Modal Analysis Of Xihou Bridge And Fagigue Analysis Of Steel Bridge Decks

A 3D finite element model is built based on ANSYS platform for Xihou Suspension Bridge, the natural frequencies and mode shapes are obtained by performing prestressed modal analysis considering large-deflection since this bridge is a highly deformed structure. The shape finding of main cables is conducted in ANSYS using APDL command stream, and the suspender forces used are acquired by continuous rigid-support beam method. Then the cable forces obtained from shape finding and the suspender forces are transferred to initial strains and given to corresponding elements of bridge model respectively. After proceeding nonlinear static analysis, the coordinates of bridge's key nodes are very close to those of final survey and the maximum discrepance is 6.5cm. 15 frequencies computed by finite element model, and frequencies identified from the signals of deck acceleration and strain, are compared respectively. The results show that the inaccuracies of corresponding frequencies are all below 2.5%, except 1st mode frequency whose inaccuracy is 3.9% and 2nd mode frequency whose inaccuracy is 3%.Stress histories produced by structural health monitoring system mainly comprise three parts, stress caused by temperature, structural dynamic response and noise. To extract the real structural dynamic response is essential for precise fatigue analysis. Through time domain and frequency domain analysis of stress histories chiefly caused by vehicles, the upper cut frequency 14Hz for excluding noise from raw stress signals is selected. After obtaining the structural dynamic stress histories, the rain-flow counting method is used to determine the stress spectrum, and Miner's law is used to calculate the increment fatigue damage. Some valuable results are obtained by analyzing the increment fatigue damages. Firstly, the effection of temperature to fatigue analysis is limit but the effection of noise may be higher than 20%. Secondly, the key position of fatigue performance can be noted, which always underneath the central carriage way of bridge decks. Lastly, daily fatigue damage increment of each measured point varies significantly, that shows the traffic volumes are not stable yet at the bridge's beginning period of service, so that precise estimation of fatigue life needs to accumulate more structural data. This paper also pays some attention to buffeting-induced fatigue problem, rain-flow counting results show that the contribution of buffeting-induced vibration to fatigue could be neglected, at least in the wind range of this paper researched.