[TowerTalk] Designing a "receiving cradle" or jack stand for my HDX 572MD US Towers tilt over

[Grant Saviers]

For safety reasons, it seems like a good idea to support a laid over 
tower by more than the erecting cable when working on it.  One reason is 
the erecting system load (mast and cable) is the highest when the tower 
is horizontal.  However, what has not been mentioned in this thread is 
where to place the support and how strong it needs to be.

For a 1000# laid over crankup a support placed at the "top" would take 
one half the load, 500# plus the cantilevered load of the mast and 
antennas.  I'm not aware of any A frame ladder (e.g. Little Giant) that 
has an adequate rating.  Then every foot the support moves towards the 
base pivot geometrically increases the support load and tower bending 
stress. Get too close to the pivot and the tower bends.  I've seen this 
trick performed.  I think supporting at 2/3 the nested height is a good 
tradeoff, vs support loading, tower stress, and height of the antenna.  
If the tower is extended a bit for working on the rotator, then the 
support loading goes up.

Here are some numbers - my HDX-589 weighs 2300#,  plus cables, rotator, 
mast, and two antennas would need a torque of about 40,000 ft-lbs to 
raise the nested structure from horizontal.    Being unsure where my 
1964 "Statics of Solids" textbook is, I found a calculator on the web 
that can handle many tower laid over models:

https://instruct1.cit.cornell.edu/Courses/arch264/calculators/example1.1/index.html 
(so far nothing bad resulted from ignoring the security warnings)

What this calculator shows is the support load for my 23' nested 
configuration would be 2300# at 16' from the base (neglecting the cosine 
reduction in the 16' load from the elevation of the tower). If I support 
it at the 23' end of the nested tower, then that support load is 1620#.  
I modeled the tower at 100#/ft and additional loads for two antennas 
(100 & 80#), mast(90#), cables & rotator(150#), combining some point 
loads to fit the calculator inputs.

To get the 28' long boom lower antenna14' off the ground I would need a 
14' high support capable of 1620# at 23' out.  Using my support 2/3 out 
rule of thumb a 10' high support capable of 2300# at 16' along up the 
tower would also get the boom 14' off the ground. To me, these are big 
requirements for a fairly tall support and the ground it rests on.  If I 
use a 4' high support, it needs to be 6' from the pivot and the load is 
6200#.  Given that two outside legs will share that point load, that 
support position sounds very risky to me re bending the tower.  (The 
pivot reaction is upward 2000# since this support is inside the balance 
point).

This is one reason I've elected to rent a 40' boom lift when I need to 
work on the antennas or rotator.

YMMV, etc.

Grant KZ1W