Ernest Christley wrote:
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.


A little flexing isn't all that bad. The retractable ladder on F-4E
flexed quite a bit yet had no trouble supporting my 220 pounds plus
anything I was carrying.

Dan, U.S. Air Force, retired