Natural frequencies of concrete bridges in the Pacific Northwest

Analyses of field ambient vibration were performed on 50 concrete bridge spans along interstate highways 1-5 and 1-405 in Washington State. These 50 spans included 21 pretensioned concrete beam (PCB) spans, 19 reinforced concrete box-girder (CBOX) spans, and 10 reinforced concrete slab (CS) spans. Eight measurement transducers were used to record ambient bridge vibrations at three location on each span: midspan, one-quarter point, and one support. These records of bridge motion versus time were each subjected to a fast Fourier transformation, and plots of amplitude versus frequency were generated for each record. The plots of amplitude versus frequency were used to determine the fundamental vertical and lateral frequencies for the bridge spans measured. These fundamental frequencies were used with the bridge design parameters to derive empirical formulas that will be used to estimate the fundamental vertical and lateral frequencies of other PCB, CBOX, and CS bridge spans along I.5, I-205 and I-405 in the Pacific Northwest.