|Paul Wescott obit:|
|Check out our new
[award-winning; see below]
Versa2une field-tunable FM antenna!
... and our new Model 6025 log periodic antenna!
4/11/13 at NAB:
New Cool Stuff Award for our
Versa2une field-tunable FM antenna!
Shively Labs is Maine's Small [Business] 2012 Exporter of the Year!
Our parent company, Howell Labs, was just named SBA's 2013 Region I Subcontractor!
|Shively Labs has won a techINK 2010 Innovation Award for our Graphical Power Meter system!
Our Model 6020 antenna brings home the Radio World Coolstuff Award!
New technical bulletin: "Tips and Tricks to Make 222-G Work for You"
November-December 2011 Radio Guide article: "A Really Xtreme Build"
September-October 2011 Radio Guide article: "Shively Labs - Where FM Antennas Are Not Just a Sideline"
May-June 2008 Radio Guide article:
Tips and Tricks to Make 222-G Work for You
by Adam Jones, P. E., 11/12/2011
ANSI/TIA-222-G (222-G) is a complicated standard. Approved by the TIA committee on January 1st, 2006, and fully adopted by the International Building Code in 2009, 222-G may represent the single largest change to broadcast tower and antenna structural design, ever. Hardest hit by the changes are sites subject to icing conditions, locations on hills, mountains and coastal areas, but no site is immune to the impact of the new standard. And, while it absolutely allows more accurate design of structures and appurtenances at a particular site, it also requires a far-greater amount of user-supplied relevant data than any previous version of the standard. Getting all the proper information into the process is essential; as overstating the conditions will cause unnecessary complexity and cost, while any understatement could lead to a catastrophic failure, and unwanted liability.
Tower and antenna manufacturers, as well as owners and tenants alike are struggling to keep up with all the permutations covered under 222-G. Many designs that were considered sufficient under previous versions will not pass the new criteria when extreme sites are specified. Surprises can be ugly: an antenna that, under TIA/EIA-222-F (222- F) was designed for half inch of radial ice, could suddenly now need to incorporate the loading of three inches of radial ice, at the same site. A tower rated as Class II, may suddenly become Class III  simply with the addition of a small whip antenna designated for 911 Communication. Why, and more importantly, what does that mean, and how does it impact the project?
This presentation will provide a bit of an overview of some of the big changes of 222-G, but more importantly, what effect the changes can have on your projects. What information is now essential to have on file, provide to your suppliers, and why; as well as some tips for a safer, and more cost-effective navigation through the process of conforming to the new standard.
Myths and Facts
Accurate Appurtenance Loading Information
There have been several major changes from 222-F to 222-G regarding appurtenance Effective Projected Area (EPA) calculations. It is expected that antennas, brackets, mounts and lighting EPA values are calculated per the criteria given in the standard. In some cases, the resulting EPA can be very different from what was done in 222-F. The following sections will review some of the major changes.
Appurtenance Drag Coefficients
In order to maximize capacity, customers need to be proactive and gather applicable 222-G specifications for all the appurtenances on the tower. Antenna systems, side mount and pole mounted, built during the 222-F era were designed and specified with drag coefficients that are higher than today’s 222-G standard. While this was appropriate for 222-F analysis, using these same values for a 222-G analysis could literally DOUBLE the loads of the appurtenance in some cases.
Generally large diameter, cylindrical antennas, such as Dielectric’s TFU-DSC series antennas or Shively’s radomed 6814, will see reduced EPA values from 222-F to 222-G. When these same antennas areas are calculated per 222-G, their wind areas may be cut in half compared to 222-F. While this change could be viewed as favorably with regard to the antenna mounts, the bigger impact would be seen if you used the larger 222-F EPA values in a 222-G analysis.
Therefore, prior to a running a 222-G analysis, it is critical to evaluate all the appurtenances on your tower and determine if they were calculated using 222-G and what their changes might be. If they were not, you may be underestimating your tower capacity or you may be re-enforcing more than you need to.
An example of this major change can be seen in the calculated area for a typical Shively 6814 6 bay FM antenna with Radomes (Table 1).
Without a doubt, running structural analyses of towers designed under earlier versions of the 222 standard requires the engineer be very well versed in antenna types and construction so that they know how to properly convert old mechanical parameters to current practices. For many antennas such as side-mounted TV’s, you can usually simply back out the radome/pipe diameter and re-calculate to G, but the engineer would need to consult the antenna manufacturer to ensure they have the proper dimensions for more complex arrays. It is inappropriate to simply reduce a 222-F wind area by a given percentage to get a 222-G value as the appropriate amount is a function of the appurtenances’ geometry. This can be an issue for antenna manufacturers if a significant amount of time is involved in locating archived prints and in recalculating mechanical specifications on complex systems such as panel antennas with escalated ice. To complicate matters even more, for certain antenna geometries, it is possible for the EPA to be different at various wind speeds (V, mph), Height above ground (z, ft), Exposure Categories, Site Classifications and Topographic Categories. The reason this occurs is because the drag coefficient, Ca, is a function of all these inputs and the geometry of the element. You cannot calculate the EPA of an object without these inputs. If an antenna manufacturer has reported EPA values for 222-G, they have made assumptions for all these inputs. While these may be perfectly valid for most comparative situations, if you are seeking cost control and reliability if may be worth this precision. The following is a simple example of a ten foot long appurtenance 4 1/16” in diameter. The EPA calculated at 130 mph is 31% lower than at 90 mph.  Obviously when this starts being factored in on larger systems the total area change can be significant.
