Steel as a Material for Bridges
Steel as a Material for Bridges
Steel is widely used around the globe for the construction of bridges of sizes ranging from from the very large to the very small. It is a versatile and effective material that provides efficient and sustainable solutions. Steel has long been recognized as the economic option for a range of bridges. It dominates the bridge constructions for long span bridges, railway bridges, footbridges, and medium span highway bridges. It is now increasingly the choice for shorter span highway structures as well.
Early bridges were made of stone, wood and concrete. The arrival of the steam train in the mid-18th century ushered in a new era in bridge design. A stronger material was needed as bridges were required to carry heavier loads over longer spans. Iron was first used to bridge the ‘Tees’ river in England in 1741. By the 1880s, steel had become a material of choice.
Amongst bridge materials steel has the highest and most favorable strength qualities, and it is therefore suitable for the most daring bridges with the longest spans. Normal building steel has compressive and tensile strengths of 370 N/sq mm, about ten times the compressive strength of a medium concrete and a hundred times its tensile strength. A special merit of steel is its ductility due to which it deforms considerably before it breaks, because it begins to yield above a certain stress level.
Steel is an ideal material for bridges. It is an essential part of modern bridges because it is strong, can flex without fracturing and has a long life, even in the harshest conditions. It can be used to build bridges of any length because of its durability and ease of manufacture and maintenance. New grades of steel increase the economic advantages of steel, while ensuring that it meets the increasing demands for high performance.
Steel is a most versatile and effective material for bridge construction, able to carry loads in tension, compression and shear. Structural steelwork is used in the superstructures of bridges from the smallest to the greatest. There is a wide variety of structural forms available to the designer but each essentially falls into one of four groups namely (i) beam bridges, (ii) arch bridges, (iii) cable stayed bridges, and (iv) suspension bridges. Some typical examples of bridges made from steel are shown in Fig 1.
Fig 1 Typical example of bridges made from steel
Steel offers many advantages and this has led to it being widely used for all forms of bridge construction around the world, from simple beam bridges up to the longest suspension bridges. The advantages offered by steel bridge solutions are not only from the material itself, but also from its broad architectural possibilities. The advantages which the steels offer are (i) high quality material, (ii) speed of construction, (iii) versatility, (iv) variety of good designs, (v) feasibility of modification and repair, (vi) recycling, (vii) durability, and (viii) aesthetics.
Modern steel bridges taking advantage of the latest advances in automated fabrication and construction techniques are able to provide economic solutions to the various demands of bridge construction. Steel also scores well on all the sustainability measures, and offers a broad range of benefits addressing the economic, environmental, and social priorities of the ‘triple bottom line’ of sustainability.
Economic priorities are economic use of resources, minimum disruption, durability, and adaptability. Environmental priorities are minimum CO2 and energy burden, minimum waste, recycling and reuse, and light weight construction. Social priorities are sustainable communities, health and safety, minimum impact, and aesthetically pleasing bridges.
Steel bridges are an essential feature of a country’s infrastructure and landscape. Few man-made structures combine the technical with the aesthetics in such an evocative way.
Advantages of steel bridges
Regardless of the type of bridge construction, a material with good tensile strength is essential and steel is effective and economical in fulfilling that role. There are several potential advantages that bridge structures made of steel can offer. These include but are not limited to the following.
- Lighter weight than concrete for superstructures of comparable spans, reducing foundation requirements and, more significantly, reducing the inertia effects induced by seismic events.
- Reduced depth of structure for comparable spans, thereby reducing the significant approach-roadway costs for the large number of overpasses used in big cities.
- Ability to repair the component to full strength whether the need for repair is generated by collision forces from over-height vehicles or environmental causes. These repairs can generally be made without affecting traffic flow on or below the structure;
- Corrosion-resistant materials that lower first and life-cycle costs for virtually all bridge environments with proper detailing.
- Flexibility for complex geometries, including horizontally curved and skewed alignments, longer spans, odd span arrangements, and bifurcated structures
- Ductility and toughness of material to allow absorption of loading well above design values without catastrophic failures.
The advantages of using steel as a material for the bridges are described below.
