Even if the construction industry is presently not performing well in many European countries, structures made of steel are becoming more and more popular. The advantages of this building material (such as high resistance, safe and high-quality fabrication and sustainability) are becoming more and more appreciated by architects and civil engineers. Further, steel structures have important advantages over concrete structures, such as short erection time and the ability to fulfill high architectural demands. The steel industry has supported this process through the development of more efficient steel products. This article focuses on such new developments in the steel products used in construction and heavy steel plates. In particular, it will demonstrate through examples of ultra-thick plates and high strength grades how heavy steel plates can be used for efficient and architecturally demanding steel structures.
Since the first application of steel to structures in the 19th century, the development of steel construction in civil engineering has been closely linked to developments in material properties and production methods. Significant achievements concerning strength, economy, design versatility, fabrication, erection techniques and service performance would not have been possible without substantial improvements in steel, especially through the application of new production processes for carbon steel grades with improved characteristics (strength, fabrication properties, durability and so on). Today, the application of these grades is mainly driven by economy, architecture, environment and safety.
By increasing the strength of steel, the cross-sectional area of the steel structural elements used can be reduced. This may reduce fabrication and erection costs, an important advantage in high-wage economies.
The size of the structural elements can be reduced with higher strength grades, allowing for special aesthetics and elegant construction that integrate well into the environment.
Steel-saving construction through use of efficient steel products also means a more efficient usage of our world’s limited resources.
Modern steel grades do more than simply offer high strength values. Special grades combine this strength with excellent toughness so that a high level of safety is applied both in the fabrication and application of the structures. This holds true particularly for modern offshore steel grades performing at the lowest service temperature.
This paper focuses on recent developments in the steel industry to offer tailor-made steel products for application in the fields of civil engineering (such as bridge building, high-rise buildings, stadiums and hydropower stations). Some outstanding examples will be given.
Dimensions of Heavy Steel Plates
Extra-wide Steel Plates
Steel fabricators in Western Europe are facing the fact that, in the long term, wages increase more quickly than the prices of their semi-finished products. Therefore, it is advisable to use plate products so that manufacturing efforts (in particular welding) can be reduced.
Today, it is possible to produce heavy steel plates in widths of up to 5200 mm. By the use of these extra-wide plates, additional butt welds in longitudinal or transverse directions can be avoided, thus cutting manufacturing costs.
For instance, on the new high-speed train bridge across the Hollandsch Diep between Brussels and Amsterdam, plates up to a width of 4550 mm were applied in order to reduce the number of weldings. Of course, this not only results in more efficient and quicker manufacturing, but also in a reduced sensitivity to fatigue, as the fatigue strength of a weld is always poorer than that of the base material.
Another example of the profitable usage of extra-wide plates is the impressive roof construction for the Olympic Stadium in Athens, designed by the famous Spanish architect Santiago Calatrava. On both longitudinal sides, the roof is carried by a lower arch with an inner tube diameter of 3.25 m and an upper main arch with an inner tube diameter of 3.60 m. The two main arches were assembled from 260 tube segments of 5 m length each. These tubes exhibited thicknesses of up to 100 mm. By using plates in widths of up to 5000 mm, it was possible to assemble the tube segment from only two semi-shells formed on a 6000-t press. Thus, the number of costly and time-consuming longitudinal butt welds was reduced to a minimum on these thick-walled tubes.
Ultra-thick Steel Plates.
Whereas the production of extra-wide plates only has a secondary influence on the mechanical characteristics of steel products, these values may be quite significantly influenced by the thickness of the plates. On the other hand, the thicker the steel material used, the higher the requirements on toughness in order to avoid the phenomenon of brittle fracture. This results in the selection of superior steel grades for tendentiously thicker constructions.
