Safety Assessment Of Bridges Due To Vessels Impact

With increasing number of bridges crossing inland waterways, strait and gulfs, and rapid development of vessel transportation, the collision accidents between vessels and bridges occurred frequently, which caused bridges damage or collapse. However, the laggard design concept and the rough methods of bridges' vessel impact design in our specification are difficult to meet the increasing needs of bridge construction in our country. On this context, this paper focuses on the safety assessment of bridges due to vessels impact, in order to establish perfect assessment flow and reasonable assessment methods. A more systematic research on basic framework, key fundamental problems and basic assessment methods, is conducted in this paper, which help us understand the risk level of bridges and provide support for vessel impact design of bridges. The main contents are as following:(1) Based on the present research of vessels impact bridges and application of risk assessment, a framework of safety assessment system of bridges due to vessels impact is proposed in this paper, in which the risk concept is introduced. This system consists of five sub-systems: Database for vessel-bridge collision assessment, Bridge safety assessment module, Determination of risk criteria, Active collision-prevention scheme and Passive structure-protection scheme. The logical relationship among these sub-systems is clarified.(2) The work on establishing database for vessel-bridge collision assessment is carried out for the first time in our country, for various data needed in the system. The basic function and contents of database is defined. The information encoding and detailed items of seven sub-databases are given, including bridge basic information, navigation information, vessel characteristics, environmental information, accidents records, active protection measures and passive protection measures.(3) The calculation model about vessel-bridge collision probability is further studied. The update-kunzi model is put forward in this paper after researching and comparing various models applied in the world at the present especially kunzi model. Compared with kunzi model, the update-kunzi model can take transit path lateral distribution of vessels into account, and it can meet the condition lack of statistical information in our country. The sensitivity anslysis of model parameters is proceeded and the rationality of this model is verified by case study. Depending on three bridges crossing Yangtze River in three-gorges reservoir, the acquisition procedure of model parameters is explained and some parameters' probabilistic models are given. The vessel transit path lateral distribution can be expressed by normal distribution and the vessel aberrancy angle distribution can be expressed by normal distribution or maximum I distribution. At the same time the suggested parameter values for three-gorges reservoir are given. The correction factor of active protection for vessel-bridge collision probability is studied preliminarily as well.(4) The probability assessment method of bridges safety after vessels impact is studied. The assessment method based on reliability theory is put forward through researching on probabilistic distribution of vessel impact force, structure response and structure resistance by monte-carlo method, and the simplified computation method of bridge substructure reliability is given. According to research result, the vessel impact force accepts normal distribution when vessel tonnage is uniform distribution, impact velocity is normal distribution and impact angle is maximum I distribution. The moment and shear response of non-linear structural system accepts normal distribution when pile-soil action adopts p-y curve. The yielding moment and shear capability of component section accept normal distribution. At the same time the correction factor of passive protection for vessel impact force is studied preliminarily as well.(5) The risk criteria of vessel-bridge impact are studied. Based on comparing and researching relevant norms about vessel-bridge impact, risk levels of various industry and natural disasters, and existing accidents in our country, as a suggestion, the risk acceptance by individuals take 1.5×10^-5, the risk acceptance by society take 10^-4 for 10 fatalities and 10^-6 for 100 fatalities in our country. The qualitative evaluation concept of environment risk and the optimum cost method is represented in the paper. Based on these, the grade assessment standard is proposed by bridge collapse frequency per year for more than 10^-3 as high-risk, 10^-3～5×10^-5 in the middle risk, 5×10^-5～5×10^-7 in the low-risk, less than 5×10^-7 as the negligible risk. And the preliminary risk countermeasures are given.(6) case study. A systematic assessment for Nanjing 4th Yangtze River Bridge and Zhongxian Yangtze River Bridge is presented, in which the specific assessment method is explained. This study provides useful references for application and development of safety assessment system of bridges due to vessel impacts in practical projects.In the finality, some prospects of bridge safety assessment due to vessel impacts are given. The contents need to be further researched are generalized.