Re: ATM 2 Piece Poles - and how trusses really work

[atm@misterg.fsnet.co.uk (Andy Gray)]

On Tue, 19 Aug 2003 18:45:19 -0700 (PDT),  Ross Sackett
<rsackett00@yahoo.com> wrote:

>
>--- Emery Greg <greg.emery@mohawkcollege.ca> wrote:

>> If so, are the bolts not in double shear?  It has been a long
>> time since Mechanics 1 and Strength of Materials - I
>> never thought I would use that stuff again :)

{aside] You and me both, Greg !!

>The connector bolts as I imagine them are in line with
>the tubes (on axis in fact) and loaded in tension-compression.

You can think of it either way:-

You're either going to butt the tubes end - end, or have an
overlapped joint of some sort. If they're butted together
over the full cross sectional area, the fastener needs to
provide some clamping force, and provided this isn't
exceeded, the joint will be as stiff as the tube.

If there is an overlap, it's difficult (but not impossible)
to make a joint without any change in cross sectional area,
so there may be a minor effect on stiffness (+ or -). The
function of the fastener (I would argue) is still to clamp
one part of the joint against the other. It is therefore the
friction between the two parts that matters, rather than the
fastener strength or stiffness. 

The commonly seen arrangement of a sleeve (internal or
external) to join two tubes is actually a good idea, but it
is usually spoilt by being a poor fit on/in the tubes,
and/or being secured by sticking a bolt across the diameter
and tightening it down.

At *best* this will clamp the sleeve on one diameter, and
turn it oval, spoiling whatever fit there ever was and
allowing wobble and slop. If the sleeve were precisely
fitted and could be clamped uniformly to the tube, then this
would be a good joint.

There'd a very readable book about building race-cars called
"Engineer to Win" by Caroll Smith which has a whole chapter
on this sort of thing, and has sound advice from cover to
cover (mostly applicable to building *anything*)

Regards, all.

Andy

Andy Gray, Dyserth, N. Wales, UK.

P.S. - The query about using Young's Modulus for hollow
tubes : It works the same. You just need to use the cross
sectional area of the metal/etc that there actually is in
the tube.