Steel offers many advantages to the bridge builder, not only from the material itself, but also from its broad architectural possibilities. The following are some of the advantages that steel can offer.
The high strength to weight ratio of steel minimises substructures costs, which is particularly beneficial in poor ground conditions. Minimum self-weight is also an important factor in trasnporting and handling components. In addition, it facilitates very shallow construction depths, which overcome problems with headroom and flood clearances, and minimises the length of approach ramps.
The Newark Dyke rail bridge, shown above, comprises a 77m span bowstring truss with 820 tonnes of S355 steel. The selection of steel was made because of its high strength to weight ratio, which permitted a shallow construction depth and minimised the total weight to be slid into position. The low self-weight also minimised foundation works adjacent to the existing rail line.
This bridge was the first UK steel bridge to be designed for the next generation of 225 Km/hr trains.
Steel is a high quality material, which is readily available worldwide in various certified grades, shapes and sizes. The testing regime carried out at the steel mills should give confidence to all clients and engineers who specify steel for their project.
Prefabrication in controlled shop conditions leads to high quality work at minimum cost. The quality control extends from the material itself and follows on through the processes of cutting, drilling, welding, fit-up and painting.
The prefabrication of components means that construction time on site in hostile environments is minimised. The speed of steel bridge construction reduces the durations of rail possessions and road closures, which minimises disruption to the public using those networks. The light-weight nature of steel permits the erection of large components, and in special circumstances complete bridges may be installed overnight.
For example, the Hallen Bridge, is a 500T truss bridge that carries a single-track railway over the M5 near Bristol. Sections were shop-fabricated and transported to site where the bridge was fully assembled off line. The whole structure was then transported into position using multi-wheeled ‘Econofreight’ vehicles during an overnight closure of the M5.
Steel suits a range of construction methods and sequences. Installation may be by cranes, launching, slide-in techniques or transporters. Steel gives the Contractor flexibility in terms of erection sequence and programme. Components can be sized to suit access restrictions at the site, and once erected the steel girders provide a platform for subsequent operations.
Steel bridges are adaptable and can readily be altered for a change in use. They can be widened to accommodate extra lanes of traffic, and strengthened to carry heavier traffic loads.
For example, the Tamar suspension bridge in Plymouth needed widening and strengthening due to increased traffic loads and volumes. The solution was to replace the concrete deck with a new lightweight steel one, and add steel cantilever sections. The result was that the widened 5-lane bridge was only 25 Tonnes heavier than the old 3-lane structure, and was able to accommodate 44 Tonne trucks.
Steel bridges can readily be repaired after accidental damage. The photograph above shows a steel composite bridge over the M5 near Weston that was struck by an over-height lorry. The outer girder was deformed by 400mm over a length of 2.5m, and initially considered “Beyond repair”. A costly beam replacement scheme would have taken many months and caused severe disruption to the motorway.
However, a heat treatment technique, based on the theory of restrained expansion, was proposed by a UK fabricator as a more economic and less disruptive solution. Heat was applied locally in a carefully controlled manner, and over a period of only two days the girder was straightened to within 18mm of its original line. No external forces were applied to bring the girder back on line, and the contract was completed for considerably less than the beam replacement option.
Steel is a ‘sustainable’ material. When a steel bridge reaches the end of its useful life, the girders can be cut into manageable sizes to facilitate demolition, and returned to steelworks for recycling.
Steel bridges now have a proven life span extending to well over 100 years. The potential durability of steel may be summarised in the following quote by a Mr J.A.Waddell in 1921:
“The life of a metal bridge that is scientifically designed, honestly and carefully built, and not seriously overloaded, if properly maintained, is indefinitely long.”
Steel has a predictable life, as the structural elements are visible and accessible. Any signs of deterioration are readily apparent, without the need for extensive investigations. Corrosion is a surface effect, which rarely compromises the structural integrity of a bridge, and any problems may be swiftly addressed by repainting the affected areas. In addition, the latest coatings are anticipated to last well beyond 30 years before requiring major maintenance.
An alternative form of corrosion protection is the use of weathering steel, as on the Westgate bridges in Gloucester, as shown above.
Steel has broad architectural possibilities. Steel bridges can be made to look light or heavy, and can be sculptured to any shape or form. The high surface quality of steel creates clean sharp lines and allows attention to detail. Modern fabrication methods have removed restrictions on curvature in both plan and elevation. The painting of steelwork introduces colour and contrast, and repainting can change or refresh the appearance of the bridge to appear as new.
Bridges are an essential feature of a countries infrastructure and landscape. Few man made structures combine the technical with the aesthetics in such an evocative way.
This arch-truss bridge connects Incheon international airport to Seoul in Korea (above), and has a main span of 540m. Steel was considered the only option for such a high profile site.