e-mosty March 2019 Long Span and Multiple Span Bridges | Page 17

IV. DESIGN
The bridge is a multi-span suspension bridge. It will
be 2,750 long, with two main spans of 1,055m
(south span) and 1,150m (north span), one
suspended side span of 324m and one viaduct of
220m.
wire strand) method. Preformed wire strands will
be delivered to site in spools, unwound, and
installed on site.
The Chacao Bridge will have a vertical clearance of
50 meters, plus a 2.5-meter 'safety space.' The
navigation channel for the 2.8-kilometer bridge will
be 600 meters wide.
The bridge will have the capacity to accommodate
four traffic lanes. The rock formation in the middle
of the 125m-deep channel provides a base for the
mid-channel.
When work began in 2014, the project team
confronted several extreme challenges in
designing the multi-span suspension bridge across
Chacao Channel. The project site is in a remote
area of the Los Lagos region, 1,100 kilometers
south of Santiago.
The new bridge is designed to have a lifespan of
more than 100 years.
The bridge will feature three concrete towers, with
an inverted Y-shaped central tower measuring
175m-high.
The channel separates the 200-kilometer-long
island from the mainland, but both land masses
are part of the coastal range noted for high
seismicity.
The remaining two towers will be 157m and 199m-
high respectively. The central tower is designed as
an inverted Y to increase the stiffness of the
system and provide a fixed point to the main cable.
The bridge is just 80 kilometers from the fault zone
where a disastrous 9.5 magnitude earthquake
struck Valdivia, Chile, in 1960. In 2010, an
earthquake of 8.8 magnitude struck offshore of
Concepción, about 650 kilometers north.
Seismicity was the most challenging design criteria
for the team.
Because the bridge is asymmetrical, with two
spans of different lengths, the three pylons carry
the burden of balancing the uneven loads.
The central, 175-meter tower became the focus of
intense mitigation efforts when subsidence of the
mid-channel rock formation created a construction
challenge. The problem was solved performing 3D
computational analysis aided in evaluating
alternatives and improving the design of the
central tower.
Additionally, the channel is prone to critical winds
of up to 208 kilometers per hour, and ocean
currents reach 9.7 knots or 18 kilometers per hour,
with 8 meter waves.
The deep channel plunges 120 meters to the sea
floor.
The deck is an orthotropic steel box girder with a
total steel weight of 20,700 tonnes. The bridge
girder is continuous from the South pylon to the
North abutment.
Mid-channel, rock protrudes enough to provide a
base for the central support tower, but it
presented problems with subsidence.
The main cables will be 561mm in diameter with
total weight of 8,900 tonnes, and are expected to
be erected by the PPWS (pre-fabricated parallel
Figures 8 – 10: Renderings of the Bridge. Courtesy of MOP Chile
1/2019