Pentakis Dodecahedron Dome Trusses

A dodecahedron dome (technically a Pentakis Dodecahedron - https://en.wikipedia.org/wiki/Pentakis_dodecahedron) is a wonderful, strong, lightweight, resilient, minimal structure. The trusses and connectors I designed here are used to build a small-scale model of the skeleton of a dome design I intend to build full-scale some day.

This could be compared to a “2v geodesic” dome, but based on the dodecahedron as the base polyhedron rather than the icosahedron.

There are two truss variants; an “A” truss (short) and a “B” truss (long). The “A” trusses are asymmetrical, with a 5-way connection at one end, and a 6-way connection at the other. The “B” trusses are longer and are symmetrical, with a 6-way connection at both ends. Both trusses have a 5-dot or 6-dot mark at each end to clearly distinguish how they need to connect to adjoining trusses. Each 5-way connection uses two 5-holed “Disc5” connectors, and each 6-way connection uses 2 6-holed “Disc6” connectors.

Parts:

To build the model as pictured (3 ground-level openings), you'll need:

• 30 x “A”-trusses
• 21 x “B”-trusses
• 12 x “Disc5” connectors
• 32 x “Disc6” connectors
• 102 x M3x18mm hex socket bolts

To have only one opening (i.e to fill in two of the openings), add an additional:

• 8 x “A”-trusses
• 4 x “Disc5” connectors
• 18 x M3x18mm hex socket bolts

As assembled and pictured (printed in PLA), this structure easily withstands over 100 kg applied on top (and would probably take a lot more, though I haven't tested it to destruction) demonstrating the immense strength of the the dome shape.

Printing:

The trusses, to obtain the best strength, need to be printed on their side. However, with the minimal bed contact and steep slopes at the ends, this makes them very prone to warping and detaching from the bed during print. So a reasonably wide brim (3-5mm, outside) is pretty much mandatory, but the post-processing effort is worth it. A deburring tool makes quick work of that, and since it'll take a while to print all of the parts, it's only a few at a time to do.

The connector discs don't need any special handling, just print them with enough top and bottom layers so they come out solid for best strength. They're quick and easy to print, and a great candidate for ‘sequential’ batch printing.

Assembly instructions:

• Insert a 5-holed Disc5 connector in each of the slots on the 5-dot end of an “A” truss, lining up the holes so that an M3 bolt will secure the discs in place.
• Attach the 5-dot end of four more “A” trusses to the same connector disc, forming a pentagon's internal ribs.
• Attach two Disc6 connectors to each of the 6-dot ends of each “A” truss.
• Attach 5 “B” trusses around the perimeter to enclose the pentagon.
• Repeat these steps 5 more times, connecting each new pentagon to the preceding structure. Of course the adjoining “B” trusses between neighbouring pentagons don't need to be repeated.

Future upgrades:

One issue I have with this design is that a single truss cannot be removed from the structure without disassembling a large area. I have a v2 design that resolves this limitation while being a bit more elegant (and can use shorter M3 bolts), but makes a weaker connection at the vertices, so I'm going to re-think that one a bit before releasing it. I'm also working on a revision that would drastically reduce the number of fasteners required (to one per vertex). Stay tuned.