近几十年来世界各国公路桥梁建设发展非常迅速，车辆荷载和车速均不断提高，由桥梁结构功能老化和车辆超载引发的桥梁破坏甚至倒塌事件屡见不鲜。准确评估既有桥梁的安全状况，严格控制超载车辆，是保证桥梁安全的关键。世界各国（包括我国）学者已指出桥梁设计规范中使用的冲击系数低估了老桥上的实际冲击系数，原因在于桥梁规范旨在指导新桥设计，并未考虑到老桥桥面损坏等情况。因此，使用规范中的冲击系数可能高估桥梁实际承载力，误导安全评估甚至酿成灾难。在我国，作为现行桥规制定依据的实测冲击系数样本很可能已经过时而不能准确反映桥梁结构和桥面以及车辆荷载的真实状况。针对以上现状，本项目拟通过数值模拟和现场试验相结合的方式来获得比较准确客观的既有桥梁的冲击系数统计特性，通过概率和可靠度分析方法提出适用于既有桥梁承载力评估的科学合理的动力冲击系数。这将对准确评估我国桥梁安全进而保证国家财产和人民生命的安全具有十分重要的意义。

Recent years have witnessed rapid development of highway bridges all over the world along with steady increase of vehicle load and speed. Increasing number of bridge damages and even collapses have also been seen due mainly to structural deficiency and functional obsoleteness of aging bridges and vehicle overloading. Therefore, accurate evaluation of the health condition of existing bridges and strict enforcement of control over overloaded vehicles would be critical to ensure the safety of highway bridges. Researchers around the world, including China, have noticed that most bridge design codes may have underestimated the dynamic impact factors on old bridges. One main reason is that the dynamic impact factors prescribed in the bridge design codes are aimed to guide design of new bridges and have not taken into consideration the deteriorated road surface conditions of old bridges. Therefore, bridge capacity rating using the code-specified dynamic impact factors may overestimate the bridge's true load-carrying capacity and lead to potential disaster. In China, the collected in-situ dynamic impact factor data used in developing the dynamic impact factors prescribed in the current bridge design codes may have become obsolete and cannot truly represent the current bridge conditions and vehicle loading. To address these issues, the proposed project aims to obtain accurate and objective statistics of the dynamic impact factors for existing bridges in China by means of numerical simulation and field test, and to propose reasonable dynamic impact factors for capacity rating of existing bridges in China using statistical analysis approaches and reliability methods. The proposed project is expected to have significant effect in guiding the capacity rating of highway bridges in China and therefore ensuring the safety of China's infrastructure and people's lives.