New tower standards coming: are you ready?
Did you know existing standards for broadcast and communications towers are currently in the midst of the biggest revision, some say, since their inception in 1949? The structural engineers in our field are very aware of this, but many Chief Engineers are not. The new standards have been in the works for six years. The standards committee, made up of members from TIA and EIA, is labeled "TR-14.7." The number of the standard is 222 and we're on revision F right now-our existing standard. The last major revision was revision D in the late 80s. The next revision of the ANSI/TIA/EIA standard "Structural Standards for Steel Antenna Towers and Antenna Supporting Structures" will change the loads and design criteria for communication towers including broadcast structures. It will also have an impact on the load carrying capacity of existing structures.
We continue from yesterday, with more of the specific details:
Existing structures will be evaluated in accordance with this standard, regardless of the design standard used for original design, when there is a change in type, size, or number of appurtenances such as antennas transmission lines, platforms, ladders, etc.; a structural modification, excepting maintenance, is made to the structure; a change in serviceability requirements or a change in the classification of the structure in accordance with Class I, II, or III Categories (Hazard Classifications-explained later).
Revision G has been prepared by eight different task groups, covering technical issues such as wind and ice loading, seismic loading, design stresses, safety and climbing, and geo-technical requirements.
As an example of revisions in geo-technical definitions, the term "normal soil" (for determining lateral load capacities, bearing load capacities and resistance to pull-out) has been eliminated in the G revision and replaced with values that are representative of a soil type, similar to those used in building codes (i.e. values representative of sand, clay, etc.).
The committee also voted to approve the new gin pole standard. Gin poles are used in tower erection and this standard will allow tower erectors and designers to merge these technologies and ensure efficiency and safety.
Other highlights of the standard: (1) it uses the gust wind speed instead of the fastest mile wind speed. This is consistent with the language used in current building codes and should help in the CP process. "Old wind speeds were referred to in fastest mile where as new wind speeds are defined in (3 sec gust)," John Wahba, SVP/Engineering Radian Communication Services, who has been actively involved in the development of this standard since 1998, explains. "For example a 75 mph wind fastest mile (language of current revision F) is equivalent to 90 mph -3 sec gust (language of the new revision G) so the tower cost would be comparable but if that zone was 70 mph (fastest mile) in the old map and it is now 90 mph (3 sec. gust) as per the new wind map, the new tower will be more expensive."
(2) It now includes a method for checking the towers under earthquake loads, which is obviously important for some parts of the country. The standard uses the latest state-of-the art knowledge on wind, ice and earthquake loads and thus raises the level of this standard to same, or even above, other recognized international standards
(3) the standard allows for different reliability classes. Instead of using a single factor of safety for all towers, the revised standard allows the owner to rate the tower to a lower class knowing that the consequences of failures are tolerable for some towers based on height, use and location. So classifying structures will now be based on their location and importance of use, or importance factors. Importance Factors allow a reduction in strength requirements for towers located in an area that a structural failure would not likely result in injury and/or towers that have non-critical uses, i.e. emergency services.
A Class I tower is considered to pose a low threat to human life or damage to property and delay in return of services is considered acceptable. This might include home use receive-only towers, HAM radio towers, or any tower located in a rural setting away from people.
A Class II tower assumes substantial hazard to human life but failure of services could be restored by other means if the tower were lost. This type of tower might be located next to manned facilities or in suburban areas where people are regularly under the towers' fall radius.
A Class III tower assumes high risk to human life and that the tower is used primarily for essential communications such as police, fire, rescue, etc. Examples under this class include towers located in urban areas where people are typically located under the tower and loss of the structure would result in a high probability of human death or injury.
Notes Craig Snyder, President/Sioux Falls Tower, Chairman of the TIA/EIA TR14.7 (Standard 222-F) Committee and Chairman of Board/National Association of Tower Erectors: "Depending on the class category, a structure is analyzed under the results and tower design can vary widely. It is worth noting that not all existing towers will require reanalysis when the new standard becomes public. Only towers where a new load or some other significant change is being proposed will require reanalysis under the new standard."
More specific changes/additions to the standard, courtesy of Ernie Jones, VP/Engineering, ERI Structural Division:
* G - Standard Uses ASCE 7-02 (50-yr. 3-second peak gust) Wind Map-The number for Florida, for example, is 150 mph!
* G - Standard Uses ASCE 7-02 (50-yr. Ice Thickness and Wind Map
* G - Standard Uses Design Criteria from AISC-LRFD-99
(Limit states approach, without an allowable stress design alternative). This version will require both a "Minimum Wind Design" and "Minimum Wind With Ice Design"
* Guyed structures to incorporate dynamic response potential by introducing a version of patch loading on upper tower mast spans
* Exposure categories expanded to B, C, and D over former versions use of mainly C
* Wind speed-up over hills, ridges, and escarpments will be required where older versions did not specify this design as a requirement
* Expanded criteria provided on wind loading for feeds, conduits and appurtenances attached to towers (improved wind shielding guidelines)
* Wind loading guidelines for antenna mounting frames
* Specific earthquake loading section added with seismic analysis procedures specified.
* Compete section on requirements for effective slenderness ratios for tower mast compression members (legs and bracing members)
* Section added for base and guy insulators (non-metallic)
* Former reference to a "Standard Soil" changed to a "Presumptive Soil" which will have significantly reduced soil strength.
* Expanded protective grounding section with reference to a minimum structure grounding resistance requirement of "10 ohms"
* A climbing and working facilities section is proposed
Tomorrow: How are broadcasters most likely to be affected?
Are you reading this from a forwarded email?
