Cantilevered floor joists

[azgreg]

What are the pros and cons of cantilevered floor joists? It just seems to me that putting the wall directly over the foundation wall or beam is much stronger. What am I missing here?

[MountainDon]

The IRC has a section that covers allowable cantilevers when building to code. Anything that varies from this requires engineering. IMO, when building in an area with lax code enforcement it may still be a good idea to follow code as the physics do not change simply because the inspector does not show up.

R502.3.3 Floor cantilevers.
Floor cantilever spans shall not exceed the nominal depth of the wood floor joist. Floor cantilevers constructed in accordance with Table R502.3.3(1) shall be permitted when supporting a light-frame bearing wall and roof only.

Link to that section  here.

[azgreg]

I saw that Don and thanks for the reply. I guess what I'm asking is when is a cantilevered floor appropriate over the alternative?

[MTScott]

I think in most construction, you see cantilever joists for making cute "bumpouts", perhaps on a second floor.  I think the only reason is to make the building seem more interesting, and I suppose it does add a minor amount of space in the given room (although, probably not worth the PITA, to me anyway).

If your question is about cantilevering your main floor system over your pier foundation beams, I too have the same question.  I see it done often.  I could only come up with two benefits myself: it tucks the piers in under your floor, thus making it easier to skirt the building.  The other I could think of, is it narrows your main spans, possibly allowing for smaller floor joists.  I'm totally speculating on both points, as I really don't know - hopefully someone smarter will show up .

[Don_P]

We use the term cantilever for this but that is incorrect, this is where the distinction becomes more important. A cantilever is a beam rigidly attached to something and extending out. Think of an I beam post with an I beam welded to it sticking out. In wood we really don't cantilever. This is a beam overhanging a support, there are things going on in that backspan area.
There is a benefit that John has mentioned with regards to his overhangs. When a beam overhangs supports not only does it shorten spans, as long as the seesaw is strong enough the loaded overhanging span helps balance and stiffen the main span. The bending moment and shear over the support climb pretty fast, that is why it goes to engineering when the cantilever gets deeper than joist depth, or for engineered materials whenever you go off their tables.

This is the beam diagram for a uniformly loaded beam overhanging one support, not quite this situation, (there is a point load on the outboard end of the overhang from above and a uniform load on the entire joist) The drawing give a sense of what is going on. Mentally I flip the moment diagram so it deflects the way the joist does. Notice the magnitude of the bending moment over the support, also notice the large shear difference at the post, a good amount of stress there. The max bending moment in the main span would be deeper (greater) if the overhang weren't acting as a seesaw and lifting on the beam. One other thing to notice, see the vertical line from where shear passes through zero stress on the main span is also the point of maximum bending stress... if you can find one you know where the other is occurring.


I'm more comfortable simply supported  .

[akwoodchuck]

Gravity can be a real female dog....in heat....all builders would do well to remember it....