[TowerTalk] Re: N7ZZ BX tower

[Stan Griffiths]

I asked Stu, N7ZZ, to comment his experiences with BX tower.  Here are his
uncut remarks for your information:

>Here it is, late Sunday and a weekend past my target for a tower reply.  I
>don't have the engineering data you probably want but will try to answer in
>anecdotal form along the lines of your question.  I've lost two towers,
>both to wind.  I remain confident that Rohn has a very conservative view of
>the tower's strength.  According to them it is not supposed to handle more
>than a ten foot boom.  Many hams besides me have put large tribanders on
>the tower, I recall N7TT back when he was W9IRH with some significant
>antenna loads on the tower although he did use guys.
>
>Anyway, the first tower I lost was a BX56 which started with an AX7 section
>and tapered to an AX4 then an SX3, AX3,AX2   It was genuine Rohn.  I've
>worked on a Canadian clone that VE7WJ installed, it was just as hard to
>climb but appeared every bit as sturdy and well galvanized.  I've no other
>direct experience with non-Rohn versions of the design.
>
>My tower had been up for a couple of years at my Seattle home before I
>moved down here and installed it in the spring of 1976.  It was not guyed,
>the concrete pour was 5' deep and more than 4' diameter.  It was only
>poured to ground level which was a mistake that came back to haunt me over
>20 years later.
>
>The first installation was a TH6DXX.  It was only a few inches above the
>slop bearing that is standard on these towers.  There was additional wind
>load above the tribander, a 40 meter vertical made of three telescoped 12'
>lengths of 6061 alloy which was insulated with teflon rings from the 4'
>length of pipe that protruded above the beam and supported the boom with a
>stock guy wire and turnbuckle device.  The 40 meter vertical used the boom
>and elements as a groundplane and seemed better on DX paths than a dipole
>at the same height.  It did add windload above the tower which was
>certainly outside Rohn's reccomendation.
>
>The tower buckled and folded at the AX3 level in a wind so strong that my
>house mounted meter was gusting against the pin which is 90 mph.  It was
>the same night W7EJ and W7ZR and other friends lost antennas.  A south wind
>that also took down trees, destroyed part of my house roof and the gutter
>system, and scared the family pretty badly.  I was home and watched the
>beam beat itself to pieces after the tower fatigued and about 15' of top
>tower plus the remaining parts of the beam were supported by two coax
>cables and the tail-twister control line.  Gusts of wind would push the
>mess away from the tower to an angle of more than 25 degrees, the gust
>would subside and gravity would bash the tower.  After a while the coax got
>tired, broke, and down came the top section.  
>
>Clean-up was easy because the bottom part of the tower was solid.  I
>climbed it the next day, saved the AX4,5,6,7 sections and as far as I know
>they are still in use at Ron's place. (W7GJS then, N7EA now)
>
>Insurance re-roofed my house and paid me well enough that I replaced the
>TH6DXX with a Telrex TB6 and the tailtwister with a RC5.  You sold me a
>HDBX that was 8' shorter and one size bigger than my old tower.  I was sure
>it wouldn't fail and the installation was great until 2 winters ago.  I
>didn't get to watch the second one destroy itself.  I landed at PDX during
>the height of the storm and other than a queasy stomach I was fine until
>the road home became an obstacle course of limbs and some pretty solid
>trunks.  I got home to find my tower and an 80' fir lying perfectly
>parallel across our private road.  I can't say how strong the wind was but
>anecdotal evidence says it was close to the earlier blast.  Once again I
>had roofers, lost several ornamental trees, and felt very lucky because we
>had a neighbor with a crushed house.  His house was north of some tall
>firs, fortunately the south wind blew everything away from our house.
>
>My belief is that the following sequence of events took place.  The tower
>buckled in the AX3 section, below the rotator.  The antenna pointed down
>like the earlier time but did not fatigue the metal and fall.  Instead it
>acted like a box kite at about 40' above the ground.  The lateral force
>must have been huge and focused on the south leg at the point where it
>jointed the concrete pour.  This is where my error of pouring the concrete
>more than 20 years earlier caught up with me.  Soil and standing water had
>bathed the legs in mud, probably for years.  The weakened steel tore on the
>south leg and the 40' of standing tower crashed.  I think that if the
>corrosion had been controlled at the concrete pour the tower would have
>stayed erect.  I think the top would have fatigued like the last time and
>fallen.  As it was, the tower was in one messy piece and a hack saw was
>needed to clear the road.
>
>I won't but up another HDBX but not because I think its a weak tower.  If I
>put up another tower it will be Rohn 45 or 55 so I can side-mount something
>at 50' and stack another beam well above that point.
>
>I don't have a Rohn manual, my memory may be bad on the section
>designations.  You mention BX3 sections, I thought they were AX3 but for
>sure they were the stout version and tapered at the top of the 8' piece. 
>One other thing that makes this location bad for wind is the shape of the
>hill.  Being at the top, wind from all directions tends to lift the
>elements of the beam when it gets much above 40 mph.  It also lifts house
>roofs so badly that after the first problem the south roof support was
>strengthened to hold things down, a job normally left to gravity.  Folks
>like N0AX and W7JNC have the same tower in more sheltered settings with as
>much or more antenna and zero problems.  Again, I think its tough tower.  I
>do hate to work on it, not for fear of it collapsing but because my feet
>are always at an angle and my hands are always on a sharp edge.
>
>Hope that wasn't too long and it captured enough of what you were looking
>for to help your discussion.  Feel free to quote me or include any parts of
>this note that might interest others.
>
>73  Stu
>
>


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