This isn’t meant to be a treatise on steel technology, and it wouldn’t even be presumed to be, but a few notes are essential to understanding the ancient roots of this material that has accompanied man throughout his evolution, and which is only recently finding widespread applications also for the construction of residential units and complexes (science fiction until a few years ago).
Historical notes on steel constructions
Steel is a derivative of iron, and is therefore the name given to ferrous or steel-based material obtained by smelting in a blast furnace and then further processing. Iron has gradually and consistently accompanied human evolution.
Suffice it to say that even the ages that temporarily mark prehistory are named after the metals that humans had learned to use. From the Stone Age (3 million years ago to 5000 BC), to the Copper Age (to 3000 BC), to the Bronze Age (to 750 BC), to the Iron Age… which accompanied the advent of the great civilizations of the past, with profound social changes largely dependent on divergent agricultural practices, made possible by the availability of tools and implements, once again made of iron.
Ferrous materials are essentially alloys composed almost exclusively of iron and carbon (in small percentages). Special steels are composed of alloys containing other elements that give the steel different characteristics.
Depending on the alloy and carbon content, they are classified as:
1 – Cast iron (carbon content between 2.06 and 4%)
2 – Steel (carbon content < 2.06%)
2.1 – Carbon steels (common steels)
2.1.1 – Structural steels (Fe 360, Fe 430, Fe 510)
2.1.2 – Low-alloy steels (containing alloy elements less than 5%) (Corten steel, Ex-ten steel)
2.1.3 – Alloy steels or special steels (containing alloy elements greater than 5%) (stainless steel)
3 – Ductile iron (carbon content < 0.1%)
The cast irons
Molten iron is obtained from iron ores, which occur naturally in the form of magnetite (Fe3O4) and hematite (Fe3O4). The ground ore must be heated to high temperatures, and originally, carbocoke (coal and limestone) was used. In this type of process, a chemical reaction occurs, producing carbon monoxide (CO), while the limestone binds with impurities, forming slag that floats on the molten metal and can therefore be disposed of.
However, the underlying property of iron production means that the molten material tends to absorb carbon. The resulting material is called cast iron (when the carbon content is greater than 2.06%). This material is extremely hard and resistant when used in compression, but brittle when used in tension due to cracks between the grains of the crystalline structure, and therefore limited its application in the construction sector. The first examples of cast iron being used as a construction material occurred in England.
Crystal Palace. Made of cast iron for the Universal Exhibition of Works of Industry and Nations in London in 1851. Note the use of the arched structure and the close positioning of the vertical uprights which tends to eliminate all tensile (bending) stresses.
Common steel
Steel begins to be considered when its carbon content drops below 2.06%.
Its application to the construction industry is relatively recent.
Steel was widely used in the construction sector between the two industrial revolutions, thanks to the discovery of mass production methods that eliminated slag from iron and the intensification of coal mining.
In 1856, Henry Bessemer invented the Bessemer converter, which reduced the carbon content in molten iron through a process in which air (oxygen) was blown into the converter. This process combined with the carbon present in the molten pig iron poured into it, thus reducing the carbon content through the generation of carbon dioxide (CO2) as a byproduct (dispersed into the atmosphere).
The reduction of carbon allows pig iron to transform into steel.
From the simple production of nails, tie rods, and brackets, its scope has expanded to include the construction of bridges, industrial structures, railways, construction tools and equipment, shipbuilding, and land and air vehicles.
Most ferrous products used in construction are made from steels with varying carbon percentages (or contents). Depending on the processing they undergo, steels can be divided into two main categories:
Construction or carpentry steels, produced in the form of profiles, bars, or sheets
Steels for reinforced concrete
This material has overcome the construction limitations of the past (and even the concepts of traditional proportions, since the only limit was the technical limit), allowing for new shapes and dimensions to create large-scale buildings and artifacts, right up to the advent of skyscrapers.
The Eiffel Tower (by Gustave Eiffel), completed in 1889 for the Paris International Exposition, was designed as a temporary structure, but has become a symbol of Paris and France.
It was the Chicago School that first began to systematically use steel in construction.
The first skyscraper, 10 stories above ground, dates back to 1885 and was built in Chicago by architect William Le Baron Jenney, who introduced the use of a metal frame.
With the innovation of electric furnaces (early 1900s), the environmental impact of the steel industry, which in turn is responsible for the emission of a significant portion of greenhouse gases, was significantly reduced.
The progressive use of scrap as a raw material has helped make its use completely sustainable.
