Study On Seismic Performance Design Theory Of Highway Bridges Based On Probability

Since the concept of performance-based seismic design (PBSD) put foward by American experts and engineers in 1995, the related researches have been the centre of great interest among the current international earthquake and structure engineering in rencent years. It is widely recognized that PBSD is the future development direction of seismic design codes for engineering structures. There are large uncertainty in PBSD for structural associated with earthquake ground motion, material and geometric properties of the structure and so on. Therefore, the performance objectives of PBSD pursued can not be guaranteed. The need for the incorporation of reliability into the design philosophy of PBSD has been recognized. In the context of structural seismic design based on the probability and performance, this paper described the seismic performance design framework of highway bridges on the basis of probability from earthquake design levels, performance levels of the bridge structures, reliability of performance objectives and probability analysis method. It is focused on the study of the establishment of probabilistic seismic capacity model, probabilistic seismic demand model and the performance limit state equation with prescripted reliability for regular bridges. The main research works and conclusions are as follows:(1) A practical framework for probability and performance-based seismic design of highway bridges was proposed. In the framework, four earthquake design levels were recommended in terms of the exceeding probability of 86%,19%,10% and 4% in 100 years, to solve the inconsistency of seismic hazard for highway bridges with different seismic fortification classification at the same earthquake zone in current Chinese Guidelines for Seismic Design of Highway Bridges; four seismic performance levels were recommended, i.e. immediately operational, limited operational, emergency traffic only, and closed to traffic, respectively, according to the expected functional requirements of highway bridges after an earthquake shock; the seismic performance objective is expressed as the conditional probability of exceeding a specified performance level at the given earthquake design level, to solve the problem that the reliabilities of seismic fortification goals of highway bridges have not been specified; according to reliability analysis method, functional relationships among partial factor, probabilistic characteristic value of basic variable and target reliability index were derived. At last, general expression for the probability based seismic performance design of highway bridges were established.(2) Four seismic performance levels were defined for the capacity of regular highway bridges, along with the explicitly physical damage state of reinforced concrete bridge pier. The empirical formula of drift ratio at each performance levels was developed with the equivalent plastic hinge theory. On the basis of experimental test results of 183 rectangular and 77 circular flexure dominant reinforced concrete piers, the corresponding coefficient of aspect ratio, axial-load ratio and reinforcement characteristic value were determined by multiple regression analysis. Considering the epistemic uncertainty of the empirical formula, the probability distributions of drift ratio were examined at different performance levels. It is suggested that they are reasonably lognormally distributed with K-S test at the 5% significance level. On the basis of experimental test results collected in this paper, the limit values of displacement ductility factor and normalized absorbed hysteretic energy at different performance levels were calculated for each of the piers. At last, the probabilistic capacity model of regular highway bridges which is convenient to perform the probability based seismic performance design and evaluation was established.(3) Because a structure is damaged by a combination of high stress excursions and stress reveals under earthquake action, a modified damage index is established considering the combination of normalized plastic deformation and absorbed hysteretic energy. The modified damage index has a value of 0 at the beginning of yielding and a value of 1 at failure and can solve the non-convergence problem existing in the Park-Ang damage index. On the basis of experimental test results collected in this paper, the limit values of the modified damage index at different performance levels were calculated for each of the piers. Then the probability distribution and characteristic values of the modified damage index were obtained by probability analysis. It is suggested that the scatter of the modified damage index at different performance levels is significantly reduced compared to the Park-Ang damage index.(4) The probabilistic seismic demands of 8 representative regular highway bridge samples were calculated using the increment dynamic analysis (IDA) method by selecting 2390 earthquake records at 3 different site conditions. These demands were expressed in terms of one intensity measure (IM), i.e., the spectral acceleration at the fundamental period with 5% damping and four engineering demand parameters (EDPs), i.e., drift ratio, displacement ductility factor, normalized absorbed hysteretic energy and modified damage index. The probability distributions of the EDPs were established at different IM levels. It is found that the selected EDPs. i.e., drift ratio, displacement ductility factor and normalized absorbed hysteretic energy are reasonably lognormal distributed at different IM levels. The probability distribution model of the modified damage index obeys a standard beta distribution at different IM levels. A practical probabilistic seismic demand model based on the EDPs are proposed by using the regression analysis method. The provided probabilistic seismic demand models are suitable for three different site conditions and can largely simplify the calculation of seismic demand in the probability-based and performance-based seismic design of regular bridges.(5) The demand capacity factors design-checking equation which account for the uncertainty in the seismic demand and capacity of regular bridges are obtained based on the probabilistic framework for seismic performance design of highway bridges proposed in chapter 2. According to the probabilistic seismic demand model and probabilistic capacity model of regular highway bridges established in chapter 3 and chapter 4, the common probabilistic analysis tool of first order second moment method is used to derive the performance limit state equation. The demand capacity factors are prescribed under the conditional of given performance objective reliability index.(6) The seismic hazard curve that in coordination with the seismic zone of China was obtained. Then an analytical expression was deduced with total probability theory to describe the mean annual rate of regular bridges performance level reaching or exceeding a specific limit state. In order to convenient for engineering application, it was converted to design-checking equation with demand capacity factors. The probability-based and performance-based seismic design procedure was demonstrated by an example regular bridge. It reveals that the proposed method can be practically applied to regular bridges with given performance objective reliability index or mean annual rate of performance level reaching or exceeding a specific limit state which makes the performance objective more secure.