A Combined Geophysical/Engineering Approach for the Seismic Safety of Long-Span Bridges

The spatial distribution of the resonance frequency of the sedimentary cover in the Cologne area was estimated by the horizontal-to-vertical spectral ratio method using ambient noise measurements. A comparison with the eigenfrequencies of selected long-span bridges crossing the Rhine river indicated an overlap in frequency and therefore potential damage in the case of an earthquake. Consequently, we conducted a dynamic vulnerability analysis using the finite-element method. In addition to accelerograms from design spectra, ground motion scenarios with sources located on the most proximate fault (Erft fault) were simulated using a numerical hybrid method that takes into account source, path, and site effects. The results indicated that due to the low frequency content of these scenarios, consistently higher responses – compared to the recommended loads in the German seismic code were obtained at all selected bridges. Usage of both scenarios revealed specific failure mechanisms. While cable supported bridges seem to be secure, grave failure was detected for a box girder bridge.

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