Of course it is possible to weld up any set of steel parts you like, but
the challenge is to *design* something that will handle the loads and
not fall down. So, without some *engineering* analysis tools and an
understanding of steels, it seems to me that the "homebrew tower" is a
dangerous undertaking.
For those inclined to make some effort to design something safe and
effective, I have found tools on the web that analyze simple
structures. For some tools that are particularly useful, see
http://www.engineeringtoolbox.com/statics-t_63.html Simple beams,
masts, cantilevers, etc. can be analyzed with a variety of load
conditions. Getting a used copy of an engineering "Statics" book would
be a good idea. My college text
http://www.amazon.com/Vector-Mechanics-engineers--Part-Statics/dp/B0000CLFGR/ref=sr_1_2?ie=UTF8&qid=1435533660&sr=8-2&keywords=harry+nara
$4 on amazon, free ship with Prime. It's mostly high school geometry.
A tapered welded lattice structure is something in an entirely different
league from an analysis perspective and a high power software tool such
as finite elements analysis is needed. Backyard engineering NOT.
Re "black steel (iron)" pipe being better quality than galvanized, not
according to the specifications: (from McMaster web site)
Galvanized steel pipe nipples and pipe provide very good corrosion
resistance in low-pressure plumbing applications. Also known as Schedule
40, all have welded construction, so there's a weld bead on the inside.
Meet ASTM A53 and A733, as well as ANSI/ASME B1.20.1.
Black steel is the most popular choice for pipe nipples and pipe in
low-pressure plumbing applications. Also known as Schedule 40, all have
a welded construction, so there's a weld bead on the inside. Meet ASTM
A53 and A733, as well as ANSI/ASME B1.20.1.
https://en.wikipedia.org/wiki/ASTM_A53 ASTM A733 is for finished
pipe nipples & B1.20.1 is for pipe threads Chinese ripped free A53 spec
at http://www.hysteelpipe.com/pipe-standards/astm-a53-standard-free/
After reading the A53 spec, I stand corrected: although there are
several variants, the steel is graded and suitable for structual use,
but who knows what is on the racks of the big box stores? I also note
per wikipedia that DOT now requires a drawn A53 pipe (DOM) so that
provides more consistent properties. It is also mentioned that A500
higher strength steel is often the choice in structural applications for
round pipe. A500 is the common steel spec for square tube and what I
see on the racks at my suppliers and what I see being used if a building
is being earthquake retrofitted.
Prep of black iron pipe for painting is a real b*tch since it often has
hard to remove coatings. My local steel supplier stocks plain steel
pipe, it appears to be machine sandblasted which is a very good surface
for adherence of paint, and there is nothing to contaminate the weld.
Threaded couplings are a bad idea as the thickness of the pipe wall is
substantially reduced and thus the pipe's tensile strength. In a three
legged structure, wind loads can be 100% of max tension in any leg so
staggering the couplings is useless.
Grant KZ1W
On 6/28/2015 13:22 PM, J Chaloupka via TowerTalk wrote:
What would I do as a non engineer?Materials, if you can get it, ungalvanized pipe,
comonly called "black pipe", it is a better quality than galvanized, which is
typically full of defects and camoflaged by the galvanizing.Make a three leg tower, with
legs made up with three lengths of pipe (your spacing ) and lattice braced to the
strength you desire) couple the individual leg elements with threaded couplings to your
desired height, stagering the couplings to each vertically (no joints opposite each
other). Taper each leg having the three elements meet at the tower top, thus making
stronger legs than possible with a single pipe. Situate the triangular legs to your
desired dimension at the base. This will be a three legged tower with a triangular plan
view. Taper the tower up to the size apex plate you desire.Lattice brace the three legs
to each other on up the tower.Paint the tower heavy, with a brush, for corrosion
protection.I would paint every year for three consecutiv
e years and then stand back and see when it might need it next.There is a paint
called rust bullet, although expensive, possibly you can get some,
JIM, W4QBQ
From: Roger (K8RI) on TT <K8RI-on-TowerTalk@tm.net>
To: towertalk@contesting.com
Sent: Sunday, June 28, 2015 3:33 PM
Subject: Re: [TowerTalk] Homebrew tower
I would be careful adding extra braces. Much of the brace strength
comes from the angles between the brace and legs. Using lesser angles
reduces the strength, requiring still more braces. OTOH. I doubt the
extra braces would cause the tower to collapse under its own weight.
The braces serve to keep the tower legs parallel, which adds a lot of
strength in the lateral plane (resistance to bending in the wind) It
adds little to the ability support a load. IE: dead weight. They also
keep the tower from bending when "tipped up" in one piece. I believe
"X" braces bolted, or welded together where they cross will add strength.
This tower is only going to be about 40 to 50 feet tall and will support
a small beam/tri-bander. I have my doubts about the wind load and
leverage from a mast supporting a 5L 6-meter Yagi and a small 40 meter
beam at the required spacing. I'd be more inclined to use a series of
sloping dipoles on 40 and limit the wind load to a single, small tribander.
I'm assuming that the limitations are materials and budget as they are
for most hams and suitable used towers are not available. That adds
another dimension to the requirements: Durability. To me, it sounds as
if failure of the tower is not an option. Here in the states, we have
the options of used towers and lots of suitable materials to build and
rebuild towers. This does not appear to be such a case. If budget is
the problem, then I'd go conservative on the antennas.
73
Roger (K8RI)
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