Construction Concerns: Steel Joists

Article and photos by Gregory Havel

Shortly after steel stud framing became common in the late 1960s, steel joists were developed to be used with them. Steel joists are larger (deeper) and of heavier-gauge sheet metal than the steel studs that often support them, because they are used as beams to support floors and roofs. They are usually manufactured in dimensional lumber sizes, as are steel studs, so that they are compatible with common framing methods.

Steel joists can be used in some fire-rated floor-ceiling assemblies provided that the proper floor and ceiling materials are used and they are attached with the proper fasteners at the required intervals. Steel joists and studs, like dimensional lumber, have no fire rating if they are not part of a rated assembly.
 
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Photo 1 shows steel joists supported by a steel stud wall frame and supporting the plywood subfloor of the room above. These are traditional C-channel galvanized sheet metal steel joists with holes punched at intervals along the length of the joist for ease in handling and to allow cables and pipes to pass through. After all utilities are installed, a gypsum drywall board ceiling is often screwed to the bottom of the joist.

Engineers have found that shaping thin metal can give it more strength. This extra shaping is often found as ridges rolled into steel studs and joists parallel to and near the corners, to make a thinner metal shape stronger.

Engineers have also found that even more metal can be saved by removing parts of the web of a steel joist and shaping the remaining metal, turning it into a sheet-metal truss.

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Photo 2 shows the steel roof joists installed in December 2009 in an addition to a school building. These steel joists have the same C-channel shape and dimensions as those shown in photo 1, except that most of the web was punched out to reduce the weight and cost. The remaining metal in the web was pressed into channel shapes to add rigidity. Note the sheet-metal bracing that was installed between the joists; the sheet-metal strip that was added below the joists; and the screws that attach the steel roof deck at specified intervals; All these steps were taken to prevent the joists from twisting under a load. This joist system is promoted for its ease of installing pipes, cables, and ducts, which are protected by plastic grommets to prevent their contact with the steel joists.

Traditional steel joists have little mass and, therefore, little inherent fire resistance. The steel trusses shown in photo 2 have even less mass and inherent fire resistance. These joists support a steel roof deck attached to them by screws, six inches of polystyrene foam insulation board, and a rubber roof membrane with gravel ballast. It will also support a gypsum drywall board ceiling attached by screws. According to the architect’s drawings, this system is designed to support a dead load (joists, roof deck, insulation, roofing, and drywall ceiling) of 24 pounds per square foot and a “live load” (snow) of 30 pounds per square foot. Firefighters are not mentioned as part of the live load.

This will be a one-hour-rated roof-ceiling assembly (UL file BXUV.L590; http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/index.html) after the resilient channels and drywall board are screwed to the bottom of the joists and all of the joints are taped and after the roof insulation, membrane, and gravel ballast have been installed.

We do not want to be on this roof with a fire below. Since this roof is an assembly of many parts connected to and supporting each other, we do not want to cut ventilation openings in it while we are working from it. We need to make a note of this type of construction on our prefire plan for this building so that we will not get ourselves into trouble.

For more information on this new development, search the Internet for “cold formed metal framing” and “steel joists.” Several manufacturers advertise products of this type.

Gregory Havel is a member of the Burlington (WI) Fire Department; a retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II and fire officer II, an adjunct instructor in fire service programs at Gateway Technical College, and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 30 years of experience in facilities management and building construction; and has presented classes at FDIC.

 

Subjects: Building construction for firefighters

FE Category: Prevention and Protection, Building Constructio

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