Overhang & Cantilevers

[Jim Morrison]

Based on the AWC's prohibition on attaching decks to cantilevered floors, Hmiller asked:

"Rarely, if ever, have I seen these free standing. Does anyone ever make a recommendation to add an additional inner support beam with posts and piers to take the load off the ledger, and the cantilvered floor system?"

Drawing a distinction between 'cantilever' and 'overhang' may be akin to picking fly shit out of black pepper for most home inspectors, most of the time. However, if we are going to read technical manuals and use them to inform our work, then we need to understand the words they use. The truth is: I can't recall ever seeing a true cantilevered floor in residential construction. They're impractical. I see a lot of overhangs.

I honestly don't care how everyone else uses -or misuses the word. My point is simply: It may just be that the AWC isn't saying what he thought it was.

So, here's my answer to hmiller's question:

I don't think I've ever seen a deck attached to a cantilevered floor system. As I mentioned, I can't recall ever seeing a cantilevered floor system in a residential home. I do not criticize decks attached to overhangs unless I see them causing a problem. I suspect that the AWC uses the word cantilever the same way I do. If you're still interested, you should write them and find out exactly what they meant.

[StevenT]

I am almost afraid to jump in, but I will.

Putting it very simply, isn't a cantilever basically a "bracket?"

If so, wouldn't an overhang, whether it be the flooring system of a house or the roadway of a bridge, that is supported by these "brackets," be considered "cantilevered?"

When I think of a beam that is overhanging, and the forces that it creates, I think of a lever. The longer the overhang, the stronger the force... just like any lever. The cantilever supports against these forces, so that the beam cant lever.

[kurt]

So, what would a torsion box be considered?

There seem to be conflicting descriptions of what these things are. On both hands, there are distinctions that seem to contradict this idea that it's all one or the other, all by respected and credentialed experts in their field.

It's edging into a hierarchical tit for tat, and supporting one's belief system with the idea these are "facts".

How can that be? Are the notables shown by Kibbel all wrong? In their confusion, they are jumbling up definitions like a bunch of HI's gone haywire?

Is a cantilever bridge actually an overhang? How do the facts come down on the definition of a cantilever bridge?

[hmiller]

I am talking about the connection detail on page 7 of this document (9 if you are reading the Acrobat page counter)

http://buildingcodes.jocogov.org/docume ... 20Book.pdf

I really don't know where you would find a situtation in residential construction where you would have a detail like Jim is talking about. A structural member supported only on one end?

[hausdok]

Ich bin ein Berliner! []

OT - OF!!!

M.

[Phillip]

Y'all blowing this redneck country boy mind. Some of this is out of my league.

What I was taught and for what I read in some of the books and guidelines a chimney, bay window and any item that is setting on a few joist that runs out and support it is a cantilever. There is guidelines on how this is suppose to be done.

A overhang is like a second floor that runs out pat the 1st floor.

If I am wrong let me know in a way that most of us can understand. I what to get it right.

Each system is designed to carry a certain type of load and when you added something to it like a deck you are adding a weight load that it was not designed for.

Just trying to learn.

[mridgeelk]

http://www.ilevel.com/literature/TJ-4000.pdf pages 8, 12 and 13 agree with you, Bill.

I'm not sure if butting in will put an end to Jim & Kurt's playtime, but their debates always go over my head.

In engineering, the term cantilever originally was used to describe a projecting beam, anchored at one end.

In architecture, cantilever was also used to describe a projecting beam used as a bracket to support another structure or member.

Since well over a century ago, the portion of a beam (or truss) extending beyond a support has been commonly referred to as a cantilever (or cantilever arm). If the beam is supported at two points but extends beyond one of the supports, the portion extending beyond a support is still a cantilever (or cantilever arm). The portion of that beam spanning the two supports is then called the "anchor span".

Mod - This thread should be split.

[Jim Katen]

Try looking at it this way:

The following two diagrams represent two different types of beams. Let's call them A & B. They both have sections that project beyond their supports and that bear weight out at the unsupported end but they resist that weight in different ways.

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Would you agree with me that these two different types of beams ought to have different names?

Oddly enough, pretty much everyone calls them both "B", even though B is far more rarely used than A. I find that curious. Does anyone else?

After some consideration, I think that the word cantilever has gained wider accepetance mostly because it sounds cool. Kurt asked about cantilever bridges. In my lexicon, there are two types. Some are made up of genuine cantilevers that are embedded in the earth at the banks. Others use overhangs at each side to support a center section. But all of them are called cantilever bridges. Might it be because no one would want to spend $250 million on something as dorky sounding as an "overhanging bridge?"

