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17 Jan 2010
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?
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.
“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.
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 second-order 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 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).
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.
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 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 sea-change 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.”
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