## 2 Analysis of Traffic Load Effect of a Super-Long-Span Suspension Bridge  
In this study, a super-long-span suspension bridge with a main span of 2300 m is selected as the main background project, and the relevant problems regarding traffic load are studied. The Zhang-Jing-Gao Yangtze River Bridge is located in the lower reaches of the Yangtze River in China, connecting Suzhou, Taizhou, and Nantong. The South Passage Bridge (hereafter referred to as the \"ZG Bridge\") is a two-span continuous suspension bridge with a main span of 2300 m, as shown in Figure 1. The cable layout is 660 m + 2300 m + 1220 m, and the beam layout is 2300 m + 717 m. The bridge adopts an integral steel box gird, and the bridge deck is equipped with eight two-way lanes. The Chinese specification (General Specifications for Design of Highway Bridges and Culverts, JTG D60) is chosen to decide the loading criteria on the background project, considering the location of the bridge and the optimization potential of the traffic load model. The mechanism of the suspension bridge has obvious geometric nonlinear characteristics, however, for traffic load, the difference between the effects considering geometric nonlinearity and not considering is acceptable, and the effect without considering geometric nonlinearity is higher, which is conservative [27; 28]. Therefore, the traffic load effect of the suspension bridge components can be obtained according to the principle of linear superposition. To be specific, the calculation of the traffic load effect of the bridge  
Figure 1: The layout of the Zhang-Jing-Gao Yangtze River Bridge. (**a**) The front elevation of the bridge (unit m); (**b**) The layout of the standard main beam section (unit: mm). components is mainly based on the loading of the effect influence line, and there are two commonly used loading methods, which are as follows:  
(1) Loading the specification load model on influence lines. The uniformly distributed load of the lane load is fully arranged on the influence line range with the same sign that causes the most adverse effect on the structure, and the concentrated force is imposed on the peak value of the influence line. When there are vehicles in multiple lanes, the multilane reduction factor should be considered, and the longitudinal reduction in the traffic load on the long-span bridge should also be considered. (2) Loading the random traffic flow on influence lines. The effect under this type of loading is calculated at every fixed time interval. In each calculation, the coordinates of the points on the influence line with an axle load are multiplied by the value of the corresponding axle load. The quantity of the traffic load effect at a certain time is the sum of all axle loads multiplying corresponding coordinates of the influence line. When random traffic flow loading is used to obtain the load effect, considering that the duration of the traffic flow simulation is obviously shorter than the service time of the bridge, the calculated effect value must be extrapolated in order to obtain the extreme value of the load effect corresponding to the service time of the structure. The effects of the main components obtained by the above two loading methods are defined as the specification value (\(E_{\text{CODE}}\)) and the extreme load effect (\(E_{\text{EV}}\)), respectively. In existing studies, the ratio of the two (\(E_{\text{EV}}/E_{\text{CODE}}\)) is commonly used as the evaluation index of the actual effect level of the component or the applicability of the specification load model [29; 30]. Specification load model loading and random traffic flow loading are carried out on the ZG Bridge.