- High strength to weight ratio – The high strength to weight ratio of steel minimizes substructures costs, which is particularly beneficial in poor ground conditions. Minimum self-weight is also an important factor in transporting and handling components. In addition, it facilitates very shallow construction depths, which overcome problems with headroom and flood clearances, and minimizes the length and height of approach ramps. This can also result in a pleasing low-profile appearance. The lightweight nature of steel construction combined with its strength is particularly advantageous in long span bridges where self-weight is crucial. In case of bridges with modest spans the reduced weight minimizes substructure and foundation costs, which is beneficial in poor ground conditions. Minimum self-weight is also an important factor for lift and swing bridges, as it reduces the size of counter-weights and leads to lower mechanical plant costs.
- Material with high quality – Steel is a high quality material, which is readily available in different certified grades, shapes and sizes. Prefabrication in controlled shop conditions has benefits in terms of quality. Trial assembly can be done at the fabrication shops to avoid fit-up problems on site. All these activities lead 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. These testing procedures provide confidence to the bridge builder and engineers who specify steel.
- Speed of construction – The prefabrication of components means that construction time on site in hostile environments is minimized which results into economic and safety benefits. The speed of steel bridge construction reduces the durations of rail possessions and road closures, which minimizes 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.
- Versatility – Steelwork can be constructed by a wide range of methods and sequences. . Installation may be by cranes, launching, slide-in techniques or transporters. Steel provides the erection contractor flexibility in terms of erection sequence and construction programme. Girders can be erected either singly or in pairs, depending on plant constraints. Components can be sized to suit access restrictions at the site, and once erected the steel girders provide a platform for subsequent operations. Steel also has broad architectural possibilities. The high surface quality of steel creates clean sharp lines and allows attention to detail. Modern fabrication methods facilitate curvature in both plan and elevation. The painting of steelwork introduces colour and contrast, whilst repainting can change or refresh the appearance of the bridge
- Modification, demolition and repair – 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. When the bridge is no longer required, the steel girders can easily be cut into manageable sizes and recycled, which is a benefit in terms of sustainability. Steel bridges can readily be repaired after accidental damage. In case the bridge is damaged, the affected areas can be cut out and new sections welded in. Alternatively, girders can be repaired by heat straightening. Heat straightening is by a heat treatment technique, based on the theory of restrained expansion. It is an economic and less disruptive solution in case a deformed bridge girder is to be straightened.
- Recycling – Steel is a sustainable material. Steel is the most recycled construction material and choosing it for bridges represent a sustainable management of natural resources. 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 steel plants for recycling. Some 99 % of structural steel either finds its way back into the steelmaking process where it is used to create new steel products or is reused. There is no degradation in the performance of recycled steel. Alternatively, component parts of steel bridges can be reused in other steel structures.
- Durability – JA Waddell has quoted in 1921 that ‘The life of a steel bridge that is scientifically designed, honestly and carefully built, and not seriously overloaded, if properly maintained, is indefinitely long’. Steel bridges now have a proven life span extending to well over 100 years. 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.
- Aesthetics – 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.
Steels used in bridges
Steel derives its material properties from a combination of chemical composition, mechanical working and heat treatment. The yield strength is probably the most significant property that the designers need to use or specify. Modern steel making and rolling processes have developed steels of suitable high yield strength without any deterioration of other properties in the steels.
Structural steels for use in bridges generally have more stringent performance requirements compared to steels used in buildings and many other structural applications. Bridge steels have to perform in an outdoor environment with relatively large temperature changes, are subjected to millions of cycles of live loading, and are often exposed to corrosive environments containing chlorides. Steels are required to meet strength and ductility requirements for all structural applications. However, bridge steels have to provide adequate service with respect to the additional ‘Fatigue’ and ‘Fracture’ limit state. They also have to provide enhanced atmospheric corrosion resistance in many applications where they are used without expensive protective coatings. For these reasons, structural steels for bridges are required to have fracture toughness and often corrosion resistance that exceed general structural requirements.
Steel used for bridges may be grouped into the following categories.
- Carbon steel – This is the cheapest steel available for structural users where stiffness is more important than the strength. This steel has yield stress value around 250 N/sq mm and can be easily welded.
- High strength steels – These steels derive their higher strength and other required properties from the addition of alloying elements. These steels need special welding techniques for welding.
