Friday, July 4, 2014

Guy Stations - the Good, the Bad and the Ugly

After a brief hiatus I have returned to the building of towers. This brings me to guying since the DMX-52 tower I am putting up will be guyed rather than free-standing, which would typical for this class of tower. In particular I want to discuss guy stations (or guy brackets): what they are, what they do and why they are needed.

In brief, a guy station is the structure that attaches to, or is integrated with, the tower to terminate the upper end of a set of guys. Guyed towers have one or more of these at regular intervals specified by the tower manufacturer to support the tower under a set of static and dynamic loads.

In this article I will not delve into the mechanics of guyed towers and guys, which is a large topic of its own. I will assume that the tower guying is being done according to engineering requirements for the structure.

Why Guy Stations?

Why not just attach the guy wires to the tower directly and save the cost and trouble of using guy stations? This is a good question. Certainly that is what I did with my Golden Nugget tower.

Should I have done so when I knew full well that it was a bad idea? I judged that in that particular case it would be acceptable. There is no bright line between good and bad, just a continuous band of gray shading. It is unlikely that I'd advise anyone else to do what I did since I have no control over others' practices and situations. Prudence dictates using the proper guying hardware when you cannot easily or reliably do the stress calculations.

Let's take a short, mainly qualitative look at the forces on a tower at the point where the guy wires are attached. Keep in mind this is an example directed to a conventional guyed tower, not the tower I am currently erecting.

In the standard configuration the tension in each guy wire (pre-load) is 10% of the guy wire's breaking strength. In the case of ¼" EHS guy wire this is 600 lb. Since the tower is laterally stabilized by 3 or more guys with equal tension there is a vector transformation of that tension into a vertical (downward) compression of the tower. With 30° guying (angle between the tower and guy wire) the compressive force on the tower is ~500 lb (cos 30° x 600), multiplied by the number of guy wires at the guy station, or ~1,500 lb for the typical 3 guy wires, at each guy station.

There is also a normal stress on the tower at the guy station (horizontally outward in the direction of the guy wire) of sin 30° x 600, or ~300 lb per guy wire. Since each of the 3 guy wires points in a different direction there is a large force that is essentially trying to pull the tower apart!

Dynamic loads (ice, wind, vibration, etc.) can increase the various forces and stresses. For example ice on the guys wires increases the normal stress and tower compression. Wind has a more complex affect since the tension increases in the windward guy(s) and decreases in the leeward guy(s), and not necessarily in exact proportion.

Towers that are specifically engineered for guying often integrate a high-strength set of cross braces, typically located at the centre of each section. It is designed to withstand the normal stress of static and dynamic guying loads, thus ensuring the tower is not torn apart. Guying hardware attaches directly to these braces. On other towers, especially those typically those that are tapered for free-standing applications, a separate guy station (bracket) must be installed to strengthen the tower to withstand the guying stress.

That is all that a guy station really is: an integrated or supplemental structural component to increase lateral strength of a tower where guy wires are attached. Now we can look at some examples, including those that I will be using on the DMX-52.

GS123 at the top of the DMX-2 tower section
The Good

A good guy station (or bracket) is one that is built by the tower manufacturer for that application. This is almost always the most expensive option but also the best. Provided that the tower is built and loaded within the rated height and capacity, and the manufacturer's guy stations and guys are similarly installed per specification, you can be assured of a safe and reliable tower.

Add-on guy stations are most often of two types: attached at the joint between sections, or; attached mid-section, usually resting on a horizontal cross brace. Wade (Delhi) guy stations are of the former type. Rohn guy stations are of the latter type.

The Wade guy station (in my case, the GS123) attaches to the lower of the set of bolts that bind the sections. As should be obvious (and can be seen in the picture) there can be no cross braces where the sections join so the guy station must supply the lateral strength on its own.

Rohn guy bracket
Rohn and Trylon guy stations grip the tower from the outside. In both cases the lateral members are heavy-gauge steel to withstand the guying stresses and ensure that when there is lateral force or motion that the tower moved as a single unit, not allowing the legs to move relative to one another. Unlike the Wade guy station the Rohn guy bracket complements existing tower cross braces, and in fact rests upon them.

Whether by gripping the legs, abutting the cross braces or bolted to the legs the guy station effectively transfers compressive force (parallel shear) to the tower legs without slipping on the legs or "hinging" the legs.

