Effects of Tower Standard 222G

May 1, 2010

The ANSI/TIA/EIA-222 tower standard, now in its 61st 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.

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Dig deeper

Previous revisions of the standard have discussed “normal soil,” however, G eliminates this designation. In G, an annex is included that lists parameters for various soil types that may be used for bid purposes in the absence of a boring report. Before a final design is completed, however, borings should be performed and this data considered. This is one area in which corners should never be cut. While the potential exists that adequate geotechnical information could reduce cost of a project, Murphy would tend to dictate that structure failure, the other end of the spectrum, will result if proper borings are not obtained.

These design items among others are applied to structures depending on the applicable categories in which they fall. Broadcasting towers would typically be considered Class II structures: Those that represent a significant hazard to human life and/or property should they fail. In addition, each structure is also considered under exposure categories, which are used to adjust wind loading based on the terrain roughness in the vicinity of the structure. There is an additional topographic category that corrects for sites on hills or elevated locations other than buildings. The result is a much more specific solution for each structure rather than a cookie-cutter approach based on broad regional assumptions.

Other important items have been included as part of the latest revision. G has taken a much more comprehensive look at corrosion protection. Previous revisions have listed hot-dip galvanizing as the minimum corrosion protection, and this is continued in G. What is so important in the latest revision is that the requirements for protection of anchors in corrosive soil are expanded. Guy anchors are kind of like icebergs in the sense that what you see above ground is not the full enchilada, and not necessarily representative of their overall condition. There have been numerous failures in recent years of towers as a result of corrosion of steel guy anchors going unnoticed. If your periodic inspections have neglected comprehensive examinations of the guy anchors, now is the time to get them checked before the tower becomes horizontal.

Not just the tower

Revision G contains a significantly greater number of climbing and working requirements. The minimum spacing between rest platforms on towers greater than 500' in height has been reduced. Warning signs are required if a structure does not comply with the provisions of the standard pertaining to climbing and working facilities. Safety climb systems must also now carry a stamped or engraved metal tag at their base indicating the size and type of cable. This is to insure compatibility with the safety equipment of personnel. A 3/8" cable is defined as the standard to minimize the equipment that must be maintained by climbers.

So although Rev G winds up being more detailed, it offers many advantages affecting the design of a structure and related pricing. A better understanding of loads on a structure may allow for more capacity, although this should not be expected in every case. Nevertheless, by considering more detailed parameters, a given structure becomes a truly custom solution, and allows for increased capacity without over or under designing a tower.

It should be noted that the use of G is required when analyzing an existing structure for applications beyond the original scope of design performed under previous revisions. So if your tower was built during the era of the C revision and you propose the addition of an auxiliary antenna not initially considered, your engineer must study the structure under G not C. Most jurisdictions and insurance companies will, however, require use of the most current revision regardless.

Although the face of our industry is changing somewhat, the necessity of towers is an undeniable fact. Failure of one or more structures can clearly result in significant economic impact. While some of the design portions of Revision G may have limited applicability to older towers for which no changes are proposed, the safety and protection portions certainly apply, and should be considered regardless of the age of your tower. The bottom line is that Rev G is a good thing, and the standard authors have crafted an excellent standard.

Ruck is a senior engineer with D.L. Markley and Associates, Peoria, IL.

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