I-joist

Engineered wood joist

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An engineered wood joist, more commonly known as an I-joist, is a product designed to eliminate problems that occur with conventional wood joists. Invented in , the I-joist is an engineered wood product that has great strength in relation to its size and weight. The biggest notable difference from dimensional lumber is that the I-joist carries heavy loads with less lumber than a dimensional solid wood joist.[1] As of , approximately 50% of all wood light framed floors used I-joists.[2][clarification needed] I-joists were designed to help eliminate typical problems that come with using solid lumber as joists.

The advantage of I-joists is they are less likely to bow, crown, twist, cup, check, or split as would a piece of dimensional lumber. I-joists' dimensional soundness and little or no shrinkage help eliminate squeaky floors.

The disadvantage of I-joists is very rapid structural failure when directly exposed to fire (much like trusses), reducing the time available for residents to escape and increasing the danger to firefighters.

Design and manufacture

An I-joist has two main parts, the web and flange. The web is sandwiched between a top and bottom flange, creating the "I" shape. The flange can be made from laminated veneer lumber or solid wood finger-jointed together for ultimate strength. It is grooved on one side to receive the web. The web is typically made from plywood, laminated veneer lumber, or oriented strand board. After sizing the webs and flanges, they are assembled with water-resistant glue by pressing the web into the top and bottom flange. After assembly, the I-joist is end-trimmed and heat-cured or left at room temperature to reach approximately equilibrium moisture content. Sizes vary according to the I-joist's intended load and span. Depths can range from 9+1&#;4 to 24 inches (230&#;610 mm) and reach up to 80 feet (24 m) in length, although 40 to 42 feet (12&#;13 m) is more common. The intended use for an I-joist is for floor and roof joists, wall studs, and roof rafters in both residential and commercial construction.[1]

Installation

I-joists require correct installation. The most common mistake is misplacing or improperly sizing holes in the web, which can compromise the joist's strength, potentially leading to structural failure.

Common mistakes made with installing I-joists include cutting or chiseling the flange, improperly sized joist hangers, improper nailing and wrong-sized nails. The rim joist depth must match the I-joist size. Mismatches can strain the joist.

A similar situation occurs where the I-joist crosses a main beam. Installing squash blocks (2×4 materials 1&#;16 in or 1.6 mm higher than the I-joist) alongside the I-joists transfers the load from the I-joist onto the beam. Missed nails and glue setting too fast can lead to an uneven or squeaky floor.[3]

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Safety

The lightweight nature of I-joists makes them more vulnerable to fire than dimensional lumber.[4] A report by Underwriters Laboratories found that structural assemblies composed of I-joists fail significantly sooner under fire conditions than those composed of dimensional lumber.[5] Fire-induced failures of lightweight trusses and I-joists have led to the deaths of several firefighters.[6] In order to use i-joists in a fire-rated assembly, additional detailing is required to ensure building safety.[7]

See also

References

I built a house last year and used them. I did not want a bouncy floor either and if designed right, it won't be bouncy. I sized them myself on the plans from span tables available. I am a mechanical engineer but didn't try to do real calcs as that is done in the span tables. My builder thought I over-sized them so we ran it by the civil engineer at the supplier and he agreed with me. So, left to some builders and others, I could see how you could end up with a structurally safe but bouncy floor. The floor is flat as a flitter (that might be a local saying). I am happy with them. If you use them make sure the builder installs "squash blocks" at all load bearing points above which is typically the ends where exterior walls are and other interior load bearing areas. They should not pass framing inspection without the squash blocks. They are readily available in stock lengths and delivered faster than floor trusses. The negative compared to trusses is running duct work, pipe, wire, etc through them. I would not frame with 2X lumber again for floors.

I will add that mine are above minimum allowable in the tables. They are 14 and a fraction tall, on 16" centers, with a large LVL beam at midspan and span from end to center beam is about 16'. The floor is glued and screwed with 3/4" Advantech subfloor. We have been here about 18 months now and the other day my wife commented "have you noticed there aren't any floor squeaks anywhere". The house we moved from was 2x12 joist, glued and nailed plywood subfloor with hardwood, built in and had floor squeaks lots of places.

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