| Stiffening truss and streamlinzed closed box girder are two commonly used forms in suspension bridges. Due to the large stiffnees of stiffening truss and its adaptability for doube-deck systems, the stiffening truss is the most prefereable for highway-railway combination suspension bridges. It is also adopted in suspension bridges built across mountains and valleys. The stiffening truss has merits of good veltilation and disruption of the well-orgnized shedding vortices which otherwize will potentially induce severe vortex-induced vibration; the flutter characteristics can also be well improved with installation of central stabilizers. However, the large drag coefficients, in combination with the considerable area of structural components exposed to wind, make suspension bridges of this kind more vulnerable to wind buffeting. In addition, some aerodynamic measures for improving flutter stability also intensity buffeting responses. In this study, by taking the Lishui suspension bridges as background, the buffeting responses of stiffening truss suspension bridges are studied through wind tunnel tests and frequency-domain method. The contents and results as follows:(1) The calculation of mass interia of the stiffening truss is essential for the development of equivalent fishbone type model of suspension bridges. However it is not straightforward to calculate directly the mass interia of the stiffening truss in developing such models. In this study, a simplified procedure based on modal analysis is developed for such purpose. The change in modal frequency of torsional mode after attaching fititious mass interia is used to calculate the mass interia for the stiffening truss. The accuracy of this procedure is proved with numerical examples.(2) The buffeting responses of the Lishui bridge in the horizontal, vertical and torsional directions are evlauted by SRSS method following the Davenport’s quasi-steady buffeting theory, and the aerodynamic damping are incorporated in such analysis. It is shown that considering the fundamental mode in each direction is sufficient for dispaclement responses in that direction while the more modes are necessary to compute the acceleration responses with good accuary. The time domain analysis confirms the accuracy of the frequency-domain method.(3) A parameteris study is made to investigate the effects of aerodynamic admittance, quasi-steay and unsteady self-excited forces, spatial correlation of fluctuating wind as well as the co-spectrum of horizontal and vertical fluctuating wind on buffeting responses of the Lishui bridge. It is shown that (a) buffeting responses are significantly reduced when aerodynamic admittance was taken into consideration, and spatial correlation and cross-spectrum of fluctuating wind speed will also significantly affect buffeting response; (b) quasi-steady self-excited force captures sufficiently accurate aerodynamic damping for vertical and lateral modes while it failes to desribe correctly the aerodynanmic damping for torsional modes.(4) Terrain roughness has two opposite effects on buffeting responses: rougher surface retards the mean velocity while in the meantime increases the turbulent intesnsity. The turbulent intensity profiles for the four typical terrains specified in the Chinese Wind Resistant Design of Highway Bridges are derived on the basis of horizontal gust spectrum, and the results are compared to those in the exising codes. The derived formulae are used to calculate the buffeting responsess of the Lishui bridge situated in the four typical terrains. It is shown that the terrain type“A”, while with maximum wind velocity, does not necessarily produce the maximum buffeting displacement due to its low turbulent intensity. Cares should be taken of choosing apporiate terrain type for wind-resisant design and wind tunnel tests. Key Words: Stiffening; Truss suspension bridge; Davenport buffeting theory;... |
PDF Full Text Request