Using 9.5" I-joist for the 20' wide if cantilevered?

[hnash53]

Could 9.5" engineered I-joists be use in the 20' wide floor construction if the I-joists were cantilevered over two beams about 16 feet apart?

If so, what kind of connectors would need to be used to tie the joists to the beams?

Thanks.

[MountainDon]

Engineered joists would be engineered to whatever size they would have to be for the application you described. Maybe there is someone here who can/will have an answer. I think that's a question the joist company would be well suited to answer.

[glenn kangiser]

I think and engineer would have to say also, but at the least they will make you stiffen the ends and sandwich the web with plywood or something to stiffen it.  I think that is more of a stretch than they would go though.

[hnash53]

I don't understand why this question is so hard to answer.  The I-joist span tables say that a 9.5" I-joist can span over 16'.  So if the joist is cantilevered about 2' at each end then it should work because the span will be less than 16'.

In the 14x24 Little House plans, John uses 2x6s that are cantilevered on each end by over 2', and he even says that it results in a stronger structure when built this way.

So wouldn't the same be true using I-joists as I have described?

[Don_P]

See if you can get this far without registering;
http://www.apawood.org/level_c.cfm?content=pub_joi_libmain
Once inside the second publication will have what you need, you'll need to register to download it but they don't spam or sell your info.

The cantilever info starts with detail 5a on page 16.

The help desk at apawood.org is staffed by engineers, they are somewhat limited in how they can respond but I've found them to be as helpful as they possibly can be. After that its up to the mfr to help you use their product.

[PEG688]

  #1: I don't understand why this question is so hard to answer. 

  #2: The I-joist span tables say that a 9.5" I-joist can span over 16'.  So if the joist is cantilevered about 2' at each end then it should work because the span will be less than 16'.

  #3: In the 14x24 Little House plans, John uses 2x6s that are cantilevered on each end by over 2', and he even says that it results in a stronger structure when built this way.

  #4: So wouldn't the same be true using I-joists as I have described?

#1: It isn't that hard and it has been answered.

#2: In theory that is true, BUT again the "engineered" part of "I" joist is the variable factor.

#3: Yes, for lumber joist that is true. Your asking a apples to oranges question, and using logic to bend it to your desired answer. Engineering is not pure logic.

#4: Ah , maybe , BUT the type of I joist , read brand , would also be a factor.

So back to question #1: Ask the I joist people IF it will work the way you drew it up, they're the ones who have all the facts and can crunch the # for the design your asking about.

Like Glenn  said they'll require some squash blocks and they will tell you what , where and how to tie the joist to the beams.

This should be a no cost to you design provided by the Manf. you choose to buy from.

G/L PEG 

[MountainDon]

   John doesn't cantilever any of his plans that are wider than 14 feet, IIRC.  I'm not sure of the reasons for that. He recently came out with a modified foundation that expands a little house version to 16 feet wide. That places the beams out under the side walls, rather than continue the cantilever design.

I am guessing that one of the reasons for not cantilevering the wider designs has something to do with heavier loads that the wider building will have to transfer down through the structure to the ground.

The info on the 16 ft wide Little House foundation may be found HERE.

And I'll repeat what I stated before, the manufacturer of the joist should have the answers and tell you if their product would be warranted with that particular design use.

[John_C]

I can give you one reason off the top of my head why an engineer would want to look at it.

If the ends of the I-joists sit on an exterior foundation or beam there would be a joist header and the wall/roof load would be evenly distributed to the foundation.

Cantilever those same joists over a 16' wide foundation and the wall/roof loads are on the ends of the I-joists and become concentrated where each I-joist is supported by the beam. 

The span tables have to do with the floor loads.  There would be some other loads that would need to be considered.

[Don_P]

Yes, the roof load is on the end of the overhang, the majority of the floor load is inboard, the bearing is taking both.
Snow load info would be one missing variable, you will also need to inform them if it is anything other than a uniform load on any of this other than the outer wall load.

I have found design values and can provide the math to answer the question if you want to do it for fun but it needs to run by the engineers at the mfr before you build either way.

[glenn kangiser]

Another way to look at it. 

The OSB used it the center web of the I beam does not have the strength to take the concentrated load of supporting the house out on the cantilever.  Load it outside of the beams on the top and it will crush above the beam.

