Hexagonal Geodesic Domes - Isometric Struts

Using struts of equal length

After reading my "dimpled domes" essay, Dick Fischbeck directed me to Joe Clinton's [Equal Central Angle Conjecture] - which suggests that it is possible to construct hexagon-based domes using struts of precisely equal length in the hexagonal layer.

While this essay doesn't contain a proof of the hypothesis - or even much concrete evidence favouring it - it does contain some suggestive drawings - which illustrate that it's possible to at least get pretty close to struts having equal lengths in hexagon-based domes.

From the perspective of building domes, this is probably good enough: the question of whether you need a few odd-length struts sometimes - if you want the dome to avoid all deviations from the surface of the sphere - can be largely left to future theoreticians.

This leads to the question of whether using equal-length struts in domes is desirable:


  • Fewer resources spent on cutting strut material - sometimes;
  • Less wasted strut material;
  • Avoidance of the issue of long struts being more prone to buckling;
  • Struts can more easily be recycled in subsequent structures;
  • The resulting panels may be of more uniform area;
  • Structural strength is more evenly distributed over the dome's surface.


  • Panes covering the surface are less flat - possibly a problem if glazing is being used;
  • Hubs become more variable - possibly increasing hub costs;
  • Forces in struts become more variable - possibly increasing the required strength of the strut material;
  • Existing works on dome buliding are not quite so relevant to the new structures.

Hub types

Whether there are cost savings associated with using hubs of equal length depends to some extent on the type of hub used to connect them.

If the hubs are of the first type, the struts may not need much cutting.

However the second type of hub makes using a mitre saw much more desirable - and if you have to saw the ends of the struts anyway, using struts of the same length doesn't save you as much effort.

On balance

I'm not sure the distortions present in an icosohedral dome are large enough to be too concerned about - so the practical differences in strength in the resulting domes are probably fairly minor.

The other issue with using isometric struts is the fact that the panels deviate from all being in the same plane more than they do with a conventional structure. If you are using a flexible covering, this may not be an issue - but if you have plans to cover the dome with something rigid - such as glass - then these deviations from the plane may well be a problem.

Tim Tyler | Contact | http://hexdome.com/