e-mosty March 2019 Long Span and Multiple Span Bridges - Page 24

Chile is one of the most seismically active areas in the world with a history of major earthquakes, and the project site is near a boundary between the Nazca and South American plates. The May 2, 1960 Valdivia Earthquake with a magnitude of 9.5, which was described as one of the largest earthquakes ever recorded worldwide, resulted in major damage to Valdivia, approximately 200 kilometers from the project location, as well as nearby Puerto Montt. Aside from damage to structures, the 1960 event resulted in landslides. In recognition of this and other major Chilean earthquakes, MOP required that the sole bridge connecting Chiloe with the remainder of the country be serviceable for emergency vehicle access following large major seismic events or tsunamis throughout its 100-year design service life. In response, CPC developed a probabilistic seismic hazard analysis considering contributions from multiple subduction zone sources and shallow crustal sources. This analysis included two major recorded historical megathrust earthquakes: the 1960 Valdivia earthquake with a magnitude of 9.5 and the 2010 Maule earthquake with a magnitude of 8.8. Bedrock response spectra were developed for the Maximum Probable Earthquake (MPE) with a 1000-year return period had a 10% chance of occurrence in 100 years (1000-year return period), and Construction Event Earthquake (CEE) with a 10% chance of occurrence in the expected construction duration of 4 years. In addition, a deterministic seismic hazard analysis was performed for a potential local fault passing under the north main span of the bridge (the Gulf of Ancud fault). The bridge was designed to accommodate a transverse permanent offset between the central and north pylon due to rupture of the potential Gulf of Ancud fault and a vertical settlement of the Roca Remolinos due to a major subduction earthquake. Along with the strong ground motions, the 1960 Valdivia event produced tsunami waves with heights of up to 20 meters that resulted in major damage to coastal areas as far away as Hawaii and Japan. The subsequent Maule Earthquake of 2010 with an 8.8 magnitude resulted in tsunami waves with heights of 15 meters. The Puente Chacao's location at the narrowest section of the Chacao Channel between the Pacific Ocean and the Gulf of Ancud is at risk from tsunami waves. As a result, the project was designed to resist a design tsunami equivalent to the tsunami resulting from the 1960 Valdivia earthquake. A related issue is volcanic effects. Many volcanoes are located near the project site and some are active: Volcan Calbucco, approximately 100 km from the project site, had a significant eruption in 2015. CPC's design studies concluded that volcanic eruptions were not a significant risk at the project location Other project location related design requirements were for tides, currents, and wind. The open site is subject to winds of over 200 kilometers per hour. Currents through the channel reach nearly 10 knots, with 8-meter waves, sufficient to interrupt ferry service. These conditions create major challenges for marine construction in the Chacao Channel, as well as demands on the long span bridge elements. CPC established data collection programs and studies as part of the project Basic Engineering program which informed the development of Final Design criteria. A CPC wind tunnel test program included sectional models and full bridge aeroelastic model tests to confirm design wind demands and evaluate stability of the structure. Project requirements also included establishment of a site seismic monitoring program to provide an on-going record of ground motions. 1/2019