Los Angeles’ Sixth Street viaduct is one of the most well-known bridges in the United States.
Made famous by appearances in countless movies, TV shows, commercials and music videos
– the 1,066-metre-long structure was an engineering marvel when first constructed in 1932.
Making use of an on-site plant to produce concrete during construction, it was later
discovered that the concrete developed had a high alkali content.
This lead to alkali-silica reactions taking place
and cracks forming throughout the structure.
Though the bridge underwent numerous costly restoration attempts over the years, no permanent
solution to its problems was ever found.
In 2004 a study of the structure revealed that it was seismically unstable and had a
70% chance of collapse during an earthquake in the next 50 years.
With the bridge spanning the Los Angeles River, the 101 freeway, the I-5, and numerous railyards
and local streets, Los Angeles’ authorities had no choice but to mark the historic structure
for demolition and replace it with a bridge that could stand for 100 years in the face
of LAs strongest earthquakes.
After an international design competition in 2012, Michael Maltzan Architecture’s
design dubbed the “Ribbon of Light” was chosen to replace the Sixth Street viaduct structure.
The new bridge features 10 pairs of illuminated concrete arches that pay homage to the earlier
structure, while enabling longer spans and fewer ground supports.
This reduces the number of points that can be damaged in the event of an earthquake and
opens-up development opportunities beneath the bridge.
Work to demolish the existing viaduct began in February 2016 and an estimated 37,000 cubic
metres of concrete and 4,200 tonnes of steel were removed over the subsequent nine months.
With the original structure removed, work began on the new bridge and, in December 2017,
erection of the falsework that will support the new structure during its construction commenced.
To support the arches and create the bridge’s unique “Y Bents”, prefabricated falsework
is being trucked-in from as far away as Indiana and Toronto.
As concrete gives off heat as it cures, liquid nitrogen is poured into the concrete mixers
during the summer months to control the temperature of the mix and prevent it from curing too
quickly in the Los Angeles heat
which could lead to cracks forming on the finished structure.
With the arch structures complete, the new road deck will be suspended by cables from
these concrete structures.
With Los Angeles located in an area of high seismic activity, the city has developed some
of the world’s most stringent building codes.
To ensure that the new bridge remains standing in the event of a severe earthquake, engineers
have worked to exceed the latest seismic safety codes.
Along with its deep foundations and reinforced concrete arch design, the columns of the bridge
have been fitted with seismic isolation bearings at their midpoints.
The design of these bearings consists of three main parts – with each element able to separate
and break free from the other once certain thresholds are reached.
These bearings allow each of the columns of the bridge to move up to 76cm (30 inches)
during an earthquake event, with the bearings – rather than the bridge – absorbing the
majority of the quake’s energy.
When the shaking stops, the internal concave shape of the bearings allow them to re-centre
themselves and return to the shallowest point within the bearing.
By fitting the bridge with 26 of these isolation bearings – most of which have a diameter
of over 2 metres (around 6.5 feet) – the bridge should be able to provide a high level
of protection during a seismic event.
When complete in 2020, the new Sixth Street viaduct will serve as a vital link in LA’s
vast road network while opening-up a large area beneath its structure to the wider community
– where 12 acres of new parkland, art installations, and other community facilities are planned
to reinvigorate the space.
At a cost of USD $428M – the largest bridge project in the city's history, and with
its impressive architectural design that nods to the past whilst insulating the structure
against seismic threats in the future, the new viaduct is firmly on its way to becoming
an icon of Los Angeles.
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