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I-beams are beams with an I- or H-shaped cross-section. The Euler-Bernoulli beam equation shows that this is a very efficient form for carrying bending and in the plane of the web, as well as shear. It is not as capable in the transverse direction, and it is a poor choice for carrying torsion. Hollow structural section beams are usually used to carry 3D loading. The I-beam has very little margin for plastic bending.
There are two standard I-beam forms:
- I-beam, formed by rolling or extrusion (depending on material).
- Wide flange or H-beam, formed by welding plates.
I-beams are commonly made of steel and may also be known as Rolled steel joists or (RSJ) - sometimes incorrectly rendered as "reinforced steel joist". British or 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 an equal, or near-equal, width and depth while UBs are deeper.
They 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. In horizontal applications, the minimum suitable size is usually determined by maximum permissible deflection rather than by ultimate failure capacity - a sagging floor is unsightly.
I-beams engineered from glued laminated timber are also becoming increasingly popular in construction 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.
Extruded plastic I-beams are occasionally used in modelling.
Wide-flange steel materials and rolling processes (U.S.)
In the United States the most commonly referred to I-Beam is primarily now a wide-flange shape. While maintaining the general I-shape, these beams have wider flanges more appropriate to modern construction techniques. Wide-flange shapes are available primarily in grade ASTM A992 which has generally replaced older grades ASTM A572 and A36.
Minimum yield strength:
- A36 - 36 ksi
- A572 - 42 ksi to 60 ksi (50 ksi most common)
- A992 - 50 ksi to 65 ksi
Wide-flange shapes are produced by the electric arc furnace method and will generally contain more than 99% recycled content.more on recycled content
AISC, or American Institute of Steel Construction, produces a "Steel Construction Manual", that is used to design structures using various steel shapes and documents the common methodologies, ASD and LRFD, (as of 13th ed.) for designing steel structures.
See also
External links
- A list of British Standard sections provided by the Corus Group
- Wood I-joists
"I-beam"