Protection of sprinklers against
damage by earthquakes — Part 2

By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu

In last month’s column I attempted to discuss when seismic protection is required. Also, discussed was the basic approach to providing seismic protection, i.e., incorporating measures to both 1) protect the sprinkler system from displacement due to story drift and 2) keep the piping fairly rigid when supported by a wall, floor or roof that is expected to move as a unit.

The article concluded with describing methods used to protect from displacement due to story drift, including introducing flexibility with flexible couplings and seismic separation assemblies and by providing adequate clearance for pipe penetrations.

This article will deal with the measures needed to keep the rigid — sway bracing and restraint.

Seismic sway bracing is intended to secure the piping from moving in certain directions. The three categories of sway bracing used in NFPA 13 are lateral, longitudinal, and four-way. Lateral sway bracing is intended to prevent the piping from moving perpendicular to the piping run in the horizontal plane. Longitudinal sway bracing prevents movement back and forth along the axis of the pipe which is also in the horizontal plane. Four-way sway bracing in intended to prevent movement of the piping in a horizontal plane and up and down vertically.

Let’s start looking at sway bracing design with some of the basic criteria. First sway braces must not be installed in a manner where it will interfere with sprinkler spray discharge (you may recall this requirement from an earlier discussion on obstructions to sprinklers).

Lateral sway braces must be provided on all feed mains and cross mains and on branch lines with piping 21/2 inches in diameter or greater. For feed mains and cross mains the bracing must be provided regardless of pipe diameter.

Spacing of lateral bracing shall not exceed 40 feet on center. In practice you will find that much lower spacing is normally needed to bracing piping to reasonably meet load limits on the bracing and fasteners (more about loads next month).

A lateral brace must also be provided within the last six feet of piping and the last section of piping on a feed or cross main must have a sway brace. A lateral brace also may serve as a longitudinal brace for perpendicular piping. The 2010 edition of NFPA 13 clarified this allowance by requiring the brace to be within 2 feet of the pipe needing longitudinal bracing and the piping on which the brace is installed must be equal to or larger in diameter.

As noted last month, any flexible couplings installed on mains other than those required to meet seismic flexibility requirements must be provided with a lateral sway bracing with 24 inches of every other coupling but may not exceed the 40-feet spacing. To avoid this requirement rigid couplings must be used. Thus it is important to check in the field to ensure that the proper couplings are installed. It is not uncommon to see all couplings on the job are either all flexible or all rigid. Some manufacturers have tell-tale markings on their couplings so that inspectors can identify the type of coupling without requiring a close-up look.

Now as with many other requirements there are exceptions which allow us to avoid the requirement for lateral bracing. Piping attached with wrap around U-hooks or U-hooks that attached the piping directly to the underside of the structure. Another exception is for piping attached with rods that are less than 6 inches long. This distance must be measured from the point of fastening to the top of the pipe, not just the length of the rod.

Basic requirements for longitudinal sway bracing include installation on maximum 80-feet intervals with a longitudinal brace within 40 feet of the end of the pipe. This requirement applies to feed mains and cross mains. Similar to lateral bracing, a longitudinal brace may also serve as a lateral brace for perpendicular piping provided it is located within 2 feet of the pipe needing lateral. There is no requirement that the piping on which the brace is installed be equal to or larger in diameter, but I believe this is an oversight and will be corrected with the next edition of NFPA 13.

It is also noted that 4-way sway bracing also can be used to serve as both a lateral and longitudinal bracing for mains attached to the riser served by the brace. Four-way braces are normally provided for risers. They are required at the top of risers that are exceeding 3 feet. They are not required for riser nipples serving sprinklers. The braces are to be installed on risers at 25 feet intervals. Bracing may be attached to horizontal mains as long as they are installed within 24 inches of riser centerline and account for horizontal as well as vertical loads.

Four-sway braces are exempted from riser nipples and from risers penetrating floors with clearance through the floor not exceeding NFPA 13, paragraph 9.3.4 limits. (Question: How does one not exceed the requirements of 9.3.4 when 9.3.4 states minimum clearances?)

It almost goes without saying that piping connecting buildings expected to move differentially must not be bracing at the connection and that bracing may not be attached to seismic separation assemblies.
In addition to bracing there is another method of protection intended to rigidly connect the piping to the structure. This is the seismic restraint. A restraint as intended by NFPA 13 provides a lesser degree of protection for piping as it is not required to meet all the load requirements pertaining to bracing. Restraints are intended to be used to protect branch lines and sprinklers on branch lines. They are intended to prevent whipping of branch lines that can cause sprinklers or piping to impact with the structure.

Restraint is required at the end sprinkler on a branch line and at intervals as specified in tables contained in NFPA 13. Restraints are also required on sprig ups that are 4 feet or longer. Restraints may be omitted where hanger rods are less than 6 inches measured from the fastener connection to the top of the pipe. I often get the question about whether single sprinkler branch lines need restrict. The answer is maybe. If there is a possibility the sprinkler can impact the structure during an earthquake restraint is needed. Often people will argue that a line serving only one sprinkler is not a branch line. NFPA 13 is not clear on this item so I recommend one must do what the AHJ requires.

Restraints are also not required on sprinkler armovers or drops. One must be careful with very long drops to sprinklers. One intuitively can reason that long drops should be restrained, and some members of the NFPA 13 Technical Committee on Hanging and Bracing would agree. However, no one has figured out how to do this practically since what is there that the long drop can be restrained to. That is why NFPA 13 is currently indicating no requirement for drops.

In the “old days” many designers incorrectly provided earthquake protection by laying out sway bracing at maximum permitted intervals and meeting the other requirements described above and called it a day. Now, of course, we know better (or do we). Now we also must carefully consider the loads involved when sizing and spacing the bracing. This will be the topic of next month’s article.

 

Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer and past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates, Inc. He can be reached via email at SDannaway@ssdafire.com.