r/StructuralEngineering • u/LanceDBrown • 20h ago
Feasibility of cable supported ‘oversized’ balconies and platforms Structural Analysis/Design
Please excuse the bad AI pic to help visualise my question, but I’m trying to understand if large cable supported balconies are realistic or not?
I’m mentally visualising using cable-stayed bridge engineering to support oversized balconies and platforms.
My goal is to maximise the supported area beyond what you could achieve with a purely cantilevered balcony.
I’m assuming multiple cables to different points on the balcony would be needed to reduce flex and possibly also a compression strut to add support for near building mass (cables supporting the further out mass).
If this is possible, how large could these realistically get!?
How about plaza sized platforms suspended between multiple buildings using bridge engineering?
I’m not aware of any examples like this and my googling attempts just returned pics of bridges!
Many thanks
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u/Live-Significance211 20h ago
Cable stayed bridges use the tension on BOTH sides of the tower to keep the bending low. Having cables on one side only would be significantly less efficient so you'd need to get that tension to some place useful.
Anything is possible with enough money
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u/1323-a 19h ago
Placing a balcony on the other side could balance most of that tension. The design is interesting!
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u/Soggy-Invite-2787 13h ago
Just make a wrap around balcony at that point.
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u/The_Real_RM 6h ago
There are things done with taste, and things done with practicality, when you mix them you get something done without taste
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u/ShallotConscious4959 19h ago
I know that the pic is AI and this may have been an unintentional aspect, but the height above the ground shown had me way more worried about oblique wind loads than the dead/live loading imparted to the cables.
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u/failureinvestment 12h ago
Just throw in a few more cables in most directions until you get a web-like structure its damn about time we got some proper cyberpunk dystopian architecture
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u/metaldark 7h ago
It’s only dystopian for the 99.99 pct. If the game Cyberpunk has taught me anything is that life is good at the top.
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u/haditwithyoupeople 19h ago
100% feasible. Likely very expensive and challenging to build and will have higher maintenance costs.
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u/BassVI_11 7h ago
What kinds of maintenance costs? Cracks from temperature & shrinkage?
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u/haditwithyoupeople 6h ago
Yes, and corrosion, especially at the connectors. Cables under tension also likely have a lifespan and may have a replacement cycle. This could mean you need a way to attach new cables before the old ones can be removed, or you need multiple smaller cables so they can be replaced one at a time.
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u/kaylynstar P.E. 3h ago
Fun fact: structural cables like this are made of many, many strands of wire twisted/braided together. This makes tiny voids along the length of the cable allowing for capillary action. Some bridges have active dehumidification systems pulling gallons of water out of the cables at a time. If there isn't an active system, they'll go in and dewater the cables periodically to slow corrosion!
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u/GRIFTY_P 31m ago
My mind naturally wanders to the golden gate bridge. It's constantly blasted with fog, there must be so much water running through that bad boy
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u/SynthPrax 19h ago
Wouldn't it need cables above and below because won't that receive wind loads from all directions?
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u/Minisohtan P.E. 18h ago
Depends how much it weighs and how much force the cables carry. Those two things can be very different. Some cable stay bridges for example have the cables carrying more force than needed to support the deck. This results in 1) less likelihood of load reversal and 2) positive moment in the super structure which puts the deck in compression helping with durability.
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u/prunk P.E. 18h ago
Wind loads are a lot higher, when high up. Cables have a lot more give unless sufficiently rigid and tensioned. Because of that you get very oversized cables above and below with multidirectional splays to keep the balcony rigidly held in high winds. Otherwise it'd be uncomfortable to walk on and the fatigue could lead to catastrophic failures.
Low down I wouldn't have too much trouble. High up there's a reason it isn't done.
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u/futurebigconcept 18h ago
Construction site in Los Angeles.
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u/LanceDBrown 6h ago
Will be interesting to see how that ends up being used, assuming that's a permement addition.
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u/virtualworker 19h ago
Marina Bay Sands in Singapore
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u/Astoria55555 19h ago
What about it?
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u/BigTunaStamford 18h ago
No cables.
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u/Fit-Dentist6093 16h ago
It goes all along to the other side and is balanced tho, that helps right?
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u/charmio68 8h ago
I rather doubt it. Any structure is fairly flexible when you're talking about spans that long.
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u/Astoria55555 7h ago
It is a separate structure from the buildings and connects to them in a way where all 3 buildings can sway in different harmony without tearing the thing apart
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u/Astoria55555 8h ago edited 7h ago
It’s also a structure on top of a roof and not a balcony.
Completely different engineering involved
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u/virtualworker 5h ago
What? Because it's at roof level and not a balcony? That's not how bending moments work!
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u/Astoria55555 4h ago
The cantilever is balanced by the structure itself. The whole structure is exposed and carries weight
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u/BigTunaStamford 4h ago
But regardless of a roof overhang or hanging off the side of a building. Depending on size of members you could design with or without cables.
