I-beams (also known as double-T esp. in Polish and German) are beams with an I- or H-shaped cross-section. 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. There are two standard I-beam forms:
- Rolled I-beam, formed by rolling or extrusion (depending on 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, 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.
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Design of I-beams
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 minimise 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
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 (W) shape. While maintaining the general I-shape, these beams have flanges in which both flange surfaces are nearly parallel. Other I-Beams include American Standard (designated S) shapes, in which flange surfaces are not parallel and H-piles (designated HP) which are typically used as pile foundations. Wide-flange shapes are available primarily in grade ASTM A992[1] which has generally replaced older grades ASTM A572 and A36.
Ranges of yield strength (where 1 ksi = 1,000 pounds per square inch):
- A36 - 36 ksi
- A572 - 42 ksi to 60 ksi (50 ksi most common)
- A588 - Similar to A572
- A992 - 50 ksi to 65 ksi
Wide-flange shapes are produced by the electric arc furnace method and will generally contain more than 95% recycled content.[2] 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.
Steel I-Beam Designation and Terminology
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.
See also
External links and references
- A list of British Standard sections provided by the Corus Group
- Wood I-joists
