I installed a Rohn HDBX48 tower nearly 20 years ago. When I applied for the
tower building permit, I followed the requirements outlined in the Rohn
literature, since they needed to be accounted for in the permit application.
The City Permit Office was familiar with an HDBX48, and already had approved
them since they had all of the required wind load calculations on file (for 80
MPH), which had been supplied by a local ham radio club.
I believe the HDBX series towers had a maximum specified antenna wind load of
about 10 sq. feet, and a maximum allowable boom length of 10 feet. I'll have to
get out my catalog to be sure, but I remember that a "Standard" 3-element
tribander yagi like a Hy-Gain TH-3 or a Mosley TA-33 had a boom length of
either 14 or 16 feet - much longer than the allowable boom length specified by
Rohn for the BX-Series. So I installed a 2-element 6-band quad (20 through
6-Meters) that had a boom length of 8 feet.
The tower plans were approved with no problems, and City Building Dept.
inspectors came over several times during the course of the project to sign off
the various stages, including the final inspection with the antenna and rotor
installed on top of the tower.
I did see a BX-series tower that had been used to support an antenna like a
24-foot long boom TH6DXX tribander yagi. The remains of the BX tower was
crumpled up lying in a pile. It was too much antenna for it at this particular
location - a home that I was seriously looking at buying from a ham, until I
found an even better location.
>From: K8RI <K8RI-on-TowerTalk@tm.net>
>Sent: Dec 6, 2011 9:19 PM
>Subject: Re: [TowerTalk] Fwd: HDBX-48 figures for 90 mph zone
>On 12/6/2011 11:13 PM, HansLG@aol.com wrote:
>> In a message dated 12/6/2011 4:43:51 P.M. Eastern Standard Time,
>> firstname.lastname@example.org writes:
>> As I recall wind pressure "on a flat surface" at 70 mph is 20.6 lbs/sq
>> so if the tower is rated for
>> 18 sq ft at 70 mph, would it be too much of a stretch to say to the zoning
>> board that since wind pressure at 90 mph is 34 lbs/sq ft, by simple math,
>> the tower should be good for 10.9 sq ft at 90 mph ?
>> Gene / W2LU
>Regardless of wind area isn't there a boom length limitation on both the
>BX and HDBX series?
>> Nop! It is not that easy. You have to take the wind surface of the tower
>> into the calculation as well which makes it "a little more complicated".
>> Hans - N2JFS
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