Research on Values of Rayleigh Damping Parameters of Long-Span Steel Truss Arch Bridge Considering Higher-Mode Effects
This study focuses on the influential mechanism of higher-mode effects on seismic responses of the long-span steel truss arch bridge and how to construct appropriate Rayleigh damping matrix when using direct integration method.Based on a steel truss arch bridge spanning 490 m,an elastic dynamic calculation model was established in SAP 2000.Given the axle forces and bending moments of the upper and lower chords of the arch,the seven modes(1st,4th,8th,11th,13th,23rd,49th)of the largest mass coefficients are analyzed in terms of the ratio of mode reaction to total reaction,and the methods to select characteristic frequencies for the establishment of Rayleigh damping matrix are discussed.As per the analysis,the axle forces in the arch springing zones of the upper and lower chords are controlled mainly by the 1st-mode vibrations,which is transitioned from arch springing to arch crown and gradually changed to be controlled by the 2nd to 4th modes of vibrations,with remarkable higher-mode effects.Vibrations of a particular mode exert different effects on the contribution ratio of axle forces and axle force distribution of the upper and lower chords of main arch.For the complex steel truss arch bridge,the Rayleigh damping matrix built with the 2nd-mode characteristic frequencies that offer relatively large vibration participation mass coefficients along the vibration direction may overestimate the higher-mode damping ratio,thus resulting in smaller calculated axle forces in the truss members besides the arch springing zones,and making the structure to have narrow safety margin.When the direct integration method is used to analyze the seismic responses of the complex steel truss arch bridge,it is recommended to establish the Rayleigh damping conditions by selecting multiple-mode vibrations,and take the envelope of the calculation results for seismic design.
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