Article and photos by Gregory Havel
When manufactured wood I-joists are used in place of dimensional lumber joists in wood-frame construction, they must be supported properly so that they will not collapse under normal conditions. Photo 1 shows the most common method used to support I-joists from wood beams. The I-joists are set in galvanized steel joist hangers that are nailed to both the beam and the I-joists, with the tops of the I-joists flush with the tops of the wood beams. All of the nail holes in the joist hanger provided by the manufacturer must have nails to achieve the design strength of the system. The most common defect in I-joist assemblies is missing nails, which can significantly weaken the assembly and is often concealed by the floor deck above and a gypsum board ceiling below.
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Photo 2 shows a common method used to support I-joists from structural steel or masonry or concrete foundations. Dimensional lumber is bolted to the top flange of the structural steel beam or to the top of the foundation wall. The I-joists rest on and are nailed to this lumber. Bracing or blocking is frequently placed between the joists on top of beams at these points, to prevent the joists from twisting under load. The most common type of blocking is cut-off pieces of I-joist that are nailed in place. Note that this type of blocking is not firestopping, since it does not have enough mass and does not completely close the opening between the joists. Where I-joists bear on the top of a foundation wall, their ends will be braced by a continuous “rim board” of laminated strand lumber (LSL) or other material that encloses their ends.
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Photo 3 shows a method used to support I-joists on top of a wood-frame wall. These I-joists will support the floor of the top half-story of this house and the gypsum board ceiling of the rooms below and are nailed to the top plate of the wall. They are also attached to the dimensional lumber rafters with wood blocking and nails and form the bottom tension member of a simple triangular truss, with the two rafters in compression. This I-joist and rafter assembly will also be attached to the wall framing with galvanized steel “hurricane clips” (seismic clips) and nails as required by the building code, to stabilize the roof during high winds.
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Firefighters should be concerned about the types of connections used in construction with I-joists and that they are made correctly. Also be concerned when I-joists or wood trusses are used in remodeling older structures built with dimensional lumber joists, since these buildings will no longer behave in the same way during a fire as they would have behaved before. A working relationship with your building inspectors can alert you to when and where these lightweight components are being used and that they are properly installed, as will visits by firefighters on duty to buildings during construction or remodeling. Note improper, homemade, or missing connectors in this type of construction during overhaul after fires, and bring them to the attention of the company officer as well as fire investigators and fire marshals.
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Subjects: Building construction for firefighters, I-Joists
