Securing a Mast in a storm

I can calculate the wind loading if I knew the actual dimensions and what manner of antennas were attached. If the antenna is small, I suspect what I'll find is that the 1.25" OD standard size mast, may have been adequately guyed for 150 km/hr (93 mph) winds if the guy wires were sufficiently thick, and the endpoint anchors sufficiently strong.

If the antenna bent in half, it is obvious that one of the guy wires came loose or broke. When that happens, a buckling type failure is probably inevitable. You can do better with a proper tower (Rohn 25 or bigger), but 1.25" OD should be sufficient for a small panel antenna.

Before you can re-engineer the structure, you will need to analyze what caused the past failure. Did the guy wire(s) break? Did the saddle clamps slip? Did the guy wire anchors rip out of the ground or roof? Was there a corrosion problem? Were the guy wires too tight or too loose?

I've seen a few mast and tower failures caused by the wind. Most common is installing the saddle clamps the wrong way. See Pg 15: and badly designed anchors: Wind load from a 20 ft mast with an antenna on top is trivial compared to the wind load from a wind turbine generator.

If you need help with antenna towers and masts, I suggest you ask on the TowerTalk mailing list for ham radio operators.

Jeff Liebermann wrote in news: snipped-for-privacy@4ax.com:

Jeff almost ALWAYS has the good answers. Thanks Jeff for your time and good contribution to this group.

What exactly is a backwards clamp?

It's a saddle clamp (or wire rope clamp) installed backwards. They should be installed like this:

There are two sides to the cable loop. The live side is the one under tension. The dead side is the one that is NOT under tension. When you tighten down on the saddle clamp, you'll see that the U-bolt makes a dent in the cable, while the cast part of the saddle clamp does not. You do NOT want a dent in the live side of the cable as it might weaken the cable.

If something is going to slip, it will be the cast part of the clamp, and not the U-bolt part. With the cast side on the live side of the loop, if the cast part slips, then the assembly just tightens up against the thimble. No problem.

However, if you assemble it backwards, and put the U-bolt on the live side, the dead side will slip and cause the whole mess to fall apart. Usually, what happens is that it slips partially in a high wind because of the increased cable tension. Then the mast or tower starts to bang around because of all the slack in the guy wires. Towers are designed for static loads, not dynamic inertial pounding, which will finish the job of disassembling the saddle clamps.

Loose guy wires can be a bigger danger than having the saddle clamps fall apart. I know of an 80ft tower that came down in high winds because of slack guy wires. The owner was worried that his guy anchors weren't strong enough, so he left the guys slack. When the wind caused the tower to bang around, the anchors and saddle clamps stayed in place, but the guy wires snapped. Sorry, but the owner doesn't want me posting photos.

Hint: I use preforms, thimbles, and only a few wire rope clamps.

Sigh... I washed my car and now it's raining. It NEVER rains in California in summer.

On Mon, 02 Sep 2013 20:30:32 -0700, Jeff Liebermann wrote: (...)

If you're into this kind of stuff, I suggest you download and read through the Rohn and Valmont/Microflect catalogs: Lots of goodies and clues on how things should be done in both including TIA-222 "Structural Standards for Antenna Supporting Structures and Antennas". While this is probably massive overkill for a 20ft pole, it does offer a clue as to the calculations and saftey factors involved.

Yep. I drove to the park to do a hike and it started to rain.

I understand the backwards clamp now. Not the best description. Very obvious once someone points it out. I have only wired up trees, but will review my handy work. Nothing has failed thus far.

I don't get the video. The guy is twisting the wire so there is no "correct" side.

Next time I'm at a transmitting site, I'll take a look at the guy wire, but I'll you, the powers that be don't seem to like guy wire supported towers very much these days. I see mostly free standing towers. I have hiked the hill where those asses cut the guy wires and crashed the tower. Now I was thinking, well if it had a VHF pager this could be justified...but nah, even then, it is still a terrible idea. There must have been a dozen wires on it. How he/they knew what to cut and not kill themselves is a good question. [I left out "she" because I can't envision any woman doing something that stupid. To knock down a 200ft tower, you need a good dose of the hormone of assholes: testosterone.]

I've seen my share of Microflects. They make sense in the boonies, but I was surprised to see one on Volmer Peak. Well the FCC website is down, but I will look up to see who uses the Microflect on Volmer. They show up as PR (passive radiator) in the database.

