1. Metal structures - must knows

1.1.  Basics of steel structures

Thanks to their numerous advantages, steel structures have been in a power struggle with other common building materials, especially concrete, for more than a century. However, steel structures are still used for building large public buildings, especially multi-storey buildings, sports halls, and other halls with large rafter spans. Due to a fairly high price of steel and heavy rafters, investing in steel structures makes sense for such high-rises, but not for smaller buildings.

1.2. Steel or metal structures?

First, lets look at the difference between the two. The term metal structures includes those made of aluminium, iron, and steel. Therefore, its not enough to say that a building is made of metal, you have to specify which type. Iron structures have become obsolete, and are replaced by steel structures, while aluminium is only used for certain parts of a building, due to its high price.

1.3. Steel

In addition to understanding the difference between metal and steel structures, its also important we distinguish iron and steel. Steel is refined iron. This is an alloy of iron and primarily carbon, in addition to other elements. Although its present in small portions, usually less than 2%, its enough to have a tremendous impact on steels properties.

1.4. History of steel use

Widespread use of steel began in the late 19th century, when technological advancements and metal processing led to discovery of iron with improved properties, i.e. steel. Before this, steel structures didnt even exist. In the 19th century, steel first found its place in Art Nouveau, when it was paired with iron to form decorative features. Floral and geometric patterns were very popular, and they were used both as structural members and ornaments. In the late 19th century, there was another breakthrough in construction - discovery of reinforced concrete, which wouldnt have happened if it hadnt been for steel. French engineer François Hennebique was a pioneer in exploring the possibility of combining concrete and steel reinforcement and gave guidelines for reinforced concrete structures. Steel reinforcement, which is meant to withstand tensile stress and tensile deformations of concrete, enabled concrete slabs and rafters to span across enormous distances, which ultimately led to discovery of the free plan in the early 20th century, during modernism.

1.5. Steel structures in construction

Steel structures are assembled on the construction site by putting steel profiles together. So, a skeleton is made up of various independent elements. All steel profiles are prefabricated and transported from the factory to the construction site. Steel structures are comprised of steel rafters, columns, and purlins.

1.6. Advantages of steel structures

Steel structures have numerous advantages. The biggest advantage is the ability to reach incredible spans, much greater than other building materials can (reinforced concrete, wood). Another advantage is the variety of profiles with different characteristics, which leaves room for a wide choice of designs in line with the structural analysis requirements. It is these two properties that make steel one of the best building materials in complex high-rises. The third advantage is that, with proper protection, steel structure is almost indestructible and extremely hard-wearing. The fourth advantage, also worth mentioning, is the fact that steel profiles are prefabricated and when transported to the construction site, theyre ready to be assembled and installed. Unlike with reinforced concrete, which is cast in situ, the construction process takes little time, and the construction site remains clean. Also, assembly allows careful control of the quality of steel profiles. Last but not least, contrary to the popular belief, steel is an eco-friendly material as it is completely recyclable.

1.7. Disadvantages of steel structures

The most obvious shortcoming of steel structures is the high price of steel profiles. This is why steel is only used to construct buildings which need physical and mechanical properties that only steel has, but other building materials dont. This is especially true for high-rises. Another important disadvantage of steel is its weight, which wouldnt be a problem if it didnt place tremendous pressure on other materials in the structure. The third weakness of steel is that, despite fire proof coatings and contrary to the popular belief, its not a fire-resistant material as it melts at high temperatures. Finally, although theres a slim chance that the alloy has a poor composition, the fact that it does exist and may lead to the building collapsing is a flaw. This is what ultrasonic and radiographic tests are for - to detect such deficiencies on time.

2. Properties of steel

2.1. General physical and mechanical properties

Steels modulus of elasticity is 2.0-2.2×105 N/mm², which makes it an elastic material. Pure iron has hardness of only 60 HV, while in steel, this value can be as high as 800 HV due to processing (carbon addition), or even 2000 HV if chemically treated. Steels tensile strength can be up to 4000 N/mm², 20 times greater than that of iron.

