The vertical challenge

The vertical challenge

Jan 1, 2006 12:00 PM, by David Nicholson and William A. Griswold Jr., PE, SE

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Revision G of the TIA-222 standard � the document that defines the industry-accepted practices and minimum standards for the design of steel antenna-supporting towers � has just been released by the Telecommunications Industries Association (TIA). Revision G is the first modification to the standard in nearly a decade. As broadcasters rush to prepare for adoption of the new standard, the time is ripe to step back to consider the state of the radio tower industry and the effect Revision G will have on it.

New tower construction has increased in 2005. Much of this activity can be attributed to the introduction of new technologies in radio and other transmission markets. Cell phone carriers are increasing bandwidth and speed with the rollout of advanced wireless technologies that enable feature-rich services, TV broadcasters are shifting from analog to digital transmission and radio broadcasters are implementing HD Radio.

With all these technology upgrades converging at one time, there is a lot of capital investment flowing into tower construction and modification. That makes the timing of Revision G particularly critical, because the code uses a new set of calculations that primarily affects the required strength of a new tower � and thus its cost � and the remaining capacity of existing structures.

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Note that all existing towers are grandfathered, so even if they fall short of Revision G guidelines, they will not be required to catch up. Changes to an existing tower's structure or the addition of antennas that increase its load, however, will require full agreement with Revision G.

The good news for radio broadcasters is that they're likely to have a relatively easy time adopting Revision G, as replacing analog antennas with new digital antennas that are often smaller and lighter rarely adds to a tower's load. In addition, the long-time practice of maintaining a backup radio antenna has made it easy for stations to use that spot on the tower for HD Radio broadcasts without having to make room for a new antenna. While this is good news, it doesn't mean that radio broadcasters won't feel the effects of Revision G.

Revision G is the seventh revision to the TIA-222 standard first published in 1949, which provides guidelines for tower design, fabrication, installation of new towers, mapping (measuring) and modifications of existing towers, and maintenance � basically covering a tower's life from cradle to grave.

Each revision updates the original standard with new and improved calculations � and the lessons learned from failure. Revision C was the first to consider tower height and geographical location in wind load ratings. Revision D introduced a new wind speed calculation and Revision E added county-level detail to the wind-loading map. Revision F, which was adopted in 1996, introduced ice loading to the mix.

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Photo courtesy of Radian Communication Services.

But none of these revisions compare to the magnitude of Revision G. While it was approved on Aug. 2 by a coalition of structural engineers, fabricators and contractors, the committee pushed for and won a delayed effective date of Jan. 1, 2006, for the new standard. The unusual five-month discrepancy in date was designed to provide a head start with the new guidelines, giving tower manufacturers and broadcasters more time to prepare for the adoption of such a complete change in philosophy.

A new tower philosophy

One of the most novel changes made in Revision G was the introduction of three factors that take into account the role each tower plays in the community as well as its physical and environmental surroundings. These new calculations mean that some broadcasters will be affected by Revision G more than others, in large part depending on the location of their tower sites.

The first factor, dubbed the Importance Factor, takes into account the nearby population density as well as the functional importance of the tower itself. The idea is to adjust the design of the tower according to the degree of human or collateral damage that could occur should the tower collapse. A tower that supports several radio stations and is located in a densely populated area will have more stringent design requirements than one that is located in the middle of a field � even if the two towers are of the same height. Also, towers that are used for essential law enforcement or emergency response functions must be able to survive a bigger beating.

Revision G also analyzes a tower's Exposure Category and Topographic Category, which together encompass the structure's "environmental loads." An analysis of a tower's exposure factor will involve consideration of the regional climate, obstructions such as trees and buildings and the proximity to open water. Topographic categories encompass atomospheric conditions related to the position of the tower in relation to the elevation of the land around it.

The nuts and bolts

Revision G is now in line with the 2002 version of the International Building Code (IBC). It also brings the United States in line with the rest of the world in terms of tower building codes. One way it does this is in the new approach it takes to measuring the various stresses placed on a tower. Under Revision G, towers are analyzed under four specific types of loading: wind, ice, environmental and seismic.

