Patrick-
Having erected some large towers in my work on microwave radio sites, I can
tell you the correct (NOT cheap) way to design your foundation: First have
a soils engineer perform a soils investigation. They drill several holes
and determine the soil conditions down to a certain depth. Then give the
soils report to a structural engineer with a list of all the loading you
plan to put on this tower, including the new crank-up, antennas, feedlines,
etc. He can then design a foundation to support those loads in your actual
soil. The three piers resist the overturning moment on the tower mainly
through friction with the undisturbed soil, not through dead weight.
Sometimes if the soil is not great, they'll design a flare (bell) at the
bottom of the pier to add more pullout resistance. Then again, if you're
over granite, sometimes you can anchor directly into the rock without a big
excavation. And if your soil is really bad, ie. sandy, the engineer might
call out a mat foundation. This is a big pad of concrete bigger than the
footprint of the three legs which in this case does hold them down with
sheer mass.
To be safe, start with a local structural engineer who has a PE license in
your state. He can refer you to a good soils engineer, and also give you an
estimate of costs.
Otherwise, if you choose to wing it, be sure to make sure your house is
outside the fall radius of your new tower!
GL es 73,
Steve
N6SJ
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Hans
Hammarquist
Sent: Monday, March 04, 2013 7:32 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Fwd: Relatively large tower base install questions.
Hi Patrick,
I think you have some major engineering job in front of you. You can
probably use your truck to erect it if you add a "falling derrick",
something used in many cases raising a tower. I used one myself.
Regarding all the adds-on you definitely will need some engineering inputs.
I also strongly suggest you get recommendations regarding the tower base.
Just dig something down does not necessary guaranteeing a proper support. I
imagine you will get some lifting forces on the legs when you add structures
to the top of the tower. Most towers take care of these forces by the guy
wires but I imaging you should be able to use some nice size anchors at the
tower legs for the self-supporting tower.
You might check out my tower and its erection here.
https://www.facebook.com/media/set/?set=a.10150198503693539.328964.579753538
&type=1&l=1c649b80ab
Good luck with your project and keep us updated what's happening.
Hans - N2JFS
-----Original Message-----
From: Patrick Greenlee <patrick_g@windstream.net>
To: towertalk <towertalk@contesting.com>
Sent: Mon, Mar 4, 2013 12:09 pm
Subject: [TowerTalk] Relatively large tower base install questions.
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
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