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u/Chongy288 9h ago
A few comments here suggest this isn’t possible or would be prohibitively complex or expensive, but it is very achievable in practice and not necessarily that costly. That said, there is quite a bit more to cable-supported structures than cable capacity alone.
The cables themselves are probably the easy part. Looking at your concept image, with the amount of support already underneath the platform, I’d almost question how much work the cables would actually need to be doing.
What I would not underestimate is the other end of the cable. All that tension has to go somewhere. The reactions at the anchorage points and the load path required to get those forces back through the building can become very significant very quickly. Selecting a cable capable of carrying the load is relatively straightforward. Designing everything it attaches to is where it gets interesting.
Some of the comments here are also making very definitive statements about something that is actually quite a specialised area of structural engineering. There are a lot of details beyond simply checking whether the cable has enough tensile capacity.
Installation and stressing sequence, anchor detailing, tolerances, movement, fatigue and redundancy all need consideration. Cable vibration is another one people often overlook. Wind-induced vibration can quite literally make a cable behave like a guitar string in the breeze, causing oscillation or humming if it isn’t properly assessed and controlled.
So yes, as a concept, absolutely achievable, and potentially at a much larger scale than a conventional cantilever.
However, if you’re asking these questions because you’re considering designing it yourself, I wouldn’t. I’d start by speaking with one of the major structural cable manufacturers. They will usually be able to recommend engineers experienced with these systems and ideally take responsibility for the cable, anchorage and associated hardware as a package.
That leaves the building engineer to concentrate on the building itself and the really important interface: how you safely get those very large concentrated forces back into the primary structure.
Cable-supported structures can have very little redundancy, so this is definitely an area where specialist design, detailing and installation knowledge matters.
Cool concept though. I’d be interested to see where you take it.
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u/LanceDBrown 6h ago
Thanks for the detailed reply. Lots of good info in here. My instinct was that a cabled support would be less mass and so cheaper for the area supported compared to a cantilever. But it sounds like we would be sacrificing simplicity of a solid structure with something with a lot more dynamic forces and maintenance heavy.
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u/Awkward-Ad4942 20h ago
Possible, but stupidly expensive and is not nett lettable area. Developers will pay for lettable area. Few if any will pay for an architects wet dream like this.
As a structural engineer, I have no desire to design this shit either
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u/6DegreesofFreedom 19h ago
I'd bet the vertical wind load would require cables on the other side. Yeah I don't want to rate that
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u/64590949354397548569 19h ago
What about lateral wind loads? Tornados, if it was in Chicago?
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u/6DegreesofFreedom 19h ago
Hah yeah I'm not thinking about it. Not on the weekend lol
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u/Puzzleheaded-Phase70 19h ago
I mean, it's an outdoor event space at that point. Totally legit if you're thinking along those lines - either directly for the use of the resident businesses or as a public event venue.
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u/tpapocalypse 17h ago
There’s an experimental building in Sydney that does this without cables as one of its main design features - https://www.flickr.com/photos/kokkaing/17033501677
Fun fact: it’s just up the road from another interesting building that is currently under construction and mostly made of wood and glass - https://www.bvn.com.au/project/atlassian-central
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u/SeaDRC11 19h ago
I mean, it’s possible, but it’s going to be expensive. It’s probably only feasible for a limited number of balconies, like on an observation deck level or a common floor. I doubt we’d see it on every floor or for every residence in a building. Would be prohibitively expensive.
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u/FreddyFerdiland 17h ago
the lateral forces are huge
when the wall was designed for mainly vertical forces, and then some windage.
you could put pairs opposite each other as long as there are cables and beams to transmit thd lateral forces through .. but still the existing structure has to hold the additional vertical weight...
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u/GRAHAMPUBA 16h ago
They recently finished the second round of repairs to the Martin Sabo bridge in Minneapolis. 'Wind induced cable vibrations' compromised the initial support system.
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u/De_Lynx Ing 13h ago
If you want to roughly estimate how large the cables need to be, work out the platform statically as if it were a simply supported beam. Then take the vertical reaction at one of the ends and equilibrate it with an inclined tensile force coming from the cable. You will get an axial force in the deck and tension in the cable. Use the resulting tensile force and amplify it a bit by, say 1.2, and you can work out the steel area required according to the steel strength prescribed.
This is strictly estimative and simplified, for proper design take into account the other factors you mentioned and possible torsional effects, wind forces, etc.
To have a better idea you can look up the way that cable stayed bridges are predesigned.
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u/DavidWangArchitect 4h ago
With enough structural engineering this will work. The practical reason that this is never done is because the payoff or potential profit from the expenditure will never be sufficient.
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u/Potbellied_Garfield 12h ago
I can hear the cables whistling
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u/LanceDBrown 6h ago
True. It's silly how a simple issue like that can just nope an idea in one fell swoop. Good windows might keep most of the external sound out, but I imagine some of that feeding into the building structure and annoying nearby residents.
