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

The FE model of the cable-stayed bridge was subjected to the three-component groundmotions of each earthquake specified in Table 6.1 as well as the rock motions of the May1 2005 earthquake at the bridge site. The peak deflections in three directions at midspanof the bridge were obtained under each earthquake. They are listed in Table 6.2. Theresponse time histories at midspan of the bridge for different earthquake records aredepicted in Figures 6.7 to 6.10.Table 6.2Peak displacement at midspan of the bridge (mm)Earthquake Longitudinal Transverse VerticalMexico City 21.40 389.7 129.2El Centro 58.00 176.9 63.10Pacoima 144.0 82.00 253.0Station D1-36.65×10-36.73×10-32.39×10From Table 6.2, it can be seen that the peak displacement at midspan of the main bridgeunder the Mexico City earthquake is the largest in transverse direction, though the PGAof the Mexico City earthquake is the lowest among the three earthquake records in Table6.1. In fact, both transverse and vertical peak displacements are higher that those underthe El Centro earthquake. These results indicate the significance of frequency contents inground motion records. A closer examination on the ground motions appears that thedominant frequency of the Mexico City earthquake is much lower than that of the ElCentro earthquake. The frequency bandwidth is much narrower than that of the ElCentro. As a result, almost all modes of vibration of the cable-stayed bridge that resonatewith the dominant frequency of the Mexico City earthquake are in vertical and transversedirections. This results in larger displacements in vertical and transverse directions. Onthe other hand, the dominant frequency of the EL Centro earthquake corresponds tohigher modes of vibration of the cable-stayed bridge, leading to smaller amplifications inpeak displacements. For the same reason, even though the Pacoima earthquake issignificantly stronger, it still induces a smaller maximum value of the peak displacementsthan that of the Mexico City earthquake. The same argument is true when the peakdisplacements due to the El Centro and Pacoima earthquakes are compared. Although thePGA of the Pacoima earthquake is several times of that of the El Centro, the maximumdisplacements induced by them are in the same order.It should be noted that the earthquake records at Station D1 are very weak, and themaximum response caused by this earthquake is thus small. Under the Mexico City, ElCentro, and Arkansas earthquakes, the maximum response occurs in the transversedirection while it occurs in vertical direction under the Pacoima earthquake. Thisindicates that the maximum response does not necessarily occur in a certain direction, butdepends on the characteristics of earthquake ground motions.81