70' dome

[Brennan Oleary]

Hi all, 

First message did not seem to post so trying this again. 

I am arguing with AI:)

AI does not think I can safely build a 70' diameter 3V dome with 2x4s. I stated it should be robust enough if I use 1/4" or possibly 3/8" carbon steel star connectors and reinforce the struts with 2x4s by subdividing every triangle into four smaller triangles. I saw Paul Robinson do this, he says it hacks a 3V dome into a 6V dome. 

Need to hear from human experts on this lol

https://www.youtube.com/watch?v=3CuJgrEicDc

https://vikingdome.com/pages/star-connectors?srsltid=AfmBOoqdtCxfqQupsK1hfLMDbh-jf0hv_39QqCd2RbdvHveJRuZg6ZnV

[Dx G]

I'll do some calculations when I get some free time, but in the mean time, here are a few items that might help.

1) Do a search on the group list for "slenderness".  I believe there was a recent reference to the "slenderness ration" Fuller suggested for dome struts.

  

2) More than most people want to read here, but does contain useful insights

http://www.rwgrayprojects.com/rbfnotes/fpapers/tensegrity/tenseg01.html

 Kenner also discusses this in his book. P 68 refers to length/diameter as 24:1 for wood, so one can do the math and see how close or how far (under or over that ratio) you'd be with a given strut length at a given frequency.

3) Even if using plain 2x4's does not appear to provide adequate strength, there are lots of alternatives, such as doubling up, going to 2x6 or larger materials, going to a floor truss or I-beam (Daystar used to build domes from these) and various other options with wood, as well as going to other materials.

  Also, don't over look the wood species.  If you look at load capacity tables (of which there are many) you will be surprised how much the wood strength can vary.  A few examples of many -

https://woodbin.com/ref/wood-strength-table/

https://extension.okstate.edu/fact-sheets/strength-properties-of-wood-for-practical-applications.html

https://awc.org/wp-content/uploads/2021/10/AWC_NDS2018-Supplement_20200827_AWCWebsite_Chapter4.pdf

4) If you are going to use plate type hubs, in case its not already on your radar screen, you could consider the gain in strength for the hub if you used at least 2 plates for each hub, one on the upper strut surface and one on the lower.  There is also the option of setting the hub up like a circular truss, such that a cross section of the hub would look more like an "A" rather than a "^". This could help prevent the hub from flattening which is an important failure mode, especially in high frequency domes.

Hope that helps some.

Dx G

[Robert Clark]

https://www.allard.org/house-as-a-product

A gentleman in Stockbridge, MA built a 42.6 foot diameter geodesic home. It is a much higher frequency than 3.

The hubs are disks he had water-jetted out of aluminum plate.  He put two angled slots into the ends of each board strut.

He then slid the ends of the boards onto the disks (two disks per vertex). Once the boards were properly positioned, he drilled from the interior through the boards and disks, but not breaking through the outside. Then, he screwed in lag bolts. I'm not sure if he angle cut the ends of the boards or not.  Here is a crude diagram of his method:

[Dx G]

Oops. Please forgive the typo in that previous message.  The term is "slenderness ratio", not ration...

Dx G

[Dick Fischbeck]

This person says his hack makes a 3 v dome into a 6 v dome and that this alteration would actually make it stronger than a normal 6 v dome is a questionable claim imo, even though as he says it would be simpler which is true. I mean, how can a vertex with a zero angular deficit be stronger than one with a positive deficit?

https://youtu.be/3CuJgrEicDc?t=198

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[Dx G]

Did he actually claim this was stronger than a 6f?  I think his position is that adding the interior triangles increases the strength of a 3f. Looks like he assumes this strength comes from the dihedral angles of the added interior triangle. Seems like having those interior triangles would resist the tendency of the 3f hub areas to invert.  I suppose an actual structural analysis would allow better measurement of any added strength, especially in situations where the 3f triangles have no triangular decking, and remain open, such as a screen house.

Dx G

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