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