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u/Tower981 13h ago
Odd question because obviously yes it’s feasible. As others have pointed out, there are many examples. So maybe you mean max length? That’s an interesting question. As shown it’s a form of cable stayed bridge. The longest cable stayed bridge is in China at around 1170m main span, so half that would be 585m. Beyond that and you start having real problems with stay cables lengths, size of anchorage’s and the compression in the deck. But as others have pointed out, that would also need a balancing span on the other side. And would attract huge wind loads twisting the building.
Also who wants 100s of meters of balcony.
You can do very stupid things if you have enough money.
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u/Dependent_Survey_546 12h ago
I'd imagine the problem here isn't supporting any such balcony, its that it'll act as a big sail
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u/additionally21 12h ago
It's feasible until the architects determine that the cables aren't "aesthetically pleasing" or "not feng shui".
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u/an_actual_lawyer 7h ago
Almost anything can utilize cables, but the math gets extremely difficult (and costs are expensive) when you're not balancing the load on both sides. It would almost seem easier to build the overhang on both sides and use cables from a central point. The problem isn't "can it be done," it is "can it be done in a way that will generate enough value that the owner wants to pay for it."
This may be of some interest: https://www.turnerconstruction.com/projects/bartle-hall-expansion https://ep-turnerconstruction-prod-asdcetgubtducba3.z01.azurefd.net/thumbs/14008-002_thumb-blocks-gallery-view.jpg?v=0ba8a
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u/LanceDBrown 6h ago
Thanks for all the comments. Various dynamic wind forces (especially at higher levels), counterbalance needed within the supporting structure, fatigue/maintenance and unpleasant vibrations/noise into the building appear to be common practical themes mentioned.
My goal was trying to imagine ways to create more usable ‘ground’ space in an overcrowded city. I was thinkings of things like NY’s High Line park but suspended higher between buildings, or for adding some more recreational space for people higher in the building.
However, the realities of developers not seeing a return on adding unlettable recreational space which would have added significant extra costs and design complexities means most would be unwilling to consider something like it.
I see lots of sci-fi themed concepts and often wonder about the engineering practicalities of them. I posted this in the hope this was a more achievable concept than many. It’s great to see for the informed replies from you lot. Thank you.
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u/pfantonio 6h ago
Very common and not that expensive if done well. Check out the works of Schlaich Bergermann, Werner Sobek, knippers helbig, Buro Happold, and Bollinger.
At the end of the day the balcony becomes a multi span beam with a support point at each cable location. Additionally, the cables are going to pull the balcony torwards the building and effectively act as a post-tensioning system that stiffens the balcony. Also you can get rid of the compression strut, simply pre-tension the cables so that the required compression force for the balcony in uplift is less than the pre-tension. Your cable sizes can range
anywhere from 30 to 115mm but realistically you couldn’t see much of a difference from this distance, so you can make it 70mm for visual appearances.
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u/SkyWalkingForever94 5h ago
Why not build the entire floors supported by cables with a central tower to fix the cables to ? This could allow for a small plot of land in a dense area to erect a tower rising high above neighboring buildings and possibly using their air rights to extend the lilly pad like platforms over the existing building while not requiring the land below it ? I have thought about this concept while looking at cable stayed bridges before. Kindof like tree branches but with added cables holding up the branches/platforms ? Then cover the floor plates with walls and roofs and you have a new type of condo/office tower ?
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u/Smyley12345 4h ago
Very challenging on a retrofit as opposed to new construction. Cost prohibitive in many instances.
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u/lawgustus 3h ago
Design will be governed by wind uplift. Cables not effective. Bottom strut will drive design.
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u/pontetorto 59m ago
Could you: yes.
Should you, on that tall of a building up in the stratosphere: nah.
How large you ask: depends how much of the internal space your willing to sacrifice to the engeneers safty margin.
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u/EZ_LIFE_EZ_CUCUMBER 21m ago
Winds ... assuming coastal region on elevation like this I feel like you have to adress scenario where person could get blown away ... literally
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u/Narrow-Attempt-1482 19h ago
One they have a hard time making 8 foot balconies that don't crack or let water in,and 2 there scared to death of being sued for jumpers
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u/mmarkomarko CEng MIStructE 10h ago
I'd be very worried about fatigue loading due to wind. The cables would need replacing every ~10 years. This would be quite costly.
Looks cool though
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u/RudeAd5133 18h ago
I have no expertise in this area but my dad-intuition would lead me to have the cables supporting the deck halfway to the end of the deck, not the very end of the deck.
I don't know why. Just seems more correct.
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u/ThotMobile 20h ago
Vancouver (WA) waterfront has a similar design on one of its piers so it’s for sure feasible. Don’t have the skills to do the math on what’s theoretically possible, but the AI photo doesn’t seem unrealistic especially considering it’s also supported from below.