# Seismic Performance Evaluation of Highway Bridges under Scour and Chloride Ion Corrosion
## 1 Introduction  
Bridges play an important part in the road network and are vulnerable to damage caused by earthquakes, hurricanes, and scour and vessel collision [1]. Effects of various hazards were separately considered in the traditional design of bridge structures, which ignored the interaction of different hazards [2]. In recent years, a large number of bridges have collapsed as a result of the combination of multiple or successive disasters, showing the shortcomings of traditional bridge design methods for meeting the design needs of bridge structures in multi-hazard conditions [3]. The design of structures under multi-hazard conditions involves consideration of the uncertainty between structural capacity and demand and the balance between reliability and cost [4, 5]. Therefore, it is of great theoretical significance and value of application to improve the capacity of bridges in terms of comprehensive disaster prevention and mitigation at the level of design, reduce the risk of catastrophic damage to bridges, and establish a reasonable design method for bridges under multi-hazard effects. Earthquake and scours are two common natural disasters that easily lead to bridge damage and destruction [6, 7]. Several procedures for seismic analysis of bridges have been developed in recent years to realize the seismic risk evaluation and damage control [8, 9, 10]. Especially in soft soil sites where pile foundations of the bridge are less able to resist horizontal loads, lateral restraint of soil on pile foundation is weakened due to scour. The pile foundation can be changed from capacity-protected element to failed element by factors such as sand liquefaction under seismic action, which may result in the transfer of damage from the assumed plastic hinge area on the pier to pile foundation. The failure probability of bridges crossing rivers in seismic zones may increase further as the depth of scour atthe foundation changes, at which point the seismic damage pattern of the bridge needs to be re-evaluated [11; 12]. It is essential to establish a framework of risk-based research that considers the combined hazards of earthquakes and scours. In addition, the increasing depth of the scour will result in more parts of the bridge substructure elements being exposed to adverse environmental conditions. As the service life of the structure grows, especially in coastal areas, saline areas, and areas with extensive utilization of de-icing salts, corrosion of reinforcements in peers can occur due to the primary cell reaction caused by chlorine salts. The spalling of the protective layer of concrete easily happens owing to the resulting rust swelling which will further accelerate the rate of corrosion of internal reinforcement. As a result, a significant deterioration in seismic performance of bridge pairs can be induced. Hence, it is of great theoretical importance to study the seismic performance of bridge structures crossing rivers in earthquake zones under the action of scour and Cl\({}^{-}\) corrosion. [PERSON] [13] provided a systematic discussion on the method for solving failure probability of bridges with high pile bearing system under the combined effect of two extreme loads by means of the theory of multi-hazards and full probability. [PERSON] et al. [14] studied the effect of scour on seismic failure probability of bridges and analyzed the seismic losses of bridges at a defined scour depth from an economic point of view. However, the probability of failure under combined scour and seismic action was not considered. [PERSON] et al. [15] investigated the seismic fragility curves of bridge piers under different scour conditions, while no consideration was given to the seismic fragility of pile foundations under scour action. [PERSON] et al. [16] analyzed the failure probability under combined action of scour and earthquake by taking bridge columns and pile foundations as the object of study.