For instance, the coming European construction code EN 1993-1-10—selection of material for fracture toughness and through thickness properties—defines a maximum allowable plate thickness of 65 mm for a conventional S355J2G3 (that means a minimum yield stress of 355 MPa and Charpy-V-test of minimum 27 J at -20 °C), for a reference temperature of -30 °C and a medium load . For the application of thicker constructional elements, a higher grade with improved toughness has to be chosen—for instance, an S355NL (i.e., a minimum yield stress of 355 MPa and Charpy-V-test of minimum 27 J at -50°C).
Due to the lack of steel products providing sufficient toughness properties even if thickness is increased, thicker plates were almost banned from use in most countries. For instance, the German construction code DIN 18800 defines a maximum thickness of 50 mm for flange plates. In order to compensate for this shortage, thick flanges were designed comprising additional, reinforcing lamellae on the top of a basis lamella.
Today, the steel industry succeeds in producing thick plates with appropriate toughness properties due to:
- Modern steel-making technology (vacuum degassing, argon stirring etc.) reducing the amount of residual elements (such as sulfur and phosphorus), and minimizing the number of non-metallic inclusions;
- The availability of slabs and ingots with large thickness, as the deformation degree (i.e., the ratio between initial slab thickness and final thickness of the plate) strongly correlates with toughness and through-thickness properties;
- Up-to-date rolling technology, for instance HS-rolling (high shape factor rolling) characterized by high deformation degrees during the first rolling passes to improve the core properties of the product.
Therefore, steel plates featuring sufficient toughness properties to resist brittle fracture are currently available. For instance, fig. 1 illustrates toughness values of plates measuring more than 80 mm in thickness. It can be seen that adequate toughness values can be obtained even with extreme thickness.
Thus, it becomes possible for steel construction to benefit from these products by substituting the classical multiple-lamallea-type flange construction. Furthermore, the use of thick plates simplifies the fabrication process.
A typical example from steel construction is the French fast railroad network (TGV). A kind of composite bridge was developed that has proven to be competitive with classic concrete structures. This system mainly consists of two longitudinal girders, each with a girder depth of up to 5000 mm, connected to each other by diaphragms arranged at distances of 8 to 12 m. A concrete deck was cast onto these girders . Plates with thickness of up to 150 mm form the flanges of the girder. In order to meet the regulations for brittle fracture, steel grade S355NL (with a Charpy-V toughness tested at -50°C) is employed.
A bridge system like this represents a convincing alternative to the pre-stressed concrete bridges usually prevailing in the high-speed train bridge market. Note that this composite design was applied to all major bridges within the latest French fast railroad line, the TGV Est from Paris to Metz/Nancy, which will be opened for traffic mid-2007. The bridge across the Mosel Valley is one example from this new line.
An example of the application of ultra-thick plates to multi-story buildings is the twin towers of the new Munich Business Towers in the northern part of the city. They have a height of 126 m and 113 m respectively. The structure consists of an innovative steel-concrete composite structure. The main supporting elements are composite columns built out of massive steel profiles that are inserted into concrete-filled steel tubes. The steel core of the composite columns are formed by joining plates with a thickness of up to 160 mm.
This construction stands on foot plates measuring up to 250 mm in thickness. These plates serve to distribute the forces into eight high-strength anchoring bars. Thus, the plates are dimensioned properly to transfer loads of up to 93000 kN (compression) and 30000 kN (tension). It is essential to ensure best deformability in thickness direction (Z35 according to EN 10164) due to the high load in the through-thickness direction.
High-Strength Steel Grades
Benefits of steel construction.
The development of new steel grades for steel construction was driven by the users’ demand for materials showing good mechanical characteristics, such as yield strength and toughness, as well as proper manufacturing properties ensuring efficient manufacturing in the workshop and during on-site erection of the steel structure.
Though the classical constructional grades S235 and S355 (i.e., steel grades with a nominal yield stress of 235 MPa and 355 MPa respectively) are still most widely applied to steel buildings and bridges, higher strength grades have gained significant market shares during the past decade. The benefit of using high strength steel in steel construction is obvious: In comparison to normal strength steel, the size of the cross-section of the structure can be reduced, resulting in a decrease in the dead weight of the structure (whereby the substructure and erection profit). Simultaneously, the cross-sections of welded joints are reduced, thus lowering both manufacturing and inspection costs and paving the way for special architectural demands .
