How Do I-Joists Work?

A blog written by our own Senior Structural Engineer, Tom Rickerby.


Date: Tuesday 24 Mar 2020

What are I-joists?

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

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

The flanges are located at the top and the bottom of the 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 e-joist, is 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).

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 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 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 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.

Find out more

To find out more about Wesbeam e-joists, including how they can be designed in your next residential floor structure, click here. 

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