CHANGES ON THE WIND MARK MALOUF, P.E. AGL (ABOVE GROUND LEVEL) AUG/SEP 2005 pp

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1 CHANGES ON THE WIND MARK MALOUF, P.E. AGL (ABOVE GROUND LEVEL) AUG/SEP 2005 pp August/September

industry standards ANSI and EIA are set to publish the first joint revision of the 222 tower structural and construction standard in nine years.

2 industry standards ANSI and EIA are set to publish the first joint revision of the 222 tower structural and construction standard in nine years. What changes does 222-G portend when it goes into effect in January? fter six years in development, the new G revision of the ANSI/TIA-222 standard, Structural Standards for Antenna Supporting Structures and Antennas, will be published this summer with an effective date of Jan. 1, This revision, produced by the Telecommunications Industry Association (TIA) in cooperation with the American National Standards Institute (ANSI), represents the most radical change to the standard since its first publication in It brings the tower industry standard in step with the major national building codes (ASCE 7 and IBC) by using the threesecond gust wind speed. It also corresponds more closely to the AISC steel-design code by using the limitstate approach. In addition to changes in the design 40 above ground level

criteria and in the loading criteria (which now include seismic loads), this revision is more comprehensive in sections such as appurtenances, climbing, grounding and existing structures.

3 criteria and in the loading criteria (which now include seismic loads), this revision is more comprehensive in sections such as appurtenances, climbing, grounding and existing structures. It incorporates state-of-the-art understanding relating to the special structures it addresses. This revision will affect the load-carrying capacity of existing telecom structures and how these towers are designed. New design standard The 2005 G revision is based on limit-states design ; prior revisions used the allowable-strength design approach. The structures are checked for two major limit states: (1) strength limit states that ensure that structures are safe under extreme loading conditions, and (2) serviceability limit states that verify that structures are capable of providing service under normal conditions. The new standard accounts for sitespecific conditions more accurately and in more detail and includes: classification of the importance category of the structure based on location and use. wind exposure categories to reflect surface irregularities. topographic effects. ice thickness by county location. These factors are combined to reflect the particularity of the structure and its location. Environmental loads Structures classification Structures are classified according to reliability requirements in three categories. Category I structures have the lowest reliability requirements and include structures posing little hazard to life and minimum risk of property damage in the event of failure. This classification is intended for structures where a delay in returning the services would be acceptable. (Ice loading does not apply to this category.) Category II structures represent a substantial hazard to life and a substantial risk of property damage in the event of failure. These structures are intended for services that could be provided by other means. Category III structures are essential facilities. Category II structures use nominal 50-year-return wind and ice loads, the same return period as the TIA-222-F standard. Category III increases the return to 100-year-return loads, and Category I reduces it to a nominal 25- year-return loading. These classifications, based on certain criteria, will allow a tower owner to have the environmental loading (by adjusting the return period) more closely match the by E. Mark Malouf, P.E. August/September

Figure 1. Standard version G basic wind speed values, without ice factors, for the continental United States. Velocities are given in miles per hour (mph) and meters per second (m/s).

4 Figure 1. Standard version G basic wind speed values, without ice factors, for the continental United States. Velocities are given in miles per hour (mph) and meters per second (m/s). The standard calls for mountainous terrain, gorges, ocean promontories and special wind regions (orange) to be examined for unusual wind conditions. 42 above ground level

