steel construction

from Wikipedia, the free encyclopedia
Riveted framework of a bridge

The steel construction refers to the portion of the civil engineering in which for the construction of, structures primarily steel is used.

Overview

In steel construction, rolled steel girders , sheets and pipes made of structural steel are connected to one another to form a supporting structure by screwing , welding or riveting . Another central structural element of the steel structure is the gusset plate , which connects the individual bars of the structure. In addition to pure steel construction, there is also steel composite construction , which connects steel elements with concrete , and steel frame construction . The design of steel structures is usually carried out according to Eurocode 3: Design and construction of steel structures (EN 1993)

The steel construction combines the advantage of the comparatively short planning and construction time with a flexible design of the supporting structure. This flexibility results, for example, from the use of relatively light and slim, heavy-duty components and a high, as well as precise degree of prefabrication and thus shorter assembly times . Components made of steel that are exposed to the weather must be protected from corrosion by surface coatings or galvanizing . If necessary, fire protection can be ensured by means of fire protection cladding or fire protection coatings. In recent years, the importance of planning, building and operating buildings sustainably has increased. The actors in the construction and real estate industry develop a holistic view of their projects. Hardly any other building material is as suitable for sustainable construction as steel: Due to its high strength, it can easily support entire high-rise buildings even with a low construction weight and filigree structures . If these are dismantled later, the steel used can be separated from the demolition mass with magnets. 11% of the construction steels collected are already being reused directly in new buildings, the rest can be converted back into high-quality steel as secondary raw material (scrap). The new steel can even be given a higher strength than the starting material. The slightly increased cost factor for structural steel is often put into perspective by a quick construction phase , flexibility of the supporting structure due to wide spans and the reusability or recyclability of steel structures compared to ostensibly more cost-effective structures such as e.g. those made of reinforced concrete. In principle, they appear to be sensibly used wherever high strength requirements are placed on the construction, for example with large spans of roof structures in steel frame construction or, for example, when aesthetic, formal design reasons require slim constructions.

Steel construction is divided into

Cross-section classification according to Eurocode 3

In steel construction there are 4 cross-sectional classes, which can be calculated differently, whereby class 1 is so compact that not only the theory of plasticity is applicable, but also a sufficiently large rotation capacity that the plastic hinge theory can be used, which enables an economical calculation . Cross-section classes 3 and 4 often allow economical dimensioning, as they are slimmer and thus generally allow more efficient lever arms with a smaller cross-section (lower material consumption).

  • Class 1: Plastic on both the cross-sectional and system level
  • Class 2: Plastic at the cross-sectional, but not at the system level
  • Class 3: Elastic
  • Class 4: The plastic calculation is not permitted due to local dents.

Corrosion protection

Car park under construction with hot-dip galvanized or duplex-coated (hot-dip galvanized + coated) steel frame
High-bay warehouse with hot-dip galvanized steel elements

As a rule, steel structures have to be protected from corrosion. This is usually done by coating the structure with anti-corrosion paint or by hot-dip galvanizing . Corrosion protection is regulated in the standards of the EN ISO 12944 , EN ISO 14713 and EN ISO 1461 series . Since steel has a high affinity for oxygen, oxidation occurs, i.e. a transition from an energy-rich metal state to an energy-poor oxide state. In the case of other metals such as aluminum and zinc, the formation of a very dense oxide layer protects the metal from further oxidation. In the case of atmospheric steel corrosion, rust or iron (III) oxide hydroxide is formed in the presence of oxygen and water (at a humidity of over 65%) ; chemical FeO (OH) , which is additionally accelerated in aggressive atmospheres (salts, especially chlorides or acids). Rust ( FeO (OH) ) has 25.37 cm³ / mol, 3.6 times the molar volume of iron (7.1 cm³ / mol). Therefore, the volume of iron increases by at least this factor due to corrosion, see Pilling-Bedworth ratio . The increase in volume can also be significantly greater due to porosity and water retention. This increase in volume causes coating materials to flake off around defects in a coating .

A distinction is made between two systems for corrosion protection:

  1. by coating and
  2. through metallic coatings.

Coatings consist of a production coating, a base coating (previously mostly zinc chromate or red lead , today mostly pigmented (zinc dust, zinc phosphate ) synthetic resin coatings) and a top coating (at least 2-layer application, as protection against moisture and UV rays), the coating materials of which consist of pigments, Binders and fillers exist. Metallic coatings consist of a metallic protective layer, in the case of structural steel mostly in the form of hot-dip galvanizing in immersion baths. Due to the nature of the process, steel parts to be galvanized must be designed for hot-dip galvanizing prior to being immersed in the approximately 450 ° C hot zinc melt. Another corrosion protection for steel components are so-called duplex systems, which combine hot-dip galvanizing or sherardizing with a subsequent coating. Duplex systems are used when steel needs to be protected from corrosion for an extremely long time.

In the case of ropes, internal protection is provided by filling the cavities during the stranding process with linseed oil and red lead paste, while external protection is provided by thick, elastomeric plastics that do not hinder the relative movements and bending of the individual links.

In addition, the shape and arrangement of the steel components should protect them from possible corrosion: Prevention of water pockets and dirt deposits, free access to the steel parts, or air and water vapor-tight sealing.

