I-beam

An I-beam, also known as H-beam (for universal column, UC), w-beam (for "wide flange"), universal beam (UB), rolled steel joist (RSJ), or double-T (especially in Polish, Bulgarian, Spanish, Italian and German), is a beam with an or -shaped cross-section. The horizontal elements of the are flanges, and the vertical element is the "web". I-beams are usually made of structural steel and are used in construction and civil engineering.
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 I-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.
The method of producing an I-beam, as rolled from a single piece of steel, was patented by Alphonse Halbou of the company "Forges de la Providence" in 1849.
Bethlehem Steel was a leading supplier of rolled structural steel of various cross-sections in American bridge and skyscraper work of the mid-twentieth century. Today, rolled cross-sections have been partially displaced in such work by fabricated cross-sections.
Overview.
There are two standard I-beam forms:
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 which are seldom now rolled in the UK. 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 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 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.
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:
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 formula_1 and the maximum expected bending moment is formula_2, then the required section modulus is given by
where formula_4 is the moment of inertia of the beam cross-section and formula_5 is the distance of the top of the beam from the neutral axis (see beam theory for more details).
For a beam of cross-sectional area formula_6 and height formula_7, the ideal cross-section would have half the area at a distance formula_8 above the cross-section and the other half at a distance formula_8 below the cross-section. For this cross-section
However, these ideal conditions can never be achieved because material is needed in the web for physical reasons, including to resist buckling. For wide-flange beams, the section modulus is approximately
which is superior to that achieved by rectangular beams and circular beams.
Issues.
Though I-beams are excellent for unidirectional bending in a plane parallel to the web, they do not perform as well in bidirectional bending. These beams also show little resistance to twisting and undergo sectional warping under torsional loading. For torsion dominated problems, box beams and other types of stiff sections are used in preference to the I-beam.
Shapes and materials (U.S.).
In the United States, the most commonly mentioned I-beam is the wide-flange (W) shape. These beams have flanges whose inside surfaces are parallel over most of their area. Other I-beams include American Standard (designated S) shapes, in which inner flange surfaces are not parallel, and H-piles (designated HP), which are typically used as pile foundations. Wide-flange shapes are available in grade ASTM A992, which has generally replaced the older ASTM grades A572 and A36. Ranges of yield strength:
Like most steel products, I-beams often contain some recycled content.
Standards.
The following standards define the shape and tolerances of I-beam steel sections:
AISC Manual.
The American Institute of Steel Construction (AISC) publishes the Steel Construction Manual for designing structures of various shapes. It documents the common approaches, Allowable Strength Design (ASD) and Load and Resistance Factor Design (LRFD), (starting with 13th ed.) to create such designs.
Cellular beams.
Cellular beams are the modern version of the traditional "castellated beam" which results in a beam approximately 40–60% deeper than its parent section. The exact finished depth, cell diameter and cell spacing are flexible. A cellular beam is up to 1.5 times stronger than its parent section and is therefore utilized to create efficient large span constructions.