5 importance of the structure and the associated risk taken by the owner. Wind loads Revision G provides a basic wind speed map (see Figure 1 on page 42). A load factor of 1.6 is applied to nominal wind loads for strength limitstates design. A directionality factor is applied to the factored wind loads to account for the probability of wind blowing from the worst-case direction. Highly wind-direction-dependent structures have a lower directionality factor. Latticed towers are assigned a directionality factor of 0.85, and pole structures are assigned a directionality factor of Wind speeds are escalated with height according to a given site s terrain characteristics. Exposure categories are the same as those contained in ASCE 7 for Exposure B (urban or hilly areas), Exposure C (flat open areas) and Exposure D (non-hurricane shorelines). The revision provides simplified equations for determining wind speed-up effects caused by topographic features such as hills, ridges and escarpments. Existing towers on mountaintops or other topographical features will be affected by the new provisions and are expected to see reduced support capacity. Gust-effect factors vary based on structure type. For self-supporting latticed towers, the gust-effect factor varies from 0.85 to 1.00 as height increases. A constant gust-effect factor of 1.10 is used for pole structures, and a 0.85 gusteffect factor applies to guyed masts. A 1.35 amplification factor, to account for dynamic-interaction effects, applies to the gust-effect factor for cantilevered spines on guyed masts or latticed selfsupporting structures and for all structures supported on flexible buildings. A gust-effect factor of 1.00 is used for determining the strength requirements of appurtenances. By using the limit-state loading, the applied loads will be amplified, which will expose any overall stability issues within a tower structure. For some slender towers with long guy spans, it will be difficult to get the analysis model to converge on a solution under the ultimate loading conditions. Some of these overall stability issues may not have always been detected using the older loading provisions. This issue may not be significant for wireless towers shorter than 400 feet, but it may affect some slender broadcast towers. The revision s appurtenances loading provisions allow for reducing the drag factors under a supercritical flow condition. These provisions also allow for reducing the effective projected areas based on the locations of the appurtenances. Appurtenance loading, especially regarding typical support platforms and mounts, and clusters or bundles of transmission lines, has specified calculation methods that prior versions did not adequately address. For a wireless tower with August/September

6 Figure 2. Wind speed (mph) with ice and design-ice thickness standards for the continental United States. 44 above ground level

7 numerous carriers, or a broadcast tower with large waveguides, this difference could result in a significant loading effect from these appurtenances. Ice loads Revision G includes an ice map (see Figure 2 on page 44) and a U.S. counties listing of mandatory ice thickness that escalates with height and corresponding simultaneous wind speed. A load factor of 2.0 is applied to the nominal radial thickness of ice. The weight of ice on a member is calculated by considering the factored radial thickness of ice around a cylinder that circumscribes the member. The projected area of ice is calculated by considering twice the factored radial thickness of ice. The additional projected area caused by ice is considered round for the purposes of calculating drag factors. Nominal three-second-gust wind speeds that are to be considered to occur simultaneously with ice are provided. A load factor of 1.0 is applied to wind loading for the ice condition because wind pressure is applied to a factored ice thickness. Ice loads are escalated with height because ice accumulation is known to increase with wind speed. This provision is intended to reflect the limit-state condition of heavy icing and the related lower simultaneous wind speed when these parameters are combined. This approach, which is a change from the way ice loading was accounted for in previous versions, is based on the latest statistical data provided by the U.S. Cold Region Research and Engineering Lab. It also includes learned failure experiences. Older towers designed without considering ice-loading will be negatively affected, and other towers designed for higher wind speed combined with a significant ice thickness may see their rated support capacity increased. Earthquake loads The design of telecommunication structures is rarely governed by earthquake loads. Nevertheless, special consideration of towers response characteristics is required in regions of frequent seismic activity. The G standard provides design criteria to ensure sufficient strength and stability to resist the effects of seismic ground motions for self-supporting structures and guyed masts. In general, this provision should not significantly affect wireless and broadcast towers unless they have structural irregularities and are located in high-risk seismic zones. Then, either a modal analysis (for self-supporting structures) or a time-history analysis (for guyed structures) would be required to properly account for seismic loading. Serviceability limit states Limit-state deformations under service-load conditions are found in the G standard. The service-load condition is defined as a 60- mph wind speed without ice using an importance factor of 1.00 and a directionality factor of 0.85 for all structures. Structures are limited to four degrees August/September

8 Do not attach lanyard around bracing members without engineering verification. DETAIL A B Recommended attachment points A Loop the laynard around the tower leg above the sideplate (gusset). Recommended minimums without engineering verification: Plate length is 1". Minimum weld size is 5/16" fillet or grove J applied to both sides of the plate. DETAIL B Figure 3. Examples of suitable climber attachment anchorages. twist or sway rotation and a horizontal displacement equal to five percent of their height. In addition, the revision provides more stringent rotation requirements for structures supporting microwave antennas. Analysis methods A new section in the G standard includes minimum acceptable models of analysis for self-supporting lattice towers and pole structures, and for guyed masts. It has requirements to consider the effects of displacements on member forces (P- effects). Pattern loading To account for a latticed tower s susceptibility to the dynamic effects of wind gusts, wind-loading patterns are considered in conjunction with minimum shearresponse requirements. This simplified method provides a loading pattern and is intended to simulate the dynamic wind-loading effects on such structures. It more closely matches the 46 above ground level

