Article and photos by Gregory Havel
Manufactured wood I-joists have been in common use since the 1970s. They are advertised as providing the same or greater strength as dimensional-lumber joists while using less material and being lighter in weight. They are also advertised as being straighter than dimensional lumber and therefore better for level floors and ceilings. These statements about this material are true under normal conditions but not in structure fires.
The top and bottom chords of a manufactured I-joist were originally 2 x 4 or 2 x 3 dimensional lumber, with a groove cut in one face of each for attaching the web with glue. The web of the manufactured I-joist was originally plywood, 1/4-inch or 3/8-inch (6mm or 9mm) thick, depending on the weight to be supported.
Photo 1 shows a close-up of the top chords and webs of today’s I-joists, still bundled after delivery to a construction job site. The top and bottom chords are laminated veneer lumber (LVL), like plywood but with the grain in all of the laminates running parallel with the long axis of the joist and cut into dimensional lumber sizes rather than into sheets. The webs are of oriented strand board (OSB), a product made of wood shavings and glue that is pressed into sheets and that is advertised as stronger than plywood. Both LVL and OSB are cured under pressure with heat. The adhesives used to manufacture OSB and LVL, and to assemble I-joists, are usually either urea-formaldehyde or phenolic resins.
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Photo 2 (left) shows I-joists with LVL chords and OSB webs in place on top of a wall framed of 2 x 6s (to provide space for the insulation required by the building code). These I-joists will support the floor of the top half-story of this house, and the gypsum board ceiling of the rooms below. The roof rafters shown are of sawn dimensional lumber. Note that I-joists are sometimes also used for rafters, in place of dimensional lumber, usually with a ridge board of LVL, dimensional lumber, or plywood.
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Photo 3 shows the I-joists that will support the first floor of this house. Note that the short sections of I-joist between the ends of the joists on top of the steel beam are not firestopping. They are blocking that is required by the I-joist system and provide lateral bracing to keep the I-joists from twisting.
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Also note that the one-inch board shown bolted to the top flange of the steel beam is common in this method of construction; it allows the I-joists to be nailed to keep them in position.
See also the splice in the web of the I-joist blocking section in the center of photo 3. Since OSB sheets are usually only eight feet long and I-joists can be many times longer, the OSB sheets are trimmed with zigzag or scalloped edges, which are interlocked and glued together when the joists are assembled. Expect to see several of these splices in the web of a long I-joist.
In all cases, the floor deck (either plywood or OSB) will be glued and nailed (or screwed) to the top of the joists, and the floor finish (carpet or tile) will be applied. In all cases, the gypsum board ceiling (usually 1/2-inch) will be screwed to the underside of the joists, unless the joists are permitted by code to be left exposed in a basement or cellar.
I-joists can be part of a fire-rated floor-ceiling assembly. In this case, the ceiling will be 5/8-inch Type X gypsum board attached to the joists according to the listing specification in the Underwriters Laboratories Directory, including the number, length, and spacing of the screws. The fire rating is tested with fire only on one side of the floor-ceiling assembly–not inside it. Fire inside a fire-rated assembly, especially of manufactured wood products, will cause it to fail quickly and catastrophically. Since the adhesives used to manufacture I-joists are cured with heat, they can begin to weaken when heated by fire, even though the temperature may not yet be high enough to ignite them.
For a detailed comparison of the behavior of sawn lumber, I-joists, and wood trusses under fire conditions, view the Underwriters Laboratories online short course at http://www.ul.com/fire/structural.html or at http://www.ul.com/global/eng/pages/offerings/industries/buildingmaterials/fire/structural. From the UL tests, manufactured wood I-joists fail more quickly under fire conditions than wood trusses assembled with metal gusset plates.
Whichever type of framing is used (sawn lumber, manufactured joists, or wood truss) and whichever type of ceiling is attached (creating fire-rated assemblies or not), the finished products will look the same when completed: carpet or tile floor above, and painted gypsum below. The only way you will know what might be supporting you if there is a fire in this building is a prefire plan for the structure based on any notes you took while it was being built.
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Subjects: Building construction for firefighters, I-Joists
