Analytical Approach to Characterize Tornado-Induced Loads on Lattice Structures

This paper presents a quasi-steady technique that combines aerodynamic force coefficients from straight-line wind tunnel tests with empirically developed tornado wind speed profiles to estimate the time history of aerodynamic loads on lattice structures. The methodology is specifically useful for large and geometrically complex structures that could not be modeled with reasonable scales in the limited number of tornado simulation facilities across the world. For this purpose, the experimentally developed tornado wind speed profiles were extracted from a laboratory tornado wind field and aerodynamic force coefficients of different segments of a lattice tower structure were assessed for various wind directions in a wind tunnel. The proposed method was then used to calculate the wind forces in the time domain on the model lattice tower for different orientation angles with respect to the tornado’s mean path based on the empirical tornado wind speed profiles and the measured aerodynamic force coefficient of each tower segment, where the wind field at the tower location was updated at each time step as the tornado went past the tower. A tornado laboratory simulation test was conducted to measure the wind loads on a scaled model of a lattice tower subject to a translating tornado for the purpose of validation of the proposed method. The moving average of the horizontal wind force on the lattice tower model that was calculated with this method compared very well with that measured in a laboratory tornado simulator, which paves the way for the use of straight-line wind tunnels to assess tornado-induced loads on a lattice structure and possibly other similar structures.