Wood I-joists are composed of two horizontal components called flanges and a vertical component called a web. Wood I-joists are used as a framing material primarily in floors, but may also be used as roof rafters where long length and high load capacity are required. Wood I-joists offer the advantages of exceptional stiffness, light weight, and long span capability. Holes may be cut in the web according to manufacturer’s recommendations, allowing ducts and utilities to be run. I-joists are dimensionally stable, and thus are unlikely to warp, twist or shrink; they are uniform in size; and are consistent in their manufacturing.
The “I” shape allows the most efficient use of wood necessary to carry design loads. This is achieved by placing the material with high strength and stiffness in the flanges. Similarly, the web material is high quality, but with different structural properties. I-joists utilize the geometry of the cross-section and high strength components to maximize strength and stiffness of the available wood fiber. Flanges are manufactured from end-joined, solid sawn lumber or structural composite lumber (SCL), while webs typically consist of high strength plywood or oriented strand board (OSB).
Manufacture of I-joists
All web and flange materials are carefully graded to ensure they will perform properly in I-joists. The flanges range from 1-5/16″ to 1-1/2″ thick and from 1-1/2″ to 3-1/2″ wide.
Today, the web material used almost exclusively is OSB. I-joists manufactured prior to the early 1990s also had webs made of plywood. Web material in typical residential I-joists is either 3/8″ or 7/16″ thick.
Performance Requirements for Code Acceptance
Although I-joists differ among manufacturers, they are all manufactured to an industry consensus standard. I-joist performance is closely monitored by the manufacturer and an independent third-party quality control agency. The quality control program assures the product continually meets code-recognized test standards. The two most important performance characteristics are strength and serviceability.
Fire Incidents with I-Joists
Specific fire incident reports indicate that when directly exposed to fire (unprotected), the loss of strength of I-joists often occurs in conjunction with burn-through of floor sheathing. Within the I-joist itself, the web is consumed first (because of reduced mass). Once the web is consumed, the bottom flange is no longer attached to the joist and falls from the system. Numerous fire incidents have been reported where the only remaining structural components in the floor system were the top flange and floor sheathing. The resulting floor systems, while remaining intact, had over 12″ of deflection. Similarly, many reports indicate that firefighters either felt a floor become “soft” or “spongy” or visually observed deflection and exited the structure.