e-mosty March 2019 Long Span and Multiple Span Bridges - Page 50

tower can cause problems with slip at the cable saddles at the tower top. The Oujiang River North Estuary Bridge, China, with twin 800m main spans is currently being built with truss girders. The research indicates that an intermediate stiffness of tower is beneficial when used in combination with other methods of stiffening. Traditionally the deck girder articulation is a series of simple spans with expansion joints at each tower. For the 3 tower Taizhou Bridge 3 only the central tower was stiffened, it was not a full A-shape but a λ-shape with the legs spread only from below deck level. The continuous girder has more stiffness and reduces rotations. The 3-tower Maanshan Bridge has a continuous deck that is also integral with the central tower. This partial tower stiffening was used in conjunction with cable clamps and a continuous deck fixed at the central tower. The deck and cables of a multi-span bridge cannot in practice be infinite. The design development of the Chacao twin-span suspension Bridge (see this issue) has taken a similar form from early A frame to the current central tower form. The effects of temperature need to be considered. For the cables, temperature variations are accommodated by a small increase or reduction in cable sag. Truss girders have traditionally been used on suspension bridges and are often used today, they usually give a stiffer deck than a box girder. For the bridge deck temperature will cause a change in length, the greater the length of the deck the greater the temperature movement and the larger the expansion joint. With the increased cable flexibility of a multi span bridge the additional stiffness of the deck girder is relatively more important (Figure 8k). A length of about 3km is the current practical limit for continuity. The large expansion joints in current long span and multi span bridges are the main limiting factor when determining the length of a bridge. Dynamic performance of the bridge is an important consideration in multi-span suspension bridges and is related to stiffness. Since the dynamic failure of the Tacoma Narrows Bridge, due to wind induced torsional flutter instability, the knowledge of bridge dynamics and wind instability has increased significantly. The dynamics of the 2-span Taizhou Bridge and the dynamic parameters of a 4-span suspension bridge with 2000m spans have been studied and are shown in figure 7 comparing them with more conventional suspension bridges. Figure 11: Suspension bridge vibration frequencies with multi-span bridges highlighted Multi-span bridges are slightly more flexible dynamically than classic single span bridges, but the dynamic performance can be improved using similar stiffening techniques as used for static analysis i.e. increased tower stiffness, the use of cable clamps, etc. 1/2019