They don't want that heavy compressive load  without web stiffener wood plates on each side glued, attached and designed per the engineer to say what it can take and how you must do it.

Lets say you make a guess that a 3/4 ply glued each side would support the cantilever and the crushing load.    That will stress the web at the stopping point of the stiffener.  Will it hold or will it snap off there when the load is put on the cantilevered end? hmm

That is the engineered wood portion - the engineer knows the characteristics of the wood and what you have to do to make it support the load without breaking.  They are not as simple as solid wood.

John limited the cantilever to the small house due to loading being too much on larger ones as I recall .

[Don_P]

Those details are in the manufacturer literature linked to above  .

I think english timberframes started using the jetty around 1500 when houses were taxed based on ground floor dimensions. It was not uncommon to double jetty, bumping out the next floor as well. This is a neat shot of a small German jettied building
http://en.wikipedia.org/wiki/File:Kunstdrechslerei_Zettler_(Schwerin)_cleaned.jpg

But, the general rule of thumb, engineered products go by manufacturers engineering. Solid sawn wood products go by the engineering principles outlined in the Nat'l Design Specification for Wood Construction.

[hnash53]

Alright, thanks for all the explanations regarding I-joist engineering and the need for consulting the providers of the material.

Does anything change here if I decided to use "regular" 2X material and cantilevering instead of engineered I-joists?

Thanks.

[Don_P]

This will get you started and give an idea of the engineering, it is overhanging on one side only. If you want to go this route I can write one up for overhanging both sides. The diagram and math are from the NDS I just put them into javascript to avoid my typical dumb math mistakes.
http://www.windyhilllogworks.com/Calcs/ohangunild.htm

The I joist calcs are along the same lines with a few more checks related to what the guys have mentioned above.

[hnash53]

Wow Don P. ...a little too much for me.  I'm just a simple guy looking for simpler ways of doing things that often are overly complicated but don't have to be.  Thought that floor joist sizing, cantilevering, etc wouldn't be so complex.

When I built my 16x28 cabin (see www.countryplans.com/nash.html ) the A-frame fiasco switched us to a regular framed cabin.  I had a bunch of oversized rough-cut 2x8s that we used up in building the floor.  The span wasn't quite 16'...more like 15' between the inside edges of the two beams.  These 2x8s were so "beefy" in size they contained about 40% more material than a regular milled 2x8.  The forum here said they thought I was fine using them.  So far there's been no deflection, sag or anything in about 8 years.

I've been thinking about either adding on a 16x40 addition to my existing cabin for fulltime living, or just starting completely over, buying new land and building a 20x40 1.5 story somewhere else.  It's the 20' wide thing that's prompted my questions.  I was hoping to build just 2 40' beams and put the joists between them.  I could if I used the 11 7/8" I-joists according to John's 20'-wide plans.  Then I started thinking...what if I cantilevered the I-joists over beams 16' apart... I could use 9.5" depth I-joists.  There I go again ... thinking.

I've got more flexibility where my cabin is now... rules, regs, codes are relaxed (non-existent?).  But if I go somewhere else, I'll have to deal with inspectors, etc.

[Don_P]

I doubt the carpenter in 1500 crunched the numbers. And it may very well work as you describe, I think by the time you do the extra reinforcement shown in the literature I linked to, the wider spread and deeper joists will be simpler and cheaper. The I joist folks will do the engineering check at no charge and then you can pricew your options both ways.
For solid sawn material you may overhang up to the depth of the joist prescriptively, that is, without an engineer's blessing. If the overhang is greater than the depth of the joist then the inspector may ask for engineering and it would be a good idea, especially if you have large snow loads. so joist sizing is simple, cantilevering up to about 9" is simple, hanging out further is technically entering into the engineer's territory.

edit... I shoulda scrutinized the new codebook further;
https://www2.iccsafe.org/states/Virginia/Residential/PDFs/Chapter%205_Floors.pdf
There is a new table of cantilevers in the 06 edition that hasn't been there before, check out R502.3.3(1) 

I seem to have rambled on to everyone's frustration, good luck with your projects  .

[harry51]

Seriously cool calculators, Don. Thanks for sharing them!