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.

causing flooding that affected over 100,000 people.

In the Valdivia around 40 per cent of houses were

destroyed and about 20,000 people left homeless.

In the Valdivia region is the Chacao Channel, a

waterway connecting the Pacific Ocean with the

Gulf of Ancud and which separates Chiloe Island

from the Chilean mainland. Chiloe Island has an

area of over 8000 km2 and a population of more

than 150,000 people, all of whom currently rely on a

car-ferry system to get to the mainland.

But by 2021, in the area that is famous for

producing the world’s most powerful earthquake,

a road bridge will be built to connect the two land

masses. Once completed, it will be the largest

suspension bridge in South America, at 2.75 km in

length. And amazingly, if the engineers get it right, it

won’t become the world’s shakiest.

Building in a shaky zoneJohn Steele, Executive Bridge Engineer at Jacobs,

has specialised in the design and construction of

bridges for over 20 years. While he’s not involved in

the Chacao Bridge build, he has worked on bridge

builds in New Zealand, another nation famous for

its seismic activity.

“Building a bridge in a highly earthquake-prone

region is not impossible provided the designer

understands the dynamic characteristics of the

earthquakes the bridge could experience during its

design life, how the bridge will behave during these

earthquakes, and most importantly, that the bridge

can absorb or dissipate the large energy generated

during the earthquakes,” Steele says.

Ground accelerations are constantly being

monitored around the globe, he notes, and the traces

from major earthquake events are regularly

made available to relevant authorities and

their bridge designers. This data is used

in the preparation of earthquake load codes

and project-specific design criteria to define

the earthquake ground accelerations and

frequencies that the designer should adopt for

their bridge design.

Modern bridge analysis software can establish

the modes of vibration of the bridge structures and

how these modes of vibration will interact with the

vibration of the ground during the earthquake to

induce force in the bridge structure. With this input,

Steele argues, designers can better predict how a

bridge will behave during an earthquake.

“Bridge design techniques for earthquake loading

have evolved over the years with the learnings from

On 22 May in 1960 the people

of southern Chile suffered the

most powerful earthquake ever

recorded. At 3:11 pm local time, and

continuing for 10 minutes after

that, the earthquake was measured at 9.4 to 9.6

on the moment magnitude scale. Since that date,

several famously destructive tremors including

Christchurch (maximum 7.8 magnitude), Nepal (also

7.8 magnitude) and many others destroyed buildings

and took lives, but Chile’s Valdivia Earthquake still

holds the record.

The quake caused a tsunami that swamped

shorelines as far away as the Philippines, New

Zealand and Australia. Waves of up to 25 metres

smashed the coast of Chile. Numerous landslides

were triggered in the Andes, blocking rivers and

John Steele, Executive Bridge Engineer at Jacobs.

ENGINEERSAUSTRALIA.ORG.AU40 ENGINEERS AUSTRALIA | AUGUST 2017 41

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Inspiring STEM+

Actively promoting STEM in schools and encouraging more girls to consider engineering as a career is at the heart of Laing O’Rourke’s new school engagement program.

Laing O’Rourke has teamed up with North Sydney based secondary school Monte Sant’ Angelo Mercy College to pilot the program with aims to address the significant underrepresentation of women in the industry.

“We are committed to creating a more diverse and inclusive workplace, not only for our people but also for future generations.”

Cathal O’RourkeManaging Director, Australia Hub

Engineering the futurelaingorourke.com

July 2017 Create Magazine-STEM.indd 1 21/6/17 9:51 am

The simplest way to understand displacement

limit is to think of the dynamic behaviour when

you push a child on a swing, Steele explains. As the

child swings higher, the period lengthens and you

instinctively slow down so the application of the

energy is always applied when the swing is at its

highest on the backswing. This continues until you

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

vibration so if the bridge structure can be designed

to remain stable at large enough displacements, the

period of vibration of the bridge will get to a point

where it is so far beyond the period of vibration of

the ground movement that the energy in the bridge

is unable to increase further.

The designer then embarks on an iterative

process of adjusting the bridge form, articulation

and member sizes and then rerunning the

earthquake analysis until they are satisfied that

they have the required strength to resist the design

earthquakes and have optimised the design to

minimise cost and simplify construction.

“Suspension bridges perform well in

earthquakes,” Steele says. “Their decks are typically

made of steel to reduce weight for their long spans

and this has the benefit of reducing the forces that

are generated during earthquakes compared to

other bridge forms with concrete superstructures.”

