If one were to put up two saw horses 10 feet apart and lay a steel I beam across them, which direction would you want to lay the I beam to give the best support when a heavy weight was layed on the beam midway between the two saw horses? Which direction would you want to lay the I beam so that the I beam deflected the least amount and resisted being bent by the weight?
I am going to try and keep this in as simple terms as I can as some people will loose interest in this thread if it is too complicated.
Referring to the "Manual of Steel Construction" most engineers refer to it as "The Green Book"..... it has tables for the properties of many structural shapes..... including I-beams. Two particular properties listed in the tables that engineers refer to..... to "pick" a particular beam size based on a load.....are "moment of inertia" and "section modulus" These values directly relate to the beams particular strength in a particular direction..... i.e. your hypothetical situation on the I-beam on the sawhorses above. There are separate values for relative strengths about the "x-x" axis and the "y-y" axis.
A general note: In the green book.... the "s" beam is what most people call an "i-beam". From this point forward... I'm going to refer to the beam as an I-beam so as to hopefully not confuse anyone.... I hope.
If you break an I-beam down into it's basic parts that make up a shape..... there are two "flanges" and one "web"..... with the web separating the two flanges. I-beams are significantly stronger if turned right for the given load..... in the case of the sawhorse example..... you would want one flange laying flat on the sawhorse.... and the web sticking up vertically... with the top flange parallel to the bottom flange..... make sense?
Anyway.... to give you some idea of the relative strengths of i-beams in general if you want to compare the "values" of section modulus or moments of inertia.... you can easily see how much stronger the i-beams are if loaded correctly....
Looking at a small 4" x 7.7 I-beam.... (7.7# per foot).... the moment of inertia in the "strong axis" which would be loaded like above with one flange laying flat on the sawhorse.... is 6.08 in^4. But if you loaded it on the weak axis.... the moment of inertia is only .764 in^4.
This should give you some sort of "feel" for how much stronger an I-beam is in one direction vs. loading it in the other direction.
In the case of the "twist" that I mentioned in my post above.... I would think you would find after some lengthy engineering analysis....that the "h-beam" rods are weaker when the load moves from loading one axis to another.... i.e. a twisting load....
One observation is that when you compare the two different rods and how they are installed in a motor.... the i-beams have their "flanges" turned 90° from the way the h-beam rods are when installed in the motor. How does that come into play? I am not real sure without some 3-d modeling..... which I don't have time to do at the moment...
I hope I didn't loose anyone...