Cantilevered Wood Beams 1

[medeek]

Where in the building code, ASCE 7-10 or the AWC NDS does it specify the min. backspan for wood beams? From somewhere I have the 1/3 cantilever rule but I need to nail this down.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[JAE]

Not a code provision that I've ever seen....just pragmatism in controlling deflections and uplift connections on the back end.

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[mike20793]

I've seen it listed as a manufacturer's recommendation and I believe it is in the IBC light gage framing provisions, but I can't remember for certain of the top of my head.

 

[dhengr]

Medeek:
I think back spans are called out/specified in prescriptive codes like the IRC, because many nail benders would put the fixed end of a canti. in a joist hanger and if it didn’t immediately fall down, call er good. Seeing it in the IRC and some product lit. is for self protection, liability protection, because many builders just don’t seem to know any better. Most codes allow engineers some latitude because they should know better, and actually be able to design the back span or the moment connection, to fit the actual conditions and their needs.

 

[KootK]

If we're talking about your recent window wall, I wouldn't actually consider those joists to be cantilevered. I'd consider them two span with a pin at the ridge.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

What JAE said. The typical rule of thumb from lay people around here is 2:1 backspan:span

 

[medeek]

This is for a different application where I have a beam jutting out of a wall supporting a couple of monopitch trusses. The overhanging roof will be about 4' but the designer and owner do not want to put a post down for this overhanging portion of the roof. My idea is to replace the top plate of the wall with a cantilevered beam (4x10) and then strap and holdown the backspan with a post to the foundation/stemwall. The snow load is 50 psf so even though the overhang is not ridiculous I still need to take a serious look at it and engineer the whole mess. I will also connect the truncated dbl. top plate to the beam with a horizontal strap to maintain diaphragm chord continuity.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

I've done what you're describing here often enough. depending on the length of the wall I've just made the entire top plate an LVL beam and still strapped it down at the back corner.

 

[KootK]

This might be a good application for an upset beam with a 3.5" wide by 9.25" deep pocket built into the mono trusses. That way you could just put the 4x10 on top of the wall and keep much of the wall detailing looking conventional. Whichever system you use, I'd take care to prevent the formation of a hinge between the bottom of the beam and the top of the wall. This might be another reason to go upset: you'll have stuff near the beam to tie it back to.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

I've never heard of an upset beam before, that is a tricky way of doing it. I've done a similar sort of thing where I've put beams inside of walls so they don't hang down into the living space. The only problem I see with this is the need for the truss manufacturer to create these same pockets in the other trusses running back along the wall, these will be raised heel attic trusses so that might cause some problems.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[BSVBD]

medeek,

First, to address the question, IBC 2308.12.6.1.2, references the 2:1 back span. It MAY be elsewhere.

(Wisconsin is neither IBC 2015 nor ASCE 7-10 yet, so, I'm not there either.)

Notwithstanding the entire "Irregular structures" portion of 2308.12, prior to the "back span" reference, it is stated, "Such irregular portions of structures shall be designed to resist the forces specified in Chapter 16 to the extent such irregular features affect the performance of the conventional framing system." This sounds like an "empirical" statement putting the ultimate responsibility and freedom on the EOR.

I believe this agrees with JAE and specifically the last sentence of dhengr.

But, why do you "need to nail this down"? (assuming no pun intended)

Is a code reviewer or inspector requiring or requesting this of you?

I don't have a specific example, but, i KNOW I've (intentionally) "violated" the 1/3 or 2:1 "rule-of-thumb" in necessary situations. I believe i've even had less of a back-span than the cantilever; therefore, the "back-end" connection is critical to resist the reaction.

 

[medeek]

Somewhere I assumed there was a hard and fast rule about cantilevers but I could not find it so I thought it would be good question for the board since someone would probably know off the top of their head which reference to go to.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

these will be raised heel attic trusses so that might cause some problems.

Depends. If the attic trusses don't cantilever, other than the top chord overhang, then you could just cut them short 3.5" and hanger them from the beam. You incur the cost of the hangers but then you have something positive to control that unwanted hinging that I mentioned. With the dropped header, I don't see how that gets resolved.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

Looks like a 4x10 DF No. 2 will work with a 4' cantilever and a 8' backspan. My down load at the fulcrum is 2090 lbs and my uplift at the backspan is only 404 lbs.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com