Multi-support seismic excitation of long span highway bridges

Gökdemir, Tunç
Seismic design and analysis are common practices in bridge engineering which require both knowledge and experience. Technical standards, such as CALTRANS (2010) and AASHTO (2007) provide guidance on the analysis with the methods ranging from Response Spectrum Analysis (RSA) to Time History Analysis (THA). These methods are usually sufficient for obtaining the seismic behavior of common structures and capture the dynamic behavior with decent accuracy. However, this may not be the case for structures which span long distances and/or subject to significantly varying soil conditions. In such cases, Soil Structure Interaction (SSI) and site-response effects should also be taken into consideration. Multi-Support Excitation (MSE) is a dynamic analysis approach which reflects such effects on analyzed models with accuracy. In this method, supports of the structures are excited with different ground motions which are amplified according to the soil conditions present at these supports. In this thesis, finite element model of a very long viaduct, which is located at Bursa, Turkey, is developed and analyzed under the effects of Multi-Support Excitation. The model of the viaduct is analyzed for four different loading cases. With each case, complexity of the analysis is increased, reflecting the non-uniformity of the soil conditions more apparently. In the last case, (Case 4) a complete Multi- Support Excitation analysis is carried out. These analysis cases are repeated with 6 different bedrock motions as input and the results are compared for each loading case. Findings of this thesis show that for complex bridges which have heterogeneous soil conditions beneath, using simplified analysis methods which does not consider local soil conditions may end up underestimating or overestimating the results significantly. Additionally, Soil Structure Interaction should also be included in models for complex and long structures to obtain more accurate results. Numerical results of this study show that analyzing long structures under Multi-Support Excitation (MSE) yields significantly different forces and displacements compared to the case where THA analysis is carried out with identical input motions applied at supports. 


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Citation Formats
T. Gökdemir, “Multi-support seismic excitation of long span highway bridges,” M.S. - Master of Science, Middle East Technical University, 2017.