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

To further investigate the relative flexibility of the bridge in three directions, the verticalacceleration record taken during the El Centro earthquake or during the Arkansasearthquake was input into the FE model in all three directions. The peak displacements atmidspan of the main bridge are listed in Table 6.3. It can be observed that the maximumdisplacement of the bridge does not occur in the same direction under the twoearthquakes, which further confirm the importance of ground motion characteristics.Under an El Centro type of earthquake record, the vertical direction appears to be moreflexible as it has the largest displacement. Under the earthquake record obtained at thebridge site, the transverse displacement is lightly larger than displacements in otherdirections, and thus more flexible. This general trend that the bridge is more flexible invertical and transverse directions is consistent with the natural frequency distribution asdemonstrated in Table 5.1.However, the peak displacements in three directions are overall in the same order whilethose due to the El Centro type of earthquake are quite different. The significantdifference between the two is attributable to the variation of their ground motioncharacteristics. As shown in Figures 6.2 and 6.4, the frequency bandwidth is muchnarrower for the vertical ground motion from the El Centro earthquake. Therefore, theresponse due to the El Centro earthquake is more sensitive to the major difference indynamic characteristics of the bridge in three principal planes (longitudinal, transverse,and vertical) as indicated by the natural frequencies in Table 5.1. On the other hand, therock motion at the bridge site has much wider frequency contents, which can excite mostof the vibration modes in all three principal planes and result in more uniform peakdisplacements in three directions.Table 6.3Peak displacement at midspan of the bridge (mm)Earthquake Longitudinal Transverse VerticalEl Centro 10.40 33.90 64.30Station D1 -32.26×106.3 Evaluation of the bridge-32.58×10 2.18×10 -3As shown in Section 3, several instrumentation stations exist at the bottom of towers,including D1, D2, and D3. At each station, three accelerometers were installed along thetransverse, longitudinal (traffic), and vertical directions of the bridge. It was observedfrom the measured data that the accelerometers at Station D3 did not work properlyduring the May 1, 2005 earthquake. The measured data at D1 and D2 indicate that theyhave similar wave forms and peak values, which are expected due to their installation onrock. The peak value of the measured accelerations at D1 and D2 are very small as it wasdue to a minor earthquake of magnitude M4.1. As a result, the components of themeasured acceleration at D1 are all scaled up by 10,000 times in order to approximatelyrepresent an M7.5 design earthquake for the bridge (Woodward-Clyde Consultants 1994).The measured earthquake records generally reflect the regional geologic conditions andseismic characteristics of the NMSZ. The three original acceleration components from84