The ANSI/TIA/EIA-222 tower standard, now in its 61^st year of existence, is in its seventh revision: version G. The current revision took effect in 2006, and has since undergone two addendums. The standard is required to be reviewed every five years, and has at some times been left as is. The venerable revision C was active for almost 20 years until being replaced in 1987 by the D revision. G and its two addendums make significant changes in the way structures are contemplated. As the next round of changes to our industry occurs, the implications of the G standard should be considered, as they not only affect new construction, but will impact the carrying capacity of existing structures.

Under G, a more comprehensive look at structure performance is taken. Two limit states, or conditions, for each structure are considered. Under the strength limit state, compliant structures are safe under the most extreme of loading conditions. The serviceability limit state ensures that the structure in question is capable of providing the desired service under normal conditions.

Gone is the fastest mile wind speed concept that had been previously utilized. Mainly the change here is due to the way the National Weather Service and other agencies measure wind speed. The new accurate methodologies to determine wind speed have translated into the change in the standard whereby wind loading is calculated according to a three-second wind gust to accommodate instantaneous loads. Most of the sites used by the Weather Service record three-second-gust wind speeds, so more accurate averages are available for G and subsequent revisions should this methodology be continued.

In addition to the changes for wind loading, we also find considerations different for ice and seismic loads. Ice loads are escalated with height, as are wind loads, and G is the first version of the standard that seriously addresses earthquake loading. In general the seismic provisions should have minimal impact on broadcast structures unless the structure is irregular in some fashion. In the case of ice loading, older towers with no such consideration will almost certainly see their capacity decrease. Conversely, towers designed for a higher wind speed combined with certain ice thicknesses may see their capacity increase. The end result, of course, is that your tower may move into an overloaded condition, or be ripe for additional revenue.

-- continued on page 2