ABSTRACT
The fundamental period of the tall tower roughly can be estimated by considering the number of anticipated stories divided by 10. According to the ANSI (ASCE-88) Code, the following periods may be calculated for translational and torsional responses of tall structures:
Steel or concrete buildings: T θ = 0.054N
where, T = fundamental translational period T θ = first torsional period
H = building height in meters N = number of stories
The aerodynamics of the tall tower are important in minimizing imposed design forces. Across-wind motion (perpendicular to the direction of applied wind load) governs the behavior of the structure. Organized vortex shedding generates the highest force levels. Vortex shedding is most organized (has the most adverse effects) with circular cross-sectional shapes, less organized with triangular shapes, and least organized with square shapes. The introduction of holes through the building cross-sections improves the behavior further. Variation of the structure’s cross-section along its height also acts to disorganize or disseminate vortex shedding. Strouhal numbers describe oscillating flow mechanisms and are based on the frequency of vortex shedding, shape geometry, and wind velocity. Strouhal numbers for common building shapes are shown below in parenthesis.