Face Zone EPA Reduction
An additional change in 222-G may allow owners of VHF and FM panel antenna systems to reduce their Effective Projected Areas (EPA). Based on the configuration of the antenna and the tower geometry, this change may allow for a reduction in the area of 20% or more. Generally for this to be allowed, the appurtenance must lie within the tower structure or lie within a face zone which extends 12” from the face of the tower. Many wrap-around panel antennas can partially meet these criteria. As in the previous section, the tower engineer would need a high level of understanding of the antenna design to properly asses if this reduction can be taken.
For the first time in 222-G, shielding is allowed under certain circumstances. However, it is quite specific in what can be allowed and would be very difficult for a tower designer to determine applicability within an antenna. 100% shielding is allowed within two widths of the feature and is eliminated four widths downstream. Interpolation is allowed. The direction of the shielding must be considered and, if used in EPA calculations, reflected in the Normal (EPAN) and Transverse (EPAT) values.
In 222-G, ice thickness escalates or gets thicker as the wind speed increases. Most antenna manufacturers have not published antenna loading information for various ice thicknesses. To properly asses the reliability of the tower, it is important to gaet updated specifications from these antenna manufacturers as the tower designer will not be able to create this information as they do not have the antenna geometry. Due to the fact that ice thickness is a calculated value, based on a variety of inputs, the ideal antenna information would report specifications at 0.0, 0.5. 1.0, 2.0 and 3.0 inches of ice.
Also changed is the method in which ice thickness is calculated. The old approach under 222-F was to simply offset the shape by the ice thickness and use this to calculate this increased EPA and ice weight. The new approach, based upon ice research done by ASCE, uses the same method to calculate EPA, but changes the method to calculate the ice weight. The new method circumscribes a circle around the shape. This diameter is generally based upon the largest out-to-out dimension for the cross section plus two times the ice thickness. See Figure 3. This change can have the greatest impact on small rectangular shapes typical to VHF and FM antenna screens and radiating elements.
Transmission Line Cluster EPA Reductions
As-built conditions can also affect the true EPA of these style antennas. Sloppy installation of cables can lead to excessive wind and ice loading. Bundling cables can reduce wind and ice loads. If cables follow structural members in the tower, their contribution to the EPA of the system can be reduced. If cables have been assumed to be individually run but are actually bundled in round or square blocks, their calculated load can be reduced.
Antennas Are Covered
Quoting directly from the standard:
“This Standard provides the requirements for the structural design and fabrication of new and the modification of existing structural antennas, antenna-supporting structures, mounts, structural components, guy assemblies, insulators and foundations.”
The standard defines a structural antenna as: a structure for radiating or receiving electromagnetic waves including reflectors, directors and screens. This means that to be compliant to the standard, antennas and their mounts must be analyzed as part of a structural review. This is especially true on top mount, cantilevered antennas. The committee recognizes that as an antenna becomes smaller, in respect to the size of the tower, the need to include them in this analysis diminishes. For example a low power FM would not require mount analysis, but a large twelve bay antenna would. The impetus behind the inclusion of antennas in the scope was to protect the public and the integrity of the tower in the event an antenna or its mount failed. In other words, if a large enough antenna were to fail, it could collapse the entire structure.
The best advice on whether or not an antenna is large enough would be given by the Professional Engineer (PE) performing the analysis. The standard does not give specific guidelines on when an antenna is considered to be a structural antenna. It defines a structural antenna as: a structure for radiating or receiving electromagnetic waves including reflectors, directors and screens.
Given this fact, it may be wise to request that your new antenna meet the standard, especially if you plan to own the structure for years to come.
Preparation is key for a smooth analysis or to purchase equipment compliant to 222-G. While some decisions will be simple and determined via a lookup in the standard, some require forethought. While this may be redundant for many readers, a quick overvew of the required inputs will be listed. These will be the basic Site Conditions any engineer using the standard must have in order to size a tower, antenna and/or antenna mounts.
Procurement and User’s Guidelines (Annex A)
Lookup values: All of the following requirements can be looked up by county within Annex B of the TIA-222-G standard:
When the maximum and minimum are different within the county, it indicates that there are multiple wind speed contous within the county. When a site lies between the contours, the higher value must be used. Further review with an engineer or building official will be required to determine the correct value. Wind, ice and frost depths can all be superseded when the county’s notes listed in Annex B indicate it is within a special region. Again, the owner would be required to investigate what the appropriate site conditions are with an engineer or building official.
