(Sending again to see if I can get the hang of GMail's formatting. Sorry
for the bad line breaks.)
I'm a licensed P.E.(Calif.) and I have helped several local hams with tower
designs 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 the
6.1 SF wind load combined cause the bottom tower leg to be just at full
capacity (see Page 14, where the Combined Stress Index is 1.00). When CSI
is over 1.0, it is deemed overstressed. Since the bottom leg is the only
tower component with CSI = 1.0, it is weakest link. The antenna weight and
surface area limits go hand in hand. If the assumed antenna weight is
lowered then the allowable SF will go up. The 200 lbs/6.1 SF should be
viewed as a pair, with neither one as a standalone limit. It's a load
combination.
The Lift Cable Analysis on page 7 does not drive the 200 lbs number. It's a
check to ensure the cable can lift the tower(and it can with lots of
capacity to spare). The presence of the lift cable force does reduce the
allowable moment in the tower somewhat (Note 1 on Page 8), but it's not a
controlling 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 when
TIA-222-G came into effect. If memory serves the safety factor for steel
using ASD is 1.67. Under ASD, safety factors are applied to the allowable
stress 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-G
standard, if you plan to get a building permit. Even if you don't plan on
getting a permit I'd recommend having it redone under -G anyway because -G
does a better job with ice loading, which could be important in NJ.
73 Jim K6OK
======= original post
http://www.ustower.com/wp-content/uploads/2014/05/TX-472.pdf
I am wondering about the spec for 200 pounds of antenna weight. Can
someone with some engineering background/experience please review the above
link and let me know:
1. Is there a "safety factor" on the 200 pounds? If so, what is it?
2. What point is the weak link for the 200 pounds?
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