To: atm{at}shore.net, adams{at}rmci.net
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


> From: "Harold" 
>
>   Some time back a gentleman who i  do not remember had an idea about
runing
> a cable from the top center of each truss to the bottom center and
> tensioning the cable with turnbuckles. .

I think Martin Cibulski was the person you refer to.

>...I assume this is to strengthen the
> truss. Is this correct?

In his case it was to avoid the use of clamps at the upper ring of his
scope.  With the cables in tension, all truss members would be in
compression, and using a socket/cup shape, the truss tubes would be held in
place, thanks to the cable tension.

From what I understand, this addition of a cable in the geometry Eric
planned to use would not really improve truss stiffness, but that was not
his intent if I remember correctly.

See:
http://overton.tamu.edu/aset/krajci/
specifically:
http://overton.tamu.edu/aset/krajci/scope-design.htm
and
http://overton.tamu.edu/aset/krajci/truss-analysis/truss-analysis.htm
and
http://overton.tamu.edu/aset/krajci/truss-analysis/truss-verification.htm

The last link mentioned above has an equation for truss stiffness that
applies to typical ATM truss arrangements.  The equation shows that if you
want to make a big change in truss stiffness, then:

A.  change length (stiffness inversely proportional to length **cubed**) B.
 change width (stiffness proportional to width **squared**) C.  change
cross section area of your truss members (stiffness proportional to cross
section area **only to the first power**) D.  change medulus of elasticity
of your truss members (stiffness proportional to modulus of elasticity
**only to the first power**)

Note that A gives you the biggest change...while C and D are more modest changes.

Modulus of elasticity of steel is 3 times more than aluminum, and modulus
of elasticity of carbon fiber tubes is about 2 times that of steel.  (In
most amateur cases aluminum is adequate.)

In your design, do you care about overall weight?  Thermal expansion? That
can also influence your choice of materials and design.

>..Is this something that would benefit enough to
> justify the work involved?

That depends on your design goals, budget, etc.

I hope this helps,
Tom Krajci
Tashkent, Uzbekistan

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