Characteristics of steel
The chemical composition and crystalline structure influence the mechanical properties of steel. Steel’s structure is compact and homogeneous in all directions, making it similar to an isotropic body. This means the material will have the same performance characteristics in all directions (including tensile stress).
Steel also features:
- Hardness, or resistance to surface impacts
- Ductility and therefore workable, although capable of deforming before breaking
- Corrosion resistance
- Good thermal conductivity
Steel manufacturing processes
All steel products are made from intermediate or semi-finished products known as slabs (if rectangular in cross-section) or billets (if square in cross-section).
The most common processes are:
- Rolling (hot, cold)
- Wire drawing (reinforced concrete bars are obtained)
- The extrusion
- The molding
- The bending
- The forging
- The profiling
- Galvanizing
We will focus on just a few of these processes to stay within the scope of steel applications (excluding reinforced concrete) in the construction industry.
The steel rolling process
It is one of the most widely used processes for producing building components. It involves passing the raw material through a series of pairs of rollers, positioned at increasingly closer intervals and rotating in opposite directions (rolling mills), which reduce the thickness and consequently increase the length and width.
Rolling can be done hot or cold.
Hot rolling of steel
Hot-rolled steel profiles are produced by heating billets to temperatures above the material’s recrystallization point (between 1000 and 1300°C), where the metal is still solid but can be shaped through repeated passes in rolling mills that progressively reduce its thickness until the required shape is achieved. The profiles available on the market are:
- H-sections (HEA, HEB, HEM),
- T-sections
- Double-T (IPE),
- Angle sections
- U-sections (UPN).
Cold rolling of steel
In cold rolling, the material (slabs) undergoes significant work hardening due to the deformation and crushing of the crystalline grains by the mechanical action of the rollers. Cold rolling produces products with improved mechanical properties and dimensional accuracy.
The main product is sheet metal.
- sheet metal (with thickness greater than 3 mm)>
- the sheet metal (with a thickness of less than 3 mm)
The sheet metal is suitable for the production of coils (sheet metal collected in rolls)
Galvanizing steel
Zinc has the crucial property of offering high resistance to atmospheric agents. It is used to coat steel sheets.
This process is called galvanizing and can be performed using various techniques, including:
- Hot-dip galvanizing is immersion (Sendzimir process) in a zinc bath at 450°C, where the metal surface intimately blends with the zinc, producing basic zinc carbonate. It can be applied to continuous strips and sheets from coils, ensuring large-scale production.
- Cold galvanizing with high zinc content paints.
- Electrolytic galvanizing, characterized by very small thicknesses and great surface uniformity.
- Spray galvanizing, which replaces hot-dip galvanizing in products not susceptible to immersion treatment
Steel bending
It is based on the use of punches and dies of various shapes, which allow steel profiles to be bent at the edges. This process is particularly used for sheet metal and for the creation of cold-formed profiles, used for cold steel frames or light steel frames (CFS – LFS).
Thin sheet metal is cold-worked through linear bending processes; when properly shaped, it acquires unexpected mechanical resistance to bending or twisting stresses, making it suitable for use as a semi-finished product known as a coil.
In this process, the metal undergoes permanent plastic deformation without being cut. Presses can be manual (with direct control of the bending by the operator) but are generally computer numerically controlled (CNC)
to produce the structural component, i.e., lightweight profiles, from a single element (the thickness of which is the thickness of the sheet metal). The final profiles are C-sections that can be combined to form an H-section. The sheet metal thickness ranges from 10/10 of a millimeter to 30/10. The material is extremely lightweight and is used to create frames that stand out for their extreme lightness, ease of assembly, and modularity.
Application of light steel frames to steel houses
Ultimately, as described, a steel house can be structurally composed of:
- Hot rolled profiles
- Cold rolled profiles
- Mixed construction systems
Remembering that in the hot rolled profiles I will have:
- Some components are produced by the carpentry shop according to the project.
- Some components include welding (additional processing).
- One galvanizing step.
- Transport to the construction site.
- Assembly.
In cold rolled profiles:
- We start with a pre-galvanized semi-finished product (sheet metal).
- Production of profiles based on the project using a CNC press brake.
- Joints and pre-assembly of the walls directly in the factory (by partitions or sections), which become components of the house that can be transported by road.
- Direct assembly on site using bolts or self-drilling screws (much faster).
It’s pretty intuitive that when I implement light steel frame processes, I see a significant cost reduction. In terms of costs, light steel frame wins over hot-rolled steel.