A properly designed space frame is made up of ONLY pure triangles. If you
make a
trapezoid or rectangle out of a triangle the loads are no longer pure
tension
and compression in the tubes. The tubes then resist the loads in bending
near the joints. This concentrates the stress on opposite sides in the
welded joints. In this case top and bottom of each of four joints.
When a tube is in pure tension or compression (pure triangle) the stress
is evenly
distributed in the tube and in the welded joints. When you weld in a
large beefy gusset or another tube between tubes like that you convert
the triangle to a trapezoid.
Make an accurate balsa wood 1/4 scale model and things will become clear.
Paul Lamar
......or imagine all tubes with rod ends, so you only can push/pull them.
Try to make up your whole mount with tubes with rod ends on both sides. You
will also find out why trapez, rectangle or square donīt work like that!!
Thomas J.
I forgot to upload these illustrations as an example of what happens
when a trapezoid is used.
Paul Lamar
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