The hardware that attaches the guy wire to the bracket must be high strength and able to accommodate a range of guy angles. The better designs allow resistance-free pivoting (see the Rohn bracket above). Many commercial towers utilize a shackle that freely hinges in the vertical plane. The Wade guy station utilizes flanges with a fixed angle, and achieves freedom of movement from the pivoting of the thimble (supplied with the guy station). This imparts a shear differential between the bottom and top of the flange. While not ideal I have yet to hear of this being a problem in the field.

I purchased a new GS123 for this tower. I wanted the best option for the guy station positioned near the top of the tower, close to the antenna load (between the DMX-1T top section and DMX-2). This will suit for the loads I am planning. The GS123 cannot be placed higher without modifying the tower.

The Bad

The worst option is to wrap the guy wires around the tower legs. The reasons are simple enough:
  • The guy wire must rest on a cross brace which may be inadequate to handle the stress of static and dynamic loads. The cross brace may also be inadequate to resist the normal stress caused by the tower legs being pulled outward by the loads.
  • Abrasive active of the guy wire on the tower leg and cross braces will first remove the protective coating on all three, then by abrasion and rust will cut through each strand of the guy wire. Failure will occur more quickly for towers formed from sheet metal than from rods and tubes.
Despite these serious problems this is how I had guyed the Golden Nugget tower. But I knew it would not be for long -- just one year -- and the static and dynamic loads would be far less than in a conventional guyed tower.

I also used polyester rope at the lower Golden Nugget guy station. Rope is a poor choice since it has a very high modulus of elasticity (stretches easily in response to increased tension) and is degraded by ultraviolet radiation from the sun. Polyester is rated "UV resistant", but don't let this fool you. After one year there is evidence of UV damage on this rope. Dacron is a superior choice if you must do this. I recall a VK who, years ago, used heavy hemp rope (around 1" or 2" diameter) to support a couple high towers. It seemed to work for him, so who knows.

The picture above shows that I am using this style of "bad" guy station to temporarily support the bottom two DMX sections. Once the third section is lifted into place it will use a more appropriate guy station at the top. When that is done the ropes will be removed.

The Ugly

I did not want to purchase two new guy stations for this tower. Rather than use a GS456 guy station at the top of the DMX-4 section I improvised a guy station. Yes, it is ugly. When I say "ugly" I mean a guy station that is up to the assigned task but looks as if it is not.

The guy station I designed and built is shown in the adjacent picture. The total price is $6, with all parts purchased new:
  • 5' of coated twisted-link, 5/32" chain, cut into 3 equal lengths
  • Four ¼ x 1¼" Grade 5 bolts and nuts, and 8 ¼" flat washers
The ends of each chain are bolted together, but not tightened. Each chain is looped around a tower leg and the other end is loosely bolted to a link on the same chain. Flat washers sandwich the chain links, acting as pressure plates.

The objective is to keep the common tie point centred within the tower and the tension in the chains high enough that some effort is required to slide the final link over its connecting bolt. As the bolts are then tightened the twisted links at each connection will shift to accommodate the pressure. When this happens the chains becomes taut. The improvised guy station is now rigid, a key requirement of a "good" guy station.

The guy station is completed by inserting guy wire thimbles into links on the outside of each tower leg. These are not yet there in the picture since I had run out of thimbles of the correct size.

Final tensioning is to be done after the tower section above this one is bolted in place. Otherwise the upper section might not easily fit into the lower one. Also, the grip of the chain is strong enough that it cannot be slid down into its resting position on the cross braces when it is tensioned.

Since the ugly guy station rests on cross braces to support the vertical load it is not the same quality as a good guy station. For this reason I am using it for the bottom guy station where the loads will be lower than at the top guy station.

The Plan

I have now been off the air since dismantling the antennas and towers in early June. Unfortunately this means I am now missing some good conditions due to the high solar flux. But all I've had time for since returning home is construction of the guy stations. I may put up a temporary antenna on the house-bracketed tower just so that I don't completely miss out.

Next up is the construction of a gin pole and lifting the section with the ugly guy station into place. It will be guyed at low tension until after the next section (the fourth) is in place. Once these guys are at working tension and the tower is plumbed the rest of the tower can be raised.

If all goes well the tower should be up later this month (July).

No comments:

Post a Comment

All comments are moderated, and should appear within one day of submission.