Selecting TJI Floor Joist Sizes

Sizing Trus Joist TJI’s

Recently I’ve recieved several emails asking me how you size TJI’s for a new home. Sizing Trus Joist brand TJI’s isn’t really that difficult once you understand the issues. Most poeple think that joists are sized based on minimum design loads. That is ultimately true, however, loading doesn’t usually govern the size you’ll select for your new home. Deflections are almost always the governing criteria in determining joist sizes in residential and even commercial applications.

First you’ll need to download the most current Trus Joist TJI Manual. The design manual contains all the span tables for different sizes and orientations of joists. The manual also includes span tables based on different deflection criteria. Also included in the manual are extremely important construction and safety details that need to be followed for a safely framed floor system.

Using TJI Span Tables

The following screen shot comes from the Trus Joist TJI Manual. It shows the standard span tables for TJI’s. The tables look overwhelming but they are actually pretty simple to use. They are organized by two main tables, one for live load deflections of L/360 and one for L/480. Within each of those two tables the information is divided again into to sections, one section for a dead load of 10 pounds per square foot (psf) and one for 20 psf.

So the question is: How on earth to I use these tables? Minimum codes typically require a live load of 40 psf and a dead load of 10 psf. These minimum design loads work well as a minimum safe loading capacity. However, those actually loads don’t typically govern the design. The minimum deflection criteria of L/360 will usually control.

L/360 means that if you have a joist spaning 10 feet you can expect it to deflect 0.33 inches at mid-span based on 40 psf live loading. I don’t know about you but a deflection of almost 3/8 of an inch while I’m walking across the floor seems too “bouncy”. That’s why the tables are also written for L/480. For the same 10 foot span you can then expect a deflection of 1/4 inch.

So my recommendation is to select your floor joists based on L/480 deflection criteria. Now depending on the room you’ll need to decide if you want to account for 10 psf dead loading or the higher 20 psf. Again for my house I went with the higher dead load of 20 psf because I wanted a really nice stiff floor that wouldn’t “bounce” as we walked around.

Example Joist Selection

So here’s how you’d select a typical TJI floor joist from the span tables. Let’s assume we’re going to have a 15 foot span (fairly typically with a 30 ft wide house). Let’s also assume we want a very stiff floor so I’ll choose L/480 deflection criteria and 20 psf dead loading. I’ll also assume we’re going to frame the joists at 16 inches on center.

I’ve highlighted several key columns in the L/480 Span Table as you can see above. First off I’ve decided I want to try and use 11-7/8 inch deep joists (most standard size). Secondly I’m using the columns for 20 psf dead load fround on the right side of the table. Now all I did was pick the lightest (I’ll explain this below) TJI in the 11-7/8 inch depth that has an allowable span greater than my design of 15 ft. As you can see I’ve highlighted the TJI 110 (left column) that has an allowable span of 17′-8″ for a dead load of 20 psf and a deflection criteria of L/480.

The second column in the table identifies the TJI size for a given depth. The sizes range from lightest (110) to heaviest 560. Once you’ve selected a size be sure to check with our local supplier. Some of the sizes in the span tables are not always available. If that’s the situation, just pick the next biggest size and go with that.

Follow Standard Construction Details

Be sure you follow all of the recommended construction details in the design manual. The details ensure safety during and after construction. The details also help you build the floor to eliminate floor noises and performance issues.

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