Note that the allowable span for 2 inch by 12 inch (51 mm by 305 mm) is 18 feet, 1 inch (5512 mm). Therefore, use 2-inch by 12-inch (51 mm by 305 mm) joists at 16-inch (406 mm) spacing.
With the 18' Span you really need 5.25 x 11.875 or 3 ply x 11-7/8". If you can easily access the joists and have the availability of a good frame contractor you could set the beam up between the joists so that there is no beam showing.
The maximum span of a 2x8 floor joist depends on several factors, including the wood species, grade, spacing between joists, and the expected load. In general, for residential construction using standard framing lumber such as Douglas Fir-Larch, a joist can span approximately 10 to 13 feet when spaced 16 inches apart.
Live Load: 40 psf (pounds per square foot) Dead Load: 10 psf. Maximum Span: Approximately 16 feet.
You should look up a joist span table. It will give you all of the details you need. For example, for a 16' span, you are going to need to use 2x12s 16" OC, or 2x10s 12" OC.
In fact, each 2x10 will support 1,200 lbs. To be clear, spans are measured as "clear spans". That is to say, from face of support to face of support.
2x4 11'-6" 2x6 17'-0" 2x8 21'-8" 2x10 25'-7" 24" O.C.
When builders need to increase the load-bearing capacity of floor joists, they often use a process known as “sistering.” This process involves attaching a new joist alongside an existing joist to increase its strength.
2x8: They're often used for mid-range floors, such as those in living rooms or dining rooms. They can span up to 10 feet and have a higher load-bearing capacity than 2x6 joists.
You can use a W6x25 (50 ksi) steel beam for your 17' clear span floor beam. The beam is 6.125” wide and 6.375” deep. It weighs 25 pounds per foot of beam length.
I-Joists are manufactured in a wide range of depths ranging from 9 ½″-24″ and can be special ordered in lengths as long as 60 feet. BCI Joists are available in standard depths up to 20″ and AJS joists are available in depths up to 24″.
You will need Double 2x10's spaced at 16" o.c., for an 18' clear span. Or you can have a spacing of single 2x10's spaced at 8" o.c. Whatever works best for you.
Is there a simple way to size floor joists? A. Carl Hagstrom responds: The rule of thumb that I like to use when sizing uniformly loaded residential floor joists is "half the span plus two." First, round the clear span of the floor joist up to the nearest foot, and divide by two. Then add two to the answer.
“A 2×6 spaced 16 inches apart can span a maximum distance of 13 feet 5 inches when used as a rafter, 10 feet 9 inches when used as a joist, and 6 feet 11 inches when used as a deck beam to support joists with a 6-foot span.”
Most building codes, including the most used, the IRC, do not directly address sistering a joist, but they do address girders, which are similar.
The allowable load for a single 2x8 joist is 400 pounds, while porcelain tile can weigh up to 275 pounds per square foot.
When a joist is weight-bearing, meaning it holds up a wall or has extra weight attached to it, it's sometimes necessary to sister multiple pieces of new material to the old one. This is usually done by “sandwiching” the old joist in the middle of the two new ones. Doing this gives additional support to the joist.
The span of a 2×8 floor joist depends on the material and spacing, but in general: For Douglas Fir, a 2×8 joist can span up to 12 feet when spaced 16 inches O.C. and carrying a typical residential live load of 40 psf (pounds per square foot) and a dead load of 10 psf.
On BestDeckSite, we recommend installing blocking on 2x8 joists too. Here's a word of caution. As you get on a roll calling out numbers for the guy at the saw while hammering in the preceding block, make sure not to throw off the joist spacing.
If the span is 6 feet or less, use 2X6's, if the span is 8 feet or less, use 2X8's, less than 10 feet, 2X10's, etc.
Generally, the customary floor dead load is 10-12 PSF (pounds per square foot) for floors, 12-15 PSF for roof rafters and 20 PSF for roof trusses. However, these may increase when a heavy finish material, such as brick veneer walls or tile floors/roofs, is specified.
Aside from providing longer spans, I-joist and TRIFORCE® floor systems are superior in strength and precision to solid sawn lumber. They are stiffer, lighter and more easily maneuverable. They also provide a bigger nailing surface to attach the floor membrane than the commonly used 2 x 10.