Their modes of vibration also tend to have long

periods so they are not too sensitive to excitation.

major earthquake events such as Loma Prieta

in California in 1989 and Kobe in Japan in 1995.

Simply increasing the lateral strength of a bridge to

withstand an earthquake can be counterproductive

as the increase in stiffness and weight of the

elements associated with the increase in the

strength will increase the earthquake loads the

bridge will experience. Bridge designers instead

look to increase the ductility within the structure to

absorb energy and redistribute loading and provide

sufficient flexibility to create a displacement limit

on the forces within the structure,” he says.

“Introducing ductility is typically achieved

through controlled local crushing of concrete and

yielding of reinforcement to form hinges within

the bridge substructure framing to absorb energy

and dampen the dynamic response of the bridge

to the earthquake. The development of the design

requirements for the reinforcement in these hinge

zones has been a key factor in improving the

reliability of bridges in earthquakes.”

“The need to control deflection of the bridge when the vehicle loading is concentrated in the main span is also a challenge.”

Connecting the island to the mainland could provide economic opportunities.

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“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

bridge than the earthquake vibration,” he says.

“The need to control deflection of the bridge

when the vehicle loading is concentrated

in the main span is also a challenge for

suspension bridge designers. When designing

suspension bridges in earthquake prone areas,

engineers typically focus on the pier design to

introduce the ductility and flexibility to control the

forces in these elements to optimise their design.”

Japan, like Chile, is highly seismic. It is also

made up of a series of islands and connecting

these islands together has made the Japanese one

of the great bridge building countries of the world.

They have a series of multi-span suspension

bridges with similar span lengths to the 1000-

metre spans of the Chacao Bridge.

The Akashi Kaikyo Bridge in Kobe, with a main

span of 1991 metres, was under construction at

the time of the Kobe earthquake. These bridges,

along with the Golden Gate and Bay Bridge,

have performed well in earthquakes with limited

damage occurring. The local failure of a section of

the upper deck of the Bay Bridge during the

Loma Prieta earthquake in 1989 was the

most significant.

The Chacao Bridge site is clearly very

challenging due to high seismicity, a risk of

tsunamis, high crosswinds and large ships

passing through the channel, Steele says.

These all govern aspects of the deck, cable and

bridge foundation design.

“It’s not impossible to design large bridges in

highly seismic areas,” he says. “Bridge engineers

just have to have the right data and tools to ensure

the bridge will be safe during construction and

in service. They would have to methodically

work through a series of iterations to develop the

optimal articulation, pier shapes and material

types, section properties and detailing for each of

the elements to achieve this outcome.”

On the ground in ChileChile’s Ministry of Public Works has shouldered

responsibility for the planning and design of the

Chacao Bridge. Dr Matias Valenzuela, Deputy

Top: A simulation of the bridge vibrating, and map. Below: Island of Chiloe.

PROJECT

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An artist’s impression of the Chacao Bridge.

Scale models of the bridge have been tested

in wind tunnels and analysed in laboratories.

Its elastic properties, including the deck, have

been measured. After the physical testing, the

entire project was taken online to be re-built and

re-tested in a digital environment.

“We designed the structure online and analysed

its spectrum, starting with response spectrum

analysis,” Valenzuela says. “The second analysis

looked at ‘pushover analysis’, which involves

pushing the structure and discovering in which

part of the structure the cracking process would

begin. From that you can read a lot of things.”

“After that we did a dynamic soil analysis

to discover what would happen in terms of

displacement when the soil around the foundation

of the structure was suffering in an earthquake.”

The testing really is exhaustive when it comes

to the threat of earthquake. Seismic waves could

come from any direction and could therefore hit

various parts of the structure at different times and

with differing levels of force, each resulting in a

unique outcome when it comes to structural stress.

Fiscal Inspector of the Public Works Ministry, has

headed up the charge.

“This is a very important project for Chile as it

will connect the south of Chile and the mainland,”

Valenzuela says. “We are trying to help this territory

grow and allow the population from other cities on

the mainland to go there and provide services and

so on.”

Conceptual engineering began in the 1990s

but the real work that led to the confirmation

of the project started in 2012. An international

consortium consisting of OAS, Hyundai, Systra and

Aas-Jackobsen was awarded the contract in early

2014. Temporary works, including seven kilometres

of road access from the nearby motorways, began in

2015 and are now complete. Right now work is set to

begin on the central, and largest, of three pylons.

