Parametric Studies of Guyed Towers Under Wind and Seismic Loads

Abstract

As the wireless communications spread there is an increasing demand of antenna supporting structures. A very common systems are lattice towers, either self-supported or guyed. The latter are chosen for economical reasons when there is enough space for their location. Radio and television employ structures that range between 100 and 600 m and communication towers for mobile phones are around 60 m though higher structures are also constructed. For large heights, guyed masts are indicated. However, it depends on the customer preferences, suppliers, budget and location. Generally, self-supported structures are preferred in urban areas and guyed masts in the countryside. Nowadays the demand for more accurate and reliable communication systems poses higher structural demands since the signal technology sometimes requires of very small motions of the supporting structures to achieve a high quality transmission. The design of these structures is, in general, carried out following the standard codes and simplified models. The dynamic actions, as wind and earthquakes, are not addressed in detail with exception of special cases, despite the large potential of adverse impact. One of the parameters significant to the dynamic response of the structure is the guy pre-load value. Although there is a recommended pre-stress of about 10 % of the ultimate stress, it is frequently found that the guys are under or overloaded, situation that can change the dynamic response of the whole structural system. In this work, a parametric study on the effect of the guy pre-load is carried out. A typical guyed tower is analyzed with a finite element model in which the lattice mast is represented by an equivalent beam-column element and the guys (or cables) by truss prestressed elements. The wind load is calculated from the design code regarding the height distribution and roughness characteristics. To account for the dynamic loads, a simplified approach is followed. The frequency content of the peak region of a typical longitudinal velocity spectrum is used to construct a temporal function. Various values of pre-loads are considered for the guys. The dynamic response is referenced to the quasi-static one and conclusions regarding the influence of the level of prestress are drawn. Additionally, analogous studies on the same tower with seismic loads are reported. It is observed that the influence of the guy pre-load variation is diverse depending on the type of dynamic load.