August 2008: Earthquake Bracing Requirements for FM Global Data Sheet 2-8

This Data Sheet provides a detailed description of the components necessary to guard against earthquakes for a water-based fire protection system. The fire protection system can include sprinkler systems, water spray systems, foam-water systems, fire pumps, standpipes, water storage tanks and reservoirs. FM Global recommends earthquake protection in those areas that are in the 50-year to 500-year zones per sister Data Sheet 1-2, Earthquakes.

These components are:

· Bracing to minimize differential movement when needed

· Flexibility to allow differential movement where designed

· Clearance to minimize impact damage

· Anchorage to minimize sliding and/or overturning

· Hangers to support the system

· Pipe and pipe joining materials to minimize pipe breaks

This month we will cover bracing.

Bracing covers include sprinkler systems, water spray systems, foam-water systems, standpipes, and piping associated with fire pumps, tanks and reservoirs. The purpose of bracing is to minimize differential movement while requiring, in effect, the piping to move at the same time and in the same direction as the nearby building structure. Though the brace is designed to force a horizontal (aka lateral) load, a net vertical uplift load must also be taken into account.

In general, bracing is needed as noted:

A four-way brace is needed within 2 ft. of the top of all risers, regardless of riser length. Intermediate four-way bracing is needed at maximum 40 ft. intervals. If more flexible couplings are needed than required, then a four-way brace is necessary within 2 ft. of every other coupling. If a riser manifold is more than 6 ft. long, a lateral brace is needed within 2 ft. of the end of the manifold. Preferably a set of braces should be provided for each individual riser, though a set of braces for two risers is allowed.

 

Vertical mains 6 ft. or longer need two, four-way braces, each within 2 ft. of the ends of the main. Intermediate braces are needed at maximum 40 ft. spacing. Similar to risers, every other additional flexible coupling needs a brace within 2 ft. of it.

 

Lateral and longitudinal braces are needed within 2 ft. of the horizontal change in direction of a horizontal main, where that main is 6 ft. or longer.

 

For dead-end mains, a lateral brace and a longitudinal brace are needed within 6 ft. and 40 ft. of the end, respectively. Seismic separation assemblies, although connecting two pieces of pipe hydraulically, for the purposes of bracing are two separate dead-in mains, and bracing is needed on both sides of the assembly.

 

Flexible couplings are sometimes added to mains and larger branch lines above what is needed. In those cases a lateral brace is needed within 2 ft. of every other coupling along a main and within 2 ft. of a coupling involving a horizontal change in direction.

 

After the installation of the above bracing, lateral and longitudinal bracing is needed on horizontal mains at a maximum 40 ft. and 80 ft. spacing, respectively.

 

Lateral bracing is needed on branch lines 2-½ in. or greater in diameter and 20 ft. or longer. Brace spacing is a maximum 40 ft. with at least one brace within 6 ft. of the branch line end. The first brace should be from 20 to 40 ft. away from the main. In lieu of lateral bracing, wraparound U-hooks hangers, and ring and rod hangers may be used if they meet certain criteria.

 

Longitudinal bracing is needed on branch lines that are 2-½ in. or greater in diameter and 25 ft. or longer. Brace spacing is a maximum 80 ft. with at least one brace within 40 ft. of the branch line end. The first brace should be from 25 to 50 ft. away from the main.

For the design lateral force, FM Global has chosen to use 0.5G, where G is the weight of the water-filled pipe, whether the pipe is normally filled with water or not. Provisions allow for a force of greater than 0.5G if required by the local authority having jurisdiction. The amount of load each brace receives is based upon its zone of influence; that is, the portion of the entire piping system that will be applied to that particular brace. It is based upon the location relative to adjoining braces and the type of brace (lateral, longitudinal or four-way).

Resistance to the net vertical uplift load can be overcome by a single brace set at a minimum angle, the use of a vertical compression strut with the angled brace, opposing diagonal braces, or opposing tension-only braces with a vertical compression strut. The angle of the single brace is dependent upon the design lateral force; for 0.5G the minimum angle is 45° measured from the vertical.

Selection of the brace is dependent upon its shape (rod, piping, angles or flats), the distance from the pipe to the structural member (brace length), the angle of the brace, where the brace is attached to structural member (underside of the member, side-perpendicular and side-parallel) and the strength of the brace attachments. Braces (including vertical struts) that act in compression and tension must have a slenderness ratio of 200 or less; braces that act in tension only must have slenderness ratio of 300 or less.

Consideration must be made for the material of the structural member:

· Wood members should be a minimum 4 in. wide (nominally) and preferably use through bolts as attachments. Lag bolts may be used but require a series of pilot holes. The bolt hole should be at least 4 diameters from the edge of the member. Washers or other metal piece should separate the bolt (and nut for through bolts) from the member.

· Brace attachments to structural steel members should avoid powder-driven studs and C-clamps. Welded studs should follow AWS D1.1. An analysis of the member by a structural engineer is needed where the member is a C- or Z-purlin, truss or joist.

· For concrete members, expansion anchors should be used instead of powder-driven studs.

In all cases, the brace should be tightly connected to its attachments, and the attachments tightly connected to the structural member.

One needs to compare the loads that are created by the zone of influence of the braces against the loads that the braces, their attachments and possibly the structural members can handle. The spacing of the braces might have to be decreased from the 40 ft. and 80 ft. so that the loads on the braces are decreased.

Risk Logic can help in determining the potential earthquake hazard that may exist to a building and its operations.