Threaded Tubes

While working on an ambitious but unfinished design, that I decided not to submit for the Makerbot giveaway contest, I came up with a way to pretension tubes based upon the idea that was raised in Forest Higgs' blog entry demonstrating that pretensioned square tubes can take a relatively large sideways load.

http://technocraticanarchist.blogspot.com/2010/03/some-crude-load-tests.html

My idea involves internally threading the tubes. A setscrew with a hole through the center would be used t