[TowerTalk] Rohn Tower Designs

[Hank Lonberg]

     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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