Towertalk
[Top] [All Lists]

## Fwd: [TowerTalk] Opinions on U.S. Towers Tubulars

 To: Fwd: [TowerTalk] Opinions on U.S. Towers Tubulars K7LXC@aol.com (K7LXC@aol.com) Wed, 28 May 1997 12:22:49 -0400 (EDT)
 --------------------- Forwarded message: From: bhorn@netcom.com (Bruce Horn) To: K7LXC@aol.com To: Date: 97-05-28 11:34:20 EDT Hi Steve, You recently wrote: > A 50 MPH wind (to which the tower is rated) exerts 10 PSF wind pressure. >A 70.7 MPH wind exerts 20 PSF wind pressure. You can see the problem already. >You'll have to take the factory rating and cut it in half to get a 70 MPH >wind load capacity. Although you are correct in this statement that an increase from 50 mph to 70.7 mph will double the wind pressure on the antennas (pressure increases with square of wind speed), this does not result in reducing the factory rating by half. It reduces the overall rating by significantly more! This is because the entire tower, not just the antennas, "feel" the additional wind force, and it takes more of the tower's strength just to hold itself up, let alone the antennas. The following chart from a portion of a posting I made to TowerTalk in August, 1996, demonstrates this effect. These are calculated values based on the engineering calcs supplied with the tower. Tower: U.S. Tower HDX-589 89-foot crankup (21-foot sections with 4-foot overlap) Manufacturer rates tower for 30 sq.ft at 50 mph wind My Assumptions: 2 inch O.D. mast from top of tower to height of antenna Mast weighs 10 lbs/linear foot Max Allowable Wind Load vs. Wind Speed: (antenna at 90 ft) Wind Speed Max Antenna Wind Force Max Antenna Area ---------- ---------------------- ---------------- 50 mph 450 lbs 44.8 sq.ft 60 354 24.5 70 209 10.6 80 48 1.9 82 0 0 Tower fails at 83 mph with no antennas As can be seen from this chart, an increase from 50 mph to 70 mph reduces the allowable wind load to less than 25% of the original speced load. Since the manufacturer rates this tower (HDX-589) at 30 sq.ft at 50 mph, the rated load at 70 mph would be slightly less than 7.5 sq.ft (allowing the same safety margin as used when specifying the 50 mph rating). Although the degree of reduction for a U.S. Tower tubular design may be different than for their triangular lattice design, the idea would hold. Unfortunately, it is not as simple as just reducing the allowable wind load in linear proportion to the wind force. 73 de Bruce, WA7BNM (bhorn@netcom.com) -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search.htm
 Current Thread [TowerTalk] Opinions on U.S. Towers Tubulars, "Dick Green". [TowerTalk] Opinions on U.S. Towers Tubulars, K7LXC@aol.com [TowerTalk] Opinions on U.S. Towers Tubulars, JC Smith [TowerTalk] MARC question, JC Smith [TowerTalk] Opinions on U.S. Towers Tubulars, "Dick Green". [TowerTalk] Re: windspeeds, k4sb@worldnet.att.net [TowerTalk] Re: windspeeds, Barry Kutner Fwd: [TowerTalk] Opinions on U.S. Towers Tubulars, K7LXC@aol.com <= [TowerTalk] Opinions on U.S. Towers Tubulars, K7LXC@aol.com [TowerTalk] Opinions on U.S. Towers Tubulars, Stan Griffiths