New design guide simplifies sprinkler details for I-joist framing

Automatic sprinkler systems in light commercial and multifamily buildings across North America are frequently paired with APA performance-rated wood I-joists in various applications. The advantages to using wood I-joists are many: they provide for longer floor spans eliminating the need for intermediate columns; they are an engineered product meeting specific standards for performance, consistency and reliability; and they are a green building product, making efficient use of available fiber resources. The typical sprinkler application involves a main line running beneath the I-joists with trunk lines running parallel to the joists. Historically, there has been a lack of information on how to attach pipe systems to the I-joists.

"Up until now, details for incorporating I-joists with commercial sprinkler systems were limited to only a few proprietary I-joist manufacturers," said Darian Wentland, market development engineer with Jager Engineered Wood Products. "Engineers were designing their own connection systems."

This led to inconsistency from job to job, poor installation practices, and confusion among building officials. The National Fire Protection Association (NFPA) does address sprinkler installation guidelines, but that is limited to dimensional lumber. To address this issue, Wentland and other I-joist manufacturers worked with APA to develop a new series of sprinkler installation details.

APA—The Engineered Wood Association has launched a new resource for I-joist technical information at www.apaIjoists.org. The site contains the Technical Note: Sprinkler Installation for APA Performance Rated I-Joists, which has more than 30 sprinkler details. In addition, the site hosts CAD details, I-joist construction guides, as well as, a list of APA member I-joist manufacturers and their product reports.

General guidelines

This article offers some general guidelines on the recently developed sprinkler system-to-I-joist attachment methods contained in the design guide. As with all design recommendations, be sure to check the structural adequacy/capacity of the hangers, fasteners, and I-joists. The design details included here are based on assumptions for permitted fasteners and hangers, design loads, and fastener capacities.

Permitted fasteners and hangers—Chapter 9 of the Automatic Sprinkler Systems Handbook NFPA 13, Standard for the Installation of Sprinkler Systems, lists the approved fasteners and common types of acceptable hangers for use with wood and other materials. The list includes wood screws, lag screws, drive screws, and bolts. Screws must be screwed into place—not pounded. Though nails are not permitted to carry loads, they are used in some of these details to simply hold a load-bearing member in place, such as when a cross member is bearing on blocking. NFPA 13R, Standard for the Installation of Sprinkler Systems in Residential Occupancies up to and Including Four Stories in Height (section 2-4.4), requires the use of fasteners complying with NFPA 13. NFPA 13D, Standard for the Installation of Sprinkler Systems in One-and Two-Family Dwellings and Manufactured Homes (section 3-4), requires only that pipes sup¬ported by structural members be supported by methods comparable to those required by local plumbing codes.

Design loads—The design loads from the sprinkler systems are based on the 2007 NFPA 13. The design load capacities shown in diagrams within the guide are based on a 15-foot maximum span between pipe supports (unless otherwise specified) and Schedule 40 pipe permanently filled with water.
Allowance is made for a short-term additional load of 250 pounds. See NFPA 13, Table 9.2.2.1(a) for specific hanger requirements and spans for other types and diameters of piping. A duration-of-load factor of 0.9 is used for permanent loads, 1.6 is used for the short-term loads, and 1.33 is used for seismic loads in the bracing designs. For seismic loads on bracing, spacing of braces depends on pipe size, pipe run length, and the seismic design category.

Design fastener capacities—For installations in lumber or into I-joist flanges, lag screw, wood screw, and bolt design assumptions are based on the American Forest and Paper Association’s National Design Specification (NDS) design methods and design capacities for Spruce-Pine-Fir No. 2 with a specific gravity of 0.42. Oriented strand board (OSB) webs are assumed to be 3/8 inch thick with a design specific gravity of 0.50 per Table 11.3.2B of NDS or APA Panel Design Specification, Form D510, available at www.apawood.org/publications.

Joist design—Most wood I-joist design tables are based on an assumed uniform load. Joists and other supporting systems must be designed to carry the added weight of the sprinkler system. This may necessitate the use of deeper I-joists, a closer spacing of the I-joists, I-joists with shorter spans and/or a different I-joist series with higher moment and stiffness capacities.

Coach rod screws—NFPA 13, Section 9.1.5.7.1, limits the use of coach screws to piping 4 inches or less in diameter, requires them to have a minimum diameter of 3/8 inch and the threaded portion of the coach screw must penetrate the wood member at least 3 inches. This precludes the direct attachment of coach screw rods to the flanges of most wood I-joists unless they have an approval for that use. Lag screws are permitted; however, the designer must remain aware that only the threaded portion is used for design and that the full length of the lag screws is not threaded (see NDS Table L2 for dimensions). While fastener penetration into the edges of the I-joist webs is permissible (through the flange face), it is recommended that web-edge penetration not be added to the withdrawal-from-flange-face value.

Conclusion

"Now engineers and other professionals have an alternative to [proprietary] code reports with these generic details," added Wentland. Figures 1, 2, and 3 (below) represent a few of the typical details that are contained within the design guide.

Fulton Desler is a senior engineer with APA—The Engineered Wood Association, responsible for assisting engineers, architects, contractors and the general public in the proper use and design of structures that incorporate engineered wood products.

Figures

Figure 1: APA performance rated I-joist—load carried by flange and web
Credit: APA

Figure 2: APA performance rated I-joist—load carried by flanges
Credit: APA

Figure 3: APA performance rated I-joist—trapeze hanger
Credit: APA