- Weathering steels – This is another variety of steel which is having enhanced resistance to atmospheric corrosion. These are also called corten steels. Weathering steels are high strength low alloy steels which in suitable environments forms an adherent protective rust ‘patina’, to inhibit further corrosion. The corrosion rate is so low that bridges fabricated from unpainted weathering steels can achieve a 120 year design life with only nominal maintenance.
- Heat-treated carbon steels – These steels are with the highest strength. They derive their enhanced strength from some form of heat treatment after rolling namely normalization or quenching and tempering. These steels can be welded with normal welding techniques.
- Stainless steels – Steel is now in general use for bridge construction but the use of stainless steel is relatively recent, 10 years to 15 years. Initially used principally for its anti-corrosion properties in safety components – guardrails, and handrails etc., stainless steel is now found in structural components, whether in the deck – in the form of beams and welded plate sections, tie-rods – or in the suspension systems – in the form of stays, cables and pylons. Stainless steels are also occasionally used to fabricate bearings and other parts for bridges where high corrosion resistance is required. However, the relative high cost of stainless steel has limited its use in primary bridge members.
The physical properties of structural steel such as strength, ductility, brittle fracture, weldability, weather resistance etc., are important factors for its use in bridge construction. These properties depend on the alloying elements, the amount of carbon, cooling rate of the steel and the mechanical deformation of the steel.
The use of uncoated weathering steel typically provides initial cost savings of 10 % or more, and life cycle cost savings of at least 30 % over the life of the structure. Initial cost savings are realized because weathering steels do not need to be painted. Life cycle cost savings come from the material’s durability. Inspections of bridges of between 18 years and 30 years old show that weathering steel perform well in most environments. Weathering steels provide environmental benefits as well. They do not require initial painting, thereby reducing emissions of volatile organic compounds (VOC) from oil-based coatings.
Stainless steels are subject to increased corrosion if they are placed in contact with regular carbon steel. This requires the use of either stainless steel or galvanized fasteners. In addition, special care is needed to avoid contact with or connections to regular carbon steel components. Stainless steels used for bridges and footbridges belong primarily to two categories of stainless namely austenitic and austeno-ferritics, also known as duplex, which combine excellent corrosion resistance and elevated mechanical performance.
The selection of an appropriate grade of steel for a bridge requires an awareness of the steel manufacturing process, an appreciation of the relevant product standards and an understanding of several issues including (i) material properties, (ii) design requirements, (iii) availability and the cost of steel, and (iv) product specification.
As the number of aging and deteriorated bridges in many places around the world has grown, so has interest in improved materials for construction, repair and rehabilitation projects. Bridge designers and builders increasingly seek materials that are stronger, more durable and less subject to corrosion or other distress than conventional steel and reinforced concrete. High-strength steel can be used to produce bridges that are more cost-effective, stronger, lighter and even more resistant to weather conditions than conventional steel. These bridges also have improved fatigue and corrosion resistance. In a study by the US Federal Highway Association, high-strength steel was found to provide lifetime cost savings of up to 18 % and weigh 28 % less than traditional steel bridge design materials. Designers and engineers can now specify new high performance steels that have yield strengths of 500 N/sq mm and 700 N/sq mm. They have superior toughness and can be welded with little or no pre-heat.
For structural use in bridges, steel products are cut to size and welded. In the structure, the material is subject to tensile and compressive forces. The steel generally responds in a linear elastic manner, up to a ‘yield point’, and thereafter has a significant capacity for plastic straining before failure. Steel derives its mechanical properties from a combination of chemical composition, mechanical working and heat treatment. The chemical composition is essentially a balance between achieving the required strength through alloy additions, whilst maintaining other properties (i.e. ductility, toughness and weldability). Mechanical working is effectively rolling the steel; the more steel is rolled, the stronger it becomes, but this is at the expense of ductility. ‘Heat treatment’ covers the control of cooling as the steel is rolled, as well as reheating and cooling processes that can be employed to influence a range of material properties.
The principal properties of steels of interest to the designer are (i) yield strength, (ii) ductility, (iii) toughness, and (iv) weldability.