2.2. Corrosion resistance of steel

Steels corrosion resistance can be adjusted to various requirements, just like its technological properties. To boost this resistance, its usually chromium thats added to the alloy (11%).

2.3. Steel shaping

Steel can be formed through metallurgical processes, i.e. hot works, including forging, casting, pressing, or rolling. Cold forming processes for steel include drawing, rolling, pressing, or cutting, but it can also be formed through powder metallurgy.

2.4. Hardened steel

As the name suggests, this is a hardened version of steel achieved through heat treatment. First, steel is heated up to its melting point, and then quenched, i.e. rapidly cooled. This is how martensite, a very hard structure, is formed. Rapid cooling as part of the tempering process causes significant internal stresses within the material, which increases the risk of cracking. To prevent this, the object being formed is quickly quenched in oil, water, or air, which gives it certain properties. The main purpose of hardening is to improve properties of steel, primarily its hardness.

3. Steel profiles

3.1. Steel profiles are semi-finished products

Because steel cant be used in its raw form, all elements made of steel are semi-finished products. This means theyre produced in a factory and then transported by lorries to the construction site, where theyre assembled into the right structure. Semi-finished products come with numerous advantages, the most prominent in this case being the high quality, as all profiles are standardized and precisely defined due to the supervised production process and pace of construction. Some architects may consider this a flaw, as they have to adjust their ideas to the material and not vice versa.

3.2. Types of steel profiles

There are plenty of standardized steel profiles with certain mechanical properties. Different types of steel profiles are distinguished based on their cross section, e.g. bar-shaped IPE profiles with I sections. In these profiles, the height is much greater than the width. HEA/HEB or H sections are similar to IPE profiles, but the difference between their height and width is smaller. If I sections have a rectangular shape, we can say that H sections are cubical. The height of IPE profiles ranges between 8 and 60 cm, while HEA/HEB profiles have the height between 10 and 100 cm. In addition to these, there are also box and pipe profiles, as well as U- and L-shaped profiles, named after the shape of their cross section. A structural analysis engineer and/or civil engineer specializing in steel usually take part in the decision-making regarding the choice of profiles.

3.3. Eurocode 3

All types of standard steel profiles, their properties, and dimension and structure requirements can be found in the standard titled Eurocode 3: Design of steel structures.

3.4. Characteristics of steel profiles

Steel profiles have excellent physical and mechanical characteristics. Due to iron properties, they can connect two very distant points, so theyre usually used for structures where this is needed, such as halls, bridges, stadiums, footbridges, etc. One of the main advantages of profiles is how easily they combine with other materials, which gives the resulting structure flexibility. Also, theyre very easy to maintain, but they do require protection from the elements to prevent rusting.

3.5. Joining steel profiles

Steel profiles can be joined into a skeleton either by welding or screwing. The choice depends on the structural requirements and the target look. Screws are always visible, unless covered with a different material, while a weld seam stays visible, although it can be smoothed out until imperceptible. Steel profiles can also be joined by soldering and riveting.

4. Steel houses

Due to the high price of steel, family homes made of steel are not that common. Nonetheless, steel houses have been around ever since the age of modernism, i.e. the first half of the 20th century. The advantage of steel skeletons is that they enable a completely free plan with large spans, which can be decorated according to the owners preferences and needs, and then completely modified in no time if need be. Some of the outstanding examples of steel homes include the Edith Farnsworth House designed and constructed by Mies van der Rohe and many houses built in the 60s as part of the Case Study House programme.

4.1. Steel & wood

Since steel structures can only form skeletons, they have to be paired with other materials to form the interior. Throughout history, steel was mostly combined with glass (one such example is the Edith Fransworth House by Mies van der Rohe). This combination is very chic and clean, though it may seem cold. This is why, in the past few decades, steel has been often combined with wood, which adds warmth to the raw look of steel. Wood makes a place look warmer and more welcoming, while steel has an air of industrial chic. Interior design often combines whitewashed Knauf walls and exposed steel elements, particularly columns.