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The Function and Aesthetics of Cantilevers
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Paul Ringstrom



Joined: 17 Sep 2005
Posts: 4210
Location: Mason City, IA

PostPosted: Wed Feb 03, 2016 10:45 am    Post subject: The Function and Aesthetics of Cantilevers Reply with quote

http://blog.buildllc.com/2016/02/the-function-and-aesthetics-of-cantilevers/
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Owner of the G. Curtis Yelland House (1910), by Wm. Drummond
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SDR



Joined: 17 Jun 2006
Posts: 18152
Location: San Francisco

PostPosted: Wed Feb 03, 2016 12:02 pm    Post subject: Reply with quote

Beautifully clear explanations of interesting architectural and structural solutions.

Note that the datum -- the continuous horizontal element which (we hope) appears inside as well as outside this house -- is placed nine feet above the floor. Wright and Schindler chose a considerably lower position for their datum, bringing the lowest ceiling planes closer to the inhabitant and permitting higher ones elsewhere without committing the sin of "architectural grandomania" -- and without leaving the occupant swimming in featureless space below unnecessarily lofty ceiling planes. (Not, I believe, coincidentally, the lower overall profile makes for a more expansive structure in drawings and photographs -- win/win, for the architect and for his client !)

Thanks for bringing this to us, Paul.

SDR
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DavidC



Joined: 02 Sep 2006
Posts: 7125
Location: Oak Ridge, TN

PostPosted: Wed Feb 03, 2016 1:52 pm    Post subject: Reply with quote

All about eaves


David
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Tom



Joined: 30 Jan 2011
Posts: 2763
Location: Black Mountain, NC

PostPosted: Thu Feb 04, 2016 10:25 am    Post subject: Reply with quote

Wright made a career out of violating the 2/3rds cantilever rule of thumb. He might even have actually used math.


(helpful observation on "datum" height ... by the way. thanks.)
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SDR



Joined: 17 Jun 2006
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Location: San Francisco

PostPosted: Thu Feb 04, 2016 7:12 pm    Post subject: Reply with quote

The 1/3 - 2/3 rule can be understood instinctively, I think: plenty of weight on the "back-span" side of the fulcrum to counter loads on the cantilevered side. As mentioned in the architect's presentation, the formula can be adjusted to suit circumstances; ". . . additional structural attachments to accommodate the increased potential for uplift" is the remedy here, as mentioned (if not specified) by the writer.

In the absence of specific information, we will have to assume that Mr Wright and his engineer(s) employed such means, as needed, in the work. A thorough study of this part of Wright's work awaits creation, as far as I know, though a few anecdotes and individual cases have come to light, Fallingwater being perhaps the best-publicized.

SDR
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Tom



Joined: 30 Jan 2011
Posts: 2763
Location: Black Mountain, NC

PostPosted: Thu Feb 04, 2016 8:09 pm    Post subject: Reply with quote

I think I disagree. I'm not an engineer, so ultimately I really don't know.

However, a beam that cantilevers 1/3 of it's length (might) should properly be classified as a cantilevered continuous beam.

Whereas a cantilever "proper" is a beam fixed at one end only.

Meaning that a "proper" cantilever relies (more)on the fixed connection, the strength of the material and the properties of the beam section, beyond the support, to resist the forces - than it does the embedded support of it's 2/3rd length.

I think the innovation and drama of Wright's work in cantilevers was seriously in this latter category and best understood that way. This is my point.
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SDR



Joined: 17 Jun 2006
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PostPosted: Thu Feb 04, 2016 8:35 pm    Post subject: Reply with quote

Interesting take. I'm not sure an architect or engineer would agree with the wording of your definition; as I see it "cantilever" properly includes an efficient and effective means of securing the extended beam and its loads. But the subject deserves investigation.

I do wonder what the layman thinks when he or she sees a cantilevered plane. It would make me quite uncomfortable to view such a structure while believing that the plane -- which I can't imagine as having no weight -- to be simply "stuck onto" the supporting structure. Is this only the result of familiarity with structures ?

Isn't there a sense of how our surroundings, the natural and the manmade, operate; a "body sense" -- built into us ? When we see a see-saw, don't we instinctively know what happens when someone sits on one end of it ? Even if that understanding of the physical world is learned after birth, on the playground of our existence, as it were, don't we as adults have such sense as a part of our vocabulary ?

I can understand the average person not knowing how buildings are put together. When you look at one of Wright's cantilevered carport roofs, do you for one minute believe that it is simply bolted, or welded, or glued to the front of the building, that it doesn't extend back into the structure for its support ?

It is certainly true, finally, that Mr Wright did not celebrate the structural means he employed; indeed I believe he intended to conceal them, or perhaps to see them as incorporated seamlessly into the fabric of a building.

SDR
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pharding



Joined: 25 Jun 2005
Posts: 2248
Location: River Forest, Illinois

PostPosted: Thu Feb 04, 2016 9:56 pm    Post subject: Reply with quote

Cantilevers are quite popular with architects right now. The bigger the cantilever the better. As FLW discovered they look very dynamic and very cool.
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Paul Harding FAIA Owner and Restoration Architect for FLW's 1901 E. Arthur Davenport House, the First Prairie School House in Chicago | www.harding.com | LinkedIn
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SDR



Joined: 17 Jun 2006
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Location: San Francisco

PostPosted: Thu Feb 04, 2016 9:59 pm    Post subject: Reply with quote

Spoken like a true architect !

