Fundamentals of Bridge Aerodynamics
The increase in span length of long span bridges results in remarkable decrease in their natural frequencies and the ratio between the fundamental torsional and vertical mode frequencies. This renders long span bridges very susceptible to the actions of strong wind. Long span bridges may experience vortex-induced vibration, turbulence-induced buffeting and motion-induced flutter instability. The world longest suspension bridge, Akashi-Kaikyo bridge with a mail span of 1991 m in Japan and the world longest cable-stayed bridge, Tatara bridge with a mail span of 890 m in Japan, and others, have indicated the significance of aerodynamic design in long span construction. Super long span bridges require advanced understanding of the wind-bridge interaction to satisfy the increasing safety and economic needs. Our research activities on the fundamentals of bridge aerodynamics include the topics on generation mechanisms of aerodynamic forces on bluff bridge sections, modeling and identification of aerodynamic characteristics, turbulence effects of bridge aerodynamics, nonlinear aerodynamics and aerodynamic design of long-span bridges.


Advanced Prediction of Wind-Induced Response of Long-Span Bridges
For most bridges, the mode-by-mode approach for the prediction of flutter and buffeting responses is valid and computationally efficient. However, this is not necessarily true for very long span bridges, for which the performance against winds has to be studied at very higher reduced velocities. Our research illustrated that without the consideration of aerodynamic coupling the buffeting response of long span bridges may be significantly underestimated. The discussion of aerodynamic coupling in terms of the contributions of the aerodynamic forces on the bridge system damping provided more insights into the generation mechanics of multi-mode coupled flutter.

Buffeting response prediction has been conventionally conducted in the frequency domain using spectral analysis approach. It is mainly due to the fact that the wind loading parameters are functions of frequency. Therefore, the frequency domain provides a convenient format for linear analysis. However, the time domain approach is most appropriate if the analysis involves structural and/or aerodynamic nonlinearities. Our research proposed a novel time domain approach, in which the frequency dependent unsteady aerodynamic force features can be accurately modeled rather than assuming frequency independence by invoking the quasi-steady theory used in most previous studies in time domain. A computationally more efficient framework was also proposed using a state-space model of the integrated loading and structural system with a vector-valued white noise input. This approach integrates the mathematical representation of multi-correlated wind fluctuations, unsteady buffeting and self-excited forces and the bridge dynamics. The state-space framework facilitates a direct evaluation of the response covariance as well as the time domain simulation of bridge loading and response, and offers a convenient formulation readily amenable to the design of motion control devices. A time domain approach based on the complex mode decomposition technique was also proposed with advanced applications.


Nonlinear Bridge Aerodynamics
Current analytical approaches for predicting flutter and buffeting responses of bridges utilize aerodynamic forces linearized at statically deformed position of bridges. Although the linear aerodynamic force model has proven its utility for many applications, it is incapable of accommodating completely the issues such as aerodynamic nonlinearities and turbulence effects. These issues may become increasingly important when the aerodynamic characteristics of bridge decks exhibit significant sensitivity with respect to effective angle of incidence and as bridge spans increase. A novel nonlinear aerodynamic force model and associated time domain analysis framework were presented for predicting the aeroelastic response of bridges under turbulent winds. The aerodynamic force model includes the frequency dependent unsteady aerodynamic characteristics that are nonlinear functions of the effective angle of incidence, and is based on experimentally determined static force coefficients, flutter derivatives and admittance functions along with the spanwise coherence at varying angles of incidence. The nonlinear structural characteristics can also be readily included in this time domain analytical framework. The proposed framework provides a novel tool to study the influence of structural and aerodynamic nonlinearities and turbulence on aeroelastic response of bridges.

A coordinated experimental investigation is in progress for further validation of the proposed approach. This effort seeks an understanding of turbulence-induced modifications of the magnitudes and spanwise coherence of both the buffeting and the self-excited forces. Incorporating such work with measurements of the effective angle and amplitude dependence of the aerodynamic forces will provide an experimental foundation for the analytical work.


Equivalent Static Wind Loading
When wind-induced response is estimated through wind tunnel test or analysis, an equivalent static wind load description is generally required for describing the dynamic wind load to fit current design practice. With the equivalent static load, designer can easily combine other environmental loads and perform static analysis to ensure the structural safety and serviceability. Traditional gust response factor (GRF) approach, which results in a load distribution similar to the static load, is in short in cases with zero mean load or response. In addition, GRFs may vary in a wide range for different response components of same structures, and may have significantly different values for structures with similar geometric shapes and wind load characteristics but different structural systems. The equivalent static load description in terms of spatio-temporal wind load characteristics and modal inertial loads provide a physically more meaningful load distribution, and may result in a convenient format for practical structural design applications. The equivalent static load for a given peak response can be expressed in terms of a linear combination of background and resonant modal inertial loads. A methodology for determining the combination factors or weighting factors was proposed, which includes the correlation of modal responses for three-dimensional coupled structural motion . Using the equivalent static load approach, full bridge aeroelastic model tests can be used to gain useful insight into the description of the wind loads. This equivalent static load approach can also be used to aid the wind tunnel tests in predicting the response components not directly measured during the test. This approach was also applied to tall buildings with coupled three-dimensional wind-induced responses.