When deciding whether to build a bridge from steel or concrete, there are a number of factors which need to be considered. Steel offers numerous advantages, including quick construction in the field, the ability to prefabricate sections, predictable material properties, the lower weight of steel compared to that of concrete – and the fact that any structural damage is readily accessible for inspection.
“Contemporary bridge designers have the choice of steel or concrete for their construction material. However, these days, bridges are often not constructed exclusively of concrete or exclusively of steel, an example being the steel-beam and concrete-deck bridges we see today,” explains Amanuel Gebremeskel, Technical Director of the Southern African Institute of Steel Construction (SAISC).
“Design of the bridge greatly affects its initial cost and naturally, the more efficient the design the lower the cost. A further consideration in the design of the bridge is its purpose. That is, whether it will be carrying pedestrians, motor vehicles or railway rolling stock,” Gebremeskel explains, adding that one of the major advantages of constructing bridges from steel is the decreased weight.
This means lower erection costs, since the bridge sections can be handled using lighter construction equipment. If designed soundly, the lighter weight of the steel bridge will also allow for lighter foundations. Generally, it is easier to make spans continuous for both live and dead loads – and to develop composite action with steel designs rather than with concrete ones.
“However, the major advantage in the construction of steel bridges is that they are considerably faster to complete, with fewer logistical challenges. Where a bridge is being constructed over a busy highway, for example, disruption to traffic is far less. With steel construction, the need for time-consuming and elaborate formwork is also eliminated. If necessary, steel sections can be prefabricated off-site and then bolted and welded together to complete the final structure,” he comments.
In terms of maintenance, historically, decks are the most vulnerable part of the bridge. To replace a concrete bridge deck requires removal of the entire section at one time; whereas steel bridge decks can be replaced one lane at a time, allowing for uninterrupted – albeit reduced – traffic flow.
Where bridges cross other infrastructure or (particularly in Africa with the often rugged terrain) geographical features such as a deep ravine or river, steel has the advantage that the section of the completed bridge can be shallower than its concrete counterpart.
“Over and above these considerations – not just locally but globally – environmental considerations are also an important factor in the use of steel,” he adds.
In this respect, today, the raw material used to construct bridges is often scrap steel. In a white paper, the (American) National Steel Bridge Alliances states that new steel bridge construction in the US annually consumes about 350,000 tons of scrap metal. And, when a steel bridge has reached its allotted lifespan, the raw material can be recycled. Although not a perfect example, the steel from the World Trade Centre has now been recycled into other products.
When bridges have to be constructed in environmentally sensitive areas, the fact that steel spans can be longer than their concrete counterparts minimises the environmental impact as fewer piers are needed to support the bridge.
In addition, high-performance ‘weathering’ steels offer greater weather resistance, and feature toughness and weldability at affordable prices. “This technology is readily available in South Africa,” Gebremeskel points out.
“Over the past two decades steel construction profiles have appeared in new forms, both internationally and locally. For example, tubular steel is now widely used, and is known for its aesthetic appeal, its light weight and its superior strength.
Furthermore, with the advent of laser fabrication, steel tube can very quickly be processed into interlocking bridge components, thereby improving the strength, speed and efficiency of welding, resulting in stronger and more aesthetic appealing structures,” he says.
The choice of whether to build a steel or concrete bridge is also dictated to some extent by which region of the world the bridge is being built in – as concrete is cheaper in some areas. In other areas, the ruling price and availability of steel may well make it the preferred construction option.
“Bridges play a mostly unacknowledged yet cardinal role in all of our lives. While offering design and economic benefits, the use of steel as a construction medium for bridges also offers the more lasting appeal of design excellence. If one thinks of the internationally renowned San Francisco ‘Golden Gate’ bridge, Millennium bridge in London or the Sydney harbour bridge, one has excellent examples of structures that are of immense practical use – and yet at the same time are highly visually pleasing examples of superb engineering design.
As with many other bridges around the world, the element which has made these enduring and iconic structures possible was the versatility and excellence of steel,” he concludes.