Angular change and secondary stress in main cables of suspension bridges

Three basic cable equations are proposed for evaluating the deflected shape of the main cables of suspension bridges subject to angular changes near the saddles of the main towers as well as the cable bands. The secondary stress on the tensioned parallel wires can be estimated via the combined use of the three basic cable equations that take into account the free and/or friction slip between wires as a function of the lateral restraints by cable bands and wrapping wires. A series of laboratory loading tests were carried out for a parallel wire strand to validate the proposed procedure. The proposed method was also applied to the Yi Sun-sin Bridge to demonstrate the level of secondary stress in parallel wires and to identify the factors that have an impact on the secondary stress in the completed stage as well as during the construction phase. The residual tension of the wrapping wire, which impacts the frictional resistance between wires of the main cable, was measured from field tests. The evaluated secondary stress was found to exceed 10% of the tensile strength of the wires. The findings reported herein also confirm that the distance between the tower saddle and the first cable band is a critical factor for secondary stress. Based on the experimental validation and application to an actual bridge, the proposed approach, despite being based on simplified formulations, is demonstrated to be useful in predicting the secondary stress that develops in parallel wire cables.