Using conditional mean spectra and uniform hazard spectra for seismic analysis of bridges

Older bridges are often not adequately designed for earthquakes. To enable a structural verification, new methods such as conditional spectra and conditional mean spectra have been developed to estimate seismic hazards more accurately. To quantify the advantage of the latter method for regions with low and moderate seismic hazards, the seismic internal forces of four Swiss existing bridges were calculated and compared based on conditional mean spectra and uniform hazard spectra. The examples chosen were two reinforced concrete and two steel bridges, one of which was used for road traffic and one for railway traffic. The bridge selection was intended to show the validity of the results for different bridge types. On average, the computations using conditional mean spectra showed lower internal forces than when using uniform hazard spectra. However, the difference was smaller than expected. In addition, practical application in an engineering office would be very time-consuming with today's software solutions. Nevertheless, the results also showed that the differences at component level can be significant.