Bill Schwab wrote:
Chris,

My cantilever would be fine if the horizontal member was attached half
way up the vertical member,


Based on what?

Based on the fact that the deflection would then be symmetrical about
the horizontal member, so the root of the cantilever would not be rotated.

It is not so much a matter of accurate and inaccurate, it is about right
and wrong.

So what's right then? In my opinion engineering is all about accuracy.
My first model may have been fairly inaccurate, but that's the way it
goes: you build one model, think about it and discuss it, then build a
better one.

Your model is fine in the absence of the diagonal tension member
(Steve's flat bar). With the tension member present it isn't a moment
which is applied to the vertical member, but a point load instead.


Actually, the model Ned cited is not "fine" in that case. It is however
reasonable, and much more so than a non-related cantelever beam.


I did some sketches and free body diagrams:

http://www.mythic-beasts.com/~cdt22/davit_calc2.jpg


What about the vertical component of the tension in the beam FBD on the
left?

We can't include it as we know nothing about the kinds of joints Steve
intends to employ in the structure. I assumed that there are pin joints
at each end of the beam because this makes the structure statically
determinate. We know so little about the structure that this is only
assumption one can make. Granted, I did not draw a pin joint. That was
an honest mistake.

I don't have a formula for this case.


Sorry, somebody has to say it: if you need a "formula" for something
like that, then you should not be giving advice on structural mechanics.

In which case, why are there big books of formulae published for
structural engineers to use? Like Roark's formula book which Ned
mentioned? Probably I could have figured out the formula from first
principles if I'd chosen to spend an hour or two on it, but I didn't
want to. Structural engineers don't begin all their calculations from
scratch.

So okay, I am not a structural engineer. I'm someone who took a few
structural engineering courses a while back. But I'm the only one who
actually tried to help Steve in the first place. Others then joined the
discussion, which I feel is a good thing if everyone remains reasonable,
but now it seems some traditional Usenet insults are being thrown in.

And no one here has yet claimed to be a professional structural engineer...

Chris