Constructional steel design for structures with a dominant wind bora load

Abstract

The design of steel structures sensitive to wind action requires an interdisciplinary analysis of wind engineering. The paper analysis the regime of Bora - a catabatic, local changeable wind, which is a rare phenomen but is characteristic for the Croatian coastal belt of the Adriatic Sea. The paper presents the computation of high, slender steel structures (steel chimneys, tower structures) with a dominant wind (Bora) load. The computation of stochastic vibrations of these structures was performed according to the procedure described in Section 3 by applying the program package NISA (Numerically Integrated Elements for System Analysis). The structure excitations were characterized as the spectrum of the white noise and the computational values of the spectral density of gusts for specific Bora cases were obtained by applying the ARIMA model-Section 2. The output values in the NISA program (structure response) are at the second moment level and are characterized by a maximum displacement (Y max) and standard deviation (expressed by RMS in the program) for the maximum spectral values of the structure response PSD. The final result of the investigations described in Sections 2 and 3 is the determination of the dynamic factor GRF or the factor of the displacement amplification of the steel structures which is used to define the equivalent static wind load, which causes displacements equal to displacements caused by turbulent wind. The results make it possible to reach important conclusions, the most significant being that the only reliable values for the Bora action on the structure can be obtained from the measurement data for the average values below 5 seconds and that the dynamic characteristics of the Bora require higher values of the dynamic factor for high structures than the values published in literature. The performed investigations are in accordance with the guidelines of Eurocode and they represent a basis for the development of national standards for the design of structures with a dynamic wind load.