Put Out the Design Fire Before it Starts


Put Out the Design Fire Before it Starts

Getting information up front on sprinkler systems can ease the design process.

Automatic sprinkler systems installed in commercial, multifamily, and residential construction provide improved fire protection to the building, its contents, and its occupants. Trusses are often used in the roofs and floors of many of these buildings and the openness provided by the webs makes trusses especially compatible with mechanical, electrical, and plumbing (MEP) and sprinkler systems. To ensure trusses are designed effectively and efficiently for use with sprinkler systems, there are three things that need to be provided to the truss designer by the building owner, building designer (i.e., architect, engineer), or general contractor prior to design. Having this information up front also reduces the number of requests for information (RFIs) down the line. Here are the three items:

1) The construction documents prepared by the building designer should clearly indicate if an automatic sprinkler system will be installed and which standard will be followed for its design and installation.

Sprinkler systems designed and installed in accordance with NFPA 13, Standard for the Installation of Sprinkler Systems and NFPA 13R, Standard for the Installation of Sprinkler Systems in Low-Rise Residential Occupancies, are stand-alone systems that introduce additional weight to the roof and/or floor assemblies. This additional weight must be incorporated into the design dead load for the top and/or bottom chord of the trusses.

Additionally, systems conforming with NFPA 13 and 13R require a 250-pound concentrated live load to be applied to the supporting structural member at each point of attachment. The 250-pound load provision is intended to accommodate the weight of sprinkler installation personnel hanging from the piping over a short time duration (i.e., enough time to regain balance). The 250-pound load is applied to any single fire sprinkler support point. Given the loading type is a short term, individual man load, the load is applied to only one location at a time and NOT simultaneously to all support points. In addition, this 250-pound installation load need only be checked as an isolated load application, NOT concurrent with other short duration loads such as floor live, roof live, snow, or wind loads. If multiple sprinkler lines are attached to the same truss, the 250-pound installation load only needs to be applied to each individual location at a time. Truss design software typically allows the truss designer to input the installation load at a specific location (if the attachment locations are provided) or a moving concentrated load applied along the truss at each panel point and at the mid span between each panel point. The objective is to check the truss to see if any of the short-term concentrated man loads control the design or change the required lumber grades/truss plate sizes.

Sprinkler systems designed and installed in accordance with NFPA 13D, Standard for the Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes, do not typically add more weight to the assembly because they use the domestic water supply and delivery system. The sprinkler pipe sizes generally do not exceed 1-1/4 inches. Therefore, the dead load, live load, and resulting forces generated by the water are considerably lower than those generated by an NFPA 13 or NFPA 13R system. Also, since the piping generally is not large enough for someone who has lost their balance to hang on to, the 250-pound sprinkler installation load is not required for NFPA 13D systems.

As a result, Section 7.4.2 of NFPA 13D allows the sprinkler piping to be supported from the structural members using support methods prescribed by the applicable plumbing code. (See digital edition for more information.)

2) The construction documents prepared by the building designer should indicate the weight of the sprinkler system and how it needs to be distributed to the trusses. 

Section of ANSI/TPI 1 lists the minimum required information that must be included in the construction documents prepared by the Building Designer. Item (d) states:

“The location, direction, and magnitude of all dead, live, and lateral loads applicable to each Truss including, but not limited to, loads attributable to: roof, floor, partition, mechanical, fire sprinkler, attic storage, rain and ponding, wind, snow (including snow drift and unbalanced snow), seismic; and any other loads on the Truss;” (emphasis added).

The weight of the system fire sprinkler includes the weight of the water-filled pipe (if a wet system is used) and should be specified and analyzed as a dead load acting concurrently with all other live and dead loads for which the trusses are designed to support. The weight of the sprinkler system is often specified as a uniformly distributed dead load in pounds per square foot (psf), but may also be specified as individual concentrated loads if the attachment locations are known. Uniform sprinkler dead load values typically range between 1.5 to 6 psf. Since this load depends on many factors specific to each project, the building codes and fire sprinkler standards (including NFPA 13), do not establish minimum design dead load requirements for fire sprinkler systems. The magnitude of these dead loads must be determined and specified by the Building Designer.        

Table 1 lists the per lineal foot weight of water-filled pipe for several diameters of schedule 40 and 10 steel pipe. A review of this table indicates that large diameter sprinkler lines (four inches or greater), risers, and lines running parallel to trusses can add significant dead load to the trusses. Systems utilizing these larger pipe sizes may require special trusses be designed specifically to support these heavier loads. Another approach is to add trusses to the layout (i.e., trusses at sprinkler locations with closer on-center spacing, double trusses, etc.) to reduce the load required to be supported by each truss.

One thing to keep in mind is trusses are capable of supporting significantly higher concentrated loads applied at panel points as opposed to in the middle of the panel between panel point locations. If the support locations of the sprinkler pipe along the span of the truss are provided to the truss designer prior to truss design, the trusses can be designed with panel points located at the sprinkler attachment locations.

The weight of sprinkler systems installed per NFPA 13D for one- and two-family dwellings and manufactured homes is typically considered part of the design dead load for the plumbing system since the sprinkler system uses the domestic water delivery system.

Additionally, the direction, location, and magnitude of any lateral forces applied to the truss from the sprinkler system may also need to be considered and provided by the building designer. If there are loads applied perpendicular to the plane of the truss, they will need to be resisted by a lateral restraint system (i.e., diagonal bracing, strongbacks, etc.) called out and designed by the building designer.

It is always best if the truss designs and the sprinkler system design can be coordinated to eliminate conflicts between truss members and sprinkler pipe runs or sprinkler head locations in order to minimize truss repairs resulting from truss members needing to be cut and removed due to interference with piping.

3) The construction documents prepared by the building designer should indicate whether the sprinkler system is to be supported from the top or bottom chord of the trusses and specify how it is to be attached to the trusses.

In general, sprinkler systems should be supported from the top chord of the trusses. When conditions require that attachments be made to truss bottom chords, this information should be clearly indicated in the construction documents so any special provisions needed for attachment and connection systems can be provided. Pilot holes may be required depending on the type of screw hanger used. Screw and bolt sizes, and other mechanical connections or parts thereof, shall be sized in accordance with NFPA 13 and the National Design Specification® for Wood Construction (NDS®). Drilling holes in tension members, such as truss bottom chords, can reduce the net section of the member and, therefore, reduce the maximum tension force the lumber can resist. If the member has a combined stress index (CSI) near 1.0, analysis should be undertaken to determine the effect drilling may have on member strength.

Additional design considerations

There are two additional considerations the truss designer should incorporate into their truss designs:

a. Locating sprinkler supports within truss panels containing chord splices should be avoided.

b. There should be no more than one sprinkler support attached to each truss panel. If more than one sprinkler support is needed in a panel, special engineering is required and the truss design engineer should be contacted.

Given these truss design considerations, the ideal situation is that the truss manufacturer is informed that the building design includes an NFPA 13 or 13R sprinkler system. From there, it is important to get as much information as possible from the owner or building designer regarding the sprinkler system design and layout prior to the truss design process. This will give the truss manufacturer a more accurate estimate of the overall load and special truss design considerations so that the process is easier and more accurate. This will help to avoid RFIs and the redesign of the trusses due to missed loading conditions, all of which will come at a significant cost if this information is not available and planned out up front.

About the Author: Jim Vogt, P.E., regularly dives into the technical details of component design to educate and assist manufacturers on engineering issues.