I-beams (also known as W-beams or double-T esp. in Polish and German) are beams with an I- or H-shaped cross-section ("W" stands for wide flange). The horizontal elements are flanges, while the vertical element is the web. The Euler-Bernoulli beam equation shows that this is a very efficient form for carrying both bending and shear in the plane of the web. The cross-section has a reduced capacity in the transverse direction, and is also inefficient in carrying torsion, for which hollow structural sections are often preferred.
I-beams are commonly made of structural steel but may also be formed from aluminium or other materials. A common type of I-beam is the rolled steel joist (RSJ) - sometimes incorrectly rendered as "reinforced steel joist". British and European standards also specify Universal Beams (UBs) and Universal Columns (UCs). These sections have parallel flanges, as opposed to the varying thickness of RSJ flanges. UCs have equal or near-equal width and depth, while UBs are deeper.
I-beams engineered from wood with fiberboard and/or laminated veneer lumber are also becoming increasingly popular in construction, especially residential, as they are both lighter and less prone to warping than solid wooden joists. However there has been some concern as to their rapid loss of strength in a fire if unprotected.
Ranges of yield strength (where 1 ksi = 1,000 pounds per square inch):
Wide-flange shapes are produced by the electric arc furnace method and generally contain more than 95% recycled content.
The American Institute of Steel Construction ("AISC") publishes the "Steel Construction Manual" for designing structures of various shapes. It documents the common approaches, ASD and LRFD, (as of 13th ed.) to creating such designs.
In the United States, steel I-Beams are commonly specified using the depth and weight of the beam. For example, a "W10x22" beam is approximately 10 inches in depth (height when the I-Beam is standing on its flanges) and weighs approximately 22 pounds per linear foot.
In Canada, steel I-Beams are now commonly specified using the depth and weight of the beam in metric terms. For example, a "W250x33" beam is approximately 250 mm in depth (height when the I-Beam is standing on its flanges) and weighs approximately 33 kg/m (kilogrammes per linear metre)