Minimum size of timber for 7 m diameter dome?

[tuk]

Hi Everybody,

I want to build a 7 m diameter 3v 4/9 Fuller-Krauscke dome out of pine
timber. Its struts (is that also the proper word when using wood?) are
between 1.15 m and 1.54 m.

I would like to make it hubless, cutting the wood in the right angles
and screwing it together, as seen in these videos:
- http://www.youtube.com/watch?v=MmuhrumkJts
- http://www.youtube.com/watch?v=yZhzYsyT9EU

The cover will be light, made of plastic or thin fabric.

My question is: What size timber do I have to use? In Domebook I/II it
is mentioned that Fuller recommended at least a width of 1/24th of the
strut length for timber domes. This gives 6.4 cm in my case (154 cm /
24), but I don't know how Fuller got to that number..

I have found 4.5 cm x 4.5 cm quite cheap, could that be strong enough?

Best regards,
Tuk

[TaffGoch]

tuk,

If the structure is to support thin-film material, you can build with fairly-delicate material, so long as it doesn't bend. Note that tent frames use fiberglass rod frames, which are thin, relative to length. The weight of the struts, themselves, will likely outweigh any film/fabric covering

The bottom-line is how much weight the geodesic frame has to support, and how much wind-load it must withstand. Perhaps, snow-load is not a consideration, but rain-wetted fabric can get heavy, at the size you propose.

I've seen dome frames of this size, made of bent bamboo, so, pretty light struts.

I've encountered poor & good quality lumber, from the building supply store, so, I know that there is variability in the density & strength of lumber of the same dimensions. Since I don't know how hard/soft is the wood you have available, I shouldn't comment on the dimensions you have described. (I also don't know how dry/wet is the wood.)

The lengths of your proposed struts are fairly short (relative to cross-section,) and that's in your favor.

You could build a test section, and test the strength, yourself. See how much it will support, without deformation. Continue adding weight, until it does fail. (This is a common approach to assessing construction strength.) 

You should note that joint-connections will likely fail before there is any problem with the length of a strut. Joints/hubs must withstand compression (which is generally not a problem,) as well as tension. As weight is added to a dome structure, the joints around the "equator" will be under the increasing tension. This is why you will often see metal straps added to horizontal, and near-horizontal, strut joints. Personally, I would not trust nails/screws only. You should, at least, include a disk or ring to each joint, on the outside, or inside, or both. (See attached image.) You can see simple joint reinforcements (and all sorts of hubs) at: http://www.domerama.com/dome-basics/geodesic-dome-hub-connectors/

-Taff

[Paul Kranz]

Taff:

 

How about the dreaded loose-pin hinge hub!

 

Paul sends...

--
You received this message because you are subscribed to the "Geodesic Help" Google Group
--
To unsubscribe from this group, send email to GeodesicHelp...@googlegroups.com
--
To post to this group, send email to geodes...@googlegroups.com
--
For more options, visit http://groups.google.com/group/geodesichelp?hl=en

[Tuk]

Thank you so much, Taff, this is all very usefull!

 

You could build a test section, and test the strength, yourself. See how much it will support, without deformation. Continue adding weight, until it does fail. (This is a common approach to assessing construction strength.) 

That is a great idea. How big a part of the dome would be reasonable as a test section? One or two base triangles?

 

You should note that joint-connections will likely fail before there is any problem with the length of a strut. Joints/hubs must withstand compression (which is generally not a problem,) as well as tension. As weight is added to a dome structure, the joints around the "equator" will be under the increasing tension. This is why you will often see metal straps added to horizontal, and near-horizontal, strut joints. Personally, I would not trust nails/screws only. You should, at least, include a disk or ring to each joint, on the outside, or inside, or both. (See attached image.) You can see simple joint reinforcements (and all sorts of hubs) at: http://www.domerama.com/dome-basics/geodesic-dome-hub-connectors/

 Yeah, I have wondered about this myself. Adding a disk is probably a good idea. Will see how the testing goes.

- Tuk

[Tuk]

Paul Kranz wrote:

 

How about the dreaded loose-pin hinge hub!

:-)

It seams there is a reason why it's dreaded..

- Tuk

[TaffGoch]

Paul, 

I've used door hinges, myself! Honest!

One thing about them: You had better make sure you have the positioning spot-on, because they are not forgiving of misalignment. The first pins go in real easy, but the last pin (at any vertex) can be... let's say... "problematic." (That's the nice word for it, without all the cussing.)

(BTW, I like that photo.)

______________________

Tuk,

To test, I would build, at least, one of the symmetrical hexagons. I prefer to build more, perhaps, two tiers of triangles (starting from the top pentagon, down.)

If tested to destruction, with satisfactory results, you can replace the damaged components, and continue with the assembly of the rest of the dome. When I say "satisfactory results," I mean that it took a much-greater load than I'd ever expect it to subsequently encounter.

-Taff

[Tuk]

TaffGoch wrote:

> To test, I would build, at least, one of the symmetrical hexagons. I prefer
> to build more, perhaps, two tiers of triangles (starting from the top
> pentagon, down.)
>
> If tested to destruction, with satisfactory results, you can replace the
> damaged components, and continue with the assembly of the rest of the dome.
> When I say "satisfactory results," I mean that it took a much-greater load
> than I'd ever expect it to subsequently encounter.

Thanks! Now on to the building! Will report when I get further.

- Tuk