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Overhang & Cantilevers


Jim Katen
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I find that in my area, many folks misuse the word 'cantilever'. A true cantilever is a beam supported only on one end. An overhang is a beam that is supported in more than one place and overhangs it's outside support by some distance. It makes a big difference. I don't suspect there are many cantilevers that are designed to also support exterior deck loads.

Heck, Jim, I thought that I was the only person who paid attention to that distinction. A tree branch is a cantilever.

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I find that in my area, many folks misuse the word 'cantilever'. A true cantilever is a beam supported only on one end. An overhang is a beam that is supported in more than one place and overhangs it's outside support by some distance. It makes a big difference. I don't suspect there are many cantilevers that are designed to also support exterior deck loads.

Heck, Jim, I thought that I was the only person who paid attention to that distinction. A tree branch is a cantilever.

The portion of the floor joist outside the foundation wall is cantilevered. It's supported only on one end.

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Hi,

Well, the distinction aside, this is good timing because yesterday's inspection had a deck secured to the overhang on a raised ranch. I knew that it's not supposed to be this way according to this document but does anyone know how far back the prohibition goes? This is a 1976 house and predates, by 20 years, my entry into this gig.

OT - OF!!!

M.

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The portion of the floor joist outside the foundation wall is cantilevered. It's supported only on one end.

Yes, I think the distinction is indistinct. It would rule out a few things as being cantilevered, like cantilever bridges and all manner of truss cantilevers.

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You are welcome to call it what you want "cantilever" or "overhang" .....essentially I am speaking about the upper floor joists that extend out above the first story wall. In the document provided by Frank it happens to call it the "Cantilevered Floor Overhang".

So I am just wondering, since I see alot of these "prohibited connections" in practical use, performing adequately, how others approach this subject.

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You can't cast the distinction aside if you're going to use the word. Katen put it very well: Tree branches are cantilevered, and an overhang is something else altogether. I am reluctant to disagree with as bright and learned a mind as Mr. Garet Denise, but I believe he is incorrect on this one. The portion of the joist that overhangs a point of supoprt is NOT technically cantilevered, though it is commonly and incorrectly referred to that way. It is, I believe, more accurately referred to as an overhang.

The distinction doesn't usually mean a whole lot in the course of everyday HI work, but tacking a 15' x40' deck tacked onto an overhang is a whole lot different than tacking one onto a cantilever. If you're going to 'call' 'em, you ought to call 'em right.

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Cast away, because you believe incorrectly.

A dictionary will explain it to you.

Or, if you will.......

An overhang is a projection of a building, as with a balcony or the currently discussed building component. It is an architectural assembly, not a structural entity.

A cantilever is, and I quote..."any rigid construction extending horizontally well beyond its vertical support, used as a structural element of a bridge (cantilever bridge), building foundation, etc....."

There is also the cantilever such as you describe, where the top is in tension and the bottom in compression, and only attached at one point.

So, a cantilever can be a number of things, including all those things we've been describing, which tends to make the distinction irrelevant at best, or simply wrong.

A mind as great as Galileo's, our father of modern physics, had a false understanding of such things. He thought, for example, that a cantilevered beam, (such as those supporting balconies) has a tendency under the action of the loads on it to rotate around the edge of it's lower surface. Leonardo da Vinci knew 150 years before him that such a beam, instead, tends to rotate around an axis halfway between it's lower and upper surfaces. This was rediscovered at the beginning of the nineteenth century by the French Mathematical physicist Navier.....

So, it's OK to misunderstand what a cantilever is. That puts us all in the same league as Galileo, except in this case it's not something to brag about.

Sure, a tree branch is a cantilever, probably the most amazing example of such a thing, a projection that can withstand hundreds of times it's original "calculated" load. Try and think of any manmade object that can do that. It's marvelous to ponder.........

A tree branch is not an argument, though.

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Friends,

I'm afraid I must respectfully disagree with Brother Mitenbuler as well; -and further, with his very loose descriptions of tension and compression, which only confuse this issue.

(Katen originally and) I offer the tree branch, not as an argument, but an example.

Perhaps I'm putting too fine a point on it, but there remains a wide, wide world of difference between a 1" overhang, and 10' long cantilever. I believe that the American Wood Council prohibition against attaching a deck to a cantilever is likely referring to one, and not the other.

