Assessment of the Bill Emerson Memorial Bridge - FTP Directory ...

1.00.986 0.952 0.9570.888 0.892 0.9050.8MAC0.60.40.20.01 2 3 4 5 6 7 8ModeFigure 5.46 Mode assurance criterion (MAC) values5.5. RemarksThe eigensolution, sensitivity, and validation of the Bill Emerson Memorial Cable-stayedBridge model have been performed in great details. Based on extensive numerical results,the following observations can be made:1. The 3-D response and behavior of the cable-stayed bridge are evident. Most of thevibration modes are coupled with others. The dynamic characteristics (frequencyand mode shapes) of the bridge indicate that the cable-stayed structure is mostflexible in vertical direction and least flexible in longitudinal direction.2. The 31 significant modes of vibration up to 14.09 Hz include more than 70%mass participation in translational and rotational motions along any of threedirections. The fundamental frequency is 0.339 Hz, corresponding to verticalvibration of the main bridge. Cables begin to vibrate severely at a naturalfrequency of 0.842 Hz or higher. The Illinois approach spans experiencesignificant vibration at approximately 3.187 Hz. The approach spans is muchstiffer than the cable-stayed span. Their interaction during earthquakes is weak.3. The 3-D FE model of the cable-stayed bridge is robust and reliable. Based onsensitivity analysis, the key parameters affecting the modal properties of thebridge are the mass density of concrete and boundary conditions. The massdensity of concrete, specified in bridge drawings, appear underestimated by 6.7%.They need to be increased in order to match the natural frequencies of the 3-Dmodel with their respective measured data. Except for expansion conditions, theuse of other boundary conditions at bases of all piers changes the naturalfrequency of the main bridge by less than 5%.4. The computed natural frequencies of the 3-D FE model agree well with thosefrom field measured data. For mode shapes, however, slight differences existbetween the computed and the measured values. One of the reasons for these76