An overview of Structural Steel

In today’s world, we see and use steel structures everywhere. The existence of the railways we use every day from home to work, or one of the world’s most visited tourist attraction point Eiffel Tower, is made possible by the use of steel as a structural material. Thanks to its unique properties, all types of steel form a basis for our modern society. But what exactly is the structural steel? What is the difference between the mild and structural steel? Why and how do people use it? All of these questions and more will be thoroughly explained in this post.

What is structural steel?

Steel, one of the most essential materials of the modern world, comes in various different grades and shapes. It is simply an alloy of iron and very low amount of carbon (up to 2.1% in weight). Versatile nature of this material enable designers and engineers to use steel in many different kinds of areas. From automotive industry to construction of structures, steel is widely used. Structural steel is a type of steel which is used as a construction material. They are designed to have good strength/weight ratio (which is also called specific strength) and to be cost effective in order to be benefited as a structural component in buildings, roads, bridges etc.

A brief history…

The very first introduction of a metal as a structural material was done in England, Shropshire in 1779. Construction of Coalbrookdale Arch Bridge over River Severn was done by using cast iron (having a carbon content higher than 2.1%). It was a material considered as four times stronger than stone and thirty times stronger than wood (1). These kind of cast iron structures have been continuously constructed in following years until 1840s, when malleable wrought iron (carbon content less than 0.15%) began to replace heavy cast iron. After that, different grades of steel with more suitable and superior properties for structural applications have been continuously developed until today. Technological advancements such as the development of Bessemer process and such made steel production easier and more profitable (2). Therefore, usage of structural steel led to huge developments in architecture and civil engineering in the past century.

Types and grades

There is not just one type of structural steel. There exists various different shapes and grades, depending on the needs for that specific application. Structural steels are classified by the shape of their cross sections, such as most frequently used I, T, C shapes (2). Besides their shape, grade of the steel directly affects the mechanical properties. So, different grades of structural steel must be chosen according to different design requirements.

Several types of steel can be shaped and used as beam, rod, plate, bar or profile. Here are the standard structural steel materials:

Carbon Steels:
Steel can be defined as carbon steel when the addition of any other alloying element (such as tungsten, zirconium, cobalt, nickel etc.) is not required, copper content does not exceed 0.4% or following elements does not exceed indicated percentages in weight (Mn: 1.6%, Si: 0.6%, Cu: 0.6%) (3). Carbon steels are generally categorized by their carbon content as low-carbon (< 0.3%), medium-carbon (0.3-0.6%), high carbon (0.6-1%) and ultrahigh carbon (1.25-2%) steels. It is mostly used in structural pipe and tubing.

High Strength Low Alloy Steels:
These kind of steels are designed to have better mechanical properties and be more resistant against atmospheric corrosion than carbon steels. They contain manganese up to 2.0%. Small portions of other alloying elements such as chromium, nickel, molybdenum, nitrogen, vanadium, niobium and titanium can be used in different combinations to alter the properties (3). Weathering steels which is a sub-type of high strength low alloy steels, have high resistance against atmospheric corrosion by forming a passive, rust-like layer on the surface, being one of the important structural steels. Mostly used in structural shapes and plates.

Forged Steels:
Forging is simply the process of shaping the metal while it is in the solid state. It is done by applying mechanical and thermal energy to steel ingots or billets. This process introduces a uniform grain structure to the material, which decreases discontinuity in the matrix by removing voids, gas bubbles and increases overall strength (4).
Quenched and Tempered Alloy Steels:
A type of structural steel (for example A514) mostly used in building constructions. As it can be understood from its name, this type of steel has undergone quenching and tempering heat treatment steps.

Why use steel as a structural material? What are the advantages?

Higher strength/weight ratio:
When compared, steel dominates every other conventional structural material such as stone, cement or wood in terms of strength/weight ratio. It means that the tolerance against poor foundations is higher.
Good ductility:
Ductility is the ability of the material to withstand loads without failure. Thanks to the elastic nature of steel, it can return to its original shape after bending. Yielding up to some point prevents premature failures. Hard and brittle materials can fail suddenly, thus are not favourable.
High Toughness:
Ability of a material to absorb energy is called toughness. Structural steels they have high toughness values; thus they are very suitable for construction applications. They are both strong and ductile. Also it should be mentioned that the main difference between mild and structural steel is that structural steels are stiffer in order to carry higher loads without needless sagging.
Architectural variety:
Steel structures make so various different architectural designs possible. All around the world, astonishing steel buildings, towers and bridges can be witnessed. One would be surprised to find out that this material was not economically reasonable to use as a structural component, just one hundred years ago.
Saving space:
In a comparison with reinforced concrete, a 40×40 cm2 steel carrier can do the same job as a 100×100 cm2 reinforced concrete carrier. This example shows that a net saving in terms of usable area can be done (5).

There exist some drawbacks which can be solved with additional costs like corrosion and fire susceptibility, or risk of buckling under heavy loads. Yet, major advantages may still outweigh these drawbacks in particular design applications.

How it is made?

Steelmaking is a rather long and complicated process to explain all in details. So, for now, we will exclude all the initial steps of the process. After hot steel is produced, it can be casted either directly into the desired shape or as slab. Slabs can be formed into different shapes by cold working or forging. Heat treatment can be also done, depending on the application. After fabrication, chemical and mechanical tests should be performed according to standards. Without proper quality testing, catastrophic accidents may occur due to flaws in material.

How to choose your manufacturer?

It can be troubling to decide on a such important matter. A wrong decision in the manufacturer can be costly. Our advices given below can be useful when choosing your manufacturer (6) (7).
Know what you are looking for.
Determine your priorities and expectations from your manufacturer before the interview with the candidates.
Take your time.
There is no point in rushing into a relationship with a manufacturer. Test multiple companies if necessary and choose the best one for you.
Ask for background.
A company with a good history is less likely to let you down, best to keep that in mind.
Think at the margin.
Things may not go so bright for you all the time. In the days like these, the amount of your orders would fall. Therefore, it is important to negotiate and come to terms on defining minimums of order before establishing a collaboration.

(1) M.H. Sawyer. World’s First Iron Bridge. Civil Engineering (New York: ASCE, December, 1979) pp.46-49
(2) McCormac, J. C. (1971). Structural steel design. Intext Educational Publishers.
(3) “Classification of Carbon and Low Alloy Steels” Retrieved in 15-07-2019 from https://www.totalmateria.com/page.aspx?ID=CheckArticle&LN=TR&site=kts&NM=62
(4) “How does forging strengthen metal?” Retrieved in 15-07-2019 from http://www.dropforging.net/how-does-forging-strengthen-metal.html
(5) ”What is structural steel?” Retrieved in 16-07-2019 from
https://apec.com.tr/en/what-is-structural-steel/
(6) “How to find the right manufacturer for your products?” Retrieved in 16-07-2019 from
https://www.sourcify.com/how-to-find-the-right-manufacturer-for-your-products/
(7) ”How to find a factory to manufacture your product?” Retrieved in 16-07-2019 from
https://www.businessnewsdaily.com/8820-how-to-find-factory.html

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