Cable Shape and Construction Parameters of Triple-Tower Double-Cable Suspension Bridge with Two Asymmetrical Main Spans

Triple-tower suspension bridges with two asymmetrical main spans can adapt to complicated terrain conditions. If the top and bottom main cables with different sags are used on the two main spans, the longitudinal constraint on the central tower can be significantly improved, which is more conducive to the promotion and application of three-tower suspension bridges. Based on the widely used multisegment catenary theory, this paper proposes an analytical calculation method to determine the main cable shape in three structural states: completed bridge state, under live load action, and free cable state. In the completed bridge state, each span is solved independently, with the longer main span being calculated first. The two main spans and two side spans require an altitude difference closure condition, while the two anchor spans require an altitude difference closure condition and a splay-saddle moment balance condition. Under live load, due to the media effect of tower displacement and splay-saddle rotation, all spans need to be solved simultaneously. In addition to the conditions of altitude difference closure and splay-saddle moment balance, conditions such as span length closure and main cable unstressed length conservation are required. In the free cable state, due to the media effect of the cable saddle preoffset, all spans need to be solved simultaneously. The solution is similar to that under live load, and thus, it only requires minor modifications of the equations corresponding to live load. During the calculation of these main cable shapes, the deformations and internal forces induced by the live load, together with key construction parameters including the unstressed length of the main cable, the saddle preoffset, the elevations of free cable, and the installation positions of cable clamps can also be calculated. The method proposed in this paper is applied to a double-cable suspension bridge spanned as 248 m + 860 m + 1,070 m + 310 m. The cable shape of the completed bridge is determined first. Subsequently, the deformations and internal forces under live load are calculated for two cases: live load covered in the longer main span and live load covered in the shorter main span. Finally, the cable shapes and construction parameters of the two free cable states, that is, the bottom-cable-only state and the double-cable state, are calculated. This example is employed to verify the effectiveness of the approach presented in this paper. Some useful conclusions are also obtained, which lays the foundation for the promotion of this bridge type.