However, it has to be taken into account that smaller cross-sections also lead to reduced stiffness in the construction. This can hamper the usage of higher strength steel, as not only maximum load criteria but also maximum deflection criteria are often applied to steel construction. Furthermore, steel manufacturers demand steel products that are easy to process. Here, weldability, even under harsh conditions (e.g., on site), is a major concern. Therefore, quenched and tempered heavy steel plates, which are today produced with yield strengths of up to 1100 MPa, are rarely used for steel bridges and buildings. As far as higher strength grades are concerned, thermomechanically rolled plates with the best welding properties and yield stress levels of up to 500 MPa are used preferentially.
TMCP Rolled Steel Plates
The aim of the thermomechanical control rolling process (TM or TMCP) is to create an extremely fine-grained microstructure. This is accomplished through a sophisticated combination of rolling steps at selected temperatures and a close temperature control. The gain in strength obtained by the fine-grain refinement allows for the effective reduction of the carbon and alloy content of the TM-steel compared to normalized steel of the same yield strength category grade. The improved weldability that results from the leaner steel composition is a major advantage of the TM-plates. The rolling schedule is individually compiled depending upon the chemical composition, the required strength and toughness properties and the plate thickness. Arranging for some accelerated cooling after the final rolling pass is beneficial to achieve the most suitable microstructure, especially with thick plates, as it forces the transformation of the elongated austenite grains prior to any occurrence of re-crystallization. For very thick plates and higher yield strength grades, a tempering process can be used after the accelerated cooling.
TM-rolled plates with minimum yield strength values of 500 MPa were supplied in thicknesses up to 100 mm for hydropower, offshore platforms and special ships .
TMCP-steel represents a material with an optimum working point in the contradictory context between strength and processing properties. As already mentioned, weldability is positively affected by the alloying content, which is usually expressed in so-called carbon equivalents in order to assess plate S460M (nominal yield stress of 460 WO) and for the high-strength quenched and tempered plate S690 (nominal yield stress of 690 MPa). It can be seen that the higher strength TMCP-steel S460M shows an even better carbon equivalent than the conventional S355J2 steel.
Application of High-Strength Steel Plates
A very convincing example of the use of higher strength steel grades in bridge building is the Millau Viaduct in the south of France, which was opened late 2004. This is the highest bridge in the world, comprising a total height of 343 m, a deck height of up to 270 m and a length of 2460 m. The deck, with six equidistant spans of 342 m each and two outer spans of 204 m each, is held by stay cables fixed on seven steel pylons. The total weight of the steel plates used for this extraordinary bridge is 43000 t—among this, 18000 t of higher-strength TMCP-rolled S460M.
The main element of this bridge is formed by a box girder that contains another central box onto which side panels are affixed. S460M plates of up to 80 mm thickness are used for the central box, whereas the side panels are formed by plates of up to 16 mm thickness. A very efficient manufacturing of the deck was possible thanks to plate widths of up to 4200 mm. From these plates and trapezoidal stiffeners, approximately 2100 stiffened panels were made in the workshop of Eiffel Construction Metallique in Lauterbourg (Alsace/France).
Following transportation to the site, these elements were welded together until a total length of 171 m was reached. This work was done at the two preassembly yards located on each abutment of the bridge. The central box sections were pre-assembled at Fos-sur-Mer. The bridge was erected by incremental launching from the north and south abutments. As soon as a new 172-m long segment was finished, this already existing part of the deck was pushed forward. Auxiliary piers bisecting the span length helped to reduce the cantilever effect during launching.
A huge amount of high-strength steel S460M was again employed for the deck structure (in particular for the inner box girder) in order to reduce the dead weight of the girder and, thus, the cantilever deformation during the launching procedure.
