Winter is approaching rapidly, and stations in many states must deal with its perils. That includes wind, snow, ice, freezing rain and cold that makes tower work miserable.
To prepare for this annual onslaught, stations must inspect the tower, paying careful attention to the lighting system and transmission line hardware. Smaller hardware items experience more trouble in cold weather. Ice tends to build, creating higher wind loads, more vibration and higher-than-normal component failure. And repairs to this hardware are likely to cost significantly more during the winter. It takes more time to get the work done, weather delays are more likely, and the hourly rate for climbers may go up. It’s far better to get everything fixed during the summer when working conditions are easier.
The presence of ice changes things in a really big way. First, and perhaps least consequential, is that it makes working on the tower virtually impossible. When ladders and tower members become covered with ice, no matter how thin, it is extremely dangerous for tower workers. Problems that occur during those conditions will probably stay unfixed until nature decides otherwise. A second and much more serious problem is simply the weight associated with a heavy ice buildup.
There are areas where significant ice becomes a problem, and those areas aren’t necessarily where one would expect. For example, there seems to be a belt across northern Missouri, central Illinois and central Indiana that calls to the ice gods. In 1964, a storm across that belt caused towers to fall, including an 1800-foot stick in Illinois, a 500-foot tower in Champaign, IL, and a 1500-foot tower east of Urbana, IL. The 1800-foot tower fell only due to the weight of ice involved. Pieces of ice more than a foot thick were falling off the tower before the legs simply collapsed. The legs seemed to telescope for about 200 feet near the top before guy wires failed and the tower collapsed. The other two big Illinois towers that fell in that storm came down when the ice built up to a really significant depth and then the winds arrived. In 1977, the same problem repeated in the same area, bringing down several towers including a tall television tower in Decatur, IL.
When dealing with calculations for wind load on towers, stations in some colder areas routinely take ice into account. They assume that either thick ice or wind will occur at any given time. Both usually don’t occur at the same time. But when they do, it rains steel. A good tower will stand a lot of ice or a lot of wind. But when a freak storm with both wind and ice hits, it can quickly exceed the tower’s design specifications. Look at it this way: When an 80mph wind hits a 2-inch-diameter tower leg, the wind load on the leg is about 18.5 pounds per square foot or 3.1 pounds per foot of element length. Add a 2-inch thick coating of ice and that load increases to 9.3 pounds per foot of element length. This is based on the force being equal to 0.0044 x V² x 0.67 pounds per square foot, where the value 0.0044 is an empirically derived constant, V is the velocity of the wind in miles per hour, and the 0.67 term converts the load on a flat surface to a curved surface. And, before the e-mail starts to arrive, no, the calculation isn’t exact, but it’s close enough for the point of this article. The tower that was designed to withstand that 80mph wind with no problem now presents a significantly larger surface and has to stand the subsequent load increase.
In the absence of wind, an extreme buildup of ice can simply weigh a tower down to the point that the legs can’t possibly support the load. That was the problem with the 1800-foot tower in Illinois in 1964. Also, several years ago, a really freak ice storm loaded two 2000-foot towers at Raleigh, NC. They were first-class towers from Kline that were built to support multiple antennas. That is to say, they were mammoth structures built to strict standards. They survived the ice buildup until the next day. Then, the sun was so inconsiderate that it shone on one side of the towers more than the other. As the ice started melting on the sunny side, the load on the towers became progressively less symmetrical. Finally, the unbalanced load caused a leg to blow out, bringing the tower down like a gut-shot cat. The staff from stations at the first failed tower called the other site and suggested that they might want to leave the immediate area of the tower base. Shortly after everyone was clear, the second tower joined its brother on the ground.
The problem in those failures wasn’t wind at all. In fact, those failures all occurred during conditions of little wind. Moreover, they were conditions for which you can’t reasonably design. The load conditions were far beyond the terms of any version of RS-222 — they were freaks. Just as one does not normally design for the wind loads involved in tornados, designs don’t normally plan on ice thickness of a foot or more.
The last problem that ice presents is that ice falling all by itself, unaccompanied by the tower, can pose a danger to people and property. Falling ice is always one of the points raised in zoning battles concerning towers. The persons who bring up the point about falling ice usually support their argument with an article from a 1936 copy of Mechanics Illustrated or some other high-level text.
Several years ago, a careful study of falling ice was made for a zoning hearing in Iowa. It determined several things that seem to be well supported by discussions with operators of several high towers. First, the really heavy stuff will almost always fall within the inner guy points. Second, falling ice reaches its terminal velocity after about 100 feet. The horizontal distance the ice travels away from the tower depends on a number of things, including the geometry of the piece of ice and the wind speed. Thin ice sheets can sail out beyond the guy points. But the mass of such ice is so slight that it poses little or no danger to persons or property.
So, what can the station do to protect itself. Stations have done lots of neat things to shield buildings, transmission lines, etc. from the force of falling ice. An excellent protective device is a heavy grade of expanded metal grating supported by a frame over the transmitter building, outside heat exchangers or air handlers, etc. This works great but it is expensive.
A less expensive technique involves covering a flat roof with concrete pavers. You can purchase the pavers at discount home-supply stores and easily replace them if and when they break. This gives the building some material to sacrifice to the energy of the falling ice. When the ice hits the blocks, the force is spread out over a relatively large area of the roof. The force may well shatter the paver, but the cost of the repair is minor and the waterproof part of the roof is undamaged. This same concept applies to thick rubber slabs. You don’t need to replace rubber slabs as often, but they are considerably more expensive.
Your author has seen stations that have constructed a steel frame over the entire building and covered the frame with a layer of used railroad ties. The ties are relatively inexpensive, they last a long time and they protect the building well. The only two drawbacks are the cost of a strong steel frame to support the heavy ties and the fact that the ties are butt ugly.
But you need to protect not only the building but any microwave dishes on the tower as well. You can protect these with ice guards. The facility should also have a protected parking area for staff cars. It’s terribly hard to keep staff when they periodically find that a portion of their car roof, holding enough ice for a summer picnic, has collapsed into their front seat.
So, for falling ice, avoid costly damages by using protective structures to deflect the ice or absorb the energy. It will also help to keep the local idiots off the site when ice chunks – with or without steel in them – are falling. For massive ice buildups or for extremely heavy ice accompanied by very high wind, get a really good insurance policy.
Don Markley is president of D.L. Markley and Associates, Peoria, IL.