Vulnerability Analysis And Reliability Assessment Of High-speed Railway Bridge Under Near-field Explosion Load

With the increasing frequent occurrence of global regional conflicts and international terrorist activities today,terrorists often target traffic infrastructure that can cause heavy casualties and economic losses.As one of the most important parts of land transportation system,once the high-speed railway bridge is attacked by a near-field explosion,and then it becomes severely damaged or even completely collapsed,which may lead to the paralysis of the transportation system.At present,most of related researches are based on the deterministic damage analysis of a single component,which is difficult to reflect the contingency of explosion load and the randomness of structural parameters,and either to predict the probability of the bridge system at various damage levels from an overall perspective.Now,the effect of near-field explosion load on bridge is not considered in the design code of railway bridge in China,and no suggestion of emergency repair and reinforcement of bridge after explosion is given.To solve these problems,by taking the Beijing-Shanghai high-speed railway bridge as study case,a vulnerability analysis method of high-speed railway bridge after near-field explosion is proposed in this thesis,based on response surface method by using ALE(Arbitrary Lagrangian-Eulerian)algorithm.Moreover,a more systematic reliability evaluation framework and emergency repair and reinforcement scheme of high-speed railway bridge after explosion are given.The main contents are as follows:(1)The research works in China and abroad in recent ten years are summarized,including structural response and damage level during explosion,structural residual bearing capacity after explosion and bridge’s anti-explosion measures.The research necessities and significances of probability-based vulnerability analysis method of bridge system after explosion and emergency repair and reinforcement scheme of high-speed railway bridge after near-field explosion is explained.(2)The characteristics and analysis method of explosion load are introduced.An "explosive-air-ground" coupling analysis model and a finite element(FE)model of reinforced concrete slab under near-field explosion are established by using the software of ANSYS/LS-DYNA respectively,by comparing with the existing experimental data,the effectiveness of ALE algorithm and the accuracy of material parameters and mesh sizes are verified.(3)According to the different types of car-bombs,the possible explosion analysis cases of high-speed railway bridge are analyzed.Taking the Beijing-Shanghai high-speed railway bridge as an engineering example,3D model of 2×32m prestressed concrete simply supported beam bridge is established by using the FE method,and then the failure modes of pier and beam under different analysis cases are analyzed.According to the statistical results of previous experimental studies,the damage of each component of high-speed railway bridge under four types of car-bombs is evaluated by using the residual axial bearing capacity of pier,the residual bending capacity of beam and the relative shear displacement of pot bearing as the assessment indices.(4)Referring to the seismic vulnerability,a definition of explosive vulnerability of structure is given.Then,the main control parameters influencing the residual axial bearing capacity of the pier,the residual bending capacity of the beam and the relative shear displacement of the pot bearing,are analyzed.A series of parameter combinations are obtained by the central composite design method,and the response surface models of bridge components are established by fitting the FE calculation results.The simple response surface model is used to replace the FE model of bridge’s each component,the vulnerability curves of each bridge component at various damage levels under different car-bomb attacks are calculated by the Monte Carlo sampling method,and the vulnerability curves of the high-speed railway bridge system are obtained by using the Boundary Estimation Method.(5)Combining with the vulnerability curve of bridge system and structural safety evaluation criteria,the damage areas of four types of car-bombs are divided,and the safety of bridges after different car-bombs are analyzed.By applying static live load of train on the beam,the vertical stiffness,transverse stiffness and torsional stiffness of the bridge are evaluated whether they could meet the requirements of the bridge design code.Then,by applying hammering force to the bridge,the natural frequency of the beam is calculated and used to judge whether it could meet the structural serviceability requirements.Finally,the emergency repair scheme of the bridge after explosion is preliminarily discussed,and the repair and reinforcement suggestions are provided respectively according to the damage modes of the bridge at various damage levels.