Hello Everyone, This is the third video in the series about
building a geodesic dome greenhouse. This time I will cover how to build the interconnecting
struts which are made from red cedar. I choose red cedar because it is rot resistant. You
could use treated lumber, which is substantially cheaper, but I was concerned about chemicals
leaching into the water for my aquaponic system.
The site I used for the calculations is acidome.ru. The entire site is in Russian, but Google
Translate does a fairly good job converting the text. The calculator lets you enter the
diameter of the dome, choose the hub type and size, and even the dimensions of the struts.
It will then calculate the angles needed at each hub and also calculate all the various
sized struts and labels them with the dimensions taking into consideration the size of the
hubs.
The best feature is the ability to have the software calculate a flat base, since a 3V
3/8th dome is not flat. One click and the struts are recalculated with the proper lengths!
With a 33 foot diameter 3V dome, each triangle will never be wider than 6 feet. This will
allow me to purchase the polycarbonate glazing in 6 by 24 foot sheets, helping to minimize
the amount of scrap.
The struts are made from red cedar 2x4’s by 14 feet. This will be enough material to
cut 2 struts from each board with some scrap. The end of each board is cut at a 12 degree
angle so it will align with the hub. Not every hub connection is exactly 12 degrees, but
there is enough flex in the structure for the angles to average out properly.
Once the struts are cut to length, they are run through the table saw to add a slight
bevel to them. When the dome is fully assembled, the polycarbonate panels will rest fairly
flat along the bevel. This will also insure that the height of each strut is the same
since it can vary slightly from the mill.
Next I remove some of the material from the end of each strut using a dado blade mounted
in the radial-arm saw. This space will provide an area for the top tab of the hub to rest
and provide enough clearance for the polycarbonate panels over the crown of the carriage bolt.
I built a jig to act as a stop and keep each strut aligned properly during drilling. This
allows for a consistently placed hole to be drilled near the end of each strut. The placement
of this hole is important so it will fit properly into the hub and provided an accurate length
for the hub and strut combination.
And now, the perfect excuse to use one of my favorite tools!
The bottom of the head of a carriage bolt has a square neck so it can grab into wood
to prevent it from turning. To get it to fit into the metal tab, I would either have to
drill the tab’s hole larger, compromising some of its strength, or turn down the neck
of the bolt in the metal lathe. I chose to turn each bolt and then will re-galvanize
the area with cold-galvanizing paint.
Here is an example of how the final assembly will fit together. The strut is sandwiched
between the two hub tabs and then tightened into place using a lock washer and nut.
This top view displays how each strut can pivot slightly on each hub. Since the angles
of each triangle section aren’t the same from one section to the next, this allowed
for me to make the same hub and allow the pivot against the bolt to make minor changes
to the angles.
This side profile shows how the dado in the strut allows the polycarbonate glazing to
clear the area without hitting the tab or bolt head.
Finally, this profile displays how the glazing will set flat into the beveled area that is
cut along the top of each strut.
I assembled the base to verify that the calculations were correct and the pieces fit together.
So far, so good!
That’s all for now. The next video I plan on detailing some of the site work. Thanks
for watching!