Electro/Telecom Industry Trends
Nov 2005
The Changing World of Tower Design
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Much has changed since the Telecommunication Industry Association (TIA) released its first standard supporting the design and construction of communication towers in 1949. And as towers continue to become increasingly prevalent, structural engineers, manufacturers, designers, owners, and standards bodies such as TIA are being forced to reexamine how towers should be designed and built.
The latest release of TIA-222, Structural Standard for Antenna Supporting Structures and Antennas, reflects some of these shifts in tower design philosophy. One example of this is the standard’s new international focus. In the past, TIA-222 was developed primarily for a U.S. audience. Now, with TIA-222-G, the committee responsible for developing the standard has worked hard to institute changes that will transform TIA-222 into an internationally recognized and accepted standard that can be implemented outside North America.
One of the most prominent changes made to the document is demonstrated by its move away from allowable stress design (ASD), which, until a few years ago, was the prevailing structural design methodology used in the United States. TIA-222-G reflects the U.S. engineering community’s move to the internationally preferred methodology load resistance factor design (LRFD), which is now widely accepted as a more accurate method for engineering structures than ASD.
TIA-222-G also reflects a greater acknowledgement among tower designers and owners of the connection between towers and the locations in which they are erected. In previous versions of the standard, towers were built to the same standard, regardless of whether they were located in the middle of a busy downtown area or in an empty cornfield. Revision G has eliminated this one-size-fits-all approach with the introduction of a classification system that treats towers differently depending upon their proximity to humans and the importance of the services they support.
The standard groups towers into three classes:
- Class 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/or are used for services that are optional and/or where a delay in returning the services would be acceptable. Services that may fit in this category include residential wireless and conventional two-way radio communications; television, radio, and scanner reception; wireless cable; and amateur and CB radio communications.
- Class II: Structures that due to height, use, or location represent a substantial hazard to human life and/or damage to property in the event of failure and/or are used for services that may be provided by other means. Services that may fit in this category include commercial wireless communications; television and radio broadcasting; and cellular, personal communications service (PCS), community antenna television (CATV), and microwave communications.
- Class III: Structures that due to height, use, or location represent a high hazard to human life and/or damage to property in the event of failure and/or are used primarily for essential communications. Services that may be included in this category include communications that support civil or national defense; emergency, rescue, or disaster operations; and military and navigation facilities.
Under the new standard, towers carrying more important services or posing a greater risk to human life or property are held to higher design and construction standards. For example, engineers designing Class I towers do not have to factor ice and earthquake loading into their designs. By classifying towers in this way, TIA-222-G helps owners save money by requiring them to construct heavier, more durable towers only where needed.
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