Near fault ground motions are believed to contain large amounts of energy and have strong spatially varying characteristics.(Ellsworth et al. 2013; Fang, Cheng, et al. 2018;Abdollahzadeh, G., et al.2018). It has been proved that the earthquake in the near fault area can bring more damage to structures than others(Guo W, et al.2020;Zhong, J., et al.2020; Zhang, C. et al. 2020; Pan, X. et al.2017). A steel box arch bridge is usually used in lifeline traffic and plays an important role because of its beautiful appearance, long span, and good economic indicators. Some lifeline traffic are difficult to completely avoid faults due to the factors of construction site and route selection, which makes some long-span steel box arch bridges inevitably affected by faults (Goel and Anil 2008;Goel, Rakesh, et al. 2014).The steel box arch bridges with large span are more vulnerable to the permanent displacement of the fault area. Due to the large stiffness, they are very sensitive to the temporary displacement in the near fault area. Once these steel box arch bridges are damaged, the regional traffic will be cut off and the disaster relief work will be seriously affected.
In this paper, the damage of steel box arch bridge under strong spatially varying near fault ground motions is studied. The occurrence of this kind of earthquake damage needs two necessary conditions. First, the energy released by the earthquake is high, and the variation of ground motion near fault is strong. Second, the long-span steel box arch bridge is located in the near fault area. Due to these above limitations, there were few cases of earthquake damage. However, due to the huge damage of strong spatially varying near fault ground motion and the sensitivity of steel box arch bridge to this kind of earthquake, once it occurs, the bridge will be seriously damaged. For example, during the Kobe earthquake in 1995, there were four steel box arch bridges in the near fault area, two of them were seriously damaged, one was slightly damaged. For the Nishinomiya port bridge with a main span of 252 meters, the superstructure of the bridge dropped due to the excessive displacement of the substructure. For the Rokko island Bridge with a main span of 217 meters, the steel bearing falls off and the wind brace yields(Wilson 2003). From these earthquake damage cases, it can be found that once strong earthquake occurs in the near fault area, the steel box arch bridge within the region will be seriously damaged. In the case of Kobe earthquake, the damage rate of four steel box arch bridges in the near fault area reached 75%, and the failure rate reached 50%.
At present, there is a serious underestimate of the strong spatially varying in the near fault, which results in the underestimate of structural damage, especially for the large span and stiffness structure such as steel box arch bridge. Generally, the time interval method takes the spatially varying into account, it simulates the different vibration of each excitation point through time interval, then to simulates the spatially varying of the ground motion. This method is very classical, which has been used in the seismic designs of many long-span bridges (Mehannyet al. 2014;Zhao 2013). In essence, the ground motion of each excitation point is assumed to be exactly the same by this time interval method, and there are only differences in the starting time of excitation. This simplification is effective for the far-field ground motion with gentle change, but for the near-field ground motion with severe change the seismic impact is obviously underestimated.
For this reason, many methods have been used to study the damage of long-span and large stiffness structures due to the strong spatially varying in the near fault area. In some methods, the strong spatially varying earthquake motion under complex conditions is regarded as the superposition of different frequency signals, so the signal analysis techniques such as wavelet technology and empirical mode decomposition are used for analysis and synthesis, and for further analysis of the long-span bridges(Konakli and Armen 2012;Dinh et al.2014). In other methods, evolution spectrum, random vibration theory, dynamic increment analysis and other seismic damage analysis methods are directly used to study the damage of long-span structures under strong spatially varying near fault ground motion(Mehannyet al. 2014.;Adanuret al. 2016, Cao, Yenan, et al. 2017). The long-span structures under strong spatially varying near fault ground motion are analyzed in detail by these methods. In recent years, the most advanced signal analysis techniques are used in seismic damage analysis. However, they are still difficult to predict the strong spatially varying in the near fault area, which makes the damage more difficult to analyze.
In this paper, another method is used to analyze the damage of steel box arch bridge under the strong spatially varying near fault ground motion. The actual records of the near fault ground motion with strong spatially varying effect is selected as input load, under which the damage of steel box arch bridge is analyzed by finite element analysis program and shaking table test. Thus, the intensity of the spatially varying of the real near fault area is revealed, and the damage of the spatially varying to the long-span steel box arch bridge is analyzed.