Prefabricated wood I-joists are made by gluing solid sawn lumber or laminated veneer lumber (LVL) flanges to a plywood or oriented strandboard (OSB) panel web to produce a dimensionally stable light-weight member with known engineering properties.
The uniform stiffness, strength, and light weight of these prefabricated structural products makes them well suited for longer span joist and rafter applications for both residential and commercial construction.
The “I” shape of these products gives a high strength to weight ratio. For example, wood I-joists 241mm (9-1/2″) deep and 8m (26′-3″) long weigh between 23kg (50 lbs.) and 32kg (70 lbs.), depending on the flange size. This means that they can be installed manually, giving advantages in labour and economy.
Factory-prepunched knock-out holes in the webs facilitate the installation of electrical services. The knockout holes also provide ventilation when the joists are used in a cathedral type ceiling with no attic above. Some manufacturers specifically offer I-joists with ventilation holes predrilled through the web for use in cathedral ceilings.
Holes for plumbing and mechanical ductwork may be drilled easily through the web, but must be located according to the manufacturer’s recommendations.
The wide flanges allow for a good fastening surface for sheathing, and the product can be cut and worked using common wood working tools. However, the flanges should never be notched or drilled and all special cuts, such as bird’s mouth bearing cuts, must follow the manufacturer’s recommendations.
In some areas there may be difficulty in obtaining large dimensions of solid sawn framing lumber. The availability of wood I-joists used as floorjoists and as deep, insulated roof joists has made them a popular product for lightweight structural members for rafter and joist applications. They are an economical alternative to open web steel joists.
Several different types of prefabricated wood I-joists are commercially available. Each type features a different combination of flange and web materials, and a different connection between the web and the flanges.
The joint between the flange and the web is a critical element of member strength and is typically protected by patent by each manufacturer (see Figure 3.20 below).
Flanges are commonly made of laminated veneer lumber (LVL), visually graded lumber, or MSR lumber.
The webs are made of either oriented strandboard (OSB) or plywood.
Web panel joints are glued and mated by several methods such as butting of square panel ends, scarfing of the panel ends, and shaping of either a toothed or tongue and groove type joint. The use of longer OSB panels is gaining acceptance as a means of lowering the number of end-to-end panel joints in the web.
Exterior rated phenol-formaldehyde and phenol-resorcinol are the principle adhesives used for the web to web and web to flange joints.