The application of modern high-strength steels offers to designers working in the fields of heavy buildings and steel bridges the opportunity to realize structures that not only fulfill the highest demands in elegance and aesthetics, but also take into consideration the need for economical and environment-friendly construction. By using heavy plates made of thermomechanically rolled steel grades, the total weight of the structure can be reduced and fabrication costs can be saved due to the outstanding strength, toughness and fabrication properties. For instance, these materials show an excellent weldability, despite their high strength.
These heavy plates can be delivered in a nearly unlimited range of dimensions, facilitating an individual solution to the design problem. Therefore, heavy plates of the steel grades S355M/L, S420M/L and S460M/L in thicknesses up to 120 mm have been widely used in the construction of bridges and multi-story buildings all over Europe. Furthermore, longitudinal profiles plates demonstrating a profile along the length (in various shapes) are available. By using these LP-plates, weldings can be avoided and fabrication costs can be reduced, while structural safety can be increased. These modern steels represent the right answer to the increasing requirements of modern constructional materials.
1. Development of Heavy Plates
At present, heavy plates are widely used in constructional steelwork—particularly the steel grades between S235 and S355 (i.e., those with a yield stress of 235 and 355 MPa respectively). The development of heavy plates for steel structures during the past few decades firstly aimed at increasing the strength level, while at the same time maintaining an acceptable weldability. In this way, the volume of steel used in the construction could be reduced. At the beginning of the ’70s, high-strength steel levels S460 and S690 (yield stress of 460 and 690 MPa respectively) were developed. However, they required a relatively difficult welding procedure and higher fabrication costs, which limited their wider use.
The demand to reduce weight (i.e., the reduction of the deadweight of the structure, and of the total volume of steel required) was the starting point for the development of longitudinally profiled plates (LP-plates) . By a special control of the rolling gaps during the rolling process, a longitudinal profile with a continuously varying thickness along the length of the plate can be given to a heavy plate. In this way, various types of LP-plates with different shapes can be produced (Figure 1). Such plates allow for an optimized adaption of the plate thickness to the actual stress in the structure.
An example of the application of these plates is presented in the Figure 2. In the Schengen Viaduct border bridge linking Germany with Luxembourg across the river Mosel, more than one third of the total steel used was LP-plates—particularly for the upper flanges of the open-box girder construction.
Today, LP-plates are applied in bridge-building all over Europe. Besides reducing the weight, the application of LP-plates also saves fabrication costs and time due to the possibility of avoiding complicated weldings. Moreover, structural fatigue behavior is optimized by avoiding fatigue-sensitive weldings, particularly in steel and composite bridges.
Aspects of fabrication and manufacturing cost savings gained more and more importance during the ’80s and led to the development of the modern generation of thermomechanically (TM or TMCP) rolled heavy plates [2, 3]. TM-rolled heavy plates in steel grades S355M, S420M and S460M, which have been available since the end of the ’80s, offer not only high strength but also an excellent weldability. Basically, the material influences the weldability through its carbon equivalent, a summary of the effects of the alloying contents on cracking tendency. TMCP-rolled steel enables much lower carbon equivalents than corresponding normalized steel grades of the same stress yield, leading to the possibility of designing even more economical steel constructions.
2. Building Construction
As far as building construction is concerned, two different fields of application may be distinguished—on the one hand, the simple and standardized multi-story and hall buildings; and on the other hand, the heavy welded constructions for industrial halls, power plants and special tall multi-story buildings. Regarding the standardized constructions, which hold the biggest share of total steel consumption in the construction market, heavy plates are only used for head plates or stiffeners for framework constructions predominantly composed of rolled beams. Steel grades S235 and S275 are usually applied here.
In cases in which high loads or large spans have to be designed, columns, piles and girders are normally welded and made from heavy plates. This design and construction method shows an economical advantage from a girder height of about 600 mm because the cross-sections of the supporting structure can be adapted individually to the constructional task through a minimal use of steel. Steel grade S355 is predominantly applied for these applications, but sometimes even heavy plates of the higher steel grade S460 are used.
