[TowerTalk] Rohn25 - different size bolt holes?

[Stan or Patricia Griffiths]

Hi Steve,

Be sure to check the tower legs for bolts stored there.  Rohn ships bolt kits in the
tower leg for 25G.  You need to look through each leg and be sure you can see
unobstructed light through each one.  If you can't see light through each tower leg,
there is some obstruction there and it has to be removed before you install the tower to
allow water drainage.  Standing water in a tower leg acts just like standing water in a
water pipe if it freezes . . . the pipe (tower leg) will burst.

I asked one time why Rohn uses two sized of bolts in tower legs on 25 and 45 sizes and
why they use two bolts of the same size on 55G.  I never got a real answer to that one.
What I was told is that the engineering decision was made so many years ago that nobody
around there now can remember . . .  I have speculated on my own answer . . . not a good
way to do engineering, by the way . . . but it is the best I can do.  There is an
engineering compromise in choosing bolt sizes for tower leg applications.  You want to
go with large bolts for more strength in bolt shear.  You want to go with small bolts so
the tower leg retains most of its strength . . . every bit of material removed by the
drill bit for tower leg bolt holes weakens the tower leg a little more.  I suspect that
using two of the smaller size bolts would not provide enough bolt shear and using two of
the larger size would weaken the joint too much.  A good compromise is to use two
different sizes of bolts in each leg.

I also asked why they used fine thread bolts in 25G but not in 45G or 55G.  Again, the
decision was made so long ago that nobody remembers why . . .  I have used coarse thread
bolts in 25G many times with absolutely no problems.  Coarse thread bolts have the
advantage that they can be "mechanically galvanized" rather than plated.  25G tower leg
bolts are well known to rust after a relatively short time exposed to the elements.  45G
and 55G bolts are coarse threaded and hot dip galvanized and hold up much better in the
weather.  Hot dip galvanized threads tend to fill up with zinc during the hot dip
galvainizing process and sometimes the nuts won't go on nicely.  Mechanically galvanized
bolts hold up like hot dipped bolts but work smoothly like plated bolts, but mechanicaly
galvanized bolts for 25G are necessarily coarse threaded, rather than fine threaded and
plated like supplied by Rohn.

I have bolt kits available and I can provide you with either mechanically galvanized
grade 5 (same grade as Rohn provides) or I can provide you with the exact bolts Rohn
specifies, and I buy those directly from Rohn.

Take a look at my web page to learn more about mechanical galvanizing and check the
prices on the bolt kits.

http://www.reprise.com/antronics/default.asp

Stan
Antronics of Oregon, Inc.
w7ni@easystreet.com

Steve/n0tu wrote:

> Curious why?
>
> Just acquired 60' some vintage of Rohn 25 ...in good shape - no rust or
> broken welds!  Why does it have different size bolt holes on the legs? That
> is, where the legs nest together and have the 2-bolts per leg  one hole is
> larger than the other. Is this a feature? Doesn't look like they've been
> drilled out cuz the galvanizing is still in tack.  Should I use 2 different
> size bolts to fit the holes.TIA Steve/n0tu




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