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Across-wind aerodynamic force
As stated above, the across-wind aerodynamic force can be obtained basically through the following channels: identifying across-wind aerodynamic force from across-wind responses of an aero elastic building model in a wind tunnel; obtaining across-wind aerodynamic force through spatial integration of wind pressure on rigid models; obtaining generalized aerodynamic force directly from measuring base bending moment using high frequency force balance technique.
Identification of across-wind aerodynamic force from dynamic responses of aero elastic building model.
This method employs across-wind dynamic responses of the aero elastic building model, combining the dynamic characteristics of the model to identify across-wind aerodynamic force. Melbourne and Cheung performed aero elastic model wind tunnel tests on a series of circular, square, hexagon, polygon with eight angles, square with reentrant angles and fillets, and tall or cylindrical structures with sections contracting along height. However, further studies showed that across-wind aerodynamic damping force and aerodynamic force mixed together make it difficult to extract aerodynamic damping force accurately. As such, the method has been seldom used.
Wind pressure integration method.
Researchers have recommended wind pressure integration to obtain more accurately the across-wind aerodynamic forces on tall buildings. Islam et al . adopted this method to obtain across-wind aerodynamic forces on tall buildings and structures. Cheng et al. experimentally studied across-wind aerodynamic forces of typical buildings under different wind field conditions and derived empirical formulas for the power spectrum density of the across-wind aerodynamic force reflecting the effects of turbulent intensity and turbulent scale. Turbulent intensity was found to widen the bandwidth of PSD of the across-wind aerodynamic force and reduce the peak value. However, turbulent intensity was determined to have almost no effects on total energy. Thus, researchers have recognized the quantitative rules of variation of across-wind aerodynamic force with wind condition to some extent. Liang et al. examined across-wind aerodynamic forces on typical rectangular buildings in a boundary layer wind tunnel using this method, thus proposing empirical formulas for PSD of across-wind aerodynamic forces of tall rectangular buildings and an analytical model for across-wind dynamic responses. Ye and Zhang decomposed across-wind turbulence excitation and vortex shedding excitation in across-wind aerodynamic forces on typical super-tall buildings. The results showed that the across-wind turbulence contributed much less to across-wind aerodynamic force than the wake excitation. Based on a large number of results, we derived PSD formulas for the across-wind turbulence excitation and the wake excitation, and further derived a new formula for the across-wind aerodynamic force. The first- and higher-mode generalized across-wind aerodynamic forces can be calculated through the integration of pressure distribution on rigid building models, which is an important advantage of this method. However, given the need for a large number of pressure taps for very large-scale structures in this kind of method, synchronous pressure measurements are difficult to make. Moreover, for buildings and structures with complex configurations, accurate wind pressure distribution and aerodynamic force are difficult to obtain using this kind of method.
High frequency force balance technique.
Compared with the pressure measuring technique, high frequency force balance technique has its unique advantage for obtaining total aerodynamic forces. The test and data analysis procedures are both very simple; hence, this technique is commonly used for selection studies on architectural appearance in the initial design stage of super-tall buildings and structures. Currently, this technique is widely used for total wind loads acting on super-tall buildings and structures, and for dynamic response computation as well. The high frequency force balance technique has been gradually developed since the 1970s. Cermak et al. were the first to use this technique for building model measurement. They initially pointed out that the balance-model system should have a higher inherent frequency than the concerned frequency of wind forces. The five-component balance developed by Tschanz and Davenport marked the maturity of balance facility.
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