INFLUENCE OF MODELING IN THE RESPONSE OF 3D STEEL FRAMES UNDER WIND LOADING

Abstract

These days there is an unprecedented rise in the number of lattice towers due to an ever increasing demand in communication. Lattice towers are 3D space frames that for design are conventionally analyzed as 2D trusses. For safety and economy, these designs need to be more rigorously analyzed considering them as 3D frames. In this study, an assessment of the effect of the mathematical modeling on the design forces in a ground supported communication tower has been made. Considering wind as the primary load, two lattice towers of heights 18m and 40 have been analyzed using Gust factor method by modeling them by three different structural idealizations namely, as 3D frame, 3D truss and as a hybrid. Further analysis of the tower due to simulated wind was adopted to assess the effects of aerodynamic behavior of the tower systems and its influence on the design estimates as obtained from various mathematical models. Differences on the structural response of the tower were also studied for angle section members with respect to tubular members. It was found that the all three models show similar lateral displacements and axial force values. The truss model gives representative values of axial forces /stresses in all members. However, the truss models underestimate the bending stresses because only the effect of out of plane bending has been considered in it. Either of the frame model or the hybrid model may be used for estimates of combined stresses for checking the design. In this study, it was found that the combined stresses necessitated the re-design of base members. Moreover similar trend of structural response was observed in both Gust factor method and dynamic analysis, though there was difference in the magnitudes. Also, irrespective of the height of the tower and the choice of section, the same trend was followed by the three structural idealizations. Keywords: Steel ground supported communication towers; Tower structural configuration; Joint connections; Mathematical models; Gust Factor: Dynamic analysis; Power spectral density; Simulation of excitation; Simiu's spectra.