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:
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
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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