Back in the early 19th century, engineers started to get serious about using wrought and cast iron for construction, but they had a huge problem to surmount: making wrought and cast iron in quantities large enough for the demands of construction was ruinously expensive. But thanks to the efforts of inventors and steel barons like Sir Henry Bessemer and Andrew Carnegie, steel is today one of the most affordable building products on the market, and it comes in a huge variety of different formulations, each developed for a specific application.
History
Prior to the 19th century, steel could only be produced in small batches at a high cost. Instead, cast iron and wrought iron were kings.
The casting of iron was a batch process, where a vat of iron was melted and poured into a mold, which became exponentially more expensive and complicated to manage as the size of parts increased. Casting iron generally produced cheaper components, but they were very brittle and had very low tensile strengths, much like concrete today.
Wrought iron required skilled laborers to work long hours, forging out red-hot iron blanks, folding them over, on themselves, and repeating the process many times. Wrought iron was expensive and tricky to make in large quantities, but it was pressed into service in many early structures.
Then along came the great inventors and steel barons of the 19th century. Sir Henry Bessemer and other inventors worked out ways of creating industrial-scale steel production processes, and companies run by Andrew Carnegie and his competitors put them to work, eagerly adapting the methods to pump out steel rails for the booming rail industry.
As time went on, industrialists and metallurgists learned more about the various elements present in different iron ore sources, and over decades of research and refinement, took steel manufacturing from a superstitious art form to the modern scientific process it is today.
Chemists and automated machinery now carefully monitor and control the levels of carbon and various alloying elements present at every step of the process. Mining operations also work to characterize their ores and sell them off to the most-appropriate processors.
Today, scores of different steel alloys exist, specialized for various applications, which are classified by agencies around the world. In the United States, the American Society for Testing and Materials is the most common specifying agency.
Types of Steel Used in the Construction of Bridges and Buildings
Structural engineers rely on steel for a wide variety of components within buildings, ranging from fasteners and connection plates up to full-sized beams, columns, and sometimes even huge cables on projects like the Golden Gate Bridge.
Mild Steel
Most of the steel used in structures is what’s known as mild steel, which has a low carbon content, high ductility, and is not very vulnerable to unintentional heat treatment from welding.
Among the most common building materials in the world, many foundries specialize only in the hot-rolling of mild steel structural shapes, making them extremely affordable compared with other types of steel.
Mild steel is used for steel beams and columns, many connector plates, nails (not screws), concrete reinforcement bars or rebar, and many other applications.
A critical feature in most structures is how much stretch or ductility the steel components have, which allows them to safely take loads much larger than those anticipated for extreme events, like a bomb blast or earthquake. Mild steel’s ductility can also help building occupants realize something is wrong if a building is overloaded because these members tend to bow, stretch, and twist in large, predictable ways, which can give an indication of problems before a collapse.
According to the American Institute of Steel Construction’s “Sizes and Grades” webpage, W-shapes are usually made from ASTM A992 steel, HSS shapes from ASTM A500 steel, and all other hot-rolled structural shapes are typically made from ASTM A36 steel.
Rebar is a less well-controlled product, with less-stringent standards. It can be produced very economically as a result, and has a proven safety record, but it can sometimes be vulnerable to becoming brittle if welded excessively.
Alloy Steel
For some applications, the basic strength of ordinary mild steel isn’t enough. In these cases, where special wear-resistance, hardness, or higher-strength demands a more complex material, other elements are added into the mix, producing what’s known as Alloy Steel.
Alloy steel is technically a bit of a misnomer, as steel is, by definition, an alloy of iron and carbon, but what’s meant here is that it’s a specialized alloy, something a little more exotic than the ordinary run-of-the-mill product.
An early example of alloy steel usage was Henry Ford’s use of vanadium steel to gain more strength on critical components of the Model T.
High-tensile cables in prestressed and post-tensioned concrete structures, and those used in suspension bridges and other cable-stay structures are some of the most common applications of these steels to large structural components.
Almost all threaded fasteners are formed from alloy steel as well, since any fine threading tends to deform rapidly with repeated use on milder steels.
Additionally, where specialized protection is required, the higher strength or wear resistance of alloy steels comes into play. Tool-resistant alloy steels, like AR500, are often used for ballistic resistance where thinner plates are required and the full thickness of mild steel required to stop a bullet wouldn’t fit.
Stainless Steel
When you add chromium and other particular alloying elements to steel, it helps to mitigate the single biggest enemy steel structures and tools face: rust.
Stainless Steel is used anywhere high corrosion resistance is required. Critical fasteners, components used in food processing or some industrial process tanks, and much of the facade of structures demand this higher resistance to rust, and this is where stainless steel shines. Most maritime structures demand it as well.
Though often not as strong as other types of steel, and often more than five times as expensive, there are many applications for which stainless steel is the only kind of steel that will suffice.
Summary
Today, more than ever in the past, steel is one of the most varied and affordable materials available.
Different applications require different formulations of steel, and specialized alloys do tend to be more costly, but they’re still often much cheaper than the more exotic metals that would otherwise be required.
If you’re interested in learning more about the design of steel structures, let me know in the comments below, or send me an email with whatever topics you’d like to learn more about. There will be many more articles, videos, and probably some PDH courses to come on these topics.
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