[TowerTalk] FW: Sources for mast material

[kb5my at starband.net]

Michael,

The 3X to 4X is called a safety factor.  It is considered good engineering
practice to use safety factors from 1.5X and up.  How low or high you go
in safety factor depends on how much the folks signing their names to the
design and/or paying the bill are willing to stick their necks out when
balancing safety of life and limb over cost.

For me, 3X to 4X is just fine when it comes to towers and masts.  I live
in a 70 MPH fastest mile/85 MPH 3-second gust county (these wind ratings
are are actually equivalent).  However, my property exposure is such that
annual sustained winds in the 80-100 MPH regime are not uncommon - and
they can last for several days running.  I can guarantee you that no
future tower that goes up on my property will be designed for the 70/85
rating with a 1X safety factor.

My wake-up call came when I witnessed first-hand the results of sustained
80-100+ MPH winds and even higher gusts (lasting several days) pushing
against a 2-year-old Rohn HBX-56 planted in 6 yards of 4000 psi concrete
with nothing but a TV antenna on it.  The upwind leg sheared off at the
base from excessive flexing, and the whole thing folded over at the base
of the other two legs, landing on my roof.  The tower was installed per
specs, but apparently it wasn't designed to withstand several days of
severe gusting at high wind speeds.  No more cheap towers for me!

73,
Dan

>
>
>>    1)  What is the meaning of the "wind speed" parameter.  The dialog
>> box
> says
> "maximum wind speed expected".  In my county, basic wind speed is 70 mph
> fastest mile, 85 mph 3-second gust.  So I figured the max instantaneous
> is probably higher.  So I figured 90 or 100 would be a good number.  But
> it really depends on what the meaning of this number is in the MARC tool
> (fastest mile, 3-second gust, other).  Can you define?
>
>     We used the formulas and specs from Dave Leeson's (W6NL, ex-W6QHS)
> book
> "Physical Design of Yagi Antennas." Maybe there's more of an explanation
> there.
>
>     The over-riding spec is your county windspeed rating from the
> TIA-222
> windspeed chart (available from
> http://www.championradio.com/windspeed.php). There are several ways of
> arriving at a figure.
>
>
>
> I don't have a copy of the above book.  Perhaps someone who has a copy
> knows the answer to this.
>
>
>
> The windspeed shown on championradio.com for my county is the fastest
> mile windspeed (70 mph fastest mile).  But the current TIA-222 spec now
> uses the 3-second gust speed (85 mph 3sg).
>
>
>
> So, as mentioned above, the question is, to which "wind speed" does the
> WinMARC application refer:  fastest mile, 3-second gust, or "maximum"
> (as the dialog box says)?  Without knowing that, the number to enter for
> "wind speed" is unknown.  70 (fastest mile)?  85 (3 second gust)?  90
> (more likely "maximum")?  And, if the correct wind speed is unknown,
> then the resulting tensile strength calculation is meaningless.
>
>
>>  2)  Also, when the calculated total is 12,290 in-lb, the lightest
> suggested
> mast type shown in the list is a 40,000 in-lb yield strength.  It's
> seems like it's only displaying mast types that are at least 3X higher
> than the calculated total.  Maybe you could shed some light on that too?
>
>
>
>     No, that's because tubing has that as a minimum rating. As opposed
> to
> pipe which doesn't; it's around 35 kpsi and we don't include pipe as a
> generally suitable mast material.
>
>
>
> But the behavior of the WinMARC application does not agree with that
> description.  For example, if I reduce the wind speed such that the
> calculated tensile strength is only 6151 in-lb, then the list of
> possible mast types also changes, with the lightest version being 25,000
> yield strength  "6063-T6 Aluminum pipe" - that's a 4X difference.  A53
> and A36 "Structural steel pipe" are also listed as suitable mast
> material.
>
>
>
> So, again, the application seems to be listing recommended mast types
> that are 3-4X higher in strength than the calculated load and the
> answers do include both tubing and pipe.  The simple question is "why is
> it picking those types, rather than something closer to the calculated
> value"?
>
>
>
>     If your design yield strength is 13 kpsi, then anything will work.
> That's why I suggested Schedule 40 or 80 water pipe or conduit. You're
> over-thinking this whole thing.
>
>
>
>
>
> Yes, this situation is relatively simple.  And thanks for the straight
> and simple answer on the sched. 40.  That helps a lot.
>
>
>
> But the problem is that the application is giving answers which differ
> significantly from what you're saying here.  So I'm trying to understand
> why - and not getting very far.  This isn't the only antenna mast I need
> to figure out.  So I'm simply trying to understand the proper inputs to,
> and the meaning of the outputs from, the application so I can use it for
> other configurations.
>
>
>
> Thanks for all the helpful info.
>
>
>
> 73,
>
> Michael - K5MEF
>
>
>
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