I'd like to hear some opinions from the engineers & architects on this forum. How do you explain the disconnect between the beam diagrams and the common broad brush use of the word cantilever?

- Jim Katen, Oregon

[Jim Katen]

Y'all blowing this redneck country boy mind. Some of this is out of my league.

What I was taught and for what I read in some of the books and guidelines a chimney, bay window and any item that is setting on a few joist that runs out and support it is a cantilever. There is guidelines on how this is suppose to be done.

A overhang is like a second floor that runs out pat the 1st floor.

If I am wrong let me know in a way that most of us can understand. I what to get it right.

Each system is designed to carry a certain type of load and when you added something to it like a deck you are adding a weight load that it was not designed for.

Just trying to learn.

Phillip,

Setting aside the names for a moment, a second floor that projects out past the first floor and a bay window are both (usually) supported the same way, by joists the run out past the outside wall. My contention is that these are overhanging joists. I call them this because if I wanted to analyze the stresses in these joists I'd use the formulae that are associated with a diagram labeled "overhanging beam." However, pretty much everyone calls them cantilevers.

A fireplace hearth extension, in contrast, is supported in an entirely different way. It's usually a monolithic extension of the hearth. My contention is that a fireplace hearth extension is a true cantilever. If I wanted to analyze the stresses in it, I'd use the formulae associated with a diagram labeled "cantilever beam." Pretty much everyone also calls these cantilevers too.

As others have pointed out, most people use the word cantilever to describe pretty much anything that overhangs. Apparently, Jimmy & I are the only ones who find this curious.

BTW, I disagree with Jimmy about the AWC's stance on deck attachment. I think that when they say cantilever, they really mean what he & I call an overhang. (Even though they publish a set of beam diagrams that defines a clear distinction between the two.)

- Jim in Oregon

[Brandon Whitmore]

BTW, I disagree with Jimmy about the AWC's stance on deck attachment. I think that when they say cantilever, they really mean what he & I call an overhang. (Even though they publish a set of beam diagrams that defines a clear distinction between the two.)

The AWC disagree as well. They specifically call the thingymajigs overhangs in this: http://awc.org/Publications/DCA/DCA6/DCA6.pdf , and specifically indicate that overhangs should not support a deck.

By the way Jim, now I need to learn how to read engineering shear/ moment diagrams. My head is gonna hurt the rest of the night-- thanks a bunch.

[Garet]

Many building components get different names by different folks in different trades/professions or in different parts of the country. A good example is band joist/band board/rim joist. Sometimes it's easy to get lost in semantics, as Jim's professor did. I've never heard a distinction between these two cases - because in a functional sense there isn't any. Every engineering community I've been in calls both of these a cantilever. For that matter, the term cantilever is even used for a vertical member with lateral loading. But let's set semantics aside.

From the standpoint of loads and stresses within the cantilever/overhang, the two cases that Jim & Jim are trying to differentiate are, in fact, exactly the same thing. Even the amount of curvature of the cantilever/overhang portion is the same. The only difference is that the back-span on the longer beam allows some rotation at the point of support, resulting in different overall deflections of the cantilever/overhang portion. The other case is theoretically 'fixed' against rotation at the point of support. In the real world of houses the deflections are insignificant. Also in the real world of houses, the 'cantilever' according to Master Katen's definition does not exist as a practical reality. No one builds that way (at least not with wood) - because they can't!

The myriad of equations Jim posted define loads and deflections both in the cantilevered portion and within the span between the supports of the extended version. If you look only at the loads, and look only at the cantilever/overhang portion, that seething mass of mathematics simplifies to the same values for both configurations (as I've highlighted below). The deflections are different (because of the rotation at the support), but the loads and stresses in the cantilever/overhang are the same.

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When I went through engineering school the 'unforgivable sin' was a failure to draw a 'free body diagram'. A FBD plucks out a discrete portion of a structure and isolates all the forces on it. One of my professors kept a large rubber stamp in his desk drawer that read "Draw a FBD". If a student came to him with a question but failed to start the calculations with the requisite FBD the stamp came crashing down (red ink, of course). Few students made that mistake twice.

So let's draw a FBD for both of these configurations. We'll use the case of a point load at the outer end and we'll cut the section a fraction of an inch outside the support.

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The resulting loads (both shear and moment) are identical in both cases. Since the loads are the same the internal stresses will be the same as well. The same is also true if we apply any other load configuration. In essence, the portion of the cantilever/overhang outside the support has no idea what's actually holding it up. It simply knows that it is being held.

