Capt. Geoffrey Thorpe wrote:
"Ernest Christley" wrote in message
...
...

My biggest concern is the inside tube not being strong enough and taking a
slight bend over time. Then it will no longer slide. Hence my question.
If it were a straight beam, I think I could run the calculations, but how
do you analyze it when the cantilevered beam isn't straight?


Divide the load in to two componants - the load normal to the beam (bending
force) and the load parallel to the beam (tension). The compression load on
the bottom face of the beam will be somewhat reduced by the tension and the
tension load on the top face will be somewhat increased and the sides of
your box will carry shear plus tension.

If your beam is 40 degrees from vertical, the bending force would be the
load (I think you mentioned 300 pounds) times the sine of the angle and the
tension force would be 30 times the cosine of the angle (I'll let you do the
math). (assuming, of course, that I am drawing the picture correctly in my
head) It would be reasonable to assume that the tension force is carried by
the entire cross section of the square tube so divide the force by the cross
section ((width + height) * thickness) and add that stress to whatever
stresses you come up with from the bending.

Or, you could clamp the bar in a vice and jump up and down on it.

--
Geoff
The Sea Hawk at Wow Way d0t Com
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Todd was correct. I was talking about the support beam and not the
step. The step actually sticks out forward from the support beam. I
apologize for not being completely clear.

I'm going to work on the calculation, just for schnitz and giggles, but
took the suggestion to heart and rigged up a test stand by nailing a
couple of blocks to the side of a sawhorse so that the outer housing was
supported at the points where it will be welded. Bounced as hard as I
could in the unstable situation, and could not detect any flexing. That
was with junk mild steel that I was using for a mock-up. I think I
might have overbuilt it by just a tad 8*)

This is the last piece to go on the airframe before the fabric.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."