[TowerTalk] For Chrome Moly and tower/wind load experts out there - Mountain top tower installation

[trappaport at austin.rr.com]

With inputs from many, I have settled on making the mountain-top 65' towers out of heavy duty Rohn 45G  SR (solid rod) with 2 guys per tower, using three 20' sections (and a 10' base section having 5' submerged in reinforced concrete). After much analysis by many people, this appears to be the most rugged yet climbable choice for a mountain top location, as it is sturdier than hollow tubed 55G in ice, and comparable to 65G in strength but easier to climb and outfit. With my 2 big antennas on top, it is engineered to withstand > 110 MPH winds and 1/2" radial ice on mountain tops, and hopefully *much* more than that. Does anyone have advice/experience/words to the wise regarding 45G SR during installation or over the long term?....The professional tower installers will be using a crane to install.

Now, the question of mast pipe arises, and the yield strength in high winds and ice becomes a concern to all I talk with.. Chrome Moly seems to be the mast material of choice, and 4130 N tubing(Normalized) appears to be readily available and strong, with 105,000 typ. PSI yield strength. Much of it is imported from China, hopefully meeting stated specifications. I will have two 65' towers on a mountain ridge, and will put 2 big antennas (and maybe a smaller VHF/UHF) on each tower: a 14 ele. Force 12 C31XR at 2' above the tower top (67' elev) and a Magnum 240N at about 8' above the tower (73') elev, and the mast pipe will go to a top height of 75' (county maximum limit).  I learned all about Chrome Moly from Charles Volgesang who runs Dillsburg Aeroplane works in Pennsylvania..He is not on email or the web, but knows how to use a fax machine, and he and his wife have dedicated their life to this, and have millions of ft. of this stuff at a reasonable price. 
 
Tower companies have expressed deep concern about the deflection issues and yield strength with such big antennas up on a mast on top of a mountain (its a huge moment arm with much torque, especially when loaded down with ice), and I have learend alot about the new 222-G tower specs that specify mountain top winds as being 3 times the normal level. The amount of ice on the antennas is a big concern for loading. Counties around the country may be adopting this new 222G tower regulation within a year or two, and this may create havoc for hams, as new, more rugged steps may be on the horizon where building permits are required. Some tower comanies have recommended I get 5" OD steel mast pipes, which will not work for my rotors that take a maximum of 2.5" OD mast!
 
HERE is the question: I have both 2.0" and 2.5" Chrome Moly Mast tubing available, each about 18' long.  The 2.0" Mast pipe (I have learned that the word "tube" is the preferred term by experts, instead of "pipe") has a wall thickness of 1/4", and the 2.5" mast tube has a smaller 0.12" wall thickness. I suppose there is a book with a formula for yield strength of tubes of different inside and outside radii, made of the same material, its probably an undergrad ME question, but I was hoping some of you out there might know off hand the answer to this question:

Which is better/stronger/wiser to use for ice/wind strength on a mountain top tower, and why?
 
a. The 2.0" OD  0.25" wall thickness Chrome Moly 4130N tube, weighing 100 lbs for 18'
b. The 2.5" OD  0.120" wall thickness Chome Moly 4130N tube, weighing 75 lbs for 18'

And a second question: would it be wise to put a little 4' or 5' lightning rod above the mast pipe to attract electrons, and if so, is there a cheap/easy way to affix the lightning rod to the top of the mast, and what should it be made of...I am thinking of a sacraficial metal element that might offer protection, etc. and would be very hard to see ("invisible") so as not to exceed zoning height requirements.

Thanks to all... 73 Ted N9NB   trappaport at austin dot rr dot com