Preliminary Study On Benefit Of Fortification Intensity For Seismic Design Of Bridges Under Framework Of Lifecycle Cost Analysis

In order to improve the determination of fortification intensity for seismic design of bridge,the benefits of fortifications with three intensities for bridges respectively on three sites with different hazard are evaluated in this thesis,by means of criterion to minimize the sum of original construction cost and seismic risk cost,under the framework of lifecycle cost analysis.Three bridges with standard design against to intensities VI,VII and VIII respectively are selected from Harbin-Jiamusi Railway,for case study.The bridges with almost the same geometrical dimensions,are modeled by Midas Civil software tool as independent pier models.In the modeling,the dead loads of the superstructures,such as gird and so on,are concentrated at the center of pallet as additional mass,and the pilesoil interaction is described by "m" method.From a natural vibration analysis,it is shown that the first modes are predominate in vibrations of all three bridge,the basic periods are about 0.56 seconds,the participating mass percentages of all first mode are all more than 72%.The capability curves of the three bridges are calculated by a push-over analysis with the Vidic's strength strength reduction model,the inelastic demand spectra corresponding to the design response spectra for six basic peak accelerations(corresponding to intensities VI,VII,VIII and IX respectively)are formed from the code for seismic design of railway engineering,the nonlinear responses of each bridge caused by the intensities are calculated by capacity spectrum method.The stiffness degradation model for seismic damage is adopted,with an improvement of the existing formula for damage index to place the yield point in between grades of intact and slight damage,damage indices of the three bridges caused by the four intensities are calculated from their nonlinear displacement responses.Probabilistic evaluation model for earthquake damage grade is established with guiding of the membership function in fuzzy set theory,processing fuzziness in partition of earthquake damage grades and referring three existing partition suggestions.The seismic vulnerability matrices of three bridges are built by combining the probabilistic model and the result damage indices.Three engineering sites with different seismic hazards are selected.The intensities at the sites with exceeding probability 10% in 50 years are VI,VII and VIII respectively.The occurrence probabilities of the three intensities in 100 years at each of the sites are calculated from their hazard curves.Increasing coefficient values of original construction costs of railway bridge for different fortification intensities,and ratio values of direct economic loss and of indirect economic loss for the five earthquake damage grades,are assigned from the available empirical data and reports in references.The additional cost in the original construction of the bridges for seismic fortification are calculated.The seismic risk costs of the bridges during the future earthquake are calculated respectively,combining with the seismic risk at each site.Contributions of both direct economic loss and indirect economic loss are taken into account in the earthquake risk cost.Finally,the benefits of bridges seismic fortification with intensity V,VII and VIII are evaluated by means of comparing the sums of additional costs in original constructions for the fortifications and the corresponding seismic risk costs in the lifecycle.The result of this thesis shows that the fortification intensities with exceeding probability 10% in 50 years are usually those with the best benefit,other than the fact that three the latter is a little bit higher than the former at site with intensity VII.It means that further study on life cycle cost analysis for bridges and other structures are noteworthy.