Defining the vortex loading period and application to assess dynamic amplification of tornado-like wind loading

Tornados are transient loading events, and several studies in the literature have shown their capability to produce dynamically amplified structure response. The present study utilizes the dynamic load factor (DLF) concept to develop the first generalized methodology to assess the possible dynamic amplification of structure having specified fundamental period Tn to tornadic wind loads. The two-dimensional impact of a rigid, circular cylinder by an impinging vortex is directly simulated; the resulting loading time history is then used to excite a single degree of freedom (SDOF) response model. The vortex load application period Tv is defined as the value of Tn for which the structure׳s response experiences greatest dynamic amplification. An expression for Tv is defined as a function of three vortex properties: critical radius, translational velocity, and tangential velocity profile, so that Tv can be computed using documented tornado-vortex properties. The range of possible tornado-vortex tangential velocity profiles is identified, and DLF curves are defined for the forcing produced by each vortex profile. A review is conducted of the range of documented tornado parameters and fundamental periods of real-world structures. Finally, the documented tornado parameters, expression for Tv, and DLF curves for the vortex profiles are used to define the possible dynamic response amplification of a structure as a function of Tn.