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