Dear Bill (and the Group),

This isn't an answer to your questions. But maybe it is, in a way.

When designing an optimized tube-frame structure such as a rocket mount or
off-shore drilling platform you are allowed to let the task drive the
properties of the material in that you can spec whatever alloy, diameter and
wall-thicness that might be required. The assumption here is that the budget
is large enough to allow you to let contracts to have the materials made to
your specs. (Don't laugh. It got us to the moon & back.)

But when applying the new software (ie, circa 1960's) to more mundane tasks,
such as the engine mount for an R-2800... or the fuselage of a Formula One
airplane, you were forced to use the materials that were commonly available.
Then you ran into an interesting problem with tooling costs and fabrication
skills, interesting in that in most cases, implementing your new, computer
optimized structure will cost millions of dollars and several years, since it
dictates the need for new jigs & fixtures, different welding & inspection
procedures and retraining your work-force.

Bottom line is that with an existing structure any benefit of structural
optimization usual fails the Practical Factors test.

Starting from scratch? Then that's a different story and there are some nice
examples of steel-tube airframes, including square & rectangular tubing (!)
that have taken full advantage of computer-aided design, MIG welding (it's
faster) and so on.

When applied to home-building I suggest you turn the equation around. Use
CAD&D to come up with a welded tube structure that uses the LEAST number of
different diameters and wall-thicknesses as well as the least amount overall,
combined in a structure optimized for unskilled weldors working without
elaborate jigs & fixtures. This speaks directly to the Practical Factors of
one-off, home-built construction, the most critical of which is cost.

The answer to your questions can be found in any number of airplanes flying
today. Unfortunately, they start at about $100k and go up. Alas, such
airplanes and the attention devoted to them virtually guarantee the demise of
grass roots aviation in America.

-R.S.Hoover