Building Codes and RS222-G
One of the realities of the broadcast business is that we all use antennas, whether mounted to a structure or simply using the structure as the antenna. In most cases that structure is a tower or pole. Those of us who have had to build a new tower or modify an old structure are well aware of TIA-222, the standards document that provides guidance in the design of communications towers. This standard has been around since 1949 and fittingly titled "Structural Standards for Antenna Supporting Structures and Antennas."
A few years ago I outlined the history of the development of the TIA-222 standard. While I'm sure the majority of you have a pretty good understanding of the document, it's worth reviewing since it has evolved dramatically over the years.
Revision E was the first iteration of the code to be defined by the TIA and Electronics Industries Association (EIA) and thusly called EIA/TIA 222-E. It further created a wind-loading map based on specific counties within each state, as well as directing the engineer to consider and design for specific conditions that might exceed the standard values.
The evolution of TIA-222
The EIA RS-222 standard was first published in 1949 and encountered only two updates until 1980, when the 222C version was published. This was an important document because it took into account more of the real-world knowledge acquired as the deployment of so-called tall towers (up to 2,000') were becoming widespread and the effects of wind and icing were becoming apparent. Not only were these towers taller, but they supported significantly more weight, particularly with TV antennas.
Version C provided a method for rating wind load based on the tower's height and location. A map of the United States was delineated into three wind zone categories labeled A, B and C. The wind loading was considered over the full length of the structure and was measured in pounds per square foot (PSF). The specific PSF rating started at about 30PSF and increased based on the tower height.
The 222-D specification made a dramatic change to the way wind loading was calculated. First, the wind speed was measured in miles per hour (MPH) and a new map was created that depicted basic wind speeds measured at 33' above the ground. The value for basic wind speed increased as a function of tower height.
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