An I-beam is any of various structural members with an Ɪ- (serif capital letter 'I') or H-shaped cross-section. Technical terms for similar items include H-beam, I-profile, universal column (UC), w-beam (for wide flange), universal beam (UB), rolled steel joist (RSJ), and double-T (especially in Polish, Bulgarian, Spanish, Italian, and German). I-beams are typically made of structural steel and serve a wide variety of construction uses. The horizontal elements of the Ɪ are called flanges, and the vertical element is known as the web. The web resists shear forces, while the flanges resist most of the bending moment experienced by the beam. The Euler–Bernoulli beam equation shows that the Ɪ-shaped section is a very efficient form for carrying both bending and shear loads in the plane of the web. On the other hand, 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.
History
In 1849, the method of producing an I-beam, as rolled from a single piece of wrought iron, was patented by Alphonse Halbou of Forges de la Providence in Marchienne-au-Pont, Belgium. Bethlehem Steel, headquartered in Bethlehem, Pennsylvania, was a leading supplier of rolled structural steel of various cross-sections in American bridge and skyscraper work of the mid-20th century. Rolled cross-sections now have been partially displaced in such work by fabricated cross-sections.
Overview
There are two standard I-beam forms:
Rolled I-beam, formed by hot rolling, cold rolling or extrusion, depending on the material. Plate girder, formed by welding (or occasionally bolting or riveting) plates. 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, shown as "W-Section" in the accompanying illustration, as opposed to the varying thickness of RSJ flanges, illustrated as "S-Section", which are seldom now rolled in the United Kingdom. Parallel flanges are easier to connect to and do away with the need for tapering washers. UCs have equal or near-equal width and depth and are more suited to being oriented vertically to carry axial load, such as in columns in multi-storey construction, while UBs are significantly deeper than they are wide are more suited to carrying bending load, such as beam elements in floors. I-joists, I-beams engineered from wood with fiberboard or laminated veneer lumber, or both, 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.
Design
I-beams are widely used in the construction industry and are available in a variety of standard sizes. Tables are available to allow easy selection of a suitable steel I-beam size for a given applied load. I-beams may be used both as beams and as columns. I-beams may be used both on their own, or acting compositely with another material, typically concrete. Design may be governed by any of the following criteria:
deflection: the stiffness of the I-beam will be chosen to minimize deformation vibration: the stiffness and mass are chosen to prevent unacceptable vibrations, particularly in settings sensitive to vibrations, such as offices and libraries bending failure by yielding: where the stress in the cross section exceeds the yield stress bending failure by lateral torsional buckling: where a flange in compression tends to buckle sideways or the entire cross-section buckles torsionally bending failure by local buckling: where the flange or web is so slender as to buckle locally local yield: caused by concentrated loads, such as at the beam's point of support shear failure: where the web fails. Slender webs will fail by buckling, rippling in a phenomenon termed tension field action, but shear failure is also resisted by the stiffness of the flanges buckling or yielding of components: for example, of stiffeners used to provide stability to the I-beam's web.
Design for bending
A beam under bending sees high stresses along the axial fibers that are farthest from the neutral axis. To prevent failure, most of the material in the beam must be located in these regions. Comparatively little material is needed in the area close to the neutral axis. This observation is the basis of the I-beam cross-section; the neutral axis runs along the center of the web, which can be relatively thin, and most of the material can be concentrated in the flanges. The ideal beam is the one with the least cross-sectional area (and hence requiring the least material) needed to achieve a given section modulus. Since the section modulus depends on the value of the moment of inertia, an efficient beam must have most of its material located as far from the neutral axis as possible. The farther a given amount of material is from the neutral axis, the larger is the section modulus and hence a larger bending moment can be resisted. When designing a symmetric I-beam to resist stresses due to bending, the usual starting point is the required section modulus. If the allowable stress is σmax and the maximum expected bending moment is Mmax, then the required section modulus is given by:
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