BTW, the comment about backfill being weak is interesting .Having run a Ditchwitch in native soil and engineered fill, fill can be mighty hard if watered and compacted.

I can see you've never used a preform. The idea is to eliminate the wire clamps. The preform has a rough surface, that cuts into the guy wire, holding it in place by friction. It's also slightly tapered to provide some level of self tightening. The guy wire will break before it will let go. It's kinda like a Chinese finger puzzle.

The problem with guy wires is that you need lots of real estate for the wires. That's available in some areas, but not in urban installs, rooftops, or crowded mountain tops. Monopoles are easier (and cheaper).

I recall that he did something crude, like bury foundation blocks for guy wire anchors. That's not strong enough for an 80 ft tower with lots of wind load. See the Rohn catalog for how to do it right.

I've learned long ago that if you are doing a "one off", just pay a professional. So if I find myself in need of installing a tower, I would just pay somebody. But it doesn't hurt to know the foo such as the right way to put on the clamps.

How do you get fires like that at the top of a cell tower? Other than those fake palm fronds sometimes used as (somewhat pathetic) camouflage, what combustible material is up there to sustain a fire like that?

I'm getting an unable-to-make-connection/timout when trying to access the site, so this is a WAG. Could there be an emergency power supply up there with a generator and

Just a guess, but it would make sense to have the actual radio transmitters and receivers up top immediately adjacent to the antennas to avoid having to run a lot of expensive low-loss RF cables from ground level.

The poly and vinyl plastics used in coaxial cables is highly flamable. A monopole fire works by convection, exactly the same as with a charcoal barbeque fire starter. It's usually started when someone wants to weld reinforcements, brackets, or mounts to the pipe with a torch. It doesn't take much to get it the cables to burn. Once the fire starts, convection insures that all the cables are suitably torched. Incidentally, depending on the size of the pole and air flow, it can burn for several days.

More: "A Clark County Fire Department spokeswoman said that Alan Bunker said he was welding approximately six-feet up on the east side of the monopole when he heard a popping sound and then noticed smoke coming from the tower."

It's kinda difficult to extract individual articles from this site, so just type in "tower fire" into the search box. Lots of horror stories, examples, and interesting reading:

All the coax and control cables in the pipe are flamable. Take a length of CAT5 cable. If possible, make it plenum rated cable (not riser cable). Hold it vertically and ignite the lower end with a propane torch. Be prepared to drop the cable. If you're somewhat insane, do the same thing inside a length of thin pipe. A bundle of CAT5 cables inside about 6ft of old 1" EMT sounded like a rocket engine for a few seconds.

That's being done. They're called "tower mounted amplifiers". The problem is stocking expensive spares and swapping boxes when the weather is not very cooperative.

More monopole fire photos:

Any fires causes other than welding? In electronics, you can account for internal problems, even surges if you are willing to throw money at the problem, but if the welder torches it, I think you are doomed.

This seems like a situation where (gasp) standards need to be set. I can hear Rand Paul screaming at that statement, but fire is a community problem. It spreads. We are all in this together, and thus government is the solution.

Back to the cables, my recollection in fire standards is there are materials that burn, and materials that are destroyed in fire but doesn't support the fire. [I'd have to dig up the technical terms.] Material science gurus surely can solve this problem.

Speaking of palm tree fires, one scenario is tree squirrels and smoldering cigarettes. Squirrels like the filters for building material. They can be ripped up and there is quite a bit of material in one. But if they pick up a lit one, woosh! Every once in a while I see the remains of a grass fire by the highway and it is blantantly obvious it started by the road at a point source, spread in a V, and then set the hill on fire. OK, I didn't get out of the car to investigate, but doh!.

But back to the burning palms, I was in the socal dessert, maybe 20 miles west of Victorville and spotted a palm tree on fire. Just the top was burning. Took a while for fire to show up since it was one of those odd places in the desert where you wonder why people live there, well other than to make meth.

Lightning!

Very cool. The original burning bush. My ancestors took 40 years of wandering around the desert to figure out how it works. Maybe you can do it in less.

Maybe they were practicing?

Incidentally, tonight is the first evening of Rosh Hashanah, the Jewish new year.