[TowerTalk] UPDATE: light standards as towers?

[Scott Bullock]

Hi Patrick, fyi this is a reply to someone else that had sent basically the
same thing.

>Hi Joe, actually no, Mass building code is pretty strict, my engineer has
been educating me on it :) You would be surprised to hear that from his
calculations, most manufacturers underspec their tower bases! Most do not
spec their towers with ice! Unfortunately we have to contend with that in
New England. I was shocked when he showed me the derated specs for ice/no
ice! It really adds significant stress to the tower and increases the
"uplift" of the base substantially! 

Many of the tower manufacturers' crankups and self supporters wouldn't
support a toothpick in 90 mph winds! Most are spec'ed at 50-70 mph, so
therefore the bases are shown appropriately very small.

FYI, We are designing my tower for minimum 25 square feet at 100 mph with 1"
radial ice, something that many towers companies don't take into account
with their specs. Some manufacturers have specs with 1/2"ice, but here in
Massachusetts, we can have significantly more that their spec.  I have seen
up to 3" ice on towers at commercial sites where we have equipment. No fun
going to a transmitter building in the winter with that on the tower!  :)

I have finalized my "stack" as being th-7dx, a3ws with 30 meter addon, and
40-2cd. This also leaves me a little loading to play with later on if I
decide to add additional fixed monobanders, while still having a reliable
safety factor built into the tower. I'll followup with the reflector when we
get the final specs done to let everyone know what the final outcome is.
73,
Scott
KA1CLX



At 11:00 PM 7/21/97 -0400, Patrick Croft wrote:
>Scott-
>You might want to review the Rohn handbook.  Base mass is very dependent on
>load size and shape round, flat, etc), but also normalized wind and soil
>type.  As an example, the base for a 120 foot SSV (Rohn)is 10'x10'x4', or 15
>cubic yards.  These are rated for 70mph (newest ANSI/EIA standard).
>Overturn foot-pounds = 141,700.  Load rating is 10 square feet at the top,
>and 14.2sq.ft 30 feet below the top, plus cables, etc.  At 90mph the top
>load drops to 8.3 sq.ft., 13.3 sq. ft. down 30 ft, etc.   
>
>The key I think is the loads at each specific location.  Midwest fairly calm
>(excluding twisters) with 70mph the norm, and I usually upgrade to the 90mph
>specs.   Your PE will be dollars well spent if he's versed in these areas.
>
>BTW - many suggest a larger width and length over extra depth on the base of
>a structure.  The improved resistance (mechanical) offered by the friction
>of the earth actually lowers the amount of mass needed.
>
>Bests,
>
>Patrick
>>>
>>>Dick,
>>> I'm just guessing - 4x4 by 6 to 8 feet deep ought to hold it I would think.
>>>
>>> I'll have the exact info pretty soon
>>>
>>Seriously doubt you'll get away that small for a 80' monopole. Would guess
>>something more in the order of 10x10 and 8 feet deep for that type of
>>application, probably 15 or more yards of concrete. My PE is looking at the
>>base size for my new tower, looks like 8.5x8.5 and 7' deep for 56' self
>>supporter according to wind loads and uplift for Mass. building codes.
>>73,
>>Scott
>>KA1CLX
>>
>>
>>
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>Bests Always,
>Patrick
>
>
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>
>
>
>


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