[TowerTalk] engineering help? Antenna weight for US Tower TX472 (JVarney)

[kr2q at optimum.net]

Thanks Jim,Interesting take on the 200 pounds of antenna weight.  I'm not concerned about knocking the tower down laterally, I'm just interested in the dead weight lifting capacity.You say that if the antenna weight is reduced the SF capacity increases.  Allow me to pose the question the other way around.  What if the SF of the antenna approaches zero (or what if the lateral wind is zero mph)...then what is the allowable antenna weight?  That's the key for me.Thanks so much!DougI'm a licensed P.E.(Calif.) and I have helped several local hams with towerdesigns and permits. I'm very familiar with the current TIA-222-G standard.>> 2. What point is the weak link for the 200 pounds?It is the tower leg in the bottom section. The assumed 200 pounds and the6.1 SF wind load combined cause the bottom tower leg to be just at fullcapacity (see Page 14, where the Combined Stress Index is 1.00). When CSIis over 1.0, it is deemed overstressed. Since the bottom leg is the onlytower component with CSI = 1.0, it is weakest link.  The antenna weight andsurface area limits go hand in hand. If the assumed antenna weight islowered then the allowable SF will go up.  The 200 lbs/6.1 SF should beviewed as a pair, with neither one as a standalone limit. It's a loadcombination.The Lift Cable Analysis on page 7 does not drive the 200 lbs number. It's acheck to ensure the cable can lift the tower(and it can with lots ofcapacity to spare). The presence of the lift cable force does reduce theallowable moment in the tower somewhat (Note 1 on Page 8), but it's not acontrolling factor here.>> 1.  Is there a "safety factor" on the 200 pounds?  If so, what is it?It appears this report uses the ASD method, which became obsolete whenTIA-222-G came into effect.  If memory serves the safety factor for steelusing ASD is 1.67.  Under ASD, safety factors are applied to the allowablestress in the tower legs and braces, not to the wind load or weight load.Please note this structural report uses the obsolete TIA-222-F standard.Your local city or county may require a new report done under the 222-Gstandard, if you plan to get a building permit.  Even if you don't plan ongetting a permit I'd recommend having it redone under -G anyway because -Gdoes a better job with ice loading, which could be important in NJ.73 Jim K6OK