SDR
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Roderick Grant



Joined: 29 Mar 2006
Posts: 9515

PostPosted: Fri Feb 05, 2016 2:59 pm    Post subject: Reply with quote

Three things must be taken into account when evaluating the cantilevers at Fallingwater:

Both client and supervising apprentice (Bob Mosher) were concerned that FLW had under-engineered the cants, so they had an engineering firm redo the specs without FLW's knowledge. As Mosher told me, he believed the house would not have survived as designed. That means the extent to which the cants failed cannot be laid entirely at FLW's feet, if at all. FLW blamed the problems on the weight of excessive amounts of steel reinforcing. Whether or not that claim is valid, the cants did show failure over time, but with the revised scheme, not with FLW's design.

The design called for a slight upslope in the cants to allow for settlement. The contractor built them dead flat, which resulted in the sagging.

In addition to the reinforced concrete beams holding up the cants, FLW expected the parapets around the perimeters to help stiffen the structure. To what extent that was done as he wished and has played a part in the overall integrity of the structure, I don't know; I have never seen any as-built structural plans. But the folded plate theory is certainly valid.

The canopy from main house to guest house is proof that FLW could do cantilevered structure properly. As far as I know, that canopy has never had any structural problems. Has it?
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SDR



Joined: 17 Jun 2006
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PostPosted: Fri Feb 05, 2016 5:58 pm    Post subject: Reply with quote

Roderick, would you identify for us the cant, perhaps using the illustration at the bottom of this page ? Are you referring to a tension member ?

https://failures.wikispaces.com/Fallingwater

SDR
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Roderick Grant



Joined: 29 Mar 2006
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PostPosted: Fri Feb 05, 2016 6:08 pm    Post subject: Reply with quote

Just an abbreviation of cantilever.
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SDR



Joined: 17 Jun 2006
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Location: San Francisco

PostPosted: Fri Feb 05, 2016 6:13 pm    Post subject: Reply with quote

Architects, what is the history of pre-tensioned and post-tensioned steel in concrete construction ? Were either of these techniques in use in 1936 ?

I take it that the Fallingwater repair employs post-tensioned rods or cables. If employed originally, is there any doubt that the cantilevers could have remained firm ?

SDR
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SDR



Joined: 17 Jun 2006
Posts: 18152
Location: San Francisco

PostPosted: Fri Feb 05, 2016 7:23 pm    Post subject: Reply with quote

The linked article answers some of my questions, and provides additional information. A comparison to the Robert Silman Associates page on the project is interesting; the latter provides only a brief but succinct bullet list of the problem and its solution.

http://www.silman.com/services/historic-preservation/fallingwater/

Mr Meek's much more forthcoming description of problem, causes, and remedies contains a couple of surprises and a disappointing summation.

One surprise (to me) is that the engineers found that the four T-bar mullions connecting the first and second floors were actually in tension, not in compression; the stiffer second-floor reinforced concrete cantilever system was able to take some of the load of the main-floor cantilever, presumably as it crept downward over time. Reinforcing plates were welded to those mullions (another surprise); they may now be in compression once again following the raising -- by 3/4 of an inch -- of the first-floor system. (Can anyone really have thought that the architect wouldn't have wanted to connect the first and second floors at their extremity, so that they would move, if at all, together ?)

I have wondered about the smooth and attractive underside of the main floor of the house -- essential, considering the inevitable views to be (drawn and) recorded from below the house. This surface turns out to be a slab in compression, poured integrally with the bottoms of the four main cantilevered beams. The failure of the system seems to have been caused by stress to the point of elongation of the rebar placed near the upper edges of those beams -- plus, perhaps, storm damage.* Wright's indignation at criticism -- which came before the reports of deflection of 1 3/4" after removal of the forms -- seems ill-supported; it is clear from this account that surreptitiously added rebar still failed to meet the need -- thus implying that Wright/Glickman's specification would likely have been disastrous ? Note Glickman's response: “Oh my God, I forgot the negative reinforcing!”. We await explanation of the term in this context . . .

SDR

*I refer to Edgar jr's description of the storm of August 1959, which attacked the house with full force: high water (and lots of sand) entering the main floor "through the bridge to the guest wing . . . the stairs became a cascade," "heavy scaffolding," irremovably attached to the house for a paint job, shaking the structure "like a terrier shakes a rat." Elsewhere he mentions storm-driven floating debris striking the suspended stair -- which was subsequently attached to the stone stream-bed.
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SDR



Joined: 17 Jun 2006
Posts: 18152
Location: San Francisco

PostPosted: Fri Feb 05, 2016 8:43 pm    Post subject: Reply with quote

This page is almost guaranteed to give you more info on the cantilever than you wanted. Tom will like the first diagram; who knew there are spring-assisted cantilevers ??

https://en.wikipedia.org/wiki/Cantilever

SDR
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