There are two other interesting examples of the application of S460M TMCP-rolled plates. The new Rhine bridge in the north of Dusseldorf (Germany), which was opened for traffic mid-2002, is a cable-stayed bridge with a central span length of 275 m. The height of the pylons had to be restricted to 34 m due to their situation in the entry lane of the nearby airport. As a result, high forces arise in the pylon heads. Selecting the high-strength steel S460ML for these structural elements was the only way to cope with this challenge. Plate thicknesses of up to 100 mm had to be used for the central parts of the pylon heads. In order to have the highest degree of material redundancy  against brittle fracture, the toughness requirement was defined by a Charpy-V test at -80 °C.
The Harilaos Trikoupis across the Gulf of Patras in Greece is a cable-stayed bridge with three central spans of 560 m and two side spans of 286 m each. S460M (TM-rolled) was used for the plated girders of the composite deck, the thickness of which was 80 mm. S460Q (quenched and tempered) was used for thicker plates. Furthermore, the pylon heads were made from S460M in plate thicknesses reaching up to 110 mm, with a total weight of 700 t.
Buildings. Other sectors aside from bridges have profited from the recent developments in metallurgy. Due to the strict requirements of architects on the aesthetics of multi-story buildings, heavy steel skeleton constructions have been used more commonly over the past 10 years. With these constructions it becomes possible to reduce the cross-section of heavy columns by applying high strength steel in order to allow for an efficient manufacturing and erection process, and also to optimize the ratio between gross and net surface of the floors. A typical example is the Commerzbank Tower in Frankfurt/Main with a height of more than 298 m . Its steel framed structure contains about 18000 t of heavy steel plates. Here, steel S355M is employed in plates with thickness exceeding 30 mm, whereas S460M was applied to highly loaded girders and columns. Thus, manufacturing costs can be reduced through the proper selection of heavy steel plates.
Examples in hydropower
Due to the globally increasing demand for energy, investments in energy production are also growing. In particular, emerging markets tend to invest in hydropower energy.
In a pump-storage hydropower plant, two fields of application are of interest for the usage of heavy steel plates:
- The penstock lining, a large-diameter pipe (up to 6 m in diameter) either installed on the surface of the dam or in a tunnel in the interior of the mountain and often embedded in concrete.
- The pipeworks and machine components in the powerhouse around the turbine, such as spiral casings, stay rings and the exit pipe.
The major requirement set on the heavy steel plate products used for the penstock pipe is efficient manufacturing, not only in the workshop but also under hard environmental conditions (for onsite welding). Today, steel plates in grades up to S690 in quenched and tempered condition are used for such pipes. In the last year, TMCP-rolled steel with up to 500 MPa yield stress was also developed for this application. Of course, this grade requires a design with thicker plates compared to S690Q. However, this disadvantage is compensated for by the much better weldability of the TMCP grades. This not only results in higher efficiency of the welding process in the workshop and on site, but also in improved safety of the weld due to better toughness properties.
Furthermore, high-strength steel plates are used for powerhouse components, such as spiral casings. These heavy steel plates, measuring up to 250 mm in thickness, are quenched and tempered. Accordingly, they offer the highest deformation properties in thickness direction, which makes them suitable for stay rings.
This article highlights the recent developments made by the steel industry in order to supply steel products for efficient construction in civil engineering. Many examples, including bridge building, stadiums and high-rise buildings, are presented.
Today, the designers of constructional steelwork can choose from a nearly unlimited range of heavy steel plates (as far as dimensions and steel grades are concerned). Designers are attracted to the nearly boundless possibilities of combining optimum dimensioning with an appealing design, capped off by efficient manufacturing properties with regard to proving a construction’s economic and competitive nature, especially when compared to pre-stressed concrete and wood. The current delivery programs for plate products consider both the demands and desires of the coming decades.
Future developments in heavy steel plate technology for steelwork are governed by the user’s desire to continue to reduce manufacturing costs. This can be accomplished by further improving heavy steel plates. In addition, more stringent requirements on the uniformity of mechanical properties, chemical composition and dimensional tolerances represent a great challenge for the processing and rolling technologies of heavy steel plates.