[TowerTalk] Max height of aluminum tubing mast?

[jimlux]

Brian Machesney wrote:
> I need some help from the more structurally-oriented members of our
> fraternity.
> 
> I need to know if it is practical to erect a 75-foot vertical element from
> aluminum tubing that has the following dimensions:
> 
> OD: 1.785"
> ID: 1.57"
> Wall: 0.11"
> Length: 48" overall; 3.25" suaged and shouldered to fit into the next
> section
> Weight: 2.7lb
> 
> I have devised a raising fixture that has allowed me to raise 45ft masts of
> this material alone and with ease. Will a 75ft element stand up, or is there
> serious risk of it buckling under its own weight? Of course, numerous guy
> levels will be needed - I'm planning on 5.
> 

preventing buckling is a matter of enough guying.  However, there's a 
tradeoff because each set of guys increases the downforce, which 
increases the chance of buckling in the next segment down.

Is that 2.7lb/ft? (It's too early in the morning for me to calculate it 
from cross sectional area and aluminum's density).. 75 feet * 3 lb/ft = 
225 lbs total weight (no guy down force).. the cross section is about 
6*1.8*0.1 or about a square inch.  Since aluminum's compressive strength 
is >10,000 lb/sq inch, at least your not going to fail from just 
compressive loads.  Buckling failure's where it's at (but you knew that 
already)..

Off hand, it seems like you're putting up something like real thinwall 
speedrail, and it's really tall.  That stuff is pretty flexible.  I 
doubt you could assemble it horizontally and then tilt it up, but you 
say have some clever scheme for erection that will distribute the loads 
(falling derrick with multiple lines?)

Is this a permanent installation, or a weekend field day sort of antenna 
(that if the wind comes up and it comes tumbling down, it's just an 
inconvenience, not a disaster)