Regardless, I'd encourage all inspectors to do their own research and satisfy themselves.

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I love to argue with Jim, especially when he's wrong......

I'm surprised, because usually Jimmy is amenable to rational thought, and avoids jumping to personal belief systems when discussing this stuff.

Loose descriptions of tension and compression is an attempt to dissuade one from thinking the idea is accurate or correct, although the description was pulled verbatim from Mario Salvadori's brilliant tome on structure "Why Buildings Stand Up". I tend to believe what's good for Mario is fine with me, and I think Brother Jim might want to think a bit before extemporizing on such things.

Of course, comparing 1" overhangs and 10' cantilevers and explaining that they're different is inarguable. But, it has nothing to do with anything........(?)........that we seemed to be talking about.

Everyone do their research, and come to your own conclusions, but beware of making up ideas about things that have been reasonably well defined by minds much more brilliant than ours for a couple hundred years........

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Ah, but you deflect and imply, or I infer, you to mean what I just said is not true, and in the same breath seem to say using credible references and quotes from reputable sources isn't something that applies either....(?).....

Evasive. And, it wasn't a Galileo quote. It was quoted from Mario's text, and was a humorous description of how these things can be misunderstood by otherwise brilliant minds.

So, since you seem welded to your original (incorrect) distinctions, maybe you can enlighten us as to the truth of overhangs and cantilevers, referenced this time with more than innuendo and personal distinctions...........[:-apple]

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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".

2009626211456_cantilever.bmp

Mod - This thread should be split.

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Few people make a distinction between an overhanging beam and a cantilevered beam and most dictionarys will reflect that. Most everyone jumbles up the terms so that any overhang is referred to as a cantilever and that's ok because most everyone else knows what they're talking about.

However in engineering and, more particularly, in the study of statics, there's a difference. An overhanging beam and a cantilevered beam are very different structures with very different reactions to loads. If you're designing one or the other, you have to use different equations to analyze the forces. When I was studying this stuff in the early '80s, I had a professor who warned us that one of the unforgivable sins in his class was to mistake an overhanging beam for a cantilevered beam. (He maintained a list of several unforgivable sins.) Perhaps that's why the distinction still sits in my head.

I've attached some pages from one of my old texts, The Manual of Steel Construction. However, you'll find exactly the same beam diagrams in any reference devoted to the structural design of beams.

Colloquially, I really don't see any downside to calling any overhang a cantilever. Everyone knows what you mean. However, a cantilevered beam and an overhanging beam are most emphatically not the same thing.

- Jim Katen, Oregon

Download Attachment: icon_adobe.gif Beam_Diags_Cant_v_Ovrhg.pdf

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I am certainly missing something in this discussion. And thinking that maybe I could learn something. So I will ask, if Bills' diagram above is that of a cantilevered floor joist system, then what does an overhanging floor system look like?

The tree branch concept has me perplexed, as it relates to nothing I can think of in home construction.

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I am certainly missing something in this discussion. And thinking that maybe I could learn something. So I will ask, if Bills' diagram above is that of a cantilevered floor joist system, then what does an overhanging floor system look like?

The tree branch concept has me perplexed, as it relates to nothing I can think of in home construction.

Bill's diagram shows an overhanging beam. It has a negative reaction at the end opposite the load. He -- and pretty much everyone else in the English-speaking world -- refers to the overhanging portion as a cantilever.

A real cantilever, however doesn't overhang anything. Look at the beam diagrams in my previous attachment. The end of a cantilevered beam is entirely captured inside a support. A tree branch, an extended human arm, and a flagpole are examples of cantilevered beams. Some spiral staircases have cantilevered supports under each tread; the supports are only attached to the center column. You'll find very few true cantilevered beams in wood-frame construction.

- Jim Katen, Oregon

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I believe that Jim and Jim are only sticking with the strict engineering definition of cantilever.

Click to Enlarge
tn_200962702541_cantileverold.jpg

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I've pointed out that the term cantilever, in building architecture, has been used as the name of the overhanging portion of a beam or truss for over 100 years. This includes well-known early published architects, engineers, university professors and Fellows of AIA, all considered experts by their piers and contemporaries. I hardly think they are among those of us who "jumbles up the terms". I've noticed a few words in the English language that have more than one definition. I've also noticed a few words now used in architecture, were earlier used to describe something completely different.

