Typical Materials
The primary material that is used to make steel framed structural systems is steel, however “steel” is a general term that is used to describe the many variations of the same metal. Steel is a purified alloy of iron, and comes in various grades based on its iron and carbon content, and forging method, among other things. Which grade, and how much of it is used for the structure of a building strongly depends on the conditions present on the site, and the magnitude of the loads that will be carried by the steel members.
Steel Members
Steel members typically come in the form of I-beams or tubes, but there is a variety of other forms that steel can take, especially in the use of connecting and supporting the larger members. I-beams are common because of their ability to take large amounts of load, shear, and moment despite the relatively small amount of material used to make them. The secret is in the “I” shape - the wide flanges at the ends in concert with the web in the middle are very good at taking bending moments and shear forces. The table below lists different examples of common steel alloys and their corresponding shapes.
(Source: http://www.aboutcivil.org/imajes/classification.gif)
Concrete
Also important in the construction of a steel frame system is concrete. Concrete is usually an essential part of the foundations due to its ability to take large amounts of compression. Like steel, concrete comes in a variety of grades dependent on the mixing properties of the concrete. It is important to choose the correct grade of concrete for the site conditions and project specifications because the various grades of concrete have different properties in order to deal with soil types and temperatures, and different load-bearing capacities. Since concrete is a very brittle material with little tolerance for tension or shear forces, it is usually also reinforced with lengths of steel rebar that help restore some of these properties to the material.