What Is Structural Steel?

Steel is everywhere in construction, to the beams holding roofs up to the sceletons of entire buildings, down to the very tools you use. Steel isn’t just one thing though—there are many different kinds of steel and different steel shapes, each with its own purpose and use. Learn more about the different kinds of structural steel how it’s used in construction.

What is structural steel?

Structural steel is metal used in construction materials. Fundamentally, it is defined as steel optimized for use in building construction—differentiated from a steel grade one might use to engineer tools, or stainless steel popularly used in kitchen surfaces and appliances.

Structural steel is generally a carbon steel, meaning it has a chemical composition containing both iron and carbon. Structural steel is any category of steel with a carbon content of up to 2.1% of its total weight. The higher the carbon content, the higher a piece of steel’s yield strength—meaning it is less ductile, or less likely to bend or warp when pressure is applied.

How does carbon content affect steel?

One might think steel used in construction should have a relatively high carbon content because it is less likely to bend under pressure. However, low-carbon steel, also known as “mild steel,” is actually the most commonly used type of steel in building materials. It typically contains anywhere from 0.04% to 0.3% carbon content, rendering it strong yet ductile. Steel buildings, especially tall steel buildings, must be strong, but also somewhat flexible. Overly rigid buildings cannot accommodate natural shiftings in the earth, which can be caused by neighboring construction or even earthquakes, nor can they bend with high winds. For this reason, mid- to high-carbon steels, with carbon content ranging from 0.31 to 1.5%, while technically considered structural steel, are generally reserved for mechanical engineering and toolmaking purposes.

Other grades of structural steel might contain other alloying elements, such as tungsten, zirconium, cobalt, or nickel.

What are the different steel grades?

Below are grades of steel commonly used in the construction industry:

  • Carbon steels. Structural steel is designated carbon steel when no other alloying element is added, copper content does not exceed 0.4 to 0.6%, manganese does not exceed 1.6%, and silicone does not exceed 0.6%. It is commonly used in structural pipe and tubing.
  • High-strength, low-alloy steels. This steel grade is designed to optimize mechanical properties and corrosion resistance. They contain manganese levels up to 2%. Trace amounts of chromium, nickel, molybdenum, nitrogen, vanadium, niobium, and titanium may be used to alter the properties. These are mainly used in structural shapes and steel plates.
  • Forged steels. Forging is the process of shaping metal while still in a solid state. This is done by applying force or heat to steel ingots or billets. The process produces a uniform grain structure to the steel, which upgrades integrity by removing voids and gas bubbles, and increases overall strength.
  • Quenched and tempered alloy steels. Quenching and tempering are processes that strengthen structural steel by heating it while simultaneously cooling in water, oil, forced air, or nitrogen. It creates a tougher, less brittle, higher-strength structural steel.

What are the different shapes of structural steel?

Below are types of structural steel sections commonly used in the construction industry:

  • Angled sections. Right-angled, L-shaped steel sections that come in equal or unequal lengths. These are usually used to support any kind of steel structure, from joining steel bars together to supporting a steel bedframe.
  • Tubular hollow sections. Circular, pipe-like steel sections with high torsional resistance, meaning they are less likely to twist under weight, and are therefore ideal for load-bearing.
  • Flat sections. Otherwise known as “plates,” flat sections can be attached to other sections to reinforce strength.
  • Parallel flange channels. Parallel flange channels are U-shaped steel sections with right-angled corners; these steel sections have a high strength-to-weight ratio, meaning they are often relatively light compared with the loads they can bear.
  • Rectangular hollow sections. Rectangular hollow sections are similar to tubular hollow sections, except they are rectangular in shape. They have high torsional resistance.
  • Square hollow sections. Square hollow sections are less commonly used than tubular or rectangular hollow sections, as their shape is not compatible for fusing with many other structural steel shapes. They are often used in the columns or posts of a structure, however.
  • Tapered flange beams. Also known as I-beams, resembling the letter “I” at its ends, tapered flange beams are typically used as cross-sections for girders. They have an overall high resistance ratio, but unlike variously shaped hollow sections, do not have high resistance to torsion.
  • Universal beams. Universal beams, known as H-beams for their resemblance to the capital letter, or wide flange beams, are similar in shape and function to tapered flange beams.
  • Universal columns. Universal columns resemble universal beams, but are primarily used for vertical building columns due to their superior load-bearing capabilities.

Pros and cons of building with steel

There are many various advantages of structural steel construction. 

  • Cheap and durable. Steel is cost-effective with significant longevity compared with other materials, and easy to produce in significant quantities. 
  • Strong. Steel also boasts a high strength-to-weight ratio, meaning it is lightweight compared with the amount of weight it can itself support. A 40 x 40 square centimeter carrier of structural steel can do the same job as a 100 x 100 square centimeter reinforced concrete carrier.
  • Ductile. Structural steel boasts good ductility. It has an elastic nature that allows it to bend and yield slightly, making sudden failure less likely. Its toughness, or ability to absorb energy, means structural steel can take a beating and carry heavier loads without sagging.
  • Versatile. Last but not least, structural steel is versatile; it can be modified, formed, and molded with relative ease.

But there are a few disadvantages of building with structural steel too. It is susceptible to corrosion, and making it resistant to such processes can be costly. It is also susceptible to fatigue and buckling, with measurable decreases in strength in higher temperatures.

MT Copeland offers video-based online classes that give you a foundation in construction fundamentals with real-world applications, like how the commercial construction industry works today. Classes include professionally produced videos taught by practicing craftspeople, and supplementary downloads like quizzes, blueprints, and other materials to help you master the skills.

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