The following information requires the owner’s cognizance:
Structure Classification (I, II, or III): The tower’s structure class was referred to in the introduction. Structure Classification is a means to segregate structures based on their usage. Class I has the lowest nominal loading requirements, while III has the highest. Per the standard the categories are defined as:
As discussed in the introduction, the Structure Class could change over the life of a tower. Consideration should be given to this fact when specifying a new tower or equipment. While a new FM antenna may not be used for “essential communications”, the addition of a fire and rescue 2 way service after installing the FM antenna would dictate the entire structure should meet the Class III requirements. Typically, the failure mode on horizontally cantilevered antennas structures is due to ice loading. A Class III structure requires an additional 25% ice load and may theoretically fail the mounts.
Exposure Category (B, C, or D): Exposure Categories are highly site specific. The default Exposure category is C. Tower and equipment designers have very few resources to assess what the most appropriate Exposure Category should be for any particular site.
Figure 5 shows how the loading changes when Structure Class and Exposure Categories are changed at a site. Above 1,000 to 1,200 feet it can be seen that the loading is the same regardless of the Class or Category. However, towers generally under 1,000 feet can be subjected to drastically different loads based upon the selections. As an example if a 700 foot broadcast tower were changed from a Category II, Exposure C to Category II, Exposure B the loads would decrease at a minimum 10%. Everything else being equal, a 10% larger antenna could be installed on the tower.
The following table was generated from the current number of towers registered the FCC and assigned an ASR number.
The data shown in Figure 5 and in Table 2 show that most towers in the US could have their loading changed significantly by changing either their Exposure Category or Structure Class.
Topographic Category (1, 2, 3 or 4)
Wind speed-up must be considered for isolated features, as shown in Figure 6 , when the unobstructed by similar features within a two mile radius of the feature. It is also applicable when the feature is two times the height of the average surrounding feature height within a two mile radius. See Figure 8: Surrounding Feature Applicability for diagrams describing this requirement. There are allowances to eliminate the increases when the grade of the feature is below 10% or the feature is sufficiently small.
This addition to the standard to account for environmental changes due to a topographic feature may have the single largest affect on tower loading and design. Loading can be increase by more the 2.5 times while also increasing the predicted ice thickness on the tower. Omitting or guessing at this factor can have dramatic impacts on the tower design. If it is over estimated costs can be increased unnecessarily. If it is underestimated or omitted when appropriate, the reliability of the tower could be severely compromised.
For example, a 300 foot, self supporting tower made by ERI was analyzed per 222-G without considering the Topographic Category and again with the category. The category chosen is Category 3 (Hill) with a crest height of 2,000 ft. It is presumed the tower is located on the top of the hill. The tower supported a small FM Broadcast FM Antenna, such as a Shively Labs 6810-8R. The results are dramatic. The shear and over turn moments more than doubled. This translates into a tower cost increase of $50K from $75K to $125K. The foundation costs would also increase at least 25% from $70k to 87.5K to accommodate the increased loading. In summary:
A possible example of compromised reliability occurred at WSPA’s digital and analog TV tower on Hogback Mountain in Greenville County, SC (Figure 8). Hogback Mountain’s peak is approximately 3,200 feet above sea level. Two miles away in the valley to the south, the elevation is approximately 1,150 feet. This is a drop of 2,050 feet which translates into a doubling of wind speed pressure and an ice thickness increase of 30%. On March 1st, 2009 the tower collapsed in a late winter storm. The tower was less than ten years old. This tower and antennas were designed to 222-F with some consideration given for the wind speed up that occurs at the site. But, without a quantifiable method to predict the increased, severe loading due to the topographical change, the ending proved to be catastrophic. As Jerry Massey, Director of Engineering for Entercom Communications stated, “The “G” specifications for this site are incredibly high, for good reason.”
While this storm could have been a 100-year event that would have brought down any tower, without consideration for topography, failures can occur.
An important fact to remember when reviewing the elevation of the feature, it is the change in elevation that is needed, as seen in Figure 6 - not the absolute value or height above sea level.
Site Class, Earthquake (A through F)
A. Hard Rock
This class is best determined by qualified engineers in areas prone to earthquakes. This paper will not address the impact on towers due to the addition of earthquake loading in 222-G.
222-G Adoption, IBC 2009
Know your “state of compliance”. There are many states that have still not formaly recognized the standard. It is anticipated that they will, however, eventually all adopt IBC 2009 or later and recognize 222-G as the standard by which new and existing towers are analyzed. The states that have currently adopted the standard are through the adoption of IBC 2009 or directly are shown in Table 3 and Figure 9.
Figure 9: 222-G State Adoption
Notes and credits:
Last updated: 03/09/12