Fire protection

Steel structures often require special fire protection , as the thin-walled cross-sections of the beams and their good thermal conductivity make them heat up quickly in the event of a fire, thereby reducing their strength. Depending on the fire load and the intended use of the structure, the failure of the structure can be prevented by overdimensioning the components to match the required fire resistance period or with special jackets. The mechanical properties of the steel are temperature-dependent, so that, for example, the yield point at 600 ° C drops by half the value at 20 ° C. The modulus of elasticity also decreases with increasing steel temperature. For fire protection, a “fire resistance period” required by law for the respective building must be observed, which is defined for normal buildings in the state building regulations of the federal states. This required fire resistance period is divided into categories depending on the structure and use, according to the German standard ( DIN 4102 - Fire behavior of building materials and components) into F30, F60, F90, F120 or F180. The numbers indicate the minimum value that the construction must withstand the fire, expressed in minutes. The "standard fire" to be assumed for the oversizing of the component or for the determination of the insulating fire protection measures is the standard temperature-time curve, also called "ETK" for short . It describes a temperature-time curve according to which the gas temperature is heated in a component test. According to the specifications of the ETK, the gas temperature surrounding the "protected" component rises steeply to over 600 ° C within the first few minutes and then increases slowly but steadily until the component fails. The time until the construction fails is rounded down to the fire resistance class of the standard. In this way, all additional measures to protect a steel component prove their performance profile.

The method of over-dimensioning (according to the European standard EN 1993-1-2) is based on a calculation. The starting point is the mathematical determination of the steel temperature in an ETK fire with the required (fire resistance) duration. By determining the steel temperature, the mechanical properties required for dimensioning can be determined. The actual dimensioning takes place similar to the “cold” dimensioning with the heat-affected mechanical properties under the fire-adjusted safety values. This design method was calibrated on the basis of tests.

Fire protection measures subsequently applied to the steel component have an insulating, shielding or heat dissipating effect.

  • Insulating fire protection measures: the profile shape following casings and cladding of steel profiles made of cement-bound spray plaster with vermiculite or mineral fibers, mostly with the necessary plaster base. Composite column systems (construction from composite construction ) mostly meet the requirements without additional measures. Furthermore, box-shaped cladding (plasterboard, thickness and fastening according to the manufacturer's approval) [⇒ F90 possible] of the steel profiles with an additional corrosion protection application. Insulating layer formers in the form of coatings (spray / brush / roller application) can be implemented with economically interesting layer thicknesses (approx. 300 to 1400 µm, corresponding to approx. 2-4 work steps) up to F60. With layer thicknesses of up to more than 3 mm (> 5 work steps), intumescent layers can now also be applied for a fire resistance class F90 (see approval Z-19.11-1794 of the DIBt - web links). The definition of the necessary layer thickness depends on the ratio of the cross-sectional circumference exposed to the flame to the cross-sectional area (U / A value), the type of profile (open or closed) and the type of component. Since intumescent coatings form a surface similar to orange peel due to their great thickness, if a high surface quality is required, complex post-processing (grinding, filling) must also be carried out. Orange peel can largely be avoided by using modern water-based systems (see approval Z-19.11-1461).
  • Shielding fire protection measures: mostly already existing, space-enclosing systems such as suspended ceilings.
  • Heat-dissipating fire protection measures: Backfilling of the steel profile cavities (supports) with pump-independent, thermally freely circulating water. Particularly suitable for high-rise construction.

Every fire protection measure has its advantages and disadvantages. Therefore, when planning, aesthetic, economic, technical and safety factors should be carefully weighed against each other.

The Cruise Center Baakenhöft in Hamburg's HafenCity is probably the first building in Germany in which R30 fire protection was implemented using hot-dip galvanizing.

Recently, corrosion protection by hot-dip galvanizing has also been used under fire protection aspects. A research project at the Technical University of Munich, completed in 2019, has shown that hot-dip galvanizing improves the fire resistance of steel. This means that a fire protection period of 30 minutes is often possible with unprotected, hot-dip galvanized steel structures.

Well-known structures made of steel

Killesberg tower in Stuttgart made of hot-dip galvanized steel

Well-known structures made of wrought iron

All malleable iron has been called steel since the early 20th century, after wrought iron , which was widespread in the 19th century, is no longer manufactured. For this reason, older structures made of wrought iron are often referred to as steel structures, which is correct according to the current definition of steel because wrought iron contains less than 2% carbon, but is historically incorrect because the wrought iron of that time contained higher amounts of undesirable accompanying elements than steel . These structures made of wrought iron include u. a .:

See also

literature

  • Frank Werner and Joachim Seidel : The iron construction. From the development of a construction method . Berlin / Munich: Verlag für Bauwesen 1992, ISBN 3-345-00466-6 .
  • Karl-Eugen Kurrer : From construction with iron to modern structural steelwork . In: The History of the Theory of Structures. Searching for Equilibrium. Berlin: Ernst & Sohn 2018, pp. 530-639, ISBN 978-3-433-03229-9 .

Web links

Individual evidence

  1. ↑ R30 fire protection through hot-dip galvanizing - Cruise Center Baakenhöft is Germany's first project. (PDF) In: Feuerverzinken Magazin 1-2020 p. 6. Accessed on May 16, 2020 .
  2. ↑ Gaigl, Ch., Mensinger, M .: Fire resistance of hot-dip galvanized, load-bearing steel structures in the event of fire. Retrieved May 6, 2020 .