Environmental and seismic loading are new considerations, but the old factors (wind and ice) are being looked at differently. Tower stresses were previously analyzed under the allowable stress design (ASD) model, which has been replaced in Revision G with the load resistance factor design (LRFD) model.

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The Revision G standard includes important considerations for new tower construction. Photo courtesy of Dielectric.

In terms of wind speed, the ASD model bases its calculations on the fastest mile speed � essentially averaging how long it takes a mile-long column of air to pass a given point. The ASD method can fail to differentiate between a day of hard gusts cutting through relative calm and a day of steadily moving air, even though the gusty day poses a much greater risk for tower collapse.

The LRFD model, on the other hand, looks at the fastest wind speed during a three-second interval. This method, which doesn't average away the fastest bursts, provides a more accurate wind speed profile around the country. It is the American Weather Service's preferred method, and because that is the source of wind speeds for tower design calculations, it was appropriate to update the TIA-222 standard to match it.

Agreement with the American Weather Service's wind speed measurement methodology was one of the reasons behind the switch from the ASD model in Revision F (and earlier) and the LRFD model in Revision G. Another was because the American Institute of Steel Construction (AISC) and the American Concrete Institute (ACI) also support it. The latter adopted the LRFD method of calculating stress in 1971, and the AISC wanted to use a similar system � in fact, its new manual includes both calculation models. Steel and concrete are key ingredients of tower construction, so there are clear advantages to everyone using the same calculation methods.

Ice calculations requirements were added in Revision F, but Revision G changes their usage considerably. In Revision F, the formation of ice was considered to be equal at the bottom of a tower and at the top, while in reality ice build-up can be much thicker around a tower's pinnacle. Some tall tower collapses in recent years can be attributed to unequal ice buildup.

Bottom line tower design

While it's tempting to assume that Revision G will result in heavier, more expensive towers because of these more stringent guidelines, that isn't necessarily the case. Such an effect is most likely to occur in northern states and mountainous areas, where ice and wind loading tend to be greatest, and in more populous regions that rank high on the Importance Factor. However, Revision G can actually result in lighter, cheaper towers in warmer areas, in places with less wind and in rural areas.

Big towers � in general, those over 1,000 feet tall � may grow proportionately more expensive to build under Revision G than smaller towers. This is primarily due to the new ice load calculations that take height into consideration, and to the fact that they carry a greater load.

Regardless of the height of the tower, the new revision makes the bidding process a bit more challenging for manufacturers. While previously they may have been able to give a single, height-based bid that could be applicable for many different sites, it's now crucial for the broadcaster to provide specific details for every intended site to get an accurate manufacturing estimate.

Choices in towers and rigging

While we've focused primarily on the design implication of Revision G, the new standards apply to the installation of towers as well. As such, the choice of a rigging company may be more crucial than ever.

The 1,000-foot point for tower height is a milestone of sorts in that tower rigging companies generally require larger, more capable equipment with longer winch cabling. Thus, it is usually the larger, better-known rigging companies that are called to raise tall towers. But for smaller towers, there are usually several companies available in any given market. Smaller rigging companies may be more price-competitive, they might be able to start work sooner, and their crews may be more familiar with the terrain and environment unique to their locale.

Because of the expense of moving heavy equipment and work crews, riggers local to a tower site � "local" usually being within a one-state radius � are usually hired to perform the work. When selecting a rigger, price is certainly a motivating factor, but disaster threatens those who make it their primary factor.

As common sense suggests, experience and knowledge really must come first. Assembling a tower isn't rocket science, but a successful installation requires the intelligence to follow blueprints carefully and a deep familiarity with best practices and building codes (now including Revision G). Riggers should be fully insured and in compliance with OSHA, which may insist on additional safety measures that drive up costs, but ultimately can save lives.

New developments in tower design include frequency-matched towers. With these advanced structures, the spacing of the interlaced steel where the antenna attaches to the tower is matched to the same spacing on the antenna, creating an integration between the two that optimizes the operation of the antenna. Dielectric Communications has developed its frequency-matched, FM-branded towers.

Nicholson is director of tower operations for Dielectric Communications, and Griswold is president and chief engineer of IETS.