Research On Urban Viaduct Seismic Behaviors In Perforcemance-Based Seismic Design Theory

Based on the profound reconsidering to modern earthquake experience and lesson, the American scholars proposed performance-based seismic design theory in the beginning of 1990s, this is an important milestone in the engineering seismic development history. The core content of the performance -based seismic design theory is to make the structure achieves a group of anticipated performance objectives under different design earthquake levels; It causes the seismic design objective changed from the traditional objective which taking the safety as the single defended goal to consider safety and the property damage two aspects requirement, emphasizing the individual design. The viaduct is the essential infrastructure in transport networks of modern cities, along with the development of urbanization process, it holds the more and more important status in the city lifeline project. Based on the simple discussion on urban viaduct earthquake design levels, performance levels and structural performance-based seismic elastic design, in this dissertation , a set of performance-based seismic design method for urban viaduct is established completely by taking the bridge pier top displacement ductility coefficient as design parameter, and several related key problems are emphatically studied.1. Proposed the purposed value of the urban viaduct earthquake design levels and importance classification levels, and by changing the design reference period method to represent the important difference; Proposed the urban viaduct 5 performance levels and 4 performance objectives; Selected the displacement ductility coefficient of bridge pier top as the performance parameter; It is deductived in this dissertation about the corresponding relations of displacement ductility coefficient of bridge pier top, the damage index and the performance levels to conventional urban viaduct.2. According to the exceeding probability under all of design earthquake levels in certain important level, the earthquake affecting coefficient maximum α_max is deductived and manufactured to table form, this is available to performance-based seismic design in elastic stage, and provided a more widespread more flexible choice to design personnel.3. In this dissertation, 434 earthquake ground motion records are selected, and be classified into 12 groups according to the soil conditions and the earthquake ground motion characteristic periods grouping in "Double-decked Urban viaduct Earthquake resistance Design Guide", The elasto-plastic time history analysis is carried on to a great deal of SDOF system with various yield strength coefficient, and obtained the ductility demand spectrum,the A_y-D_y form earthquake demand spectrum and the damping ratio influence coefficient. The research indicated that, the two kinds of demand spectra are related to yield strength coefficient and periods,and the the damping ratio influence coefficient are related to damping ratio, the above three items depend on soil conditons and earthquake ground motion characteristic period groupings strongly. Through the non-linear regression analysis, the mathematical expressions of above two kinds of demand spectra and the damping ratio influence coefficient are established that considering soil conditions and earthquake motion characteristic periods grouping, it might supply in viaduct design in plastic randge and correlative research based on "Double-decked Viaduct Earthquake resistance Design Guide" and "City Bridge Earthquake resistance Design Standard" .Specially the A_y-D_y form demand spectrum, that has the following characteristic of that structural periods represented by radial lines which crossing zero point intersecting demand spectra with different ductility factors are all the same. This has provided the convenience for the realization multiple performance objectives in displacement-based seismic design.4. In this dissertation, the lateral load pattern for bridge Push-over analysis is studied by means of elasto-plastic time history analysis to 5 single piers and 6 whole bridge structures under tens of earthquake ground motion records. The analysis result indicated that: to regular single pier (slenderness ratio is bigger than 2.5 and smaller than 10) , it is more reasonable that by using displacement-pattern-based lateral load pattern in bridge Push-over analysis than lateral force-pattern-based; The bridge pier displacement pattern is determined by the 1st vibrate mode, and may be simplified to a single displacement that loading on the center of gravity of bridge superstructure. As for the whole bridge structure, by analyzing the distributation of the displacement of superstructure, the displacement of pier top and the pier base shearing force, the Push-over analysis displacement pattern is determined by the vibrate mode and it's combination of superstructure.5. Based on the ductility demand spectrum and the A_y-D_y form demand spectrum, Elasto-plastic demand spectral method is proposed to calculate the non-linear displacement responded. The basic solving process of elasto-plastic demand spectral method is put forward. Firstly , the MDOF system is decomposed into several inelastic SDOF systems using modal Push-over analysis, and to determined related parameter; Secondly, the yield strength coefficient under difference vibrate mode and difference design earthquake levels are determined using A_y-D_y form demand spectrum; Thirdly, according to the ductility demand spectrum, the displacement ductility coefficient and the equivalent SDOF system displacement are obtained; Finally, the displacement of equivalent SDOF system is transformed to MDOF system, and be combined.6. Starting with the stress-strain constetutive relationship of confined concrete which from Mander et al., through a series of hypotheses, the deformation capacity formulas of rectangular, circular section bridge pier are proposed , which shows the relationship among mechanical stirrup reinforment ratioω_w, the axial compression ratioη_k and the curvature ductility coefficientμ_φ. At the same time , the formula is evaluated with 25 group of tentative data, and the result showsμ_φcalculated by the formula approaches to test data in the average significance.7. The urban viaduct design procedure based on performance seismic design theory is proposed: Firstly, the response spectrum method is used in the elastical design range, that is familiar with for everybody; Secondly, in elasto-plastic design stage, the main step is: (1) Calculating the elasto-plastic displacement under a certain design earthquake level by using the elasto-plastic demand spectral method; (2) Designing the deformation capacity of bridge pier under a certain performance objective. The completed design procedure is illustrated with a urban viaduct, and the elasto-plastic time history analysis is used to validate this method. It shows that the elasto-plastic demand spectrum method is effective and correct to calculate the nonlinear displacement.