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New tower standards
The onset of digital television has greatly boosted interest in broadcast towers. Existing towers that once were regarded by owners as necessary evils now receive favored treatment in their new guise as valuable vertical real estate. Most existing towers today probably have as many antennas and as much ancillary equipment hung on them as they can carry without collapsing. So the push is on for new tower construction, not only for digital television antennas but for all the new sources of communication inspired radiators that require stable sky hooks hundreds of feet in the air. This means that both AM and FM stations may be affected by the new tower standards when they are put into force.
The pending Revision G tower changes allow for some grandfathering of installations.
The broadcast tower industry has made fantastic strides since the beginning of radio. There have, of course, been a few spectacular tower collapses, generally caused by high winds and or ice, and occasionally by vandalism. I can't recall any whose failure was officially blamed on over loading; although most of us have seen towers that we would hate to be near in a high wind. In much the same manner as most critical engineering projects are safeguarded, broadcast tower construction has always conformed to specific engineering standards.
In the U.S., the joint TIA//EIA-222 standard oversees the design of broadcast and antenna supporting towers. Standards are not static and they are reviewed every five years to ensure that they keep pace with industry demands, safety requirements and new construction methods. Each successive revision carries a capital letter designator. Sometimes no changes are warranted and a revision may last for many years. An earlier revision, Revision C, was kept in use for nearly twenty years before being replaced by a new version. The current Revision F will remain in use until Revision G has been completed and approved by the various industry committees.
The engineering group that establishes the TIA/EIA standards has a two-part job. First they have to ensure that the approved design rules will satisfy the safety requirements of the new and often very sophisticated antenna systems that the new communications world requires. They also have to be familiar with the local and national building codes so that their new revisions are acceptable to these groups.
Revision G will accomplish a number of new things. In the past owners of tall towers have had difficulty in satisfying local authorities, not only about usually unfounded concerns of non-ionizing radiation, but fears of falling towers (plus general dislike of towers).
As presently proposed, the new standard will make several significant changes. Among others the safety requirement regarding climbing, working and the additional load of personnel working on a tower will be covered. The minimum safety requirements will be based on the use of qualified and experienced personnel.
In the field of safety the rather out-of-date design theories of Revision F are about 20 years old. The new Revision will take into account the latest steel design theory based on national building trends.
At the present time any “add on” structures such as FM, STL, cell or similar communication antennas are not considered as part of the original tower in their design, As a result we can expect to find structures smaller than the main FM antennas designed to the same structural limits as the tower itself.. This should help in lowering windage values and possibly in icing load conditions.
Wind loading on towers is the major consideration when planning a new structure. Ice loading is of course a serious factor in tower design, but it is generally a temporary rather than permanent consideration. The effect and control of ice loading will of course also be addressed in Revision G.
Wind loading is calculated based on wind speed. The government has changed the method of measuring and assessing the effect of wind on towers. At present, wind loading is calculated based on “the fastest-mile wind speed” encountered once in 50 years for the tower site. This means that the wind speed is determined by averaging the highest speed of one mile of wind driving the anemometer at the proposed site. Because the time required to obtain the top speed is variable and depends on wind velocity, high wind speeds require shorter recording time. This can result in incorrect or misleading wind loading figures.
Revision G is expected to base wind loading on the “three second gust wind speed.” This will also use the once-in-50-years maximum value but it will be the maximum speed measured for a period of three seconds at the tower site. Basing tower design on the peak gust occurring over three seconds is designed to take the maximum instantaneous loading in account.
Put simply, the old formula allows for variations in the speed during that time. Most of the country's weather services use the new method of measurement of gusts in three second periods. The use of three second gusts provides a broader base for tower designers. A point to remember is that the new and old wind speeds cannot be compared directly because the Revision G wind speed of 100 mph for a three second gust is not the same as a 100 mph fastest wind gust speed. The latter value of 100 mph for three seconds would seem to have more effect on a tower than a single short gust of 100mph.
The broadcast engineer's béte noire is the ice that comes without warning and drags a tower down. It is most likely that ice will now be included as a part of the tower's mandatory load, and the ice area map will be included. The currently used Revision F only calls for icing to be considered if the tower is in an “ice region.” It is felt that this is rather too arbitrary, and the new standard probably will nominate regions where icing has to be considered. Structural requirements will probably be based on ice data plus height of the tower, site elevation and the degree of exposure.
The question that many engineers will probably be asking is: “Will Revision G apply to my existing tower?” If no changes are to be made to the original load specification, and a professional engineer has certified that it was satisfactory, there will probably not a be any effect on an existing tower. But if new antennas, etc. are to be added to an existing tower that will change its loading beyond the original specification, then Revision G will no doubt apply.
E-mail Battison at firstname.lastname@example.org.
It will probably be the end of 2002 before the numerous committees involved in creating the new revision have completed their deliberations. In the meantime, Pirod has developed an interesting and useful booklet describing the efforts and effects of the anticipated Revision G. It was written by Myron C. Noble, PE., the president of Pirod, to let engineers know what will probably be changing in the tower specification world. It is available free from Pirod at PO Box 128, Plymouth, IN 46563.
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