e-mosty June 2017: Osman Gazi B. US Suspension. Hålogaland B. e-mosty June 2017: Suspension Bridges | Page 15

2.9 Aerodynamic Stability
Aerodynamic stability was extensively checked,
including numerical analysis and wind tunnel testing.
The bridge has a ratio of 1:9 span length to tower
height above the deck. Its ratio of span length to the
distance between the cables is 90 while typical values
for suspension bridges are in the range 55-60.
This combination of a slender bridge with a long main
span posed considerable design challenges in order to
fulfil the requirements of ensuring aerodynamic
stability at a wind speed of 63.1m/s at the bridge deck
level. The deck and the inclination of the bottom plate
were optimised to meet such requirements - the
bridge box section is arranged with a slope of 15.8° of
the lower inclined side plates relative to the horizontal
bottom plate.
The numerical analyses and wind tunnel test showed
a critical wind speed of 61 m/s. Wind tunnel tests
carried out in smooth flow proved that there will be
no vortex-induced vibrations, which saves potential
costs for installing and maintaining any mitigation
measures.
3. CONSTRUCTION
3.1 Tower Foundations
The towers are founded on rock at -31 and -22 metres
below sea level.
Four caissons, with a 10m diameter, were
manufactured. These caissons were filled with stone
material as ballast. The caissons were produced by
slip form casting. It was the first time that slip forming
of caissons was permitted by the Norwegian Public
Roads Administration. Slip forming is a construction
method whereby concrete is cast in layers in a slip
form that is raised at regular intervals. For production
of the caissons a total of 3m of the foundation was
cast and then lowered into the water every day.
In the tidal zone two caissons were linked by a pile
cap up to 5m thick, a little more than 42m long and
almost 16m wide.
The caissons were placed at a depth of about 30m
onto blasted bedrock on the seabed which was
levelled by concrete.
3.2 The Towers
The A-shaped bridge towers are built in concrete.
Climbing formwork was used from the caissons up to
the height of the viaducts. Above this height the slip
forming method was used.
After the pile cap was cast and cured, a climbing
formwork was rigged and the bottom part of the
tower columns was cast, in five stages. Just below the
roadway the tower columns were connected by a
crossbeam.
When the crossbeam was finished, further casting
was done by slip form casting up to the connection at
the top of the tower. The tower top was cast using
conventional formwork. Inside the bridge towers,
there is a lift in the one tower column and stairs in the
other.
Figures 16 + 17: Caisson foundations
2/2017