great
lengths
building Bridges
Life span
Bridges are, by nature, largely functional. They are built mostly to transport
people and resources from one place to another. But how can we ensure that
the bridges built today will be a pleasure to behold tomorrow, performing
their function with minimal impact on the environment?
From the masonry arch, born in Mesopotamia and
perfected by the Romans, through the first iron spans of
the 18th century, and culminating in today’s elegant steel
cable-stayed and suspension bridges, each generation has
improved on the bridge engineering and design of its
predecessors. And yet those designing and building bridges
have always tried to achieve the same basic goals: erect
something that is stable and strong; that does not cost the
earth; and that will not require undue maintenance over its
lifetime (typically, bridges are designed to stand 120 years).
But the profession now faces higher expectations
than did its forebears. Bridges must not only be functional,
they are also expected to be environmentally sustainable
and, increasingly, eye-catching, iconic and unique. This
development is being driven by several factors, including
more stringent government regulations, the demands of
clients and the expectations of society at large.
“Life has become very different in the last few years,
particularly with climate change,” says Chris Hendy, head
of bridge design and chair of the Atkins bridge engineering
network. “Whereas, in the past, our main driver was keeping
costs down, now we have to look at the big picture, especially
sustainability issues.” On the one side, achieving such goals is
easier than before. Advanced computer modelling allows
engineers to understand the pressures on structures more
intimately, and to experiment with designs that, in the best
cases, are both resilient and use less materials and labour.
On the other, growing congestion and entanglement of
urban environments mean that the engineer’s job is more
difficult. Frequently, they will need to construct or retrofit
bridges in spaces that are unforgiving, without disturbing
traffic or other vital activities. How do you balance the
practical requirements of bridge building with the desire
to improve on the past, while never forgetting the impact
these choices can have on the environment in the future?
Bridge the gap
The first thing to remember is that, despite these changes
in perspective, aesthetics still often drive bridge building
projects. Clients are increasingly likely to request
aesthetically pleasing designs, even if they add to the cost,
says Hendy. He points, for example, to the proposed River
Weir bridge, in Sunderland, in England’s north-east.
The local government is prepared to pay a premium for
something that stands out, believing the cost will be offset
by the boost to tourism and other benefits associated with
any iconic landmark. Whereas, in the past, such structures
would have been purely functional, today even motorway
bridges need to have a baseline appeal.
At the same time, Hendy points out that clients in the
UK and Europe as well as some developing countries now
expect sustainability considerations to be built in to any
proposed designs. And this can touch on everything from
the noise made during construction to the quantity and
type of materials used, and how the bridge is to be
maintained over its lifetime.
The challenge for Hendy has been how to explain to
clients the way in which different aesthetic choices can
affect a project’s environmental impact.
To overcome the problem, Hendy and his team have
recently developed a new tool to help engineers as well as
Atkins clients assess the carbon impact of bridge designs,
based on a dozen or so different factors.
“Life has become very different in the last few years...
Whereas, in the past, our main driver was keeping costs down, now
we have to look at the big picture, especially sustainability issues”
“The aesthetic appearance of
structures has become a key issue.
The ‘greening’ of bridges, by
using planters, green roofs and
architectural high quality finishes,
is now mandatory in many cases”
“It’s a very quick way to communicate
the sustainability balance in a particular
structure,” Hendy says.
Assessing the carbon impact of bridge
design involves working out the lifecycles
of different materials, including steel and
concrete, as well as looking at how bridges
are assembled on-site.
“We try to pre-cast and bolt together
the various elements of any bridge design
in lighter pieces wherever possible. The
transport to the site is that much easier
and the equipment you need to lift things
in is that much lighter. Therefore, it produces
a corresponding reduction in the carbon
footprint of the actual structure,” says Hendy.
26
Stretching the technology
Advanced computer modelling plays a key
part in reducing the amount of materials used
and thereby minimising environmental impact.
This is particularly true in the case of slender
structures, such as long footbridges, which
can be susceptible to strong “second-order”
effects (where stresses are magnified because
of the low stiffness and large deflections).
Because computers were not able to
quickly allow for such deflections
in the past, building codes forced engineers
to be conservative in their designs, adding
extra materials to structures for added safety.
Today’s more powerful computer modelling
allows designers to accurately assess secondorder effects, leading to reductions in the
amount of materials required. It also allows
for greater creativity in design.
This was the case in Atkins’ recent work
on the Medway Bridge in the south of
England: “By using a non-linear computer
model to analyse the piers on the Medway,
we reduced the amount of steel reinforcement
required by 60 per cent compared to the initial
design, which was based on the codes of
practice,” Hendy says. He estimates this
London 2012:
building a bridge for legacy
equated to a saving of about £1m for the
client, as well as making the structure simpler
and reducing build time.
Computers also played a significant part
in Atkins’ work on the recently completed
Dubai Metro, which required the rapid
construction of 42km of viaducts across the
city. Hendy says the need to account for strong
earthquake forces in the area would normally
have increased the amount of steel needed to
reinforce the concrete structure. But advanced
modelling meant conventional bearings could
be exchanged for an “elastomeric” type that
reduced the materials used, without
compromising safety.
It’s bridge design with a practical and
sustainable edge, and people are taking note.
In fact, Hendy’s work on these projects, along
with his designs for a stay cable replacement
scheme for Penang Bridge, the longest bridge
in Southeast Asia, earned him the 2009
Diploma for Younger Engineers by the
International Federation for Structural
Concrete (fédération internationale du béton).
