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 do 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 spanning 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.