9 results from a full dynamic analysis, which is quite involved. The additional loading patterns apply to guyed masts with three or more spans, with at least one mast span greater than 80 feet within the top one-third of the height. For tapered, self-supporting, latticed towers that have extended straight portions, or portions with significantly reduced leg slopes, additional wind loading patterns are also considered. This new provision will affect existing tall towers the most. For example, additional capacity may be available in the lower portion of the tower and in the guy wires and anchors, and some reduction in capacity may result in the upper portions of the tower. The minimum shear-response requirements will negatively affect towers that were originally designed to closely meet the loading requirement curve. Foundations The foundations section has been changed to be consistent with the limitstates-design methodology. It provides a more concise presentation of the design parameters required to maintain foundation stability. This revision eliminates the fictitious normal soil and instead provides presumptive soil parameters for both sandy and clay-bearing soil types for use when a geotechnical report is not available. The importance of a geotechnical report that the G standard now stresses is reflected in conservative values for the parameters of the presumptive soil and by the requirement of a report for essential facilities; that is, Category III structures. structures in close proximity to buried pipelines or electrical substations. Cathodic control and concrete encasement are specified as acceptable additional corrosion protection. When taping or coatings are used, cathodic protection is also required because of the increased risk of corrosion at cracks or discontinuities. The revision recommends that soil resistivity and Ph values be included in the scope of a geotechnical investigation. Climbing facilities The Climbing Facilities section of the G revision presents compliance requirements more comprehensively, including strength and dimensional requirements. Examples of climber attachment anchorages in the annex Grounding The minimum required corrosion protection is hot-dip galvanizing, the same as in prior versions. However, revision G introduces a requirement for additional corrosion protection for steelguy anchor shafts in direct contact with corrosive soil (resistivity less than 5,000 Ohm-cm and/or Ph values below 3 or greater than 9). [Editor s note: See related story on page 26.] Additional corrosion control methods are to be used for AM antenna structures and other August/September

10 industry standards show suitable attachment points for climbers (see Figure 3 on page 46). Warning signs are required to be placed on structures that do not meet the standard requirements. In addition, an identification tag must be posted at the base of a safety climb system indicating the size and type of safety cable, with the intent of ensuring compatibility with a climber s safety sleeve. The standard now specifies a 3 /8-inch diameter cable as the standard size to use to minimize the safety sleeve sizes required to be maintained by a climber. Existing structures The revision has more specific language regarding structural analysis of existing structures, with exemptions allowed from certain sections of the standard. The revision requires that: Existing structures are to be analyzed in accordance with that revision of the standard, regardless of the standard used for the design of the original structure, under any of the following conditions: 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. a change in the classification of the structure to a higher class. Existing structures need not be reanalyzed for the G revision of the standard unless conditions change as outlined above. On structures that may be negatively affected by the new revision, if no changed condition is to occur, the tower s compliance is considered to be grandfathered. Design and predictive advantages The new provisions of the ANSI/ TIA-222-G standard will allow the designer to use the state-of-art knowledge in the design of these special structures. It will allow the owners or users of a tower to adopt a loading pattern that more accurately reflects the tower s characteristics and its use. The revision provides loading requirements that more closely represent the current understanding of the environmental loading to which a structure is subjected, and it provides updated and more comprehensive requirements to ensure a safe structure. Acknowledgement and availabilty The author wishes to acknowledge his colleagues hard work on the TR14.7 technical editorial committee and the full committee for developing this standard. The ANSI/TIA-222-G standard will be available for purchase from the Telecommunications Industry Association, agl Malouf, president of Malouf Engineering International, Dallas, served as a member of the TIA TR-14.7 technical editorial committee for the revised 222- G standard. 48 above ground level

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