[Antennas] Fasteners

[Dave Shrader]

George, 

Tower and antenna torsion loads are primarily limited by proper guying.

Bending loads, couples and torsion introduce stresses and strains in the
overlapping 'telescoping sections'.

The moments of inertia, critical for all load bearing characteristics,
are much greater in the tower sections due to the larger radius of the
section, 1 inch diameter versus a bolt at 1/4 inch. [I = 1/2 m*R^2].
When I took my strength of material courses in the 1950s, bolts and
rivets were never used as load bearing elements in any structure design.

Rohn claims a properly guyed Rohn 25 can be erected to 200 feet.
[Properly guyed controls the torsion loads]. That surely would stress
the h... out of 1/4 inch bolt[s]. That's a 2000# tower load plus a 40#
rotator load plus a 50# antenna load. The cross sectional area of the
1/4 inch bolt is 1/16 square inch. Three bolts equals 3/16 square inch.
Vertical shear load equals ~2100/[13/16] = 11,200#/in^2 as a preload. 

Any additional wind or torsion loads would add a secondary dynamic as
opposed to static shear vector perpendicular to the first and in the
same plane. This would increase the loads even further by the vector sum
of two vectors.

Proper design and installation will couple the torsion loads into the
tower structure at an effective 12 inch diameter [high moment of inertia
= very stiff].

I'd be curious what Rohn has to say.

I suspect it's a belt and suspender mentality.

DD, W1MCE

"George, W5YR" wrote:
> 
> But they are. You are looking only at the vertical load. Consider now
> torsion and bending loads and you will see what the hardware does.
> 
> 73/72/oo, George W5YR - the Yellow Rose of Texas
> Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
> Amateur Radio W5YR, in the 56th year and it just keeps getting better!
> QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
> Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437
> 
> Dave Shrader wrote:
> >
> > I have what may seem to be a dumb question.
> >
> > My Rohn 25 uses GRAVITY to hold the sections together.
> >
> > The upper section telescopes over the lower section by about 10 inches
> > (maybe 8 inches). The bolts are installed after the sections are already
> > mated. If no bolts are installed, my antenna will still be standing
> > because gravity takes the vertical loads, not the bolts. [It is guyed].
> > Now, if gravity fails, like an earthquake, and my Rohn sees a negative g
> > force, who cares?
> >
> > My question: What is the purpose of the bolts since they are not load
> > bearing elements in the design?
> - - -
> 
> Your moderator for this list is:
> Larry Wilson KE1HZ antennas-owner@mailman.qth.net
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