3V dome entrance design by DiscoDave

3V door front view 3V doorway right isometric
front view isometric view

The geometry of the 3V dome imposes certain limitations on any entrance design.  This represents my best compromise among headroom, shoulder width, complexity, and strength.

Before After
before (detail) Dimensions for the 3V doorway

At the bottom, I replaced two long dome struts (red) with identical triplets; the center triplet lies flat on the ground and is also the bottom of the entry.  At the top, I drilled holes through two of the short dome struts (yellow) and connected them with a new strut to form the top of the entrance.  Two new long struts connect top and bottom to complete the trapezoidal frame.  New diagonal struts connect to either side at the bottom, replacing the other two red struts that had been removed.

In total, this requires making 8 new struts, plus modifying (or remaking) 2 of the short dome struts.   

Gulp!  

10 new struts to replace 7 in the original dome structure?  Is it really worth it?  Consider these features and decide for yourself:.
  1. The bottom of the entrance is flush with the ground.  The only tripping hazard is the diameter of the strut lying across the entrance.  Use a doormat and you should be OK.
  2. The entrance is a generous 3' 3" wide at the bottom.
  3. At "shoulder height" (5' 0") the width is 24", enough for an XL-sized person to pass through without turning sideways.
  4. The headroom of 6' 5" is enough for almost everyone except the very tallest people.
  5. Solid as a rock!  We've done pull-ups and chin-ups from the top bar, and we couldn't see any sign of deflection or deformation.
Because you'll have to make up some new struts, the ideal time to do this is while you are making all the other dome struts.  Installed, the entryway replaces 7 of your original dome's struts: 2 short (yellow-A in the diagram below), 1 medium (blue-B), and 4 long (red-C).  You can keep these removed struts as spares, or if you haven't made them yet, you can consider not bothering doing so.
diagram of the struts labeled

(from above) name Quantity1 normalized hole-to-hole length2 bend angle for each tab3
A short 3V strut (yellow) -2 0.8453
10
A* short 3V strut with hole  2 0.8453 10
B medium 3V strut (blue) -1 0.9785 12
C long 3V strut (red) -4 1.0000 12
D entrance bottom 3 0.6623 8
E entrance diagonal 2 0.8858 11
F entrance side  2 1.3306 16
G entrance top 1 0.3261 4
1As compared to the original, intact 3V dome.  A negative number (in red) means you remove those original struts (you can keep them as spares).  A positive number indicates how many of the new struts are needed to make the entryway.(back)
2As your dome's hole-to-hole dimensions may differ from mine, I am providing the spacings in relative terms, with the longest normal 3V dome's strut length defined as 1.0000.  Multiply your own dome's longest strut's hole-to-hole dimension by these factors to get the right sizes for your dome.(back)
3While not crucial, getting your bend angles correct, or close to correct, makes dome assembly much easier. (back)

The special short dome strut with the hole

special yellow strut
The upper two vertices of the entrance are made by drilling a hole into two of the dome's shortest struts.  The hole's position is 2/3 (or 1/3) of the way along the length between the holes in the end tabs, and is oriented in the same direction.  Drilling a hole in a strut will naturally weaken it around that spot (stresses are concentrated ~ 3X in the vicinity of the hole), so it's a good idea to remake these two struts out of larger diameter tubing.  In 2002 we used 1" EMT conduit to build the regular dome struts and 1" EMT for these drilled pieces (1" EMT is 2 times stiffer and twice as strong as 1".)  They have stood up to the abuse over the years exceptionally well.