[TowerTalk] TowerTalk Digest, Vol 142, Issue 22

[Donald Chester]

Steve     K7LXC wrote:

>The base of any tower DOES NOT sway. And  with a properly tensioned guy
system, there is very little movement anywhere in  the tower. I mean you've
got at minimum 1200 pounds of tension on one guy level  (3/16" tensioned to
10% spec or approximately 400# per guy times 3 = 1200#). It  ain't going
anywhere.>>

All towers sway in the presence of wind. Even the Eiffel Tower. I once rode all the way to the top, and could feel enough sway to make me feel slightly queasy, even though I knew the thing wasn't going to fall or collapse.  Sight up one of the guy wires of any guyed tower. They all have a degree of sag. That sag allows movement - a desirable thing,  because a slight freedom of movement at the base and at the guy sections puts less stress on the tower sections.

>>   Fixing the base solidly in the pier buried under  concrete causes
movement of the tower under wind conditions to put all the  stress on the tower
sections, possibly causing failure at the welds under  extreme conditions.

       >I'm not an engineer  but I can't find any validity in this
statement. Bigger towers use the  pier-and-pin technique because it minimizes leg
stresses on big towers  with microwave dishes which are kind of a non-issue
for your  typical amateur installation. There are tens of thousands of guyed
towers with  the bottom section buried in concrete and the incidence of that
failure mode is  non-existent in my experience.

But towers do come down in severe windstorms.  How can one tell which one was the first of the series of events that occur in a failure, which one initiated the process?  I  have seen photos of AM towers on base insulators, which have the freedom to twist and sway at the base, that have withstood hurricanes with permanent bends easily visible to the eye, yet the tower stood and continued to be used for years afterwards. Don't tell me that tower didn't have significant movement at the base during the severe wind event.

Most amateur radio tower installations with the bottom section buried, are at 60' or 70' maximum. 150' of Rohn 45 or 55 tower is not a typical amateur installation. I have never seen a tall communications or broadcast tower, microwave dish or no microwave dish, with the bottom section buried. They are always on a pier-and-pin. In fact, Rohn makes a special tapered base section for 25G and 45G, designated "25TG" and "45TG", with a 1/2" thick steel disc welded to the bottom of the tower legs, with a hole in the middle to accommodate the pier pin. In addition, those bases have holes to accommodate a base insulator if one is used. 

Just because Hammy Hambone does it and gets away with it, doesn't mean that's the proper way to do it. A friend of mine up in Massachusetts purchased some property and found a 120' heavy duty solid steel tower lying on the ground in the woods. He cut a few trees, pulled it up vertical and guyed it with plastic rope, just letting the base plate, which is similar to that of the 25/45TG, rest on the bare ground, with no concrete base at all, only a  few inches of topsoil over bedrock, and that tower has now stood for at least 20 years, and he climbs the thing.  But I would never even think about jury-rigging a tower in such a manner.

The current Rohn catalogue/installation book shows all their standard guyed tower installations; even the little 25G, using the pier pin base.  They recommend the "base cast in concrete" method only for short self-supporting installations.

>>  Besides, back filling with concrete would be  no  guarantee of a perfect
seal between the old and new concrete, and any additional  rust would then
be hidden from view - not good.

   >Wrong again. The seal is immaterial. A big plus is  that the
questionable legs will be entombed in concrete with zero chance of  failure. It'll
also put the top of the base above grade so that this situation  will never
happen again since water will just run off.

Any metal inside a concrete structure buried in the soil must be thoroughly "entombed" in concrete. That means completely sealed from water infiltration other than what may seep through the concrete by osmosis. If the added concrete doesn't bind with the old concrete base 100%, it may leave one or more microscopic cracks allowing infiltration, which could be corrosive to the metal due to electrolysis caused by dissolved minerals picked up from the soil.

Nearly all the force on a guyed tower base is vertical. The added concrete would add negligible reinforcement to the existing tower base to withstand vertical forces. It might help a little if the base were so corroded that it crumbled under the vertical pressure during a windstorm, by preventing any lateral movement as the tower dropped, probably no more than a couple of inches at most. But if the tower is in that poor a condition it needs to be repaired or replaced in any case.

Another thing to consider is, how rusty are the INSIDES of the tower legs? Even if the outside diameter looks good, they will be weakened if the tower has rusted from the inside out, which may well have happened if water accumulated and stood for long periods of time inside the legs. I have seen old home TV antenna towers from the 1950s that were little more than painted rust. Furthermore, if the area has freezing temperatures in winter, the hollow legs may have filled with water and frozen, cracking the tubes, since the soil accumulation probably stopped up the weep holes at the bottom of the legs.  If fresh concrete were piled on top of the old base for whatever good that might accomplish, it would be imperative to drill new weep holes near grade to prevent  further water accumulation.

I would check the base thoroughly to make sure it still has good enough integrity to let stand: sufficient thickness of sound metal and no cracks. Otherwise, it might be worthwhile to hire a crane to lift the tower just enough to replace the bottom section and base plate. The guy wires would need to be loosened, but not necessarily disconnected from the anchors. The pier pin should be OK even if it is rusted, since it is a solid steel rod.  This operation would be the job for a professional tower crew. Not cheap, but it should still cost a lot less than a whole new tower (put up correctly), including a typical amateur radio 70 footer.  

If the bottom of the tower looks satisfactory, be sure to paint it with a specialised rusty-metal paint, both inside and out. Rustoleum makes such a product, and I believe Cold-Galv does as well. The interior of the legs could be painted by temporarily stopping up the weep holes, temporarily removing the bolts and squirting paint down from one of the bolt holes at the bottom of each leg of the next section up, filling the legs up at least beyond the rust, then after a while re-opening the weep holes to let the excessive paint drain out.


Don k4kyv