[TowerTalk] [Bulk] Re: MASTS

Tue Mar 15 22:59:59 EDT 2016

Jeff,

Your scenario is a likely one, although cyclic loading above a certain 
threshold of yield stress greatly accelerates fatigue failures in 
steel.  It's worse in aluminum, and I saw that happen on my 80m 
irrigation pipe vertical, it looked like it was cut perfectly into two 
pieces with a laser.  A welded structure is generally more fatigue 
susceptible.

http://resources.yesican-science.ca/sts115/fatigue1.html

Grant KZ1W

On 3/15/2016 19:03 PM, Jeff AC0C wrote:
> What is possible in the worst case and what is likely seem two 
> different things.  I've not seen EMT do anything other than just bend 
> (which it does pretty easily) and buckle (if you really really abuse it).
>
> So while I can imagine a scenario where the beam is bent back and 
> forth by alternativing massive blasts of wind resulting in a buckle 
> being introduced on both sides of the mast - eventually causing it to 
> break and the thing flying off, that seems pretty bizarre.  The more 
> likely case is that it will just bend - and a guy will look out his 
> window and wonder why his beam has a big tilt that was not there 
> before last night's storm.
>
> 73/jeff/ac0c
> www.ac0c.com
> alpha-charlie-zero-charlie
>
> -----Original Message----- From: Grant Saviers
> Sent: Tuesday, March 15, 2016 8:40 PM
> To: TexasRF at aol.com ; towertalk at contesting.com
> Subject: Re: [TowerTalk] [Bulk] Re: MASTS
>
> Gerald,
>
> Could you point me to the 35kpsi spec?  I couldn't find a steel spec on
> the Wheatland EMT manufacturer site, or that matter on any other site I
> looked at that google turned up re conduit specs.
>
> Here is a section from a Q&A by the Steel Tube Institute re the
> controlling ANSI 80.3 standard for electrical conduit
>
> "Rigid conduit is intended for electrical applications, not structural
> ones. Its requirements are governed by UL and ANSI specifications, not
> by ASTM. The main issue here is that, whereas dimensions may be similar
> between the two, rigid conduit is not intended for, is not designed for,
> and is not tested for any strength or structural requirements."
>
> So while the yield number and wall thickness in the mast calculator turn
> out ok, YMMV.  And likely with the thin wall it will deform with boom
> and rotator clamps.
>
> Standard schedule 40 water pipe would be a much better choice for
> several reasons:
>
> 1. the grade steel is specified
> 2. it is specified to withstand an internal pressure, which is a control
> on the seam weld quality
> 3. it is allowable in structural applications, although used mostly in
> larger diameters.
> 4. hot dip galvanized inside and out
>
> Caveat emptor.  Unless it falls down, and your insurance asks about the
> engineering and the plaintiffs attorney finds the statement to the
> effect "I don't care if it falls down."
>
> Grant KZ1W
>
>
> On 3/15/2016 12:35 PM, TexasRF--- via TowerTalk wrote:
>> Google says 1.5" EMT is actually 1.75" OD and .065" wall thickness and
>> yield strength is 35,000 psi.
>>   Putting these numbers in a mast analysis program and using a 7 sq 
>> ft wind
>> load 24" above top of tower, failure happens at a wind speed of 84 mph.
>> Lowering  to 12" above tower top, failure is at 119 mph.
>>   So, it could be useful in a modest installation.
>>   73,
>> Gerald K5GW
>>   In a message dated 3/15/2016 2:01:59 P.M. Central Daylight Time,
>> infomet at embarqmail.com writes:
>>
>> Let’s  not call names.
>> An EMT mast is great, as long as you don’t get greedy and  make it 
>> too tall
>> outside the tower.
>> Mine is about 2’ above the tower top  and I think it will survive any 
>> wind
>> likely in my area.  And I really  won’t cry if the $150 beam is damaged!
>> It’s several years old and shows no  sign of deterioration.  It cost 
>> almost
>> nothing and weighs next to  nothing.
>> So I have a used tower, used tribander, used rotator, EMT mast, but new
>> coax.
>> I also have a HB amp made from used components, with an easy KW+ output.
>> It’s a rare pileup I can’t get through.
>> I am VERY happy,  but may change to an LPDA so I can get the WARC bands,
>> someday.
>>
>> I also  use junk tubing from a kid’s swingset for Field Day. It’s worked
>> fine  for a decade or so.
>>
>> Successful systems can be designed around most any  components, as 
>> long as
>> one doesn’t try to do too much!
>> I’d far rather see a  ham put up 30-40 feet of used tower and a used
>> tribander than envy those with  more elaborate, expensive setups!
>> A few more hours on the air will make up  for a few dB of antenna
>> performance and there will be money left for  beer.
>>
>> Wilson
>> W4BOH
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