A NEW CODE-BASED SIMPLIFIED APPROACH FOR CONSIDERING MULTI-SUPPORT EXCITATION IMPACT ON BRIDGES

Abstract eng:
During the last 40 years, many sophisticated methods have been proposed for considering the effects of spatial variability of earthquake ground motion on the dynamic response of bridges. However, the complexity of these methods as well as the false perception that SVEGM has a generally favorable effect on bridge response, generally result in overlooking this phenomenon during design. To deal with the problem of multi-support excitation, modern seismic codes (US Standard Specifications for Highways and Transportation Bridges, Caltrans, ATC-32, Japanese Design Specifications for Highway Bridges) prefer to consider increased seating lengths for the deck, thus dealing with the problem in an implicit manner. A more practically-oriented approach is described in Eurocode 8 – Part 2 for Bridges, currently the only seismic code worldwide providing a clear and detailed framework for considering the effect of SVEGM in bridge design. This paper focuses on the effects of asynchronous excitation on the seismic response demand of bridges. A comparison of the existing simplified approaches with more refined methods, based on time history analysis, using simulated ground motion histories through spectral representation-based simulation techniques, is conducted. A new approach that is based on the response spectrum modification is also presented. Parameters examined include time delay due to finite wave propagation velocity and loss of coherency along the bridge length. The impact of local site effects is taken into account using appropriate impedances. The paper concludes with the effects of various parameters on the seismic demand of bridges. The results indicate that multi-support excitation can be locally detrimental, increasing the seismic demand on specific components of the structure that are related to the anti-symmetric modes. Comparison between the existing simplified methodologies shows their superiority to conventional design through the EC8 framework while the proposed methodology can be even more efficient.

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