After thousands of hours of committee work the most comprehensive change in the 222 standard since Version D was promulgated nearly 20 years ago has been released for general use. Because of the rapidly increasing global expansion and the ever-expanding integration of American products and standards internationally the committee determined that the new standard shall stand as an example of a state-of-the-art standard internationally and domestically.
Consulting Engineer John Ericsson, P.E., a member of the working group and the editorial committee, provided me with a copy of the nearly 300-page standard. Craig Snyder, president of Sioux Falls Tower and Communications, who was also chairman of the committee, provided a great deal of additional information concerning the activities of his committee, TR14. The final draft was approved in June 2005 and released for general publication in July 2005. The specifications and requirements of the new standard may be employed at this time if desired. However, adherence to the new standard, Revision G, will not become mandatory until Jan. 1, 2006.
Most readers will probably ask if Revision G applies to an existing tower. I'm sure many engineers will be relieved to learn that the answer is generally no, if no changes are to be made to the original load specification and a professional engineer certifies that it will be satisfactory and that there won't be any effect on an existing tower. This requires careful consideration but if new antennas are to be added to an existing tower, which results in changes to its loading beyond its original specification Revision G will probably apply.
The new specification' s table of contents lists 15 main topics or objectives, each of which may have as many as eight subsections. The topics are:
- Designed strength of structural steel
- Other structural materials
- Guy Assemblers
- Foundations and Anchorages
- Protective grounding
- Obstruction and marking
- Climbing and working facilities
- Plans, assembly tolerances and marking
- Maintenance and condition assessment
- Existing structures
|I||Structures that due to height, use or location represent a low hazard to human life and damage to property in the event of failure and used for services that are optional or where a delay in returning the services would be acceptable.|
|II||Structures that due to height, use or location represent a substantial hazard to human life and damage to property in the event of failure and used for services that may be provided by other means.|
|III||Structures that due to height, use or location represent a high hazard to human life and damage to property in the event of failure and used primarily for essential communications.|
Table 1. The classification of various structures from table 2-1 of the standard.
There are 14 annexes that form procurement and user guidelines. These are intended to help engineers in their procurement of antenna supporting structures and antennas designed in accordance with the new standard. The sections referred to in the annex correspond to sections of the standard that contain an A prefix.
Although the new standard G specifications are comprehensive and have a no-nonsense air about them, they have been written with thought for the engineer and considerations for his design and operational requirements. Default design parameters suitable for the referenced sections are provided and help simplify the procurement specifications for structures. The annexes also provide amplification and clarification of many of the specifications. The annex listing follows.
A Procurement and the user guidelines
B U.S. County listings of design criteria
C Wind force on typical antennas
D Twist and sway limitations for microwave antennas
E Guy rupture
F Presumptive soil parameters
G Geotechnical investigations
H Additional corrosion control
I Climber attachment anchorages
J Maintenance and condition assessment
K Measuring guy tensions
L Wind speed confessions
M SI conversion factors
Annex D will certainly be of great interest to engineers who have to take care of STLs. It deals with one of the ever-present problems of tower distortion and twisting and swaying of microwave supports. A specific design criteria map section is included in Appendix 1 and occupies 30 or more pages. Comments on a few of the specifics that apply directly to broadcasting give an idea of the depth of this standard. Broadcast stations are located in many different types of terrain ranging from level and suburban areas to open and up mountain peaks. The effect of wind speed over various types of terrain is covered in depth.
The standard pays a lot of attention to transmission line mounting and the effect of clustering transmission lines, or blocks of transmission lines, on towers. For instance, the effect of transmission line placement is examined in detail.
A matter that is making engineers a little sensitive these days is that of earthquakes. Seismology and the special requirements of antenna and antenna mounting structures in regions of high seismicity are covered in depth, and some of the information may bring comfort to engineers in California.
|Site Class||Description of upper 100 feet of soil for the site location|
|A||Hard rock with 10 feet or less of soil overburden|
|B||Competent rock with moderate fracturing and weathering with 10 feet or less of soil overburden|
|C||Dense soil, soft rock or highly fractured and weathered rock|
|E||Weak soil (excluding site class F) |
|F||Soils vulnerable to potential failure or collapse under seismic loading|
Table 2. Site class definitions from table 2-11 of the standard.
Structures are classified according to usage, and the classifications are listed in Table 1. Site types are classified and defined, and then run through seven soil profiles ranging from hard rock to soils vulnerable to failure under seismic loading. These are listed in Table 2.
Corrosion, a topic that has long been a subject of argument among broadcast engineers, is fully covered. Structures are required to have hot dip zinc coating, preferably galvanic. All types of corrosion control are also covered in depth. As would be expected guy wires and their fittings receive a lot of attention. The subject of insulators, base and guy, receive adequate coverage. Details of required specific cathodic protection using sacrificial anodes and impressed currents are also well covered.
Another topic that can be a subject of disagreement among radio engineers is grounding. Scope and definition are both covered together with methodology, and connections between various elements are fully discussed and details provided.
Although tower climbing does not normally fall within the purview of station engineers there are always a few daredevils who like to go up towers. These fearless souls will find a lot of interesting reading in section 12 entitled Climbing Facilities. This section provides a surprising amount of information about such things as required safety equipment.
Section 15 Existing Structures addresses the evaluation and modification of existing structures. Reference to this section emphasizes the need to take great care in storing all the drawings and specifications of all the structures associated with a given station. Lack of this information can eventually result in high costs in restudying what has been done and lost.
The creators of 222 Revision G deserve hearty congratulations on the successful delivery of a complete and comprehensive international standard.
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