Kurt and reflector:
I think the issue of safety factors and allowable stress and ultimate
stress gets somewhat confusing. In regards to the Guy safety factor;
the EIA/TIA-222-F spec in paragraph 8.1.2 states:
"Safety Factor of Guys.- The safety factor of guys shall be calculated
by dividing the published breaking strength of the guy or guy
connection strength, whichever is lower, by the maximum calculated
tension design load."
So if you want a SF of 2 for guy design as Rohn states:
Max. design tension = break strength / 2
Just what safety factor and what datum strength one uses is a matter
of situation anaylsis and the uncertainty of the loading the system
will be subject. If you use breaking strength (ultimate strength) then
you are viewing the problem with a plastic design strength philosophy.
If you use yield strength then you are using a allowable strength or
elastic strength philosophy.
The 9th edition of the AISC Steel Construction Manual- Allowable
Stress Design in regards to tension members states in section D1:
allowable stress in tension only members:
Ft= 0.6 Fy (on gross member area)
Ft= 0.5 Fu (on net effective area)
For A36 mild steel Fy=36,000 psi
Fu=58,000 - 80,000 psi (typ. 60,000 used)
Ft=.6(36,000) = 21,600 psi
Ft=0.5(60,000)= 30,000 psi ( less than Fy)
or if you choose a SF of 2 (Say if for guys)
Ft=.5(36,000) = 18,000 or (based on yield)
Ft=.5(60,000) = 30,000 (based on ultimate)
It should be noted by the reflector readers that the above
values are for mild structural steel as in plate, beams,
angles, etc and NOT wire rope or EHS guys!
Either one is appropriate it just depends on the enviromental
situation of the system under consideration. In the marine arena,
nasty enviromental concerns aside, with continuous motion almost all
the time and thousands of stress reversal cycles (fatigue) I would
most likely use the lower value based on yield strength. I think this
answers your second question concerning long term conditions in the
marine arena. For antenna towers on land I would not be as concerned
with the fatigue criteria (fewer stress reversal cycles) and would go
with the breaking strength (ultimate strength) philosophy.
The true test is actual experience. There seem to be a lot of towers
out there that are standing using the SF of 2 of Breaking Strength
criteria and they seem to be fairing well. If your designs are fairing
well using your criteria then I would not be too worried. The real
killer is maintainence and the lack of it for guyed systems.
Hope this helps, Sorry to burden the reflector with the more subtle
aspects of design issues.
73
Hank Lonberg, P.E.
KR7X
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