Abstract-One of the critical issues for a long suspension bridge is the vibration induced by wind. The excessive vibration on long suspension bridge owing to bridge flexibility may cause the bridge's aerodynamic instability and vehicle accidents. Therefore, in the present study, the optimum ratio for the span and turning angle to bridge width was investigated. The analysis was carried out on bridge with total span of 470m, main span 270m and side span 100m, using variations of bridges width ranging from 9m to 22m. SAP2000 program was used to analyze the stability behavior of the suspension bridge under wind load at speed of 35m/s. The results of analysis shows that the optimum ratio of span to width bridge is 0.034L or L= 29.375b. When the turning angle was taken into consideration in the analysis, it will result in increasing the bridge's stability, decreasing natural frequency of the structure and increasing stresses on the cables. Further, it was also found that the deflection and the internal forces on the girder meet the specified limits by AASHTO and AISC standard specification.

Abstract.An aeroelastic interaction always take places in long-span bridge, i.e. the dynamic relationship between wind forces (aerodynamic) and bridge structural motion (dynamics). Wind forces may lead to serviceability and safety problems, and may even cause instability of the whole bridge structure due to the flexibility nature of long span bridge structures.The slimer girder plate will make the structure more unstable as can be seen from the occurrence of lateral deformation. This paper presents the results analysis of cable stayed bridge that has total span of 640 m, two planes configuration of harp-typed hanger cable, the A shape pylons span along 320 m (main span),and side span of 160m.Structural analysis was conducted to determine the optimum ratio between width and spans of cable stayed bridge. The bridge was modeled with various width of 7 to 22 m. Structural modelling was conducted using SAP2000 software to analyze the structural stability of cable stayed bridge under wind load at speed of 35 m/s.The influence of wind loads to the cable stayed bridge stability can be seen based on the bridge deck deformation at the Y-axis (U 2 ), in which for the width of the bridge deck of 7 m, 8 m, 9 m and 10 m, the deformation of U2 are 0.26 m; 0.17 m; 0.12 m and 0.10 m, respectively. Meanwhile, for bridge's width of 11-22 m, the value of U2 axis deformation is relatively constant between 0.08 m to 0.07 m. This finding suggests that the ratio between width and length of bridge greatly affect the stability of the cable stayed bridge. Cable-stayed bridge, especially for concrete bridge, with two plain system having optimum ratio of width and length show no sign of an aerodynamic instability when fulfills the requirements of B ≥ L / 3.