Date: Tue, 4 May 1999 09:33:25 -0400
From: Robert W Burkhardt <bobwb@juno.com>
To: Kenneth Snelson
Subject: Re: Monday morning
Hi Kenneth,
I'm glad the patent montage looked good to you. I didn't see anything comparable at your 4D site, and I think it will give immediate background to the curious on the patent.
Your description of tuning is interesting to me. I go through a process that sounds similar in designing a structure. Usually I'm working to move members apart which are interfering with each other. As far as stresses go, my problems seem to be with tendons which have very low stress and thus tend to slackness and don't contribute to the structure. I try to aim for a situation where tendons are fairly evenly stressed, although I could just as well put stronger tendons at locations with higher stress.
In my investigations, the geometry (the relative xyz locations of all the points in the structure) has pretty much determined the strut and tendon stresses. I've been working with a very elastic material (nylon) for my tendons though and I can imagine that working with steel cable very minor changes in geometry (such as would not make much difference in the maquette) might yield a very different set of stresses.
If you had the interest, I'd be interested to cast some of your structures in my mathematical framework to see if it can reflect these various tunings you speak of. I'd just need coordinate and member data such as might perhaps be available from the computer graphics renderings of your structures you've done.
I've always wondered about this curious quote I found in the New Alchemy Quarterly long ago (quoted in my preface to Fuller's letter on tensegrity at my website) where they thought one tendon failure could lead to total collapse. I'd never experienced anything like this behavior in a tensegrity, but with some of your boundary-pushing designs I can imagine this might be a problem. I don't think their concern was really relevant to spherical tensegrities and their derivatives which were what they were talking about.
In readjusting the obelisk I built last summer I abandoned the mathematics and just pulled on tendons to restore its straightness. That and the experience this spring of disassembling the dome into various half shells gave me an interesting taste of the more intuitive hands-on aspects of tensegrity design which you are a master of.
If I can interest some others, perhaps around a school or something, I'd like to get into designing with metal struts and cable instead of wooden stakes and nylon. The latter have been aesthetically interesting but not very useful as serious architectural structures.
Bob Burkhardt