Now that we've taken a (long) detour through the land of semantics and engineering mechanics, let's go back to the question at hand, which is "Can a deck safely be attached to a cantilevered floor joist?" Since virtually all decks fail at connections, let's consider how things are connected. The typical band joist is attached to the outer end of cantilevered floor joists only with nails into end grain. Attaching a deck to such a structure (even with lag bolts from the deck ledger into the band joist) would still leave the band joist nails as the weak link in the chain. That's a good enough reason for me to say it's a bad idea. FWIW, the document from Johnson County shows a way to strengthen that connection (Figure 7), but I have not seen that detail elsewhere.

http://buildingcodes.jocogov.org/docume ... 20Book.pdf

Perhaps we could get Dr. Woeste to offer his opinion?

[Jim Katen]

. . . The myriad of equations Jim posted define loads and deflections both in the cantilevered portion and within the span between the supports of the extended version. If you look only at the loads, and look only at the cantilever/overhang portion, that seething mass of mathematics simplifies to the same values for both configurations (as I've highlighted below). . . .

But of course they're the same. I never said that they were different. My point is that the supports are different.

- Jim Katen, Oregon

[kurt]

I think I just saw infinity........

I'm trying to remember back to where someone said these two things had easily identified distinctions. Now they're the same, but the supports are different....(?)

I'm blaming Morrison for starting this with his dang distinctions.

What I'm taking away from this is all the things I've always thought were cantilevers, are cantilevers. And don't go attaching a deck to them, whatever they might be.

[Jim Morrison]

It may also be that the distinction is simply outdated. I learned it from my father, whom I presume learned it at university when he was studying to become a civil engineer half a century ago.

[kurt]

See? See?....

It's the Morrison's fault.

[hausdok]

Jeez,

And folks call me anal retentive! If they awarded Oscars for anal retentiveness some of you guys would be getting multiple awards.

ONE TEAM - ONE FIGHT!!!

Mike

[kurt]

I'm holding out for the Lifetime Achievement Award........

[Jim Morrison]

More thread drift...

Why does the word "President" now appear beneath my visage to the left of my posts?

Was I elected to some lofty position while I wasn't paying attention, or (more likely) are you looking for a figurehead to hang blame on when things go wrong around here?

[kurt]

Mike, the boy'z wisin' up.........we need a new plan......

[hausdok]

Maybe I shoulda made him the Imperial Dinkledorf. Oh well, too late now.

OT - OF!!!

M.

[Garet]

Perhaps we could get Dr. Woeste to offer his opinion?

Turns out he already has. See June '09 JLC, p19. BTW, he uses the term 'cantilever' when the joist extends out beyond the foundation. (neener neener neener!)

[hausdok]

Timely discussion,

I did a 20-year old home yesterday that's had the inner end a a large deck hung off of a 10-ft. wide two story tall bumpout. Upstairs on the second floor, the floor of the bumpout actually slopes downward to the exterior wall. Below that in the dinette area where the back entrance is I could see where they'd replaced hardwood flooring equivalent to the depth of the overhang. I'd bet it's been shimmed to flatten it out.

ONE TEAM - ONE FIGHT!!!

Mike

[FrankWoeste]

Perhaps we could get Dr. Woeste to offer his opinion?

Turns out he already has. See June '09 JLC, p19. BTW, he uses the term 'cantilever' when the joist extends out beyond the foundation. (neener neener neener!)

Actually, there is a technical difference. In the case of a cantilever structural model, the "slope of the elastic curve" at the support point is zero (no rotation). In the case of a beam overhang, the slope of the elastic curve at the bearing location next to the overhang is, in general, non-zero. At the bearing location, the rotation of the beam or (elastic curve) can be counterclockwise, zero, or clockwise depending upon the loads on the entire beam. The term cantilever seems to convey the issue easier when talking about or writing about a deck being attached to one.

[Michael Brown]

More thread drift...

Why does the word "President" now appear beneath my visage to the left of my posts?

Was I elected to some lofty position while I wasn't paying attention, or (more likely) are you looking for a figurehead to hang blame on when things go wrong around here?

Mike and I have always wondered how you managed to change it? Check your profile. Can you change it back?

If not we can just leave it, then if any anything bad happens up here we can just blame the "President"... its the way its done. [:-tong2]

Michael Brown

[Jim Morrison]

I don't think I put it there. I just checked my profile and can't find the word "President" anywhere in it.

Besides, you know that if I had done it I wouldn't have written 'President', I'd probably have gone with: "HHIC"