Spanning the world
Clearly, a bridge supporting a new metro
system in Dubai will have different
sustainability issues than a transport bridge
in the south-east of England. The key, says
Hendy, is to remember that projects should
always be approached with a fresh set of
eyes – “Every project is different,” he says.
The design and types of materials used will
be determined by local customs, economics
and environmental factors.
For example, steel is used heavily in the UK
because it is relatively cheap and the UK has
a bank of specialists trained to work with the
material. By contrast, concrete is the dominant
material in the Middle East because it is
cheaper and the higher labour costs associated
with the material (due to the need for
reinforcing and pre-stressing) are easier to bear.
“In Malaysia,
there is a strong
awareness that
climate change is
an issue, but they
will be keen to
follow a lead from
Europe on how to
go about reducing
the footprint”
For visitors attending the London 2012
“The bridges will be located
Olympic and Paralympic Games, entry
side by side, with the legacy bridge
to the Olympic Park will be a defining
supported by two permanent arches,
moment. It has to reflect the scope and
one on either side,” says Nuttall.
grandeur of the event, drawing them
the bridges. As you cross, you are
the whole experience.
presented with the entire width of
Atkins, working with a team of
between the two. “They will be joined
designing the footbridge leading to the
for the duration of the Olympic and
entry point at the north of the Olympic
Paralympic Games, and then the
Park. The bridge also had to offer value
temporary bridge will be removed.”
for money and fit in with the long term
“This will be one of the permanent new
expensive to maintain because it needs to
be painted and re-painted regularly, but may
make economic sense if the initial cost is
cheaper than a concrete design, particularly
for long-span bridges or programme and site
constraint requirements. Glass Reinforced
Plastic or Carbon and Aramid Fibre supported
bridges have the advantage of being
lightweight and quick to erect, and are also
cheap to maintain. However, as a new and
emerging technology, the manufacturing
costs of these bridges are currently prohibitive
for large scale projects and are only used in
limited applications.
Douglas Simmons, divisional director for
Atkins in China, says concrete is also the most
common material for bridge building in his
part of the world. But planners in Hong Kong
and mainland China are increasingly aware of
the aesthetic value of bridge designs and the
need to incorporate environmental principles:
“There have been a lot of high impact
projects pushed through in the last 20 years
without consideration for how they blend
with the environment. As things have
become more congested and as more
channels have opened through which the
public can complain, there has been a seachange in attitudes,” he says. This concern
may not extend to a project’s carbon impact,
but planners are more willing now to take
account of the immediate impacts.
“The aesthetic appearance of structures
has become a key issue. The ‘greening’ of
bridges, by using planters, green roofs and
architectural high quality finishes, is now
mandatory in many cases,” Simmons says.
Though some countries, such as the UK,
are likely to take environmental considerations
such as carbon reduction into account today,
practices developed in Europe and elsewhere
in the western world are likely to become
more common in the developing world over
time. Hendy says environmental practice will
follow the way of health and safety, which
has improved gradually as companies such
as Atkins have taken their policies overseas.
By way of example, Hendy points to the
improvements in health and safety practices
Atkins encouraged through its work on the
Penang Bridge in Malaysia.
“It’s the same as health and safety.
There’s a lag,” he says. “In the UK, clients
understand that climate change is important
and are taking it into account from the start.
In Malaysia, there is a strong awareness that
climate change is an issue, but they will be
keen to follow a lead from Europe on how to
go about reducing the footprint. Hopefully,
our work is making a difference.”
the crossing, with an arch running
architects, has been tasked with
legacy strategy of London 2012.
A similar general distinction exists
between northern and southern Europe.
In countries like Germany, steel is the norm
for short to mid-span bridges. In Spain and
Italy, concrete is the more likely option.
Dr Ghassan Ziadat, regional head of
bridges for Atkins in the Middle East and
India, says concrete is often the more
sustainable choice in developing countries
because of a lack of the right type of
structural steel being produced locally.
“Most developing countries do not
have locally produced structural steel for
use in bridgeworks. Therefore, designing
concrete bridges reduces costs generally
and improves sustainability by maximising
the use of locally produced materials.”
In the harsh Middle East environment, the
choice of materials will also be determined
by the cost of maintenance. Steel is relatively
“One arch acts as a barrier between
in and offering a welcoming start to
The bridge also uses varying
transparency for added impact: “If you
stand on the bridge and look straight
bridges being built in the Olympic Park
through the archway, you would have
forming one of the three main entrances
an almost completely uninterrupted
into the Park during London 2012,” says
view. Stand at a more oblique angle and
Hayden Nuttall, design director responsible
the structure has been designed to close
for structural engineering with Atkins.
off the view, a bit like Venetian Blinds.
The bridge will be the first point of
It plays a bit of a trick on the eyes and
contact with the Olympic Park for those
offers great opportunities to change
arriving by coach, taking them from East
how the bridge looks and feels at night,
Marsh over a carriageway and into the
with different lighting,” says Nuttall.
Olympic Park itself.
“During London 2012, the number of
“It’s all part of the strategy to bring
the Olympic Park out and integrate it
people that will need to cross this bridge
with the surrounding urban landscape
is so great that the bridge will need to be
at all of the crossing points. It has to
about 14 metres wide to accommodate
offer an elegant design and go beyond
them all,” says Nuttall. “After London
functionality,” Nuttall concludes.
2012, however, the bridge won’t need
to be anywhere near as large.”
The solution was to create a 5.5m
“The permanent bridge is described
as a landmark bridge because of the
way in which it interacts with the
wide permanent bridge and a 8.5 wide
landscape. It’s quite a striking piece
temporary bridge.
of architecture.”