Common dimensions for heavy plates in the heavy building construction area are thicknesses from 8 to 100 mm, widths up to 1.5 m and lengths up to 18 m.
The following examples demonstrate the typical application range of heavy plates:
- Power plant buildings (e.g., the thermal power station Schwarze Pumpe, Germany, [Figure 3]) with an overall height of 161 m. The columns and beams were mainly fabricated with TM-rolled heavy plates S355M/ML and the standard structural steel S255J2G3 mod. in plate thicknesses up to 65 mm. Additionally, S690QL was used in some high-tension loaded areas.
- Commerzbank Tower, Frankfurt, Germany (Figure 4): Due to the very strict requirements of architects in regards to the aesthetics of multi-story buildings, heavy steel skeleton constructions have been used more and more over the past 10 years. A typical example of this is the Commerzbank Tower in Frankfurt, with a height of more than 298 m . Its steel-framed structure contains about 18000 t of heavy plates. TS355M steel was used for plate thicknesses exceeding 30 mm, while S460M was applied in highly loaded girders and columns. In this project, fabrication costs could be reduced through the optimal selection of heavy plates.
Mainly heavy plates of the steel grade S355 are used in European bridge-building. However, high strength plates of S420 and S460 are being employed more and more for big-span bridges. Sometimes even the higher-strength steel grade S690 is used (e.g., in the water-quenched and tempered condition). Normally the plate thicknesses are less than 50 mm, but in some instances plates of thicknesses up to 150 mm can be found in the load-bearing sections.
Typical examples of modern bridge-building are given in the following:
- Erasmus Bridge, Rotterdam, The Netherlands: The Erasmus Bridge connects the inner city of Rotterdam with the north bank of the Nieuwe Maas, Kop Van Zuid. The steel bridge has an overall length of 499 m, with a 410-m-long cable-stayed bridge composed of a 139-m-tall pylon and an 89-m-long flap bridge. In total, 6000 t heavy plates of steel grades S355M (thickness less than 100 mm, 4200 t), S460L (thickness less than 80 mm, 2000 t) and S460QL (thickness less than 125) were used for this bridge.
- Oresund Bridge, Denmark-Sweden: The Oresund fixed link consists of a 7500-m-long framework composite bridge, a 4000-m-long artificial island and a 3500-m-long tunnel. The approach bridges and the cable-stayed main bridge were built as a truss bridge with an upper concrete deck and a lower steel deck (railroad deck). Heavy plates of steel grades S460M/ML in thicknesses up to 80 mm (60000 t) were used for the shore spans, whereas the main bridge was constructed of S420M/ML in thicknesses up to 50 mm (16000 t).
Nesenbachtal Bridge, Germany: This bridge, located near Stuttgart, was constructed as a composite bridge with a welded-box girder and a concreted deck (Figure 5). In order to retain the very low construction height of 3,000 m, the highest-strength steel (S690QL) was used in the areas of high stresses, in a total amount of 200 t. Finished in 1999, this was the first bridge in Germany to use this material. Another recent example of the usage of this special steel grade was a road bridge in the GVZ Ingolstadt, in which graceful and thin pile heads were designed.
Today, the designers of constructional steelwork can choose from a nearly unlimited program of heavy plates when it comes to dimensions and steel grades. Thus, nearly unlimited opportunities are available to designers to combine optimal dimensioning and design of buildings, as well as efficient fabrication properties with regards to an economical and competitive construction. The current delivery programs of the plate products fulfill the demands and desires for the coming decades.
The future developments of heavy plate technology for steelwork are characterized by the user’s desire to facilitate further reductions of fabrication costs using further improved heavy plates. Additional, stricter requirements on the homogeneity of the mechanical and chemical characteristics and on dimensional tolerances and flatness signify a great challenge for the heavy plate processing and rolling technologies.
This article originally appeared at Dillinger Hütte and is reproduced with permission.