Time Domain Flutter Analysis Of Bridge And Program Realization

Structural flutter stability is an important factor which could affect and control the design of long-span bridges. It affects directly the wind-resistant safety of a concerned bridge during its designed lifetime. To date, there are two general methods being applied in bridge flutter-stability analysis, including that in frequency domain and that in time domain, respectively. The main purpose of this paper will, by means of numerical simulation, be focused on the time-domain evaluation of flutter stability of bridge structures.To fulfill a time-domain flutter analysis for a bridge, the key problem lies in the simulation of the self-excited aerodynamic loads in time-domain forms. Based upon the achievements reported in existed literatures, the principles of nonlinear least squares solution of parameters involved in both an indicial function and a rational function are deduced in this literature, as well as the relations between these model parameters and the self-excited aerodynamic force expressions initially proposed by Scanlan. The consistency in the method of indicial-function and that of the rational-function is also vindicated through numerical simulations. Further, with VC++ programming language, a three dimensional object-oriented program (OOP), aiming at time-domain finite element flutter analysis, is developed and verified by numerical examples. Based on indicial-function algorithm and experimently determined wind field, the flutter stability of a bridge crosses over a deep valley is investigated in time domain, allowing for taking into account the un-uniform wind velocity distribution along the bridge deck. Finally, the time-domain methodology of indicial-function algorithm is further implemented successfully with the platform of the commercial software ANSYS. The major work and conclusions are as follows:(1) A comprehensive review is given to the existed theories in bridge flutter analysis;(2) The derivation of models for time-domian self-excited-load expression, including both the model by indicial functions and that by rational functions, is provided in this paper, as well as the identification of the model parameters involved wherein. Numerical example shows a consistency between these two methods in the simulation of the self-excited aerodynamic forces;(3) An introduction is given as to some key concepts of object-oriented programming in developing finite element analysis program. Based on VC++ programming language and object-oriented programming method, a three-dimensional finite element (FE) analysis program is developed, which possesses functions including geometric nonlinear static analysis, modal analysis and time-domain flutter analysis;(4) With the developed FE anlysis program and in virtue of the wind field tested from a topography model in wind tunnel, indicial-function paramters are fitted first. Further, based on indicial-function expressed self-excited aerodynamic forces, the flutter stability of a given bridge structure can be investigated in time-domain. This method is then applied in the investigation of the effects on flutter stability due to un-uniform distributed wind along the deck length of a bridge crossing over a deep valley. Results show that the horizontal distribution mode of mean wind speed in valley can lead to an obvious impact on the bridge flutter threshold, which exhibits mainly in enhancing the critical flutter wind speed. Thus, neglection of the un-uniform wind distribution in horizontal direction may result in a more expensive wind-resistant design.(5) At last, another indicial-function based time-domain algorithm, based on secondary development on the platform of the commercial FE software ANSYS, is also achieved by the author to evaluate the flutter stabilities of bridge structures. Numerical examples show that, with the fitted self-excited-force models, it is convenient to fulfill in ANSYS-platform the recursive algorithm necessitated by the memorial characteristics of the aeroelastic forces. So it is feasible to conduct time domain aerodynamic stability analysis in the ANSYS program resorting to its function of secondary development. This method, as compared to the approach of self-developing large softwares, has the merits of efficiency and reducing the risk of algorithmic error or careless omissions in so far as the issue of flutter or flutter-buffeting of a bridge is concerned.