Seismic analysis of lattice towers

In the absence of specific guidelines for the seismic analysis of self-supporting telecommunication towers, designers may be tempted to apply simplified building code approaches to these structures. However, these towers respond to earthquakes in a different fashion than that of shear buildings. The objective of this research is to propose simplified methods for the seismic analysis of self-supporting telecommunication towers.The author studied the specific problem of self-supporting lattice telecommunication towers using numerical simulations and applying the modal superposition method and the response spectrum technique on ten existing towers, typical of microwave towers usually erected in Canada. The analyses are carried out using a set of 45 strong motion horizontal accelerograms to study the horizontal effects. Vertical dynamic effects are studied using two approaches: the first considering the same horizontal accelerograms in the vertical direction after reducing their amplitudes to 75%, the second using a distinct set of 55 vertical accelerograms.As a first stage, simple regression analyses are performed on the results to yield earthquake amplification factors for the base shear and the total vertical reaction. These factors are presented as functions of the tower's largest flexural period or largest axial period of vibration as appropriate and peak ground acceleration at tower site. They can be used by designers to estimate the expected level of dynamic forces in self-supporting telecommunication towers due to an earthquake.As a second stage, a simplified static method is proposed to estimate the member forces in self-supporting telecommunication lattice towers due to both horizontal and vertical earthquake excitations. It is assumed that the lowest three flexural modes of vibration are sufficient to estimate the structure's response to horizontal excitation accurately, while only the first axial mode will reflect the actual behavior of towers in response to vertical excitation. An acceleration profile along the height of the tower is defined using the spectral acceleration values corresponding to the lowest three flexural mode shapes or the lowest axial mode as appropriate. The mass of the tower is calculated and then lumped at the leg joints. A set of equivalent static lateral or vertical loads can be determined by simply multiplying the mass by the acceleration. The tower is then analyzed statically under the effect of these forces to evaluate the member forces. The maximum error associated with the proposed simplified static method is found to be 25% in the extreme cases with an average error of +/-7%.The effect of including antennae clusters is also addressed in the analysis and findings are summarized in the thesis.The study is extended to include transmission line towers in order to simplify the response of coupled tower-cable system by replacing the cables with an equivalent mass. Several frequency analyses were performed on the system in order to achieve a better understanding of the behavior of the coupled system, however, it was not possible to simplify this interaction in the way desired. Several observations are presented which may help in further studies