[TowerTalk] Relatively large tower base install questions.

[Patrick Greenlee]

Background:

I have recently purchased the remnants of a failed tower from a friend of 
mine. The tower was originally 100 ft tall, of triangular construction and 
the legs were on a little over 14 ft centers (that is right over 14 feet not 
inches) where they were attached to 18 inch diameter concrete piers 7 ft 
deep into the ground. The tower held a wind generator and it failed due to 
gyroscopic forces applied by the rotating mass changing directions due to 
variable wind directions.  A witness described the event as quite 
spectacular.  Anyway the bottom two sections are in great shape, just a 
little surface rust.  The legs are about 4 1/2 inches OD pipe and are joined 
at the 20 ft level by pipe flanges that are multiply through bolted.  Ditto 
the connection to the pipe sections embedded in the 18 inch diameter piers. 
The cross braces are angle iron bolted together where they cross and 
attached at their ends to pad eyes welded to the steel legs. One leg has 
welded on ladder rungs alternating on either side of that leg.  Two of the 
legs have hinged bases where they attach to the piers so the tower can be 
tilted up and down.

Over the years since installation lots of trees and brush has grown up 
around and through the lower of the two sections making tilting down for 
disassembly a real AXE MEN challenge ( hours of chainsaw work.) I have the 
chainsaws up to one with a 32 inch bar and a motor the size of a small dirt 
bike but it might be easier to just climb the tower and disassemble it from 
the top down.  I have a friend who also has tower climbing experience and 
personal safety gear (harness etc.) We expect to get it taken apart and 
hauled off with a two day weekend effort.  I have a welding trailer with 
17.5 KW genset and air compressor to support the use of air driven impact 
tools for taking the rusty fasteners apart (or twisting them off if 
required.)

So far so  good (just like the guy who fell from a 40 story building and as 
he passed the 20th floor he shouted out so far so good) Then comes the 
re-erection at my site.

Questions:

I am open for suggestions. Lacking credible objections from this august body 
I will try to assemble this 40 ft tower on the ground and tilt it up into 
place. I will have 360 degree access and freedom all around the site.  I 
have a truck with a front mounted winch of 12,000 pound capacity.  I have a 
good stock of 2 3/8 oil field pipe,k drill stem, etc and full welding, 
cutting, bending, whatever capacity in my shop.

Since in its new life this tower base is not holding up a 100 ft heavy duty 
tower what is a reasonable pier size and depth?  I can auger up to 12 inch 
diameter holes up to nearly 6 ft deep with my equipment,  Alternatively I 
have a ride on trencher (like a backhoe but self propelled not on the back 
of a tractor) that can dig a 13 inch wide trench down to 7 ft or so. I could 
block off part of the trench and back fill prior to the pour and have a more 
or less rectangular pier about 13 by xx inches up to about 7 ft deep but 
this is more effort and pending your input may not be required.

I intend to add a spiral (triangular) staircase around the outside of the 
tower to access the observation/work platform that I will install at the 40 
ft level. This deck will be larger than the tower's triangle dimensions at 
the top (40 ft level) and so will be cantilevered.  I will not extend this 
deck so wide as to extend beyond a vertical projection of the piers thus not 
risk a tip-over event due to any load of sightseers.  I want to install an 
automated crank-up tower inside this tower such that when fully lowered you 
can stand on the platform and access the lowest antenna on the tube coming 
out of the center of the top of the crank-up tower.

Any thoughts?

How much pier diameter and depth into the ground is enough?

Gin pole recommendations for tilting up after reassembly?

I assume I would install the crank-up tower inside the 40 footer prior to 
tilting the tower upright but will entertain alternative suggestions.

TIA for any help

Patrick AF5CK