Field-Measured Natural Frequencies of Delaware Memorial Bridge
The Delaware Memorial Bridge is a major suspension bridge that is critical to the tristate region of northern Delaware, southern New Jersey, and southeastern Pennsylvania. The Delaware Memorial Bridge consists of two nearly identical suspension bridges (hence, the nickname Twin Spans) that cross the Delaware River from southern New Jersey to northern Delaware. Field ambient vibration measurements of Structures 1 and 2 of the Delaware Memorial Bridge were conducted in June 2001 and September 2003. These measurements were used to derive the natural frequencies and modes of vibration for these structures. These field-measured frequencies and modes were then compared with each other and with computer-generated natural frequencies and modes of vibration derived for Structure 2 of the Delaware Memorial Bridge in 1997. Specifically, the 2001 seismometer-generated results for Structure 1 were compared with the 2003 accelerometer-generated results for Structure 1; the 2003 field-measured results for Structure 2 were compared with the 2003 field-measured results for Structure 1; and the 2003 field-measured results for Structure 2 were compared with the 1997 computer-generated results for Structure 2. Overall, these three types of comparisons indicated relatively good correlation for transverse motion but only fair correlation for longitudinal and vertical motion.
SUSPENSION BRIDGES UNPLUGGED
The `coupling effect' between the flexible, suspended superstructure and the very stiff cable anchorage in suspension bridges can produce large motions during strong earthquakes. This is especially true if the anchorage is founded in soft soil conditions. Decoupling the anchorage from the superstructure by installing viscous damping devices can control displacements and dissipate energy.
Ambient Vibration Studies of Golden Gate Bridge: II. Pier‐Tower Structure
Dynamic characteristics such as natural frequencies, mode shapes, and damping ratios of the Golden Gate Bridge tower were determined using ambient vibration data. The ambient vibration tests involved the simultaneous measurement of longitudinal and lateral vibrations of the main tower (San Francisco side). Measurements were made at different elevations of the tower and on the pier, at a total of 10 stations. Good modal identification was achieved by special deployment and orientation of the motion‐sensing accelerometers and by summing and subtracting records to identify and enhance definition of longitudinal, torsional, and lateral vibration modes of the tower. A total of 46 modal frequencies and modal displacement shapes of the tower were identified: 20 longitudinal, 15 torsional, and 11 lateral, all in the frequency range of 0.0–5.0 Hz. Finally, comparison with previously computed two‐ and three‐dimensional mode shapes and frequencies shows good agreement with the experimental results, thus confirming b...