# Review on the Service Safety Assessment of Main Cable of Long Span Multi-Tower Suspension Bridge
## 1 Introduction  
In China, bridge construction has entered the river and sea crossing era [1, 2]. The multi-tower multi-span suspension bridge (Figure 1) realizes the super long span capacity by adding middle cable tower based on the traditional two tower suspension bridge, and has advantages in terms of reductions of main cable tension, anchorage scale, main span length, and total cost as compared to other typologies of bridges [3, 4]. With the continuous improvement of the expressway network and high-speed railway network constructions, and according to resource-saving, environment-friendly, and sustainable development concepts, the suspension bridge shared by highways and railways has become a resource-saving bridge type for river and sea crossing channels, such as Qingma suspension bridge in Hong Kong and Wufengshan Yangtze River Bridge in Jiangsu under construction. Therefore, it is of great importance to explore the construction technology of long-span multi-tower suspension bridges across the river and sea, shared by highways and railways. The long-span multi-tower suspension bridge consists of a main cable, cable tower, main cable saddle, splay saddle, hanger, stiffening beam, and anchorage (Figure 1). The main cable is a flexible load-bearing component suspended on the cable tower through the main cable saddle composed of saddle groove and diaphragm, and anchored in the anchorages at both ends through the splay saddle [5]. The main cable, with the characteristics of high cost and difficult replacement, is the \"lifeline\" of suspension bridge. Once it fails, it will seriously endanger the structural safety of suspension bridge, even cause great economic losses and endanger safety of people's lives. The main cables of George Washington Bridge, built in 1931 with a main span of 1067 m, exhibit severe corrosion and a large number of fractured wires [6], which causes a maintenance cost of approximately 500 million US dollars. The main cable wires of Forsyth bridge present severe wear andcorrosion [6], which induces a maintenance cost of approximately 200 million pounds. The main cable and saddle of the middle tower of a multi-tower suspension bridge are prone to skidding accidents, which causes the structural instability and even collapse of suspension bridge [7]. Therefore, the service safety of main cable of long-span multi-tower suspension bridge is directly related to the structural safety of suspension bridge. The main cable of long-span multi-tower suspension bridge across the river and sea is subjected to the wind loads with distinct directions attributed to the effect of strong wind [8]. The roles of changing environment temperature and sunlight radiation cause the obvious temperature gradient of internal and surface temperature fields of main cable (attributed to the hysteresis of internal temperature change of main cable), and thereby induce changes of internal forces and line shape of main cable [9]. In addition, the main cable of a suspension bridge shared by highway and railway is subjected to the dead load (bridge deck system, stiffening beam, main cable, hanger) and live load (automobile, railway train) through hanger and cable clamp. The railway train has the characteristics of high speed and heavy load. The long-span multi-tower suspension bridge across the river and sea exhibits the large flexibility [4]. Ref. [4] indicates that the up and down deflection deformations of main beam of a three-tower suspension bridge are 7.65 and 2.44 times of those of a two-tower suspension bridge, respectively, under the action of the vehicle attributed to the weak restraint of main cable at the top of middle tower of three tower suspension bridge. The suspension bridge with very low natural frequency is very sensitive to wind loads, which easily induces the coupled vibration of wind-vehicle-bridge system; the lateral displacement, lateral acceleration and vertical acceleration of the suspension bridge all increase greatly with increasing wind speed [10].