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.
Vibration Frequencies of the Chesapeake Bay Bridge
The aerodynamic causes of suspension bridge oscillations, i.e., vortex theory, flutter theory, and negative slope theory are examined. Thirteen factors to be considered in the design of a suspended span are listed. Frequencies of antisymmetrical and symmetrical modes of vertical oscillation are given and also a minimum frequency in torsional oscillation. Mathematical studies are confined to the suspended span. Seismic studies give a mean fundamental period for the suspended span of 4.5 sec (13.3 cpm) and peak to peak amplitudes as high as 1.0 in. under the influence of a brisk cross wind and heavy traffic. This period agrees well with the calculated period of 4.51 sec (13.31 cpm). Other periods are 2.05 sec (29.27 cpm) and 0.35 sec (171.42 cpm). The cantilever span of extremely stiff design shows a fundamental period of 0.26 sec (231 cpm). The elastic-inertia and aerodynamic characteristics of the Chesapeake Bay Bridge are found to be satisfactory.
Wind‐Induced Response of Golden Gate Bridge
Full-scale observation data of wind-induced behaviour of the Golden Gate Bridge from a 1962 study was extensively re-examined by spectral analysis. The experimental investigation on the same bridge was also conducted using proposed taut strip bridge model with the simulation of natural wind turbulence at laboratory scale. The study shows that the response of the full-scale bridge at subcritical wind speed range is predominately gust-induced buffeting in the first asymmetric mode of vibration. As the wind speed increases, the motion shifts to a coupled torsional-vertical instability in the first asymmetric mode of vibration. The study shows that the taut strip model in turbulent boundary layer flow is capable of simulating the full-scale motion described in the first and second options within reasonable confidence. Conventional section model wind tunnel tests do not reveal the motion of the full-scale bridge at subcritical wind speeds. For the prediction of full-scale bridge behavior from the taut strip model tests results, it is important to take the mode of vibration into account.