How Do I-Joists Work?

What are I-joists?

Timber I-joists are commonly used throughout the world for residential and commercial floor structures. Wood I-joists are lightweight, with excellent strength to weight ratios and spanning capabilities.

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I-joists consist of two main parts: the flanges, and the webs.

The flanges are located at the top and the bottom of the I-joists. The flanges do most of the work in an I-joist. Therefore they are usually made out of high strength timber, which in the case of Wesbeam's innovative e-joist, our laminated veneer lumber (LVL).

The top and bottom flanges are joined by a vertical web. The role of the web is essentially to hold the bottom and top flanges apart, much like the chords or metal webs used in a truss. The webs consists of a thinner timber product; typically Plywood or Oriented Strand Board (OSB).

What is an I-joist used for?

I-joists are primarily used as floor joists or roof rafters in residential and commercial construction. I-joists have structural flanges on the top and bottom, connected by a vertical, structural web. These joists offer several advantages over traditional timber beams, including longer available lengths, lighter weight per meter than solid beams, increased straightness, and the ability to cut large service penetrations through the structural web. This makes them the ideal choice for residential floor construction.

How do they work?

If we look at a single spanning I-joist with weights spread across the top (much like a floor joist), it deflects downwards just like any other beam. When we look a little closer at what the different parts are doing, the top flange is pushing in on itself, and is in &#;compression&#;. As the bottom of the I-joist stretches outward, the bottom flange is put into &#;tension&#;.

These two flanges are then connected by the structural web, which has forces running up-and-down known as &#;shear&#;.

The forces in the top and bottom flanges are quite consistent along the length of the I-joist. This is why it is so important not to cut or damage these flanges. Just like a truss, damaging the tension and compression members will compromise the integrity of the I-joist.

What about penetrations?

With the shear forces in the web, these are much higher at the support points, where all of the vertical weight on the beam is essentially supported. At these locations, the web can be stiffened if needed and should not be cut without consulting an engineer.

What this does mean is that in the middle of the beam, the web is not doing a whole lot! The forces are low and the top and bottom flanges are being well supported from the ends. This allows large penetrations and holes to be cut into the web without compromising the integrity of the joist.

These holes can be used to pass through plumbing pipes or air conditioning ducts. For rules regarding what size and where holes can be drilled in Wesbeams e-joists, refer to our new Hole Creation - Quick Reference.

Engineered wood joist

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]

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

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