e-mosty March 2019 Long Span and Multiple Span Bridges - Page 12

Figure 12: Deck section The twin section steel box section deck reflects the technology recently used on other long span bridges. Each box is shaped to minimise the effects of wind forces and to maintain aerodynamic stability. A number of different profiles were tested to optimise the behaviour with variations to the geometry of the inner web, gap width and the use of different heights of wind screens. The design considers traffic loading as follows:   Loaded lengths < 200 m UDL = 81.8 kN/m (6 lanes) based on Eurocode 1991-2 load model 1, 2 and 3 Loaded lengths > 200 m UDL = 58.8 kN/m (6 lanes) EN 1991-2 SE-NA taking effect of long loaded length into account Flutter stability is dependent on mean twist angle of bridge girder due to mean wind loading and to ensure stability the models must prove that the deck remains stable with a wind speed of about 70m/s. The approach span sections at each end are of prestressed concrete box section construction. SHIP IMPACT The navigation clearance envelope for the bridge is 1600m wide by 70m high, centred on the main span. However the design needs to consider potential impact by shipping, and the lower sections of the towers, up to 29.5m above sea- level, are exposed to ship impact. AERODYNAMIC TESTING Aerodynamic modelling and testing of long-span bridges is essential to understand the response of the structure to the dynamic effects of wind and to optimise the design to achieve stability. Analysing of local wind data gave a basic wind speed of Vb=29 m/s, giving V=46 m/s at deck level (+86.0). The design requirement is for a 180,000 DWT ship, 370m long and 52m wide, impacting at an angle of up to 30 degrees and imparting a global impact force of 370 MN. These loads are then transmitted through the composite shafts and caissons. Wind tunnel testing was carried out in three locations, looking at specific characteristics:    Deck section model at 1:60 scale in Canada Tower section model (1:80 scale), full tower model (1:225) and tower erection stages (1:225) in Denmark Full bridge model (1:190) and deck erection stages (1:190) in China. Semi-local and local impact governs the steel tower leg design up to +29.5m. The box sections are stiffened with horizontal diaphragms and skin plate thickness has been increased to deal with these actions. Horizontal stiffeners are also added to increase local bending resistance of the skin plates. Aerodynamic stability was verified (with additionally damping of the towers) through wind tunnel tests of the full aeroelastic model of the bridge. Strict design criteria have been introduced to achieve minimal damage of non-accessible parts of foundations under accidental load. 1/2019