It is important to maintain sprinkler systems in service for fire protection following a major earthquake. Present standards for sprinkler system earthquake resistance are mostly satisfactory; however, impairment to sprinkler systems will result if the ground motion causes partial building collapse or breakage of underground water mains.
It is advisable to allow piping to accommodate itself to building movement and to its own inertia forces with a minimum of stress; however, a certain amount of damping is desirable. This can be achieved by flexible couplings and lateral braces on the feed mains. The branch lines can then be left free to adjust themselves to the racking effects of building movement. A certain degree of flexibility is also desirable, especially on bulk mains where flexible couplings are provided. Since building walls will drift inward and outward while oscillating, flexible couplings at the base and top of sprinkler risers are needed to accommodate this deflection. Flexible couplings at the top and bottom of each floor in multistory buildings, and also where piping passes horizontally through walls of separate buildings, will generally prevent damage to the piping. Clearance where risers and feed mains pass through floors, walls, and foundations are also needed. Loose fitting sleeves fitted with asphalt mastic can be provided around the pipes. Riser fittings, such as drain pipes and fire department pumper connections, may fail if they are solidly cast into a wall.
The top of the sprinkler riser is provided with four-way bracing to accommodate wall deflection in any direction. Bulk piping, if provided with U or other type braces, will not move in a direction perpendicular to the pipe. Each feed and cross main normally has at least one brace arranged to prevent longitudinal movement. Such braces are usually provided at the junctions of feed and cross mains.
It is well known that fires following an earthquake can cause major loss, sometimes exceeding that of the earthquake. Large fires are likely following a major earthquake because of impairment of water supplies from breaks in mains; loss of power to operate fire pumps; electrical arcing; gas main ruptures; flammable liquid spills; damaged process equipment; hampered manual firefighting, etc. A good example of this is the 1906 San Francisco Earthquake in which the greater part of the city was consumed by fire. Severe fires following an earthquake also have occurred in Japan.
Earthquake related strains are imparted to a fire protection system through the building or the ground to which it is attached, or through the inertial movement within the system itself. Uncontrolled differential movement can cause damage when fire protection systems are not provided in a systematic manner with the necessary features that incorporate sway bracing, flexibility, clearances and anchorage where needed. The most common type of damage, based on past experience, is water damage due to water leakage from broken overhead sprinkler piping or sprinklers, primarily due to lack of sway bracing where needed.
Common sources of water damage were broken or separated overhead sprinkler piping, broken sprinklers due to impact with nearby structural members or other equipment, broken sprinklers or pipe drops due to excessive differential movement between unbraced suspended ceilings and the pipe drops, and broken in-rack sprinkler system piping or sprinklers due to excessive rack movement. In addition to damage from water leakage, fire protection systems are often impaired due to direct damage to the systems, or due to damage to public water supplies or utilities needed for fire protection. Significant impairments to fire protection systems may expose a facility to a severe fire loss following an earthquake.
Changes to NFPA 13 on Protection of Piping against Earthquake Damage
In the past, the location, spacing strength, etc. of the protection systems (bracing, hangers, flexible couplings and clearances around the system) had to meet pre-described criteria in the standard. Alternative methods to the standards can be used with the provision that the seismic analysis be certified by a registered professional engineer and the method will perform at least as least as well as the building structure when subjected to the same earthquake forces. This change should not affect the majority of buildings that will be built in earthquake-prone areas; it will be more cost effective to follow the prescribed sections of the standard than to conduct a detailed analysis.
Previously piping hung from structural members were allowed to snug up to the bottom of the member providing a tight fit. Changes to the standard now require a minimum 2 in. clearance between the near face of the pipe and the near face of the structural member. Clearance is no longer needed around piping if it passes perpendicular through studs or joists that are part of a wall or floor/ceiling assembly.
The use of nonmetallic piping is increasing in noncombustible concealed spaces. In the past, if this type of piping did not have the prescribed clearance between the pipe and a floor, wall, platform or foundation that it passed through, flexible couplings would be needed on either side of the penetration. Now if this piping has an equal or better flexibility than the couplings both the couplings and the clearance around the pipe can be eliminated.
Risers now exceeding 3 ft. in length need to have four-way bracing. If a riser exceeds 25 ft. in length, then four-way bracing is needed at a maximum 25 ft. spacing.
Previously, the horizontal loads on the braces were based upon one-half of the weight of the water-filled piping. Now it is based upon 115% of one-half the weight of the water-filled piping. Provisions remain in place for the local authority having jurisdiction (AHJ) to use the 115% factor.
The maximum horizontal loads for sway braces (commonly known as the Load Tables) have become more conservative.
Branch lines now require restraint (a lesser degree of resisting loads than bracing) if they could be damaged through impact against the building structure, equipment, or finish materials, caused by the upward or lateral movement of the piping. The restraint, previously described in past versions of the standard, needs to be installed at a maximum 30 ft. spacing.
Powder-driven studs can now be used to attach hangers to the building structure if the design horizontal loads are at least one-half the weight of the water filled pipe. These studs must be specifically listed for use in areas subject to earthquakes.
Retroactivity of these changes on existing sprinkler systems will vary with the local AHJ. The local AHJ may also have requirements that are also more restrictive than NFPA 13 for new sprinkler systems.