PROJECT STRENGTH IN DESIGN A major engineering design challenge has been overcome in Chile as a three-kilometre suspension bridge begins construction in an area famous for producing the world’s most powerful earthquake. By Chris Sheedy. n 22 May in 1960 the people causing flooding that affected over 100,000 people. Building in a shaky zone from major earthquake events are regularly of southern Chile suffered the In the Valdivia around 40 per cent of houses were John Steele, Executive Bridge Engineer at Jacobs, made available to relevant authorities and most powerful earthquake ever destroyed and about 20,000 people left homeless. has specialised in the design and construction of their bridge designers. This data is used recorded. At 3:11 pm local time, and In the Valdivia region is the Chacao Channel, a bridges for over 20 years. While he’s not involved in in the preparation of earthquake load codes continuing for 10 minutes after waterway connecting the Pacific Ocean with the the Chacao Bridge build, he has worked on bridge and project-specific design criteria to define Othat, the earthquake was measured at 9.4 to 9.6 Gulf of Ancud and which separates Chiloe Island builds in New Zealand, another nation famous for the earthquake ground accelerations and on the moment magnitude scale. Since that date, from the Chilean mainland. Chiloe Island has an its seismic activity. frequencies that the designer should adopt for several famously destructive tremors including area of over 8000 km2 and a population of more “Building a bridge in a highly earthquake-prone their bridge design. Christchurch (maximum 7.8 magnitude), Nepal (also than 150,000 people, all of whom currently rely on a region is not impossible provided the designer Modern bridge analysis software can establish 7.8 magnitude) and many others destroyed buildings car-ferry system to get to the mainland. understands the dynamic characteristics of the the modes of vibration of the bridge structures and and took lives, but Chile’s Valdivia Earthquake still But by 2021, in the area that is famous for earthquakes the bridge could experience during its how these modes of vibration will interact with the holds the record. producing the world’s most powerful earthquake, design life, how the bridge will behave during these vibration of the ground during the earthquake to The quake caused a tsunami that swamped John Steele, a road bridge will be built to connect the two land earthquakes, and most importantly, that the bridge induce force in the bridge structure. With this input, shorelines as far away as the Philippines, New Executive masses. Once completed, it will be the largest can absorb or dissipate the large energy generated Steele argues, designers can better predict how a Bridge Zealand and Australia. Waves of up to 25 metres Engineer at suspension bridge in South America, at 2.75 km in during the earthquakes,” Steele says. bridge will behave during an earthquake. smashed the coast of Chile. Numerous landslides Jacobs. length. And amazingly, if the engineers get it right, it Ground accelerations are constantly being “Bridge design techniques for earthquake loading were triggered in the Andes, blocking rivers and won’t become the world’s shakiest. monitored around the globe, he notes, and the traces have evolved over the years with the learnings from 40 ENGINEERSAUSTRALIA.ORG.AU ENGINEERS AUSTRALIA | AUGUST 2017 41 PROJECT Inspiring STEM+ Connecting the island to the mainland could provide economic opportunities. The simplest way to understand displacement limit is to think of the dynamic behaviour when “The need to control you push a child on a swing, Steele explains. As the deflection of the bridge child swings higher, the period lengthens and you instinctively slow down so the application of the when the vehicle loading is energy is always applied when the swing is at its concentrated in the main highest on the backswing. This continues until you span is also a challenge.” can’t (or won’t!) apply additional force, meaning you can’t lengthen the period of vibration any further. Earthquakes have a relatively constant period of Actively promoting STEM in schools and major earthquake events such as Loma Prieta vibration so if the bridge structure can be designed in California in 1989 and Kobe in Japan in 1995. to remain stable at large enough displacements, the encouraging more girls to consider engineering Simply increasing the lateral strength of a bridge to period of vibration of the bridge will get to a point as a career is at the heart of Laing O’Rourke’s withstand an earthquake can be counterproductive where it is so far beyond the period of vibration of new school engagement program. as the increase in stiffness and weight of the the ground movement that the energy in the bridge elements associated with the increase in the is unable to increase further. Laing O’Rourke has teamed up with North strength will increase