Structural Steel 101: Shapes, Properties, and Applications

For reliable longevity and to bear extreme weight, structural steel must be of the right composition. Iron and carbon are two of the most vital components used by steel mills when making structural steel. The carbon lends strength to the iron ore, which is the source for the iron in steel and is quite soft on its own. To achieve load-bearing capacity, structural steel must have a higher carbon content by weight, and manufacturers can increase the amount of carbon according to the level of strength and ductility its application requires. Most construction purposes only have the need for low-carbon, or mild, structural steel, which contains between 0.04 and 0.30% carbon by weight. Medium- and high-carbon structural steel requires from 0.31 to 1.50% carbon by weight, making this steel suitable for mechanical engineering applications.

Structural steel can also contain levels of manganese, phosphorus, sulfur, and silicone, among other materials. While manufacturers can add additional metals such as chromium, titanium, and molybdenum to their steel compositions to achieve greater strength, this is typically best for non-structural steel as it can result in a brittle end-product.

Whatever the composition, manufacturers must test their structural steel for acceptable yield and tensile strengths. Part of what makes structural steel strong is its ability to yield under weight pressure without permanently changing shape. The point at which structural steel does irrevocably change shape is called its yield strength. Additional weight pressure will eventually bring the steel to its tensile strength limit, the point at which the steel actually breaks. Yield and tensile strength are measured in pounds per square inch (psi) and kilopounds per square inch (ksi).

For evaluating impact or energy absorption within structural steel, the Charpy impact test has standardized the process. Operators utilize a weighty hammer pendulum and a structural steel material sample to calculate how much energy that particular steel can absorb when the pendulum strikes it before the material reaches yield and tensile strength limits. The Charpy test can also incorporate temperature testing to mimic environmental temperature fluctuations.

Given the critical nature of construction applications, the American Society for Testing and Materials (ASTM) has over 12,000 regulatory standards categorizing steel grades and governing the material and its tolerances. These regulations provide a consistent standard across industries and guarantee that steel meeting ASTM requirements will be of the highest quality. Some of the approved structural steel grades include ASTM A36, ASTM A500, ASTM A572, and ASTM A588, with multiple shape options based on the application.