- Yield strength – The yield strength is the most significant property that the designer needs to use or specify. Yield strength reduces slightly with increasing plate thickness. Steels of 350 N/sq mm yield strength are predominantly used in bridge applications because the cost-to-strength ratio of this material is lower than for other grades. Higher strength steels may offer other advantages, but they are less readily available and the additional strength is of little benefit if fatigue or maximum deflection governs.
- Ductility – Ductility is a measure of the degree to which the material can strain or elongate between the onset of yield and the eventual fracture under tensile loading. Good ductility offers the ability to redistribute localized high stresses without failure and to develop plastic moment capacity of sections. The bridge designer relies on ductility for a number of aspects of design and fabrication. It is therefore of paramount importance to all steels used in structural applications for the bridges.
- Notch toughness – The nature of steel material is that it contains some imperfections, albeit of very small size. When subject to tensile stress these imperfections tend to open. If the steel were insufficiently tough, the ‘crack’ would propagate rapidly, without plastic deformation, and failure would result. This is called ‘brittle fracture’ and is of particular concern because of the sudden nature of failure. The toughness of the steel, and its ability to resist this behaviour, decreases as the temperature decreases. The requirement for toughness increases with the thickness of the material. Hence, thick plates in cold climates need to be much tougher than thin plates in moderate climates. Toughness is specified by requiring minimum energy absorption in a Charpy V-notch impact test, which is carried out with the specimen at a specified (low) temperature and the requirement is given as part of the grade designation.
- Weldability – All structural steels are essentially weldable. However, welding involves laying down molten metal and local heating of the steel material. The weld metal cools quickly because the material offers a large ‘heat sink’ and the weld is relatively small. This can lead to hardening of the ‘heat affected zone’ of the material adjacent to the weld pool and to the reduced toughness (often called embrittlement). The greater the thickness of material, the greater is the reduction of toughness. The susceptibility to embrittlement also depends on the quantity and nature of the alloying elements, principally the carbon content. This susceptibility can be expressed as the carbon equivalent value (Ceq), and the material standards give an expression for determining this value. The higher the Ceq, the more difficult it is to weld. Weld procedure specifications are drawn up that set out the necessary welding parameters for any particular steel grade and weld type, to avoid embrittlement.
Steel products like plates, hot rolled sections (beams, channels, angles etc.), steel cables, and hollow sections are used in the construction of bridges.
In hot rolled sections universal beams are the most used section. These beams can be fabricated or rolled but rolled beams are normally preferred.
Steel cables used in bridge construction are generally referred to as bridge strand or bridge rope. They are constructed from individual cold-drawn wires that are spirally wound around a wire core. The nominal diameter can be specified between 15 mm and 100 mm depending on the intended application. Strands and cables are almost always galvanized for use in bridges where internal corrosion between the wires is a possibility. Because cables are an assemblage of wires, it is difficult to define yield strength for the assembly. Therefore, the capacity is defined as the minimum breaking strength that depends on the nominal diameter of the cables. Since cables are axial tension members, the axial stiffness needs to be accurately known for most bridge applications. Because relative deformation between the individual wires affect elongation, bridge strand and rope is pre-loaded to about 55 % of the breaking strength after manufacturing to “seat” the wires and stabilize the deformation response. Following pre-loading, the axial deformation becomes linear and predictable based on an effective modulus for the wire bundles. Bridge rope has an elastic modulus of 140 N/sq mm. The elastic modulus of bridge strand is 165 N/sq mm.
Hollow structural sections (HSS) are commonly used in building construction and they can be considered as an option for some bridge members. Increased lateral bending and torsional resistance can make them an attractive option for cross bracing and other secondary members subjected to compression. HSS have also been used to fabricate trusses used for pedestrian bridges that are subject to lower fatigue loading. HSS commonly refers to cold-formed welded or seamless structural steel tubing. The minimum specified yield and tensile strengths of 350 N/sq mm and 430 N/sq mm, respectively. The shapes are usually formed by cold bending carbon steel plate into the required shape and making a longitudinal seam weld along the length. Both round and rectangular shapes are available with various cross sections and wall thicknesses. The suitability of HSS for bridge members subject to the fatigue and fracture limit states has not been established. Cold bending of the corners of rectangular shapes can lead to reduced notch toughness in the corner regions. Another possible concern for bridge use is the need to control internal corrosion within the tubes, since the interior of the tube cannot be accessed for visual inspection.