the earthquake loads the The designer then embarks on an iterative Sydney based secondary school Monte bridge will experience. Bridge designers instead process of adjusting the bridge form, articulation Sant’ Angelo Mercy College to pilot the look to increase the ductility within the structure to and member sizes and then rerunning the program with aims to address the significant absorb energy and redistribute loading and provide earthquake analysis until they are satisfied that sufficient flexibility to create a displacement limit they have the required strength to resist the design underrepresentation of women in the industry. on the forces within the structure,” he says. earthquakes and have optimised the design to “Introducing ductility is typically achieved minimise cost and simplify construction. “We are committed to creating a more diverse through controlled local crushing of concrete and “Suspension bridges perform well in and inclusive workplace, not only for our yielding of reinforcement to form hinges within earthquakes,” Steele says. “Their decks are typically people but also for future generations.” the bridge substructure framing to absorb energy made of steel to reduce weight for their long spans and dampen the dynamic response of the bridge and this has the benefit of reducing the forces that Cathal O’Rourke to the earthquake. The development of the design are generated during earthquakes compared to Managing Director, Engineering the future requirements for the reinforcement in these hinge other bridge forms with concrete superstructures.” zones has been a key factor in improving the Their modes of vibration also tend to have long Australia Hub laingorourke.com reliability of bridges in earthquakes.” periods so they are not too sensitive to excitation. ENGINEERS AUSTRALIA | AUGUST 2017 43 July 2017 Create Magazine-STEM.indd 1 21/6/17 9:51 am PROJECT ENTRY NORTH PARGUA CHACAO BRIDGE ENTRY SOUTH CHACAO “As learned from the Tacoma Narrows Bridge failure in the 1940s, suspension bridges are very sensitive to crosswind loading. Their design tends to be governed by the design requirements to keep the deck stable under crosswinds, which can gust and shed wind vortices at frequencies that are much closer to the modes of vibration of the the upper deck of the Bay Bridge during the bridge than the earthquake vibration,” he says. Loma Prieta earthquake in 1989 was the “The need to control deflection of the bridge most significant. when the vehicle loading is concentrated The Chacao Bridge site is clearly very in the main span is also a challenge for challenging due to high seismicity, a risk of suspension bridge designers. When designing tsunamis, high crosswinds and large ships suspension bridges in earthquake prone areas, passing through the channel, Steele says. engineers typically focus on the pier design to These all govern aspects of the deck, cable and introduce the ductility and flexibility to control the bridge foundation design. forces in these elements to optimise their design.” “It’s not impossible to design large bridges in Japan, like Chile, is highly seismic. It is also highly seismic areas,” he says. “Bridge engineers made up of a series of islands and connecting just have to have the right data and tools to ensure these islands together has made the Japanese one the bridge will be safe during construction and of the great bridge building countries of the world. in service. They would have to methodically They have a series of multi-span suspension work through a series of iterations to develop the bridges with similar span lengths to the 1000- optimal articulation, pier shapes and material metre spans of the Chacao Bridge. types, section properties and detailing for each of The Akashi Kaikyo Bridge in Kobe, with a main the elements to achieve this outcome.” span of 1991 metres, was under construction at Top: A simulation of the bridge the time of the Kobe earthquake. These bridges, vibrating, and On the ground in Chile along with the Golden Gate and Bay Bridge, map. Below: Chile’s Ministry of Public Works has shouldered have performed well in earthquakes with limited Island of Chiloe. responsibility for the planning and design of the damage occurring. The local failure of a section of Chacao Bridge. Dr Matias Valenzuela, Deputy 44 ENGINEERSAUSTRALIA.ORG.AU PROJECT Fiscal Inspector of the Public Works Ministry, has headed up the charge. “This is a very important project for Chile as it “We prefer that any damage will connect the south of Chile and the mainland,” is very slow and smooth, Valenzuela says. “We are trying to help this territory grow and allow the population from other cities on meaning afterwards it can the mainland to go there and provide services and be repaired.” so on.” Conceptual engineering began in the 1990s but the real work that led to the confirmation Scale models of the bridge have been tested of the project started in 2012.