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

significant vibration at approximately 3.187 Hz. The approach spans is muchstiffer than the cable-stayed span. Their interaction during earthquakes is weak.7. Based on sensitivity analysis, the key parameters affecting the modal properties ofthe bridge 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%.8. The computed natural frequencies of the 3-D FE model agree well with thosefrom field measured data. The maximum error of the first 31 significant modes iswithin 10%. For mode shapes, however, slight differences exist between thecomputed and the measured values. One of the reasons for these differences isthat the exact locations of all accelerometers deployed on the bridge are unknown.Nevertheless, the mode assurance criterion index between a computed modeshape and its corresponding measured one is above 0.888 for the first eightmodes. This indicates that the 3-D FE model is fairly accurate for engineeringapplications.9. Time history analysis indicates that an earthquake excitation of higher peakacceleration does not necessarily induce a stronger response. The maximumresponse does not necessarily occur in the same direction of the bridge underdifferent earthquake excitations but depends on the earthquake characteristics, themodal properties, and mass distribution.10. All cables behave elastically under a design earthquake. Their factor of safety islarger than 2.35 at all times. On the other hand, the cable subjected to least stressis always in tension, ensuring no slack occurrence during the earthquake.Therefore, cables can be simplified as linear elements for seismic analysis.11. The solid section of both towers at the lower portion is generally more criticalthan the hollow section of the upper portion above the cap beams. The in-planebehavior of two towers is always in elastic range under the design earthquakewith a wide margin of safety. Similarly, for out-of-plane behavior, the upperportion of the towers above the cap beams remains nearly elastic with asignificant margin of safety. The lower portion of the towers, however, likelyexperiences moderate yielding out of plane during the design earthquake thoughthe safety of the bridge is not a concern.7.2 Future researchThe seismic instrumentation system was installed and put in operation in December 2004.Since then, acceleration data from ambient vibration have been collected continuouslyfrom the Bill Emerson Memorial Cable-stayed Bridge. The current study only addressedone way of using these data for structural assessment of the bridge under a projecteddesign earthquake.94