# Review on the Service Safety Assessment of Main Cable of Long Span Multi-Tower Suspension Bridge
## 1 Introduction
Therefore, coupled roles of wind loads with different directions, earthquakes [11; 12; 13], temperature, as well as dead and live loads cause coupled vibration of wind-vehicle-bridge system under the temperature effect [10], and thereby induce time-varying loads and vibration states of main cable along axial, vertical and transverse directions, which causes the elastic deformation of main cable, then changes the line shape and finally affects the stability of suspension bridge system [14]. In order to realize the shape control of main cable during the construction and service of long-span suspension bridge, [PERSON] [14] proposed several analytical methods for the calculation of spatial cable curve of a suspension bridge considering the geometric nonlinearity. The atmospheric environment of river and sea presents characteristics of high humidity, more salt fog (NaCl, MgCl\({}_{2}\)), and easy formation of acid rain [15]. Therefore, electrolyte  
Figure 1: Schematic diagram of long-span multi-tower suspension bridge. corrosion solutions with different salt concentrations and PH values are easily formed. However, the electrolyte corrosion solution can invade and stay in the main cable during the erection period of main cable (usually more than one year). Meanwhile, the electrolyte corrosion solution easily invades and gathers at clearances among steel wires in the main cable [16]. Therefore, the main cable of a long-span multi-tower suspension bridge across the river and sea is always subjected to electrolyte corrosion. It was reported that electrolyte corrosion is one of main failure modes of existing bridges [17; 18]. As the main cable of long-span multi-tower suspension bridge is bent around the main cable saddle, average axial and bending stresses of the main cable at the saddle and near both sides exhibit the largest values, which indicates easier failure locations of main cable [19; 20]. The main cable is composed of parallel strands, and the strand is composed of parallel high-strength galvanized steel wires. As the main cable is bent around the saddle groove of main cable saddle, time-varying dynamic loads of main cable at both sides of the saddle easily cause dynamic contacts and \"layered sliding\" between strands and between steel wires in the main cable as well as the fatigue stresses of cable strands and wires. Meanwhile, dynamic contact and slip characteristics between the main cable strand and saddle groove/diaphrag easily occur. Therefore, those will cause the friction and wear between steel wires in the main cable, friction and wear between external steel wires of main cable and saddle groove/diaphrag, and fatigue stresses of cable strands and wires. As the difference of time-varying dynamic loads of the main cable at both sides of the saddle is enough to overcome the total friction force between main cable and saddle during dynamic slipping, the gross slip phenomenon occurs between main cable and saddle [7]. Skidding accidents between main cable and saddle easily occur at the middle cable tower, attributed to weak constraints of main cables on both sides [21], which leads to the structural instability and even collapse of the suspension bridge. Therefore, coupled effects of temperature, electrolyte corrosion solution, internal and external friction and wear of main cable, and fatigue stress cause the tribo-corrosion-fatigue damage of main cable under the temperature effect. The tribo-corrosion-fatigue induces the friction and wear, electrochemical corrosion, fatigue crack propagation and fracture of steel wires in the main cable, which reduces the effective cross-sectional area and thereby affects the load-bearing safety of main cable. In addition, the corrosion and scratch of steel wire during transportation and erection lead to the crack initiation of steel wire.