With the assistance of Bentley Systems’ RM Bridge

software, Valenzuela and his team have conducted

exhaustive testing of options and designs, materials

and systems, weather effects and seismic forces.

“Maybe it is not unique but we are applying a

very strict procedure for that,” Valenzuela says.

“We analyse several methodologies for the design.

We analyse energy dissipation systems, damper

systems, confinement detailing for seismicity

and specific reinforcement types in the concrete,

so if the concrete begins to crack then the bridge

deformation can be controlled and does not create

immediate failure. We prefer that any damage is

very slow and smooth, meaning afterwards it can

be repaired.”

“We prefer that any damage is very slow and smooth, meaning afterwards it can be repaired.”

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of not only the effects of various forces on the

bridge and the behaviour of the bridge under those

forces, but also allowed for changes over time.

“What we can check is that not only is the

structure going to be alright but also the behaviour

and the structural capabilities of the bridge across

time,” Mabrich says. “One of the most important

factors when you design a bridge in an area like

this is the effect of time on the materials, including

time during construction and how that will affect

the behaviour of the bridge.”

In the software, he says they can try many

materials to make sure, for example, that it

is not only structurally sound but it is also

aerodynamically sound.

“The structure will have to cope with winds of

up to 200 km per hour,” he says. “For an engineer

this build has everything you don’t want. It has

wind, it has ocean currents and waves, it has busy

shipping channels, it has serious seismic activity

and it even has geological conditions that are not

perfect. It has been a serious challenge.”

He feels the Chacao Bridge will be an

engineering marvel but, more importantly, it will

benefit the community, and the nation.

“All of the team is thinking about the real social

and cultural benefits this bridge will bring to our

society,” Valenzuela says. “We are looking forward

to a successful result for engineering and for

Chile. This bridge will connect about 2000 km of

our country to the mainland. There are 150,000

inhabitants and this bridge will provide them with

a better life.”

NEW BRIDGE CODE

All possibilities had to be tested and re-tested, with

the design accounting for all outcomes. All of the

materials, down to the specific reinforcement recipe

for the concrete used to create the road surface,

have been tested for the specific conditions.

“In discussing the best asphalt for the pavement

we have been researching the durability of

concrete,” Valenzuela says. “We have discovered

some very good research from Europe about the

reinforcement process that means we’re reducing

the corrosion. This can mean that if an earthquake

comes as the bridge ages, the structure’s durability

is higher.”

The software storyAlex Mabrich, a Senior Engineering Consultant

for Bentley Systems, has been heavily involved in

the design engineering side of the Chacao Bridge

project. The bridge’s location, Mabrich confirms,

makes it a completely unique construction.

As Mabrich says, the bridge must be able

to withstand a “total destruction earthquake”.

Importantly, the software gave designers a view

“One of the most important factors when you design a bridge in an area like this is the effect of time on the materials.”

A nine-part Australian Bridge Code has recently been published by Standards Australia that includes a comprehensive revision of the 2004 Code, and the introduction of two new parts.

Australia has seen large changes since 2004 in both the design and construction of bridges, particularly around climate change, sustainability and safety-in-design.

Other changes include revisions to road barrier and pedestrian barrier requirements, new loading and requirements for the protection of piers and abutments subject to collision loads from derailed

trains, the introduction of spherical bearings, new rules for design for hydrocarbon fire and the introduction of steel fibre reinforced concrete.

New concrete strengths of up to 100 MPa, structural steel up to 690 MPa and durability provisions have also been introduced.

The two new sections – parts 8 and 9 – include the rehabilitation and strengthening of existing bridges and the design of timber bridges, making bridges more sustainable and lowering the impact on the environment.

Wije Ariyaratne, Principal Engineer Bridges at NSW Roads and Maritime

Services (RMS), said there is a clear need to provide a standard that is suitable for rehabilitating and strengthening these structures, and if they are properly engineered and maintained, they could have a design life of 100 years.

“Without the standard, there won’t be a logical and consistent approach to the design,” he said.

“Providing a standard for design and understanding timber’s inherit strength to weight ratio, its sustainability and its ability to capture and store carbon will encourage more authorities and designers to promote the use of more timber bridges in Australia.”

From left: Temporary works; Matias Valenzuela with a model of the bridge in a wind tunnel; the Akashi Kaikyo Bridge in Kobe, Japan.

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