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.
Dynamic Tests of Full-Scale Structures
Basic objectives of dynamic testing are outlined, and major test types are summarized. Force generation equipment suitable for tests of full-scale structures is described, and typical results are given, with examples drawn mainly from the field of earthquake and wind excitations. Results of low-level ambient vibration tests are compared with higher-level vibration generator tests, and with excitations caused by strong earthquake ground motion. Some instrumentation systems suitable for such testing are described. Information is given on the ground motions caused by large underground explosions, and the use of such explosions to simulate damaging earthquake excitations is considered.