Geodesic dome structural analysis basics
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- čas přidán 26. 07. 2024
- This is a none technical look at some very basic structural analysis principles, hopefully it will shed a little light on a otherwise daunting topic.
Links:
Wind load tool: geo-dome.co.uk/windload_tool.asp
Snow load tool: geo-dome.co.uk/snowload_tool.asp
Other tools:
Force effect for Ipad - Jak na to + styl
Thank you for so many wonderful videos and amazing explanations. Was planning to build a traditional greenhouse but have always been fascinated with geodesic domes.
This video was very helpful in starting to understand the structural integrity of the geodesic dome. I just purchased a property that had a 20' diameter dome shell on it. It has 3 dormers for doors and/or windows and is built on quite a frame. It has been standing basically abandoned for about 20 years in the mountains of north idaho where we get 3-7 feet of snow during our long winters. It was covered with asphalt shingles and is basically dry inside, with only tiny signs of some water entry but this spring it was 100% dry so I can't say for sure that didn't happen during construction all those years ago.
We are going to reframe inside the dormers and extend one out to create a front porch. It should be a fun project, I'm going to turn into an off grid tiny home.
This video is gold - thank you! By understanding the “why” I am better able to understand the “how” and “what”.
Note: from the applied 100N load, the resultant of 350 N would be divided by 5 struts; ie reduced to 70 N
You did an EXCELLENT job of explaining the structural considerations of a geodesic dome. Well done! Graphics were very helpful and your straightforward manner was very engaging. I hope you are a structural engineering professor at some major university!
Thank-you for taking the time to put this video together! It's most appreciated. I think you're mistaken about the vertical load analysis, though. To determine the member forces you want to project that single point load force's direction to be coplanar to the triangle in question. That component is small for the dome's topmost triangles. Also, I believe you're neglecting to consider the bending moment within each timber member. I hope this is helpful. Again, thanks a million!!
Thank you! This was very enlightening!
Excellent explanation! Thank you!
Paul, thanks a lot for your useful videos and for sharing your knowledge with the world! If I may, I have a question: I´m designing a geodesic dome 2v frecuency, 10 meters in diameter. Because I want it to be taller than 5 meters, I'm designing it to be 3/4 of a sphere, rather than 1/2 of sphere. According to your experience, 3/4 of sphere is just as strong as 1/2, taking into account that in 3/4 of sphere the first row of triangles is leaned outward? Thanks in advance for your answer!
Thank you! Great explanation
Thanks for making this video. Has helped me figure out some stuff for the dome I am building.
Good video Paul
Really nicely explained, thank you. I am trying to solve for a monolithic shell. Do you have any suggestions or resources that could help? Like, should I calculate the dome as triangles and add them up or something to get an approximation?
thankyou very much great masterdome!
Very interesting analysis, I,m lerning so much about this topic, thank you so much, and gob bless you.
Outstanding video Paul! Any thoughts of using Ferrocrete, then plaster over a pvc frame? We are NOT looking for a 30 year life for the structure. We are also looking at adding circular tubed skylights at the top. Any thought anyone on keeping our cost down for a temporary structure to last maybe 5 years? Note: We are looking at a 3v size geodesic dome.
Hi Paul. Help. Really like your videos, and is planning my own project. I am struggling to find someone that could assist to check and verify the structure of the design. Would you have any to recommend? Thanks again
Thank you Paul.
Paul, world class web site and tutorials. My local codes impose a snow load factor known as 35 pound ground snow load. I assume this means we have to design roofs to hold 35 pounds per square foot? So how does that work in your snow load calculator? Also, for wind loads, we need anchor the structure for overturn resistance. When calculating the wind load, we need to know the uplift force resistance. My concern is can the bottom triangle member take the vertical load, and how to, and how often, to attach it to the foundation. Thanks again for your contributions and hard work on the subject. I hope to buy some wangers.
great vid!
Is it possible to reinforce each triangular pie shape by adding an extra strut between the center points?
Hi Paul, do you have any information regarding how loads would act on a dome made from solid hexagonal panels as opposed to the traditional frame and skin method? I'm having trouble finding any examples of this approach. Any info much appreciated.
This is a very late response. For what i've study with solid components you have a lot of disadvantages.
- More weight overall and not a lot more of structural strengh
- whole panels maybe more dificult to attach to each other than framing and skin
- the materials used need to have structural strengh a.k.a. you are limiting your self on the type of skin used (glass, plastic, etc)
- a more heavy structure can colapse and be unstable (not recomended), the ideal is a light and thermical isolated surface.
All this talking of a house dimmensions, in a dome much more small all this things do not matter as much
I don't quite understand why there are tension forces on the rings if they are linked with the hobs where compression forces act upon. Shouldn't they balance each other out or at least mix them together so that their overall compound is zero?
Even the hexagons close to the pentagons at the top should only have their outline struts under tension when a load is applied to their vertex and that seems to go against what the video says.
