At 08:16 PM 4/15/2006, Michael Braun wrote:
>Thanks everyone for your emails.
>
>The problem with the AB-577 manual is that it does not contain data of
>relevence to a professional engineer. Practically all of the manual
>describes how to use it rather than the materials involved in its
>manufacture.
Naturally.. it's a user manual. But, it was written by someone (back in
the 1960s) who had the engineering analysis (such as it is) in front of
them. Your challenge (which will consume time, if not cash) is to find it,
deep in the dusty archives of some depot. That's sort of the tradeoff for
all these surplus things. You could go out and buy a brand new tower, with
brand new, freshly wet-stamped drawings, but it would cost a bunch more.
>Does anyone know of a study done with these types of things, or
>especially one with regard to wind load? I'd need to be able to provide a
>professional engineer with enough documentation to show that my AB-577 with
>an improved guy system can survive a 90mph wind gust for 3 seconds.
Actually, the PE will need to do the analysis for him/her self, or at least
review the analysis in detail. It's not simply a matter of looking over a
sheet and "wet-stamping" it. {This is called, perjoratively, "rent a
stamp", and is illegal in most states.} You *can* make life easier for the
engineer by having a lot of supporting information, and the sources of the
information. A bald assertion that the breaking strength for the guy is
ABC pounds isn't going to fly, you need manufacturer datasheets.
If you have the inclination and skills, you might also find a PE willing to
let you do a lot of the actual grunt work of the analysis, in exchange for
a lower fee. (I know I would, in such a situation..) Have you checked with
the ARRL Volunteer Consulting Engineer program?
Practically speaking, I suspect that you'd have a tough time getting an
engineer to "sign off" on a "permanent" installation of a 40 year old
system designed for temporary installations on a battlefield, with
uncertain material quality. That's not to say that it isn't safe, but that
the problem of "proving" it's safe might be more trouble than it's worth,
or the things that the engineer will require you to do will be impratical.
Questions that would need to be answered: What sort of surface is the base
on? Will it support the downforce from the guy tension and wind loads? What
are you using for guy lines? What's the static tension? How are they
anchored? How do the segments of the mast stay together? What's the
strength of materials in those segments? What's the impact of the joins?
What you probably want to focus on is getting your local authorities to
believe that it's safe, and that it fits into some category that doesn't
require wet-stamped engineering drawings. It might be possible that a
decent analysis, short of sealed and stamped, might go towards convincing them.
Maybe you can use a "back of the envelope" analysis to show that the thing
is strong enough by a factor of 5 or 10, so any errors in analysis or
estimates of strength of materials aren't likely to significantly change
the risk of failure. The analysis itself isn't all that mindbendingly
difficult, at least, if you ignore the segmented nature of the mast. There
are several books out there on stress analysis for amateur antennas, and
some websites, as well as a few articles over the years in QST or QEX along
with Excel spreadsheets.
Jim Lux, W6RMK
>Thanks,
>Michael N3CA
>
>On 4/15/06, Joe - WD0M <WD0M@centurytel.net> wrote:
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