Seismic Design of Communications Towers

TLDR: In this article, the authors present a nonlinear analysis of a 2000 ft guyed tower with and without mass irregularities, and compare the results with the results obtained using the equivalent lateral force method.

Abstract: Beginning in 2006, the tower industry will begin using the new revision of the Structural Standard for Antenna Supporting Structures and Antennas, TIA/EIA-222-G. Revision G incorporates several significant changes from previous revisions. It moves form allowable stress design to limit state design, and incorporates the wind and ice provisions of ASCE 7. However, the most significant change is the addition of seismic provisions for communication towers. This is the first time the Standard has seismic loading requirements for tower in regions of high seismicity. Because towers are special structures, current seismic provisions in building codes do not always adequately predict their behavior in earthquakes. Revision G provides methods that better estimate the performance of communication structures subjected to ground motion. Revision G provides: methods for determining 1) when earthquake loads need to be considered in the design of communication towers, 2) the fundamental period of various classes of towers, 3) seismic forces. In general, communication structures can be classed as self-supporting and guyed. For design purposes, the response of self-supporting towers can be predicted using linear elastic methods of analysis. Pole structures fall under this category. However, guyed towers are intrinsically nonlinear. Despite their nonlinear behavior, studies at the University of Windsor and McGill University show that the equivalent lateral force method provides an adequate estimate of the seismic forces in guyed towers when using the equations for the fundamental frequency defined in Revision G. As a precaution, the writers of Revision G put a limit on the use of the equivalent lateral force method on guyed towers with mass or stiffness irregularities taller than 450 m (1500 ft) and when any guy radius exceeds 300 m (1000 ft). Under these conditions, the Revision G requires that a time history analysis be performed. Furthermore, when any guy radius exceeds 300 m, out-of-phase motion of the anchor points needs to be considered. This paper presents a nonlinear analysis of a 2000 ft guyed tower with and without mass irregularities. The analysis considers both in-phase and out-of-phase base motion for comparison. The results of the nonlinear analyses are compared to the results obtained using the equivalent lateral force method.