Solid advice Mr Robinson. Don't build on a windy day etc. Well done getting the calculation tools, did you employ the services of a programmer or do it yourself or was it just a widget? Either way, totally awesome sir.
Also paul, if you was to build a dome twice the size say 6m 3v to a 12m 6v how much would you have to increases the size of the timbers? Twice as much cross sectional area?
OK you're doing two things there, double frequency, then doubling the size. You won't need much bigger timber section because the strut lengths will stay roughly the same, you will need considerably stronger hubs though. I made a 12m dome with pseudo 8v geometry (a 4v with extra stud work inside each triangle frame) here's the video: czcams.com/video/6ERJx-BPw3w/video.html
So to answer your question, not much bigger struts but hubs that resisted inversion, you can do this with a double skin or by beefing the hubs up.
I cheat and use pseudo geometry: geo-dome.co.uk/article.asp?uname=sudo_domes
This is actually stronger that conventional geometry..
I am researching earthbag dwellings. AS I have never built one before, I was thinking about using a GeoDome as the 'skeleton' of the earthbag house. I have seen this done with hay bails but not with earthbags. Any thoughts on the weight of earthbags on a geodome skeleton?
robert...did you build with dirtbags? i like the idea for sure. i myself am thinking of building the unity dome he has come out with, but then shotcrete the outside..or aircrete it and wrap to seal..what do you think?
He’s “given the store away” he’s “left the cash register open” he’s “left money on the table” What kind of capitalist is Paul Robison? My kind. Kickstarter supporter, David F
I'm trying to guesstimate the strength and max.load of a steel struts dome (and also what type of struts would be most efficient). So I came back to this video ...
So I noticed something I apparently didn't understand the first time.
@ 4:13 you (and the Force-Effect app) are saying that at given load on a top node of lets say 100N the compression force on each downward arm is actually much higher.
How is that possible? (ignoring the construction's own weight)
Also I was wondering on the type of steel beams to use, and quickly excluded I-beams because they're designed to take load either in vertical or in horizontal (with its web being vertical) orientation, but of course in a dome most struts are at different angles, and not a single one is actually vertical.
For example, if you imagine the top pentagon made with I-Beams, since they're almost horizontal the beams should be with the web vertical and the two flat sides horizontal - that's almost a perfect position (since all the beams of the top pentagon are mostly in the same plane, at only ~12 degrees slope to be precise).
Going a row down however the angle (of the dome wall) gets much steeper (but far from vertical), but at this position the angles between connecting struts at given hub isn't irrelevant since it can be at up about 60 degr. slope, and this time the slope is in the wrong direction relative to the I-Beam's web.
The ultimate example is when you see the wall triangles at the bottom of the dome. The walls are close to vertical (especially if it's a 5/8 dome), but their I-Beams (assuming their orientation at the hubs isn't changed) will be at ~60degr. angle, and also the bottom beam (that's practically horizontal) will be in the worst orientation for a I-Beam - with their web horizontal.
However if the load force distributes only as compression & tension forces of each beam, then orientation wouldn't matter, since I-Beams (and pretty much any type of beam) is strongest vs forces on the axis of their longitude. (and load forces that land on the length of the beams - from the wall panels - is not to be ignored either)
So could you clarify in more details how (and why) the forces are distributed around a dome?
Thanks!
Thx for posting info on Dome. May I know ... What's the difference between building the Dome Top-Down (Roof first, Base last) versus Bottom-Up (Base first, roof last)?
Top down is used on bigger domes, you do need a central crane but the frames are pushed together as you build. You need a few people for this. Building from the bottom up is best on smaller domes and can be done with one person but the base has to be placed accurately, and the frames have to be pulled together as you go up. Don't fix the base until the dome is fully built and building on uneven ground can be a problem.
I want to build a 44ft 6v dome and then bury it under at least 4-6 feet of dirt or bury it inside the belly of a hill so that the only thing anyone might see is the door.
My question is what materials would such a structure need to be made of to bear that kind of weight for at least 100yrs?
Hi Paul - you made very nice videos and also a great website. It helps a lot to understand dome structures.
You sound like you are from UK and there they use imperial measurements if I am not wrong. However your calculation tools are mixed Metric and Imperial. (radius in meter but windspeed in Miles per hour). May I suggest to put a simple unit in your calculation tools. i.e. Radius/m: 6 - it would help. And since you have to touch your website again - can you share the formula you use for your calculations as well? Thanks for considering. Cheers
Thanks for that, the UK uses both, we went metric in the 1970s but the building trade often still use imperial we also still use MPH, that's why my measures are a bit mixed. You can find the formula by entering 1 unit for radius. I will add some more calculation tools though. Cheers
thank you .but can you explain more why there is no shear force if we have snow load . can you also tell us why there is a shear force in tunnels .and make a Comparison between these two
thank you so much for this video!!! love u
How high can a geodesic dome be built with steel or concrete kilometer range?
