South America’s longest suspension bridge will be earthquake resistant
In one of the most challenging engineering projects ever undertaken, Chile is building a unique steel-built suspension bridge near the site of the strongest earthquake ever recorded.
Planned to open in 2023, the Chacao suspension bridge is sited just 80km from a seismic fault zone where a record-shattering 9.5 magnitude earthquake struck in 1960 and an offshore earthquake measuring 8.8 hit the area in 2010.
This isn’t the only challenge that faces the designers, as the bridge will link the island of Chiloe to the mainland across a sea channel that comes with powerful currents and winds that can reach speeds of more than 200kph.
Set to cost more than $700m, once completed the bridge will replace the ferry service that runs to Chiloe, bringing travel times down from 30-45 minutes to just 3 minutes. This will impact massively on the island’s economy and, it is hoped, will boost tourism in the area.
Rising to the challenge
Spanning 2750m, the Chacao will be South America’s longest suspension bridge. Tackling a project of this scale required a robust design, engineering and construction team, and a consortium made up of OAS, Hyundai, Systra and Aas Jakobsen won the work from the Chilean government, with Hyundai leading on construction and bringing in Arup as a consultant.
Arup formed a multidisciplinary consultation team to deal with the challenging site conditions, with geotechnics, maritime impacts, wind and seismic engineering, and anchorage and foundation design all key parts of the planning process.
Galvanised steel wire also offered the best breaking-strength-to-weight ratio for the suspension bridge cables, allowing the designers to fully optimise the bridge’s support structures while meeting the unique site requirements
The 2.7km span of the bridge will be supported by three steel-reinforced concrete towers, with two main-spans measuring 1,055m and 1,100m, and a suspended side-span of 380m. Supporting a highway with two lanes running in each direction, the 175m tall central tower will sit on Roca Remolinos, a small reef in the middle of the channel where a rocky outcrop breaks the surface.
Due to the high seismic activity in the area, it was vital that the structure of the suspension bridge have a correspondingly high ductility to deal with potential tremors. To ensure the seismic performance of the concrete towers, steel reinforcement bars were required in the pile structure, in addition to an external 70-mm thick steel casing at their top. This steel core also gave the foundations the flexibility to deal with the surging tides that hammer the bridge’s coastal location.
Surface level strength
It’s not just in the bridge’s foundations that steel is adding unparalleled ductility and strength. Throughout the bridge’s superstructure steel is crucial to the performance of the Chacao bridge against a set of challenging environmental criteria.
Designed for a 100-year lifespan, the bridge’s 24m-wide deck is fabricated from structural steel plate that plays a part in allowing it to resist wind speeds of more than 240kph. At the heart of this is its orthotropic box girder design, fashioned from 20,700 tonnes of high-strength steel.
Orthotropic bridges have their decks stiffened longitudinally with lattice girders and transversely with floor beams. This reinforcing allows the bridge deck to carry vehicular loads while also contributing to the overall load-bearing structure.
Modern high-strength steels are allowing for lighter structures that do not compromise on durability and strength. This material performance was vital for Chacao as the bridge deck and main cable weight, as well as the number of pylons, had to be kept as low as possible to deal with the location’s seismic activity.
Galvanised steel wire also offered the best breaking-strength-to-weight ratio for the suspension bridge cables, allowing the designers to fully optimise the bridge’s support structures while meeting the unique site requirements.
To achieve a project like this that pushes boundaries and has the power to connect communities in hard-to-reach locations, the designers are squeezing every last drop of performance from their materials. It is clear that when it comes to realising the impossible, engineers will continue to reach for steel.