The Properties of Steel I-Beams
Taking Advantage of the Strength of I-Beam Construction
An I-beam is a steel beam with a cross section shaped like the letter “I”. The horizontal portions of the I-beam are the flanges, while the vertical portion is the web. This simple configuration results in a significant increase in structural strength over a solid square beam of the same cross sectional area. While even railroad track is technically an I-beam, standard structural I-beams are symmetrical in both planes and have parallel flange surfaces.
I-beams are excellent for carrying bending and shear in the plane of the web, and less so in the plane of the flanges. I-beams are not good carriers of torsion, and applications that require resistance to torsion are better off using a hollow beam shape than an I-beam.
I-Beam Production
Steel I-beams are generally manufactured using one of these two methods:
- Rolled – The beam is hot rolled, cold rolled, or extruded, resulting in a one-piece beam. This method results in a stronger beam, but may not be feasible for custom beam designs or large beam sizes.
- Plate Girder – The beam is constructed from multiple plates. The plates are connected either by welding, riveting, or bolting.
The type of I-beam construction will be chosen based on the properties required for the specific application.
I-Beam Sizing
The most common type of I-beam in the United States used in structural construction is the wide flange beam. Wide flange beams have size designations that start with the letter W, and reference the height of the beam and the linear weight of the beam. For example, a beam designated as W12X120 means that this wide flange I-beam has a height of 12 inches and a weight of 120 pounds per linear foot. Canadian and European manufacturers use this same convention, but use the metric units of millimeters and kilograms.
The yield strength of an I-beam depends on the specific steel alloy used in its construction. One of the common alloys used in wide flange steel I-beams, ASTM 992, has a yield strength between 50 and 65 ksi (1 ksi = 1000 pounds per square inch).
I-Beam Usage
Construction is by far the largest consumer of I-beams. This includes building construction, including residential, commercial, and industrial buildings, as well as highway and bridge construction. I-beams are sized depending on the requirements for loading, deflection and vibration. Flatbed truck beds are often constructed from pre-stressed steel I-beams. When the truck bed is fully loaded, the I-beam is forced into its unstressed shape, resulting in far less stress and fatigue over the life of the trailer.
Despite its deceptively simple shape, the steel I-beam provides the strength performance that heavy-duty applications require. For more information on the characteristics of steel I-beams, please read Anatomy of a Steel I-Beam.
Sources
Engineers Edge Beam Properties
Structural I-beam in residential construction.
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