or spans between independent structures, accommodating movement in all three directions (2.2.1.4.8, p. 23).
• A flexible coupling within 0.6 m of the connection for drops longer than 0.6 m that supply more than one sprinkler (2.2.1.4.6.1).
• More flexible couplings than recommended require additional lateral bracing (2.2.1.4.2); other joints are welded or rigid, and two-piece plain-end couplings without torque indication are not used (2.2.1.7).

Clearance and Impact (2.2.1.5–2.2.1.6)

• Holes or sleeves through walls and floors 50 mm larger than the pipe for DN25–90 and 100 mm larger for DN100 and above, sealed with mastic or weak mortar, or mineral wool with a pipe collar in fire separations (2.2.1.5.1).
• Sprinklers in suspended ceilings free to move at least 13 mm (preferably 25 mm) in all directions; other sprinklers at least 50 mm clear of structural and non-structural elements (2.2.1.5.3).
• Suspended ceilings with sprinklers, ductwork, conveyors and cable trays that could strike the system are restrained, and rack anchorage is verified where in-rack sprinklers are installed (2.2.1.6).

Pumps and Tanks (2.2.5–2.2.6)

• Riser piping from the pump is four-way braced and horizontal piping two-way braced. Where the pump house rests on the ground and pipes pass through the floor without clearance, flexible couplings are unnecessary; where the pump is above grade in a building, suction and discharge piping get flexibility as for sprinkler piping. Fuel lines are flexible, and the pump, driver, controller, fuel tank and batteries are anchored (2.2.5, p. 26).
• Ground-supported steel tanks are FM Approved for a seismic zone at least as severe as the site and anchored per the Approval report. On anchored tanks, a horizontal discharge pipe to a pump has two flexible couplings, one close to the tank wall and one within 0.6 m of the pump; unanchored tanks need connections accommodating 50 mm horizontal and 100 mm upward movement (2.2.6.1).

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

• FM earthquake zone (DS 1-2) and G factor established
• Four-way braces at riser tops and at each floor
• Mains: lateral braces ≤ 12.2 m, longitudinal ≤ 24.4 m
• Lateral bracing on DN65 and larger branch lines; end restraint on small dead-end lines
• Brace angles at least 30° from vertical; slenderness limits met
• Flexible couplings at riser ends and floors; seismic separation assemblies at seismic joints
• Clearance at wall and floor penetrations and around sprinklers
• Pumps, tanks and equipment anchored

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. DS 2-8 aims to keep the fire protection system working after the earthquake and to minimise water damage from leakage (2.1).

Do mains smaller than DN65 need bracing?

Yes. FM DS 2-8 2.2.1.1.4 braces feed and cross mains regardless of size, and risers are braced regardless of size as well. The DN65 threshold applies only to branch lines (2.2.1.1.5.1); smaller dead-end branch lines get a short hanger or lateral restraint within 1.8 m of the end (2.2.1.1.6).

What brace angle is required?

Under DS 2-8 2.2.1.3.5.2, diagonal members on horizontal pipe are at least 30° from vertical and preferably at least 45°. The flatter the angle, the larger the uplift VF = H / tan Θ − ½ Wp on a single diagonal brace; if it is positive, increase the angle or add a vertical member (2.2.1.3.5.6).

What is the difference between a flexible coupling and a seismic separation assembly?

A flexible coupling allows limited angular and axial movement between pipe sections; DS 2-8 calls for them at riser ends, at floor penetrations and on drops (2.2.1.4). A seismic separation assembly or FM Approved flexible pipe loop accommodates large movement in all three directions where piping crosses a building seismic joint or spans between independent structures (2.2.1.4.8).

How is the brace design load calculated?

Under DS 2-8 2.2.1.2, the horizontal design load is H = G · Wp, with Wp the water-filled pipe weight in the brace's zone of influence. G is at least 0.754 · SDS; without a site value, use 1.0 in FM 50-year zones, 0.7 in 100-year zones and 0.4 in 250- and 500-year zones, or a higher value if the local code requires it.

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