A word to the wise
An essential component of virtually all land mobile radio systems is the antenna tower, usually a galvanized steel structure, either guyed or self-supporting. The job of the tower is to provide a safe and effective structure to hold both land mobile radio antennas and point-to-point microwave antennas in all weather conditions. Ensuring the tower is strong enough to withstand the elements is the job of a licensed structural engineer — who is the only person qualified to certify the tower’s safety.
The structural engineer relies on building codes and industry standards to verify compliance. In virtually all U.S. jurisdictions, the prevailing building code is the International Building Code (IBC), which specifies a subordinate standard for towers, TIA-222: Structural Standard for Antenna Supporting Structures and Antennas. TIA-222 is a voluntary standard that only becomes mandatory when specified by the IBC or otherwise required by the local building code.
TIA-222 covers several essential topics with respect to steel antenna towers:
- How to specify environmental conditions for the tower study.
- How to analyze an existing or proposed tower to ensure compliance.
- Standards for inspecting existing towers.
- Standards for drawings, assembly tolerances and the marking of tower members.
- Minimum data that must be included in a soils report.
The first version of TIA-222 was published in 1949. The current version of the standard is TIA-222-G, with an effective date of Jan. 1, 2006. Because version G is the most current, many tower owners assume it also is the version required by the local building department. Unfortunately, this is not always true. Most building departments do not immediately adopt the newest version of the IBC. At the time of this writing, most jurisdictions are still using IBC-2003, which specifies TIA-222-F, first published in 1996. Only when the jurisdiction adopts IBC-2007 will G become mandatory. During this interim period, many tower owners are ordering studies for both F and G and designing the tower for the worst case.
If the structural engineer is the only one qualified to certify the tower’s compliance, what role does the tower owner play? The tower owner or his representative (the radio engineer or technician) has several important tasks. He must:
- Order the soils study and provide the soils report to the structural engineer.
- Assemble all of the prospective antenna load data in a format useful to the structural engineer.
- For existing towers, inventory all antennas and transmission lines.
- Specify the environmental and operating conditions, including structure class, ice conditions, topographic category, and site elevation.
- For new tower purchases, capture all of this information in a request for proposal (RFP) or bid specification.
One of the weaknesses of TIA-222-F was the lack of guidance for specifying the basic wind speed (the wind speed applied at a height of 10 meters above the ground). Annex B of the standard lists the building code wind speed for every county in the U.S., but winds on mountaintops frequently exceed the Annex B speed, so the tower owner was forced to make a best guess. TIA-222-G addresses this problem at the cost of greater complexity. In G, the standard determines the wind speed, or more precisely the wind force, as a function of the county’s basic wind speed (from Annex B), ice conditions, the structure class, exposure category and the topography of the site.
Let’s take a closer look at these parameters and how they are chosen for particular uses and environments. Note that we will paraphrase the standard. See the standard — a hard copy or PDF can be obtained from the Telecommunications Industry Association Web site at www.tiaonline.org for $441 — for exact wording.
Basic wind speed
In F, the basic wind speed was the fastest-mile wind speed, meaning the worst-case wind speed maintained for a linear mile of air. The problem with this definition was two-fold: Most wind gauges do not measure the fastest-mile wind speed, and building codes use the 3-second gust. In the interest of harmonizing the standard with the IBC, the committee adopted the 3-second gust for its definition of wind speed and a conversion formula to translate fastest mile to 3-second gust. For example, a fastest-mile wind speed of 75 mph translates to a 3-second gust wind speed of 90 mph.
Like wind speed, ice thickness and type was largely left up to the tower owner. Now, geographic ice charts are part of the standard, although the tower owner is still free to apply more stringent ice conditions. A key principle with regard to ice is that the wind speed when ice is present is significantly reduced relative to the wind speed under dry conditions. The assumption is that ice cannot accumulate and adhere to a tower under high winds.
The structure must fall into one of three classes:
- Class I structures present a low hazard to human life or property in the event of failure and/or they are used for services that are optional or where delays in return to service would be acceptable.
- Class II structures represent a substantial hazard to human life or property and/or are used for services that may be provided by other means.
- Class III structures represent a high hazard to human life or property and/or are used primarily for essential communications.
As a rule, the higher the class, the more stringent the design. If the tower owner does not specify a structure class, the default is Class II.
There are three-exposure categories:
- Category B
Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions the size of single-family dwellings or larger.
- Category C
Open terrain with scattered obstructions having heights generally less than 30 feet.
- Category D
Flat, unobstructed shorelines exposed to wind flowing over open water.
Category A is no longer used. It was originally intended to model dense urban areas with tall buildings, but assumptions about wind behavior in such an environment are not longer valid. All wind cases are covered adequately by the remaining categories.
The topographic factor addresses the problem of a county’s basic wind speeds that don’t reflect mountaintop conditions. In other words, there is now an equation for calculating a wind speed “speed-up factor” for mountain peaks using the county’s basic wind speed and the particular height and shape of the mountain or hill. There are five topographic categories:
- Category 1
Flat or rolling terrain, for which there is no speed-up factor.
- Category 2
Applies to structures located at or near the crest of an escarpment.
- Category 3
Applies to structures located on the upper half of a hill.
- Category 4
Applies to structures located on the upper half of a ridge.
- Category 5
Wind speed-up factor is based on a site-specific investigation. It is used when the structural engineer finds that the site does not fit in any of the other four categories.
There usually is some trouble distinguishing between categories 3 and 4. Consult your structural engineer for clarification.
An often-overlooked requirement in earlier versions of the standard that still applies is that modifications to existing towers must comply with the new standard, despite the fact that the tower originally was designed for an earlier standard, usually one that was significantly less stringent. Often this is a hard pill to swallow.
Jay Jacobsmeyer is president of Pericle Communications Co., a consulting engineering firm in Colorado Springs, Colo. He holds BS and MS degrees in electrical engineering and has more than 25 years’ experience as a radio frequency engineer.