1st Croatian Conference on Earthquake Engineering 1CroCEE 2021

This paper contains the results from the dynamic analysis carried out for the design of a flexible structure of a complex form for which it was difficult to estimate, a priori, whether earthquakes or winds have a more unfavourable effect upon its dynamic behaviour. The matter at stake is analysis of a telecommunication tower with a total height of 155 meters that is being built on Vodno mountain, in the vicinity of Skopje, at an altitude of 1034 meters. The central part of the tower is in the form of an irregular quadrangle designed as a reinforced concrete core from the terrain up to the level of 112 m. Above the RC core, there rises an antenna truss with a square cross-section running to the level of 140 m. Upon it, a multi-purpose antenna with a height of 15m will be mounted. The reinforced concrete core is designed to have four circular working platforms starting at 56 meters along height with the last one being at the level of 82 m with a diameter of 18 m. The first three platforms are placed on three different sides of the tower while their centres lie at distance of approximately 2.5 m beyond the face of the tower. The fourth platform is placed centrically around the tower, at the level of 82 m. All of them cover an area of approximately 200 m2 each. The structure of the platforms is designed as a 3D steel truss in the form of a cylinder that is rigidly anchored to the reinforced concrete core. Designed on the structure are also four decorative elements in the form of rafters that extend from the terrain to below the platforms on all four sides of the core. These have a complex form both in respect to cross-section and along length. Around these elements, from the tenth to the fifty fifth meter, a highly porous mesh is designed. For the considered structure, a study for definition of the seismic effect of near- and far field seismic zones as well as a study for definition of the wind effect were carried out by testing of the tower model in an aerodynamic tunnel for the purpose of defining the aerodynamic pressures (both positive and negative) in eight directions of wind effect. The dynamic analysis of the structure showed presence of many modes along the three axes. During structural design, many analyses were carried out to obtain an objective response for definition of the most unfavourable dynamic effect and appropriate proportioning of the structural elements. Some of the performed analyses will be presented in this paper, with a focus on those that are not usually carried out in traditional design. For example, the antenna truss that is highly porous is considerably less sensitive to wind effect, but the second mode of the concrete core generates large inertial forces in the truss. This and other more important analyses are presented in this paper.