Lets not forget ice loads! Most of the land mass of the United States
is subject to icing. Many areas -- including my apparently benign
location in central Maryland -- can and does accumulate 3/4 inch radial
ice loads.
Wind will also damage coaxial cables if it causes it to rub on any
surface.
73!
Frank
W3LPL
---- Original message ----
>Date: Tue, 20 Mar 2007 19:15:00 -0500
>From: Red <RedHaines@centurytel.net>
>Subject: Re: [TowerTalk] Tower Coax Support Sleeve
>To: Jim Lux <jimlux@earthlink.net>, TowerTalk <towertalk@contesting.com>
>
>Many thanks, Jim;
>
>Your calculations and experience appear quite conclusive. We don't need
>to worry too much about the weight of common coax overloading the
>tensile strength when it is supported at a single point, but we need to
>prevent the increased tensile load due to wind from exceeding its
>strength by tying it to a structure to support the horizontal load.
>That may require a clever solution on telescoping structures. It's been
>many years since I used a telescoping tower. If I remember correctly
>(and I'm not sure of that!), I suspended the coax inside the sections
>and carefully handled it to make sure it wasn't damaged while lowering
>or raising or lowering the tower. I'm sure that coax was subject to
>flapping in the wind, within the confines of the tower. I used it for
>only a couple years before replacing it with a freestanding tower with
>all coax and control cables secured to a tower leg at short intervals to
>prevent flapping.
>
>73 de Red
>
>Jim Lux wrote:
>
>> So, 100 ft of RG213 will weigh 11 lbs, which is a lot less than the
>> breaking strength of 160 lbs. Granted, you probably want to stay away
>> from the absolute limit.
>>
>> Wind loads might well be a bigger problem. A 1/2" diameter pipe 100
>> ft long could have quite a bit of wind drag. In fact, this is the
>> real limiter, I think. A quick back of the envelope shows that the
>> wind load on a 1/2" diameter tube at 80 mi/hr is about 0.7 lb/ft. The
>> analysis goes just like a catenary except the wind load is used
>> instead of gravity. Say you've got a 100 ft run, and you allow it to
>> move a foot (like sagging 1ft in a 100ft span).. The tension in the
>> coax would be 870 lb. If you allow 2 meters (6 ft) of displacement,
>> the load goes down to about 70lbs, still high. And, in that situation,
>> the cable would probably flap around.
>>
>> Moral of the story.. the weight of the cable isn't a big deal, wind
>> loads on the coax is. So you need a fairly reasonable grip to hold it
>> at top, but you need to tie the cable to the tower to take the lateral
>> loads.
>>
>> And, now actually having run some numbers, it seems to match practical
>> experience at field day. The disasters with coax are almost always
>> wind induced.
>>
>>
>> Jim, W6RMK
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|