Guyed tower turnbuckle design

[apsix]

I'm checking calculations for turnbuckle selection, using LRFD (well, the Australian version of it).

Applied tension is therefore factored.

Turnbuckle capacity is in service load WLL (working load limit). WLL is 40% of proof load and 20% of ultimate strength.

The designer has shown that the applied tension is less than 80% of the ultimate strength, and is also less than the proof load, and is therefore adequate, he claims.

Surely this is incorrect? If we were using ASD we would expect a safety factor of 5 over the ultimate strength. Shouldn't we expect a similar overall safety factor for LRDF?

I would have converted the factored load to a service load by dividing by 1.5 and ensuring that this is less than the WLL. Does anyone agree with my approach?

 

[kslee1000]

I think you need to figure out the authority and governing code for design of guyed tower components, and find out the approved design method(s).

 

[kelowna]

I agree with you Apsix.

Turnbuckles and other components for cable systems use different safety factors to find maximum service load from ultimate strength. 5 is a very common one for turnbuckles, shackles and other components. This higher safety factor is due to abuse, installation issues, fatigue, repeated use of the equipment and other factors.

I think it is incorrect to assume you can use a lower safety factor. Working load should not exceed the maximum working load specified by manufacturer, and I find strange that any engineer would take that responsibility on his or her soulders.

 

[baw1]

I agree with previous post. First define the governing code or criteria. However, having said that I know some of the ASME B30 codes for lifting devices call for a FS of 5 against yield and 3 against ultimate strength.

 

[warrenw]

apsix
A factor of safety of 3 (breaking strength/working load) is a resonable number for wire rope used for a guy supported structures.
The turnbuckle and all the rope hardware should have atleast the same capacity as the rope.

 

[apsix]

Thanks to all you replied.

I've recommended using the next size up where the WLL is exceeded.
The cost of a dozen larger turnbuckles is insignificant, and worth the peace of mind.

There was no guidance in the tower design code (AS3995).

 

[VoyageofDiscovery]

Just a caution with regard to using ultimate strength design for turnbuckles, turnbuckles are forged and do not have the yield characteristics expected in ultimate strength design. I checked with CSA Canadian Standards Association a few years ago and they do not have a standard for structural strength or specifications of these types of components.

I have used Crosby versus Korean and Chinese products as the later have a track record of failures. Every time I asked for mill certificates from Korea and China, material quality is questionable.

HTH

VOD

 

[PersonalProfile]

Ref AS1418, guy ropes etc, for a given classification, a Zp / SF is provided.

I would have thought your proposed approach would be most technically correct. As was mentioned before, why take the risk on your their own shoulders?
Engineers generally work to established standards.

Regards,
Lyle

 

[OzEng80]

aspix

I take it the 'next size up' is higher grade turnbuckle? This was encountered recently in my office and we were unable to source a practical turnbuckle with the same WWL (and diameter)as the threaded rod (due to the vastly different SF). Due to this lack of WWL correlation for a given rod and turnbuckle diameter we were forced to fabricate the component with a high strength coupler and plates.

It is certainly odd that manufacturer's cannot provide ultimate capacities and let engineers determine the appropriate safety factors to apply. It is my understanding that this is the whole intent of limit state design anyway.