Research On Site Condition And Seismic Performance Index Of Girder Bridge In High-speed Rail

High-speed rail lines cross the region of complex site types, bridges account for a higher proportion of line mileage and comparatively huge dimensions of the cross-sections of the girder and pier of the bridges in high-speed rail are applied to cater for demand of the railway smoothness and the passengers comfort. Whether the seismic performance of the girder bridges satisfied the target performance of the three level seismic fortification targets coded in the existing Code for seismic design of railway engineering (GB50111-2006) (2009 version), is an extensively concerned question in the engineering field. The main contents of this paper are as follows:Influence of site condition:Based on the analysis of the geographical strata data of 225 engineering layered soil sites, the equivalent shear wave velocity of the soil sites considering the different depths of covered soil are calculated respectively. Furthermore, the classification of the soil sites is carried out according to the different seismic codes. It is shown that the classification method resulting from the shear wave velocity is convinced irrational in current Code for Seismic Design of Railway Engineering. Considering the classification methods of soil site in different seismic code and the upcoming Seismic Ground Motion Parameter Zonation Map of China (GB 18306-2015), the influence of site condition on the characteristic period and peak ground acceleration is analyzed. Selecting a simply supported girder bridge with its span of 32m as the case bridge in Beijing-Shanghai high-speed railway, the impacts of the site classification on the translational and torsional spring stiffness of the pile-soil interaction are developed and compared in detail based on the strata data mentioned above. Moreover, the displacement at the top and shear force at the bottom of the single pier model is calculated under the different site conditions. It is shown that the site condition has a significant effect not only on the design earthquake action of the engineering structures, but also on the spring stiffness value considering the pile-soil interaction, as well as the seismic response of the piers.Performance based seismic index:Taking a typical 40m+64m+40m three span continuous girder bridge and a 32m simply supported girder bridge as the case bridges, the different finite element models are constructed for the different pier heights (10m-20m) and bridge arrangements using Opensees software. The incremental dynamic analysis (IDA) method is used to analyze the longitudinal seismic performance of the bridges mentioned above.It is shown that pier height and bridge arrangements have little influence on the displacement ductility coefficients for the moderate damage limit state, ranging from 2.14 to 2.38, meanwhile for the severe damage limit state, the displacement ductility coefficients can be close to 7.84-8.25, which are significantly greater than the allowable ductility coefficient of 4.8. It can be reflected that 4.8 is too uniform and conservative, the ductility capacity of the bridge pier is underestimated, and the displacement ductility coefficient should be given for different damage states.For pier height range of 10m-20m in high-speed rail bridges, bending moment curvature analysis could use the traditional calculation method directly, which is recommended in guidelines for seismic design of highway engineering, but the equivalent plastic hinge length should be revised.Seismic performance evaluation:According to the research on seismic fragility of case bridges based deformation criterion, it is shown that the pier height is the fundamental factor which affects the seismic performance of the pier, while the bridge arrangements factor with different pier heights is slight, within the pier height range of 10m to 20m. During the design reference period, the bridge piers in high-speed rail will be substantially slight and moderate damaged with high probability, and the severe damage probability is about 30%. However, the occurring probability of the complete damage (collapse) is as small as being neglected.The seismic fragility of case bridges is calculated by performance based deformation and energy criterion, which points out that calculated damage index DI of threshold values for different damage states with Park-Ang model exist insufficiency and the availability of Fajfar model IDA-based method. The two criteria are used to calculate the seismic fragility respectively, which comparatively shows performance based deformation criterion is sufficient for the structures conformed to the ductility check.