| Because the main cable is tensioned, the material is utilized efficiently, China's construction of long -span suspension bridge in the past 10 years developed rapidly. Suspension bridge is a composite flexible system made up of main cable, stiffening girder, tower, saddle, anchor bank, and hanger. It is very important to determine its geometric form in the bridge's completion state accurately, because main cable is the main load-bearing component. Main cable's true geometric form in the bridge's completion state is neither ideal parabola, nor ideal catenary, but it is identified by the equilibrium condition and the geometrical condition of structure in the actual load together. So far, the main methods of determination of main cable's profile of suspension bridge are parabola theory, segmental catenary theory and the both-based nonlinear cycle alternate method, virtual beam method, anti-algorithm, parameter equation method and so on.This topic uses analytical iterative method for construction calculation, mainly construction calculation of main cable. It calculates the main cable, suspender and other components in unstressed length, and initial installation location of the main cable and cable clamps. Its calculation principle is that, firstly, considers the actual situation of the additional dead load shared by main cable and stiff girder, and uses suspended-cable mechanics theory, that is the shape of main cable under the weight themselves is catenary . When it is under the action of the concentrated loads, cable between the concentrated loads is catenary . In the state of designable parameters, that formula based on the force equilibrium condition and the geometrical compatibility condition of cycle iterative method is established, obtains the unstressed length of main cable, and amends the length of main cable at the location of saddle. Secondly, the geometric form of free-cable and pre-bias of cable saddle are calculated according to constant unstressed length of cable. Finally, the location on installation of cable clamps and unstressed length are calculated according to geometric form of free-cable. This topic adopts two calculation method of EXCEL and MATLAB programming to calculate unstressed length of main cable of YiChang Yangtze River Highway Bridge respectively. That the absolute value in the difference between the two cycle of horizontal force is controlled in EXCEL method, is less than convergent error. Namely, it is considered to content accuracy requirements. The MATLAB programming method also controls these two parameters' accuracy. When the horizontal component of cable force and vertical supports force which content accuracy requirements are obtained, that is to calculate unstressed length of each cable and elevation of each location on installation of cable clamps by integral method. Then the length of main cable at the location of main saddle is amended. Eventually, actual unstressed length of main cable is calculated.Through example computation shows that the results of the two calculation methods of EXCEL and MATLAB programming are very close, and are close to the result of the literature. Then the relative error is very small, so the two method has very high accuracy.
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