[UK-CONTEST] Mastrant ropes - Caution?

Fri Oct 5 03:28:41 EDT 2012

Hi Martin,
Thanks for the comments.
I was (thankfully in some ways!) not present when the event happened and so I do not know exactly what took place. What is clear is that whatever guying arrangements were made by me, with regard to head load, they were not adequate. Every installation faces this situation and I fully understand your close interest in the discussion.
If you were following the discussions from the beginning, before I placed my order with yourselves, you would note that I had many recommendations about your site. After dealing with you and your colleagues, and the guys at Vortex, I strongly praised the way you dealt with me and I stand by those comments. Your service was excellent.

With regard to specifics, I can only add the following comments:
I used your fist grips on the rope, both at the top and at the bottom, used three grips per termination and had them spaced probably 60-70cm. I have commented to several people that these are very good items, make a very secure termination, and there was no evidence (so far) that these have slipped on the guy that 'failed'.
I was asked about the guy peg. It absolutely did not move. Rock solid.
The lower two sets of steel guys remained fully tensioned - no sign of stretch.
I seriously doubt that there was ever a gust of 124 mph.
There is no doubt that the D12 stretches - I had to shorten the guys by some cms on several occasions after installation, in order to maintain tension, and had just re-checked all the guys and set the tower up about 3 days prior to what was not forecast to be an exceptional storm (it turned out otherwise!).

Somehow, the top section of the tower moved too far past 'vertical', reached a critical point and we know the results. No doubt, what I did failed to achieve the correct balance between headload and guying performance. Too much headload or inadequate guying is all about perspective! Obviously the decision to do what I did was mine alone and I am not blaming anyone else. An interesting, if unwelcome, lesson perhaps.......

Best wishes,

Keith

-----Original Message-----
From: UK-Contest [mailto:uk-contest-bounces at contesting.com] On Behalf Of G3RAU at aol.com
Sent: 04 October 2012 11:28
To: uk-contest at contesting.com
Cc: huml at mastrant.com
Subject: Re: [UK-CONTEST] Mastrant ropes - Caution?

Martin OL5Y at Mastrant, the synthetic rope  manufacturers, has asked  to post his  analysis:- Hi,  I am Martin, OL5Y at Mastrant. We are sorry to learn of the Versatower failure recently experienced by Keith GM4YXI. As his tower was guyed  with a Mastrant product we have naturally taken at close look at what may have caused the problem.

I made the calculation of forces in the particular setup and here are  the
results:
Input conditions for calculations:
- Height of guying: 21 m
- Distance anchoring point from the bottom (guying radius): 13  m
- Antenna on the top: 41ft boom with 5el on 20m and 2 fiberglass el.  for 40m Moxon (wind area estimate 1.6 sq m, 17.2 sq ft)
- Speed of the wind: 200 km/h (55.6 m/s, 124  miles/h) Forces (at maximum):
- affecting guywire: 6755 N (corresponds to 689 kg, 1415  lb.)
- affecting tower (vertical axis): 11487 N (corresponds to 1171 kg,  2576
lb.)
The rope type Mastrant-D has an elongation of 1,2% (when it is  tightened to SWL, i.e. 30% of breaking strength). This stretch means “elastic” -  the rope shrinks back to its original length when the load is  removed.
If in the above mentioned conditions is used the Mastrant-D, 12 mm  (as used by Keith) the rope elongates 11 cm only! Of course only if it  was tightened before and not slack.
It is clear that this stretch could not cause the failure of the  mast.
I don’t know exactly what has broken or how the ropes were terminated  so I cannot say more. My guess about what might happen is that the rope slipped in the cable grips or the mast somehow fell. The manufacturers specification of  the particular mast is :  “The  80ft HD Versatower max head load at
120 mph is 9.9 sq ft or 0.92  sq m tubular aerials- which equates to 102.6 Kg / 226.9 LBs” so I guess it could be highly overloaded by the wind load weight  of up to 2576 lb. on the top. That could make the individual  section steel  support ropes stretch beyond their design limit or some other  damage.
I believe that I explained little bit more the situation, but please  contac t me if you need more help or guidance.
73! Martin OL5Y
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