[UK-CONTEST] Mastrant ropes - Caution?

[G3RAU at aol.com]

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