A quoi sa sert un toit en alum? Peut il remplacé un toit en acier pour les réservoires de stockage hydrocarbure et huile ?
Hi Paul, I was searching this kind of info today..
But I still have some doubts on dome designs.
For example in all pages it said that if you increase the frequency (lets said v3 to v4) increase the sturdiness of the dome.. But in what sense?
If I apply as you show a force in any vertex towards the center of the dome, then the angle in a v4 is lower than v3, so the tension must be much higher to counter the force.
This is the thing I never understand with fuller idea, when he said that domes becomes stronger with size.. If a dome is very large, then the angle becomes zero and the structure can not counter loads apply to the top of the dome.
Of course I must be wrong... but I dont understand the physics behind..
What about on practice? on the vertex joints? these are strong enough to counter forces towards the center?
You are exactly right, a 4v dome is more likely to have a node inversion but there are a lot more struts to share the forces, so it will be weaker with a point load but stronger with a broad load like snow on the roof. The thing about getting stronger as it gets bigger is not true, if a structure gets stronger as it gets bigger then in theory there is no limit to the size you can build, this is not possible, imagine building a small dome from paper the size of a soccer ball.. then keep building it bigger until you get to the size of a house or football stadium, the paper would crumple under its own weight. The way to prevent node inversion with high frequency domes is to use a double skin or space frame type construction. Well spotted!
You are kind of Myth Buster for domes, sir.
Yes, like B. Fuller's US Pavilion at Expo'67 in Montreal !
I believe the idea that the strength of domes increase with size is a confusing version of the principle that, for a dome of a constant "strength," the ratio of the amount of material needed to the volume goes down as the volume enclosed goes up. This is simply because the area of the enclosing shell goes up as the square of the radius, while the volume goes up as the cube. (The apparent "strong-ness" is sort of going up.) Fuller, I think, was trying to persuade skeptics that a dome coving an entire city was not really impossible for lack of materials strong enough, as people supposed, when he said things like this.
Fuller's geodesic domes were conceived as struts joined only at their ends. Struts which are "too long" fail in compression before the materials compression strength because of the phenomenon of buckling. (The struts bend sideways, before they crush.) The reason for going to higher frequency domes is to lower the length of the struts below the bucking point. (Fuller's idea was to use materials to their maximum potential.) Buckling (according to a mathematical theory) occurs at a certain critical ratio of length to cross-section of a tube.
Excellent video Paul. Would there be any special concerns when building a dome using PVC?
Technically not, though I'm not a big fan of PVC, I'll be uploading a video shortly about the build it to last concept. Is it better value to make a very expensive building that lasts a long time compared to building a cheaper one that has a much shorter lifespan.
Paul Robinson Not to mention plastic is really bad for our environment.
Paul Robinson That's a wise engineering principle. Many here are just DIY man that has no engineering education.
I think if you build another little dome on top - where you got that weak point that very top pentagonal, it would strengthen it. And to me it sort of looks like East cultural structures, domes and all....
I think you are incorrect regarding timber selection. I believe you are correct in your understanding of how force is divided. However I believe you are incorrect on how it is applied. At the point of contact you're going to have the force that is applied and the direction that it is applied. For that segment or segments. However, like an arch, every other piece will be under compression and that force would be applied basically along the plane of the triangle. This would mean that the triangle itself would see the same direction of force regardless of its position in the dome. I think you would likely be correct however if you applied external force to a segment or segments where the narrow cross section of the timber were to be supporting that load.
Which formula did you use to get wind load? force = area x pressure x Cd
Could you tell me please, is that dome 3v? Thank you for sharing your knowledge!
Yes this is a 3v, however it would be a very similar scenario for a 4v or higher frequency.
Paul Robinson. Thanks for answer my question!
This is really interesting, but the volume is so low I am struggling to hear it at full volume.
i got no problems att 30% sound.
Yes the volume is too low.
It's something a lot of youtubers don't get right, since they focus much more on the content than the way it is delivered. It's not a big deal, but I notice there are very different volume levels on the different videos. As audio engineer, I find it annoying, but as I said, no big deal.
I would suggest - friendly - that you pay more attention to the audio production part of the videos - it will make them that much better. :)
What?
mostly inaudible on my device, even with earplugs and w/o ambient sound
What is it the name of the App? Force Effect? I couldn't find it..
Most f the forces... that's deep.
I don't under stand a word you just said can it hold more weigh
If the drywall, foam insulation, glass, plywood, and plastic sheet companies can cut panels at 60 degrees instead of 90 degrees in the factories, it would be more cost-effective to make domes....Otherwise you're making triangles out of square pieces and producing a lot of waste!!!! Not very efficient....
Seems to me that it be more efficient to make concrete and spray-on foam domes.....
It's a shame the software used in this video is discontinued. There's no clear replacement either.
wireless speakers are your friends
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