Field - Measured Natural Frequencies of the Delaware Memorial Bridge

The Delaware Memorial Bridge is a major suspension bridge that is critical to the tri-state 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 that were derived for Structure 2 of the Delaware Memorial Bridge in 1997. Specifically, the 2001 seismometergenerated results for Structure 1 are 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 different types of comparisons indicated relatively good correlation for transverse motion, but only fair correlation for longitudinal and vertical motion. INTRODUCTION The Delaware Memorial Bridge consists of two nearly-identical suspension bridges that cross the Delaware River from Salem County in southern New Jersey to New Castle County in northern Delaware. Both structures are owned, operated, and maintained by the Delaware River and Bay Authority (DRBA). Structure 1 (the southern of the two structures) was completed in 1951, while Structure 2 (the northern of the two structures) was finished in 1968. Each structure has one 655.3-meter (2,150-foot) main suspension span and two 228.6-meter (750-foot) suspended side spans. In both structures, the suspended deck truss is attached at the main cable anchorages by connections that allow rotation about the transverse axis with no relative motion allowed in the longitudinal, transverse, or vertical directions. The suspended deck truss is attached at the towers by connections that allow rotation about the transverse axis and relative motion in the longitudinal direction only. Structures 1 and 2 of the Delaware Memorial Bridge cross the Delaware River closer to the mouth of the river than any other bridges and as such they have the longest main spans of any bridges that cross the Delaware River. These structures provide the critical link that connects the southern end of the New Jersey Turnpike with Interstate Highway I-295. As such, these two structures are considered to be two of the most important bridge structures in the tri-state area of northern Delaware, southern New Jersey, and southeastern Pennsylvania along the I-95 corridor. Because of their size, location, and importance, Structures 1 and 2 of the Delaware Memorial Bridge are comparable to the Golden Gate Bridge at the entrance to San Francisco Bay, the Chesapeake Bay Bridge across the entrance to Chesapeake Bay, the Astoria Bridge at the mouth of the Columbia River, and the Verrazano Narrows Bridge across the mouth of the Hudson River. FIELD AMBIENT VIBRATION ANALYSES Field ambient vibration analyses are field vibration measurements that are taken using ambient traffic and wind loads as the dynamic input. Such analyses are much more practical for major river crossings where complete closure of the structure for controlled loading is not possible. Field ambient vibration analyses have been used to derive the natural frequencies and modes of vibration for a variety of structures including concrete and steel highway bridges (1, 2, 3) and suspension bridges (4, 5, 6, 7, 8). Thus, the state of the art of field ambient vibration analysis is well established and the procedures are well known. Field Ambient Vibration Measurements The goal in the field ambient vibration measurements that were conducted on Structures 1 and 2 of the Delaware Memorial Bridge and on other bridges over the Delaware River between 2001 and 2003 was to recover the fundamental or lowest-frequency modes for each direction of motion. The main objective of the measurements that were conducted on Structure 1 in 2001 was to test the seismometers that were purchased for field ambient vibration measurements of bridges in the Delaware Valley region. The main objective of the measurements that were conducted on Structures 1 and 2 in 2003 using triaxial accelerometers was to provide data that could be correlated with the 2001 field-measured results and with the 1997 computer-generated results (9, 10). In both 2001 and 2003, TRB 2006 Annual Meeting CD-ROM Paper revised from original submittal.