On 7/15/01 10:47 AM, Chris Adams at n4vi@arrl.net wrote:
>
>Fellow Towertalkians:
>
>
>As our house rebuild is nearing completion I am starting to look
>seriously at putting in a 35-40' Rohn 25G house bracketed tower.
>Looking at the Rohn drawings leads me to a few questions:
>
>
>1. All of their drawings recommend using the HBUTVRO Heavvy Duty
>Universal House Bracket.
Yes, that's the super-heavy-duty bracket designed for Rohn 25 and 45.
>They do make two other models although the Heavy Duty does appear to be
>more "heavy duty" by the drawing. Anyone have any experience using the
>other brackets? Their model numbers are: HB25(ABC)G and HBU
The HBU is the heavy-duty bracket for Rohn 25 and 45. The HB25 series are
only for Rohn 25.
I'm currently in the middle of installing a bracketed tower using the
HB25B's.
>2. The plan is to use two brackets as specified by Rohn. One will be
>attached to the side of the building and the other will be attacked to
>the Roof trusses (not roof overhang, etc). This is detailed in Rohn
>drawing # D850221 on page MS-28 of the Rohn Consumer catalog I recently
>purchased.
You didn't specify the bracket height. Depending on the location of the
brackets, they will be subjected to considerable force. My bracketed
installation is 44' tall, with a mast to 48' topped with an A3S. The
brackets are at 25.5 feet and 17 feet.
By my rough calculations, the top bracket should see forces of about 500
lbs in a 75 mph wind. The mounting location and bolts need to be able to
distribute that force, as well as deal with lateral (twisting) forces.
You may not be able to just bolt to a truss and leave it. You may have to
add internal brackets to distribute the force in the structure.
>My concern here is tower alignment. Although the bracket has a
>significant adjustable range, it appears to be in discreet 6" steps.
The HB25 series has a series of holes about 3" apart on the arms, and two
holes on the bracket fixture. By using one or the other set of holes,
you can adjust the distance of the bracket about every inch and half.
>How accurately do I need to align the two brackets? Assume the brackets
>will be 12-15' apart.
Very accurately. They will determine how straight your tower is.
For my installation, I have temporary installed a bracket at the 10 foot
level. This is my reference point. Using a plumb bob, I spotted the
location of the tower base and dug it out. I placed two joined sections
in the hole, leaving me about 15 feet of tower up. This allowed me to
accurately position the tower to be perfectly vertical, again using a
plumb bob, before pouring the base.
When the concrete cures, I plan to stack two more sections of Rohn 25.
This would be about 35 feet (actually, not quite), which is practically
free-standing at 75 mph. (Just don't try to put any antennas on it) At
this point, I'll climb the tower and attach the second bracket at 17
feet. This assures the bracket properly aligned with the tower. Once that
bracket is secure, I'll remove the bracket at 10', and install it at 25.5
feet. Then I can finally stack the top section, add the rotator, mast and
antennas.
The HB25 brackets allow a LITTLE adjustment laterally (along the face of
the wall), but none going into or away from the wall.
Frankly, I came up with this plan to climb the tower to install the
brackets because I felt very unsafe At the end of an extension ladder up
15 feet. I'll feel a lot safer strapped on to the tower with proper
safety gear. I can't imagine trying to manipulate the HB25 brackets and
install them while holding on to a ladder. If you insist on installing
the brackets first, I suggest using a scafford or cherry picker.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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