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Architecturally, which is what I thought we talk about in TIJ, these mean mean different things than in bridge building or high steel construction.

Historically, as shown by Bill, these terms are used in ways that are markedly different. I don't know how one can take definitions from one field of study and apply them to another in the way they're being applied here, implying that any idea different than that proposed by some professor from long ago and far away is jumbled.

Ain't jumbled.

Wow, I successfully split a topic.........I think.......

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Architecturally, which is what I thought we talk about in TIJ, these mean mean different things than in bridge building or high steel construction.

No. They don't. People use and misuse the term cantilever on bridges & high steel construction in the same way they use it and misuse it for residential architecture.

The beam diagrams apply to all beams, whether they're in a high rise or in a kitchen countertop. It's all the same terminology and the same principles, just on a different scale.

Historically, as shown by Bill, these terms are used in ways that are markedly different. I don't know how one can take definitions from one field of study and apply them to another in the way they're being applied here, implying that any idea different than that proposed by some professor from long ago and far away is jumbled.

I think you're missing my point. Let me explain more clearly.

1. (Fact.) A cantilevered beam and an overhanging beam are two different structural entities. Each reacts to loads in a different way. It doesn't matter if the beams are part of a bridge, a high rise building, a residential building, or a doghouse. This isn't a matter of opinion or conjecture. This is a simple fact. Anyone who designs these beams will have to understand their different properties.

2. (Fact.) At one time, someone (I don't know who, very likely the Astronomer Royal) decided that one should be called a "cantilevered beam" and the other an "overhanging beam." These terms have been used consistently for a long time -- well over 100 years anyway. (BTW, I found an excellent collection of beam diagrams here: http://www.awc.org/pdf/DA6-BeamFormulas.pdf )

3. (Fact.) During this same period of time, people have used the term, "cantilever" to describe any portion of a beam that extends past it's suppport. They do this regardless of whether the beam is a cantilever or an overhang.

4. (My opinion.) Using one term to describe two entities can be confusing and inaccurate, especially when each entity has its own perfectly good term. I find it similar to the misuse of the term "tuckpoint" -- hey, everyone knows what you mean, so what's the problem? I've met masons who've been working with brick & mortar their whole lives and they use (misuse) the term tuckpoint. Surely it must be ok. Or the term "telephone tile" in lieu of "structural terra cotta." Hey, everyone says telephone tile.

How about "neutral wire?"

Or "cinder" block?

Copper "pipe?"

How about "level" when they mean "plumb."

Construction is full to the brim with words that are used inaccurately. For the most part, everyone understands what people really mean. But it's important to remember what the real terms are.

- Jim Katen, Oregon

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Well this has been a lesson in futility... However an interesting topic for the historians and wordsmiths among us.

I will continue to call the joist extension of an upper level floor system, over the lower story, a cantilevered floor system. That seems to be what everyone in my location knows them as, and nobody ever seems to looked confused when that term is applied. Like I say, call them what you want, I don't read your reports. :)

So anyone up for answering what to recommend with split level homes and second story decks hanging off the wood framing that hangs over the edge (I guess similar to a tree branch? ) ? :) Any way that was the real question. The answer will be interesting considering the differing opinions on the word cantilever.

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Is it possible that each joist extended past the wall below it is cantilevered and the end product is a overhang?

Kind of like when you take 2x4 studs and nail them between 2 x 4 studs, which now become top or bottom plates and the product is called a wall??

Back to the original question.

If the house was built in 1976 (32 years ago) and there is no sign of failure, what ever they did must have worked. If you see signs of repairs such as added cantilevered joist, then you may have a cause for concern.

Sorry I do not have any big words to add.

Have a great day

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Entertaining and enlightening, but this early in the morning it makes my head spin.

Need coffee!

I'd have to come down on the "overhang" side for most residential construction.

I've always thought of cantilevers as being supported only at one end, kinda like the big ole cantilevered arm holding